ALLOYS USED IN DENTISTRY

ALLOYS USED IN DENTISTRY

INTRODUCTION

          Metals and alloys play an important role in dentistry.  These form one of the four possible groups of materials used in dentistry which include ceramics, composites and polymers.  These are used in almost all the aspects of dentistry including the dental laboratory, direct and indirect dental restorations and instruments used to prepare and manipulate teeth.  Although the latest trend is towards the “metal free” dentistry, the metals remain the only clinically proven material for long term dental applications.

WORK ORIGIN / MEANING

Metal         :         Latin– metallum = mine

Wrought    :         Old English – worhte = to work beaten to shape

Eutectic      :         gr-eu=well, tectos = to melt, easily melted

Anneal       :         Old English-aelan = to burn, heat

Grain         :         Latin – granum = seed

Alloy                   :         Latin – alligere = to bind

Dendrite    :         gr-dendron=tree

Element     :         Latin – elementum = a first principle, a substance

that can not be resolved by chemical means into simpler substances

Crystal       :         Any substance having regular shape and flat

Surfaces

Lattice        :         fr-lattis=lath=bar=network of crossed bars

Space lattice = a geometrically regular, 3 dimensional arrangement of atoms in a space as it exits in a crystalline material and studied in x-rays

Ingot  =  piece of cast metal sent to the work shop for rolling etc.

Base metal =  metal which is easily oxidized when heated in air e.g. copper, lead, iron, zinc.

METALS :

Chemical elements in general can be classified as   1.  Metals  

2.     Non-metals

3.  Metalloids

          Metalloids are those elements on the border line showing both metallic and non metallic properties, e.g. carbon and silica.  They do not form free positive ions but their conductive and electronic properties make them important.

          Metals constitute about 2/3^rd of the periodic table published by DMITRI MEDELEYEV in 1868. Of the 103 elements which are categorized in the periodic table according to the chemical properties, 81 are metals.

          According to the metals hand book, they can be defined as “AN OPAQUE LUSTROUS CHEMICAL SUBSTANCE, THAT IS A GOOD CONDUCTOR OF HEAT AND ELECTRICITY AND WHEN POLISHED IS A GOOD REFLECTOR OF LIGHT”

HISTORY OF METALS

          Metals have been used by man ever since he first discovered them.  In ancient and pre-historic times, only a few metals were known and accordingly these periods were called as “COPPER AGE”, “BRONZE AGE” and “IRON AGE”.  Today more than 80 metallic elements and a large number of alloys have been developed.  Ore is a mineral containing one or more metals in a free or combined state.

PROPERTIES OF METALS :

          All metals are solids except for mercury and gallium which are liquid at room temperature and hydrogen which is a gas.  The properties of metals can be listed out as follows :

1.     They have a metallic luster and mirror like surface

2.     They make a metallic sound when struck

3.     Are hard, strong and dense

4.     Ductile and malleable

5.     Conduct heat and electricity

6.     Have specific melting and boiling points

7.     Form positive ions in solution and get deposited at the cathode during electrolysis.  E.g.  copper in copper plating.

The outer most electrons of the atom are known as valence electrons.  These are readily given up and are responsible for most of the properties.

Metals are tough and this is due to the fact that the atoms of the metals are held together by means of metallic bonds.

The chemical properties of metals are based upon the electromotive series which is a table of metals arranged in decreasing order of their tendency to lose electrons.  The higher an element is in the series, the more metallic it is.  This tendency of metals of lose electrons is known as oxidation potential.

CLASSIFICATION OF METALS :

          They can be done in many ways like :

1.       Pure metal and mixture of metals (alloys)

2.       Noble metals and base metals :

           Noble metal is one whose compounds are decomposable by heat alone, at a temperature not exceeding that of redness.  E.g. Au, Ag, and Pd.

Base metal is one whose compounds with oxygen are not decomposable by heat Alone, retaining oxygen at high temperature. E.g. Zn, Fe, and Al

3.       Case metal and wrought metal

          Cast metal is any metal that is melted and poured into the mould

Wrought metal is a cast metal which has been worked upon in cold condition

4.       Light metal e.g. Al and heavy metal e.g. Fe

5.       High melting metal e.g. chromium and low melting metal e.g. tin

6.       Highly malleable and ductile metal e.g. gold and silver

INTER ATOMIC BONDS :

          The atoms are held together in place by atomic bonds or forces.  They may be

1.     Primary

2.     Secondary

Primary bonds or inter atomic bonds :

These are very strong bonds and may be of either type :

a.  Ionic                -        These are seen in ceramics

b.  Covalent        -        They are seen in organic compounds

c.  metallic bonds -       They are seen in metals and are non

directional

Secondary bonds or inter molecular bonds :

These are weak forces and are otherwise known as Vander waal’s forces.  The various types are :

a.       Hydrogen bonds

b.       Dipole bonds

c.        Dispersion bonds

Of all these, the most important one is the metallic bond which was explained for the first time by LORENTZ, a Dutch scientist in 1916.  It can be explained by using the atomic and sub atomic structures.

The sub – atomic structures

1.     Protons – positive charge

2.     Neutrons – neutral charge

3.     Electrons negative charge

The center or the nucleus of an atom consists of proton and neutrons and are therefore positively charged.  This is balanced by the revolving electrons which are negatively charged and arranged in concentric shells with progressively increasing energy.  The electrons in the outer most shell are known as VALENCE ELECTRONS.

These are loosely bound and are therefore readily given up by the atom to form positive ions.  The cations thus formed behave like hard spheres and the electron cloud formed by the freed valence electrons roam about freely in the interstices formed by the arrangement of the solid spheres.  The electrons act like glue to hold all atoms together and are known as INTER ATOMIC CEMENT.  Because of this, the metals are strong, hard, malleable, ductile and good conductors of heat and electricity.

MICROSCOPIC STRUCTURE OF METALS :

In the solid state, most metals have crystalline structure in which atoms are held together by metallic bonds.  This crystalline array extends for many repetition in 3 dimensions.  In this array, the atomic centers are occupied by nuclei and core electrons.  The ionisable electrons float freely among the atomic positions.

The space lattice is a 3 dimensional pattern of points in space and hence called as point lattice.  In this the simplest repeating unit is called as the UNIT CELL.  The size and shape of the unit cell are described by three vectors.  They are a,b,c, and known as crystallographic axes. The length and angle between them are known as LATTICE CONSTANTS AND LATTICE PARAMETERS.

When a molten metal is cooled the solicitation process is one of crystallization.  These are initiated at specific sites called nuclei.  These in the molten metal are present as numerous unstable atomic aggregates or clusters that tend to form crystal nuclei. These temporary nuclei are known as EMBRYOS.  These are generally formed from impurities within the molten metal.  In the case of pure metals, the crystals grow as dendrites which can be defined as a three dimensional network which is branched like a tree.  The critical radius is the minimal radius of the embryo at which the first permanent solid space lattice is formed.

The crystals are otherwise known as grains since they seldom exhibit the customary geometric forms due to interference from adjacent crystals during the change of state.  The grains meet at grain boundaries which are regions of transition between differently oriented crystals.  These are regions of importance as they are sites of: 

1.     Less resistance to corrosion

2.     High internal energy and non crystalline

3.     Collection of impurities

4.     Barriers for dislocations

The nuclei can be homogeneous or heterogenous based upon whether they are developed from the molten liquid or formed as a result of foreign bodies incorporated into the molten metal.  When the crystals meet at the grain boundaries they stop growing further.  The grain boundaries are about 1-2 atomic distances thick.  Grain boundaries can be high angles (>10-15 degrees) or low angled (< 10 degree).

The grain structure can be fine where in, it contains numerous nuclei as obtained during the rapid cooling process (quenching) or refined when foreign bodies are added to obtain the fine grain structure.

EQUALIXED GRAINS

          When cooling occurs and grains are formed, the grains start growing from the nuclei peripherally. This takes the shape of a sphere and are equilaxed in structure meaning that they have the same dimensions in any direction.

COLUMNAR AND RADIAL GRAINS

          In a square mould, crystals grow from the edges towards the centre to form columnar grains whereas in the cylindrical mould the grains grow perpendicular to the wall surface and form radial grains.  Columnar grains are weak due to interferences in the converging grains.  Sharp margins have columnar grains.

GRAIN SIZE :

          The grain size can be altered by heating.  When the metal is heated above the solidus temperature to the molten state and rapidly quenched, small grains are formed whereas, when they are allowed to cool slowly to room temperature the grains tend to grow due to atomic diffusion and this results in an increased grain size and subsequent decrease in the number. The more fine the grain structure, the more uniform and better are the properties.

ANISOTROPHY :

          Alloys with uniform properties due to the presence of fine grain structure are said to be anisotropic.

METHODS OF FABRICATION OF METALS AND ALLOYS

1.       CASTING :  It is the best and most popular method.

2.       WORKING ON THE METAL :  They can be worked in the hot or cold conditions.  They are known as wrought metals.  They can be pressed, rolled, forged or hammered.

3.       EXTRUSION  :  A process in which a metal is forced through a die to form metal tubing.

4.       POWDER METALLURGY : It involves the pressing of the powdered metal into the mould of desirable shape and heating it to a high temperature to cause a solid mass.

SPACE LATTICES

          The structure of the crystal can be determined using the BRAGG’S LAW OF X-RAY DIFFRACTION.  There are 14 lattices known as BRAVIS LATTICES and these are grouped under six families.  These vary depending upon the crystallographic axes and lattice constants which are the length of the vertices and the angle between them.  The six families are :

1.                 Cubic

Simple 

Body centered

Face centered

2.                 Triclinic

3.                 Tetragonal

Simple

Body centered

Rhombohedral

4.                 Orthorombic

5.                 Hexagonal

Simple

Body centered

Face centered

Base centered 

6.                 Monoclinic

Simple

Base centered

The arrangement of atoms in the crystal lattice depend on the atomic radius and charge distribution of atoms.

The most commonly used metals in dentistry have one of the following space lattices :  body centered cubic, face centered cubic or hexagonal lattice.

SIMPLE CUBIC LATTICE SYSTEM

LATTICE IMPERFECTIONS AND DISLOCATIONS

          Crystallization from the nucleus does not occur in a regular fashion, lattice plane by lattice plane.  Instead, the growth is likely to be more random with some lattice positions left vacant and others overcrowded with atoms being deposited interstitially.  These are called defects and can be classified as :

A.  POINT DEFECTS OR ZERO DIMENSIONAL DEFECTS

1.  Vacancies or equilibrium defects :

          Absences of an atom from its position.  This can be :

Ø Vacancy

Ø Divacancy

Ø Trivacancy

2.   Interstitialcies :

      Presence of extra atoms in the interstitial spaces.

3.  Impurities

4.   Electronic defects

          Point defects are responsible for increased hardness, increased tensile strength, electrical conductance, and phase transformations.

B.  LINE DEFECTS OR SINGLE DIMENSIONAL DEFECTS :

These can be

1.     Edge dislocation

2.     Screw dislocation

The planes along which a dislocation moves is called as slip planes and when this occurs in groups it is called as slip bands.  The crystallographic direction in which the atomic planes move is called as the slip direction and the combination of slip plane and slip direction is called as slip system.

These are responsible for ductility, malleability, strain hardening, fatigue, creep and brittle fracture.

The face centered cubic consists of large number of slip systems and therefore is very ductile.  This is seen in gold.

The hexagonal closely packed system seen in zinc possesses relatively few slip systems and is therefore very brittle.

In between these is the body centered cubic with intermediate properties.

The strain required to initiate movement is the elastic limit.  The method of hardening of metals and alloys is based on the impedance to the movement of dislocations.

C.SURFACE DEFECTS OR PLANE DEFECTS OR TWO

DIMENSIONAL DEFECTS :

1.  Grain boundaries

2.   Twin boundaries :

          These are seen in the NiTi wires responsible for transformation between the austenitic and martensitic phases.  These are important for the deformation of the α titanium alloys.  The atoms have a mirror relationship.

3.     Stacking fault

4.     Tilt boundaries

D. VOLUME DEFECTS

          These include cracks

ALLOTROPHY AND ISOMORPHOUS STATE :

ALLOTROPHY

          This ability to exist in more than one stable crystalline form is called as allotrophy.  The various forms have the same composition but different crystal structure.

ISOMORPHOUS STATE

          The ability to exist as a single crystal at any atomic composition of binary alloys is known as iomorphous state e.g. Au-Ag, Au-Cu.

HEAT TREATMENT OR SOLID STATE REACTIONS

          Heat treatment of meals (non-melting) in the solid state is known as solid state reactions.  This is a method to cause diffusion of atoms of the alloy by heating a solid metal to a certain temperature and for a certain period of time.  This will result in changes in the microscopic structure and physical properties.

Important criteria are :

1.     Composition of the alloy

2.     Temperature to which it is heated

3.     Time of heating

4.     Method of cooling slowly or quenching.

The purpose of heat treatment is :

1.      Shaping and working on the appliance in the laboratory is made easy when the alloy is soft.  This is the first stage and called as softening heat treatment.

2.      To harden the alloy to withstand high oral stresses, it is again heated and this is called hardening heat treatment.

i.  ANNEALING OR SOFTENING HEAT TREATMENT

          This is done for structures that are cold worked.  These cold worked structures are characterized by :

1.     Low ductility

2.     Distorted and fibrous grains

When cold work is continued in these, they will eventually fracture.  This is may :

1.              Transgranular – through the crystals and occur at room temperature

2.              Intergranular – in between the crystals and occurs at elevated temperature

These can be reversed by annealing.  The various phase are :

1.     Recovery

2.     Recrystallization and

3.     Grain growth

Technique:

          The alloy is placed in an electric furnace at a temperature of 700° C for 10mins and then rapidly quenched.  Annealing temperature should be half that necessary to melt the metal in degrees Kelvin.

Recovery

During this phase, the cold work properties begin to disappear.  There is a slight decrease in tensile strength and no change in ductility.  The tendency for warping decreases in this stage.

Recrystallization

There is a radical change in the microstructure.  The old grains are replaced by a set of new strain free grains.  These nucleate in the most severely cold worked regions in the metal.  The temperature at which this occurs is the recrystallization temperature.  During this the metal gets back to the original soft and ductile nature.

Grain growth

          If the fine grain structure in a crystallized alloy is further heated, the grains begin to grow.  This is essentially a process in which the larger grains consume the smaller grains.  This process minimizes the grain boundary energy.  This does not progress until the formation of a coarse grain structure.

Properties of an annealed metal

1.     There is an increase in ductility

2.     Makes the metal tougher and less brittle

Stress relief annealing is a process which is done after cold working a metal to eliminate the residual stress.  This is done at relatively low temperatures with no change in the mechanical properties.

ii. HARDENING HEAT TREATMENT

          This is done for cast removable partial dentures, saddles, bridges but not for inlays.  This is done for clasps after the try in stage so that adjustments can be carried out during the try in when the metal is soft.

Technique

          The appliance is heat soaked at a temperature between 200-450° C for 15-30 minutes and then rapidly quenched.  The results is :

1.     Increased strength

2.     Increased hardness

3.     Increased proportional limit

4.     Decreased ductility

Microscopic changes

          Diffusion and rearrangement of atoms occur to form an ordered space lattice.  Therefore this is called as order hardening or precipitations hardening.

iii. SOLUTION HEAT TREATMENT OR SOLUTION HARDENING

          When the alloy is soaked at 700°C for 10 minutes and then rapidly quenched like that for a softening treatment, any precipitation formed during the earlier heat treatment will become soluble in the solvent metal.

iv. AGE HARDENING

          This is a process in which following solution heat treatment ; the alloy is once again heated to bring about further precipitation as a finally dispersed phase.  This causes hardening of the alloy  and it is known as age hardening because the alloy will maintain the quality for many years.  E.g. Duralium.

METHODS OF STRENGTHENING METALS AND ALLOYS :

          All metals possess an inherent barrier to dislocations.  This is relatively small and known as pearls stress.  This is imposed by the bonds associated with the arrangement of atoms in a given crystal structure.  Thus to improve the mechanical properties, other methods of hardening are used.  These are :

1. GRAIN BOUNDARY HARDENING OR GRAIN REFINEMENT HARDENING

          A poly crystalline metal contains numerous grains or crystals.  These meet at the grain boundaries.  The grain boundary is non –crystalline and contains impurities. These act as barriers to dislocations as it moves by slip planes from one grain to another.

          Finely grained structure contains large grain boundaries and hence the obstacle to motion of dislocations is higher.  therefore dislocation density rises rapidly due to plastic deformation.  These dislocations at the grain boundaries increase and therefore the stress necessary to continue the plastic deformation also increases.  Therefore, there is an increase in the yield strength and ultimate tensile strength.  The yield strength varies inversely with the square root of grain size (hall petch equation).

Grain refinement can be done by :

1.     Heat treatment

2.     Addition of grain refiners which act as nucleating agents.

Grains refiners are metals or foreign bodies of high melting temperature.  They crystallize out at high temperature and act as nuclei or seeds for further solidication. e.g.  iridium, rhodium.

The best method to improve properties of alloys and metals is by the addition of grain refiners.  Finely reined grains structure contain grain size >70µm.

2.  SOLUTION HARDENING OR SOLID SOLUTION STRENGTHENING

          An alloy is a solid solution ; it has a solute and a solvent.  The atomic diameter of a solute and solvent will never be the same.

          The principle of solid solution hardening is by introducing either tensile or compressive strain depending on whether the solute atom is smaller or larger than the solvent respectively and finally distorting the grain structure.  This solute can be either :

-         Substitutional

-         Interstitial

3.  PRECIPITATION HARDENING

          Another method of strengthening alloys is by means of this technique.  In this, the alloy is heated so that precipitates are formed as a second phase which blocks the movement of dislocations.  The effectiveness is greater if the precipitate is part of the normal crystal lattice which is known as coherent precipitation.

4.  DISPERSION STRENGTHENING

          It is a means of strengthening a metal by adding finely divided hard insoluble particles in the soft metal matrix as a result of which, the resistance to dislocations is increased.  This increases hardness and tensile strength.

          The ideal properties are seen when the particles range from 2-15% by volume with spacing at 0.1 – 1.0µm intervals and particle size from 0.01 – 0.1µ.

          The ideal shape of the dispersed particle is a needle like LAMELLAR SHAPE which can intersect with the slip planes.  Powdered metallurgy makes use of this method for strengthening.

5. STRAIN HARDENING OR WORK HARDENING

          This is seen in wrought metals.  The metals are worked after casting to improve their mechanical properties.  They may be forged, hammered, drawn as wires, etc.  All this is done below the re-crystallization temperatures.  This working causes vast number of deformations within the alloys or metals.  These interact with each other mutually, impeding the movements.  The increased stress required for further dislocation movement to achieve permanent deformation provides the basis for work hardening.  This result is distorted grain structure with the grains being fibrous.

ALLOYS

ALLOYS AS ALREADY SAID, MEAN IN LATIN = TO BIND

Alloys can be defined as

1.      Alloy is a combination of two or more metals which are generally mutually soluble in the liquid condition.

2.      Alloy is a metallic material formed by the intimate blending of two or more metals.  Sometimes a non metal may be added.

3.      Alloy is a substance composed of two or more elements, at least one of which is a metal.

METHOD OF ALLOYING 

1.             By melting together the base metal and the alloying element, mixing them thoroughly and allowing them to solidify. This is the common method.

2.             Sintering or powder metallurgy :  Metals are powdered, mixed and pressed to the desired shape and then heated but not melted till the powders unite to form a solid mass.

OBJECTIVES OF ALLOYING

The subjects of alloying are :

1.     To increase the hardness and strength

2.     To lower the melting point

3.     To increase the fluidity of the liquid metal

4.     To increase the resistance to tarnish and corrosion

5.     To make casting or working on metal easy

6.     To change the microscopic structure of metal

7.     To change the color of the metal

8.     To provide special electrical and magnetic properties.

The alloying treatment may be present in the main or base element as a :

1.     Substitutional type

2.     Interstitial type

3.     Chemically combined form

CLASSIFICATION OF ALLOYS

          The alloys can be classified in many ways :

1.   According to the uses      -        All metal inlays

                                                -        Crowns and bridges

                                                -        Metal ceramic restorations  

                                                -        Removable partial dentures

                                                -        Implants

2.   Major element present    -        Ferrous alloys : rich in iron

                                                -        Gold and silver alloys

                                                -        Babbit metals – tin and lead

based alloys

                                                -        Nickel alloys

3.   Nobility        -        High  noble metals      :   noble metal -  60wt%

                         gold – 40%

                             -        noble metals       :         25% wt%, no

stipulation for gold

                             -        predominantly based metal :   <25% of noble

metals

4. Principle three elements :                    -        Au-Pd-Ag

                                                          -        Pd-Ag-Sn

                                                          -        Co-Cr-Mo

                                                          -        Ti-Al-V

5.   Based on yield strength and    -        Soft

      elongation                                  -        Medium

                                                          -        Hard

                                                          -        Extra hard

6.  Based on the dominant phase   -        Isomorphous

                                                          -        Eutectic

                                                          -        Peritectic

                                                          -        Layered

                                                          -        Intermetallic compound

7.  Based on the method of             -        Cast metal

     of fabrication

                                                          -        Wrought metal

8.  Based on the number of metals          -        Binary

                                                          -        Ternary

                                                          -        Quaternary

                                                          -        Quinary

The composition of alloys can be defined by :

-         Weight percentage of each element

-         Atomic fraction or percentage of each element

Usually the alloy properties relate more directly to the atomic percentage rather than weight percentage.  The atomic % is not always equal to the weight %. 

-         In Au-Cu3, the wt% of Au is 51% of Au is 25%

-         Beryllium is present in nickel alloys in a small amount of 1.8wt%, but by at % it constitutes about 10.7%.

SOLID SOLUTIONS OR ISOMORPHOUS STATE OR SINGLE PHASE  :

          Solid solution is nothing but solution in the solid state.  The alloys of this type exist in a single phase with two or more components. It consists of a solute and a solvent.  These are completely miscible in any proportion in both the solid and liquid state.  Solvent is that metal whose space lattice persists and solute is the other metal.  By far these represent one of the simplest, most common and useful of all combinations.

E.g.   Au – Ag

          Au – Cu

          Au – Pt

          Au – Pd

          Ag – Pd

The solid solution can be either :

1.      SUBSTITUTIONAL SOLID SOLUTION

In this the solvent atoms are replaced by the solute.

This can be either :

-         Regular or Ordered

-         Random or Disordered

The ordered arrangement is one in which the atoms of solute are arranged in the solvent in an ordered fashion so that they are not distinguishable from the solvent.  E.g.  Au-Cu3 obtained when 50.2 wt% of gold and 49.8wt% of copper is cooled to below 400°C.  This causes a distorted crystal structure leading to keying it and increasing hardness.  This ordered structure is called as super lattice.

The random arrangement contains solute that is randomly distributed in the solvent.  E.g.  Pd-Ag, in which the silver atoms replace the palladium atoms randomly.  This arrangements has higher energy.

2.      INTERSTITIAL SOLID SOLUTION

In this, the solute atoms are present in positions between the solvent atoms.  E.g.  carbon is distributed interstitially in iron to form steels.  In this the atomic size of the solute atoms should be smaller than the solvent atoms.

HOME ROTHER’S RULE OF SOLID SOLUBILITY :

          For substitution solid solutions, the solubility limit of solute in solvent depends on :

1.      CRYSTAL STRUCTURE

Only metals with the same type of crystal lattice can form a series of solid solutions particularly if the size factor is less than 8% most of the metals used for dental restorations are face centered cubic.

2.      CHEMICAL AFFINITY

When two metals exhibit a high degree of chemical affinity, they tend to form an intermetallic compound on solidification rather than a solid solution.

3.      VALENCE :

Metals of the same valency and size are more likely to form extensive solid solutions than metals of different valencies.  If the valancies differ ; the metal with a higher valence may be soluble in a metal of lower valence.

4.      ATOM SIZE

If the sizes of the two metallic atoms differ by less than 15% they posses a favourable size factor for solid solubility.  If the size factor is greater than 15% multiple phases appear during solidification.  For good solubility the size difference should be less than 8%.

COOLING CURVE OF A SOLID SOLUTION

          A cooling curve of a solid solution type of an alloy is shown.

The temperature is found to drop as in the case of a pure metal from ‘e’ to ‘f’ by simple cooling of the molten solution.  At the temperature ‘f’, crystals of the solid start to form throughout the liquid.  The alloy is partly liquid and partly solid in the stage of cooling from ‘f’ to ‘g’.  During this time interval the composition of the remaining liquid is changing slightly and the temperature continues to drop slowly.  The portion of the curve from ‘f’ to ‘g’ represents the solidification or freezing range during cooling in contrast to the freezing point seen in pure metals. Portion ‘g’ to ‘h’ represented the cooling of the solidified alloy.

PHASE DIAGRAM OF A SOLID SOLUTION ALLOY

          The phase diagram of an alloy of composition X (approximately 60% A and 40% B) is shown :

          TmA and TmB represent the melting points of the pure metals A and B.  This alloy is rendered completely molten by heating it to a temperature above T1 which is the liquidus temperature for that particular composition.

          When the alloy is cooled from above T1, it remains molten until it reaches T1 where the first solid begins to form.  The composition of the first solid to form is given by drawing a horizontal line or TIE LINE to intersect the solidus.  In this case, drawing such a tie line reveals that the first solid to form has a composition Z (approx 90% A/10%/B) As the alloy is further cooled, more crystallization occurs and between temperatures T1 and T2 a mixture of solid and liquid exists.

          Selecting one temperature Tsl within this region, the composition of both solid and liquid can be predicted by noting where the tie line intersects both solidus and liquidus. Thus, at temperature Tsl, the composition of the solid is Y (approx 80%A/ 20%B) and the composition of the remaining liquid is W (approx75%B/ 25%A).  On further cooling, the alloy becomes completely solid at temperature Ts.  The last liquid to crystallize has the composition V (approx 80%B/20%A).  This confirms the previous observation for the solid solution alloy, that a cored structure exists in which the first material to crystallize is rich in the metal with the higher melting point (A), whilst the last material to solidify is rich in the other metal (B).

PROPERTIES OF A SOLID SOLUTION ALLOY

The solid solution possesses:

1.     Increased hardness

2.     Increased strength

3.     Increased proportional limit

4.     Decreased ductility

5.     Decreased resistance to corrosion due to coring

6.     Melting range rather than a point

In general the microstructure of a solid solution resembles that of the parent metals with properties that resemble an average of the two compounds. The properties keep increasing until the concentration of each compound reaches 50%.

EUTECTIC ALLOYS

          The eutectic alloy is one in which the components exhibit complete solubility in the liquid state but limited solid solubility E.g.  Ag-Cu.  The term eutectic means lowest melting point.  The eutectic alloy has the lowest melting point than either of the constituent metals.

          In silver copper system the temperature of silver is around 960.5°C and that of copper is 1083° C.  But that of the eutectic composition is 779.4° C.  In this, an intimate but heretogeneous mixture of the component metals exist when the alloy solidifies.  E.g. a mixture of salt and ice although completely soluble in each other in the liquid state solidifies as separate salt and ice crystal on solidification.

          These in contrast to other alloys do not have a solidification range ; instead they have a solidification point.  When the eutectic alloy solidifies, the atoms of the constituent metals segregate to form regions of nearly pure metals, which result in a layered structure. 

It can be written as :

          LIQUID = α SOID SOLUTION + ß SOLID SOLUTION

          It is referred to as invariant transformation because it occurs at a single temperature and composition.  The first formed grains of the above said equation are called as primary grains and they are larger than that of the eutectic composition.

          Partial eutectic is a system where in the metals exhibit solubility in liquid state and limited solubility in the solid state.

COOLING CURVE OF A EUTECTIC ALLOY

          The solidification of an alloy of eutectic composition may present the same curve as that of a pure metal, except that the solidification temperature is lower than that for either of the pure metals.  The cooling curves of eutectic alloy, pure metal and a composition between that of a metal and pure eutectic composition is given below :

          During the cooling of such a mixture, the first break in the curve represents the separation of some crystals of excess pure metal, resulting in a change in the shape of the cooling curve.  As the metal crystals separate, the composition of the remaining liquid alloy changes until the true eutectic composition is reached.  At this time, the freezing of the eutectic mixture occurs without further change in the composition and at a constant temperature.

          The cooling curve of an alloy of 50% tin and 50% lead through the temperature range from near 300°C to about 120° C is shown.  This composition does not represent the eutectic composition of lead tin.

          The cooling curve for this eutectic type of an alloy with excess lead present can be divided into five distinct parts, each of which represents a change in condition, or liquidsoid phase equilibrium of the system.  These changes in the curve may be observed by simple inspection.  The simple cooling of the liquid alloy is represented from the starting temperature of about 270°C to 210°C.  This section of the curve is the same as that found in the uniform cooling of any liquid, and the temperature drop here represents a simple function of the time of cooling.

          The second portion of the curve from 210°C to 176°C represents the separation or freezing out of pure lead from the molten mass.  Within this range the whole mass is beginning to crystallize, but the crystals that separate are pure lead floating in a liquid bath of lead and tin, which is continually becoming richer in tin as a result of the lead separation.  Lead continues to separate as a crystalline metal until a temperature of 176°C is reached.

          At this point, the change in direction of the curve represents a rise in temperature from 176°C to 183°C, which is from the under cooled condition.  This is due to the liberation of the latent heat of fusion.  The first irregularity of the curve at 210°C was brought about by the liberation of the heat due to crystallization of lead.   The final solidified mass consists of a heterogeneous mass of lead crystals surrounded by a matrix of lead tin alloy mixture of definite eutectic composition of 62% tin and 38% lead.  The matrix alloy has had its composition developed through the process of separation of 12% of pure lead (50% minus 38%) during the cooling from 210° C to 183°C.  This final matrix alloy is called the eutectic mixture.

          Finally, from the temperature of 183°C downward, the curve represents the simple cooling of the solid alloy.

PHASE DIAGRAM OF A EUTECTIC ALLOY

          The phase diagram is obtained as for the pure metal.

          In this diagram, on the left is shown the melting point of lead (327°C) and on the right the melting temperature of tin (232°C).  The melting temperature (183°C) of the eutectic alloy (62%) tin is shown to be lower than that of either ingredient metal.

By connecting the portions of the cooling curves which represent the eutectic freezing temperature and the portions of the cooling curves which represent the first separation of the excess ingredient metal in different alloy compositions, a diagram is obtained.

From this it is evident that any alloy composition will be in the liquid phase when heated to a temperature above that represented by the lines from 327°C for pure lead, to 183°C at 62% tin for the eutectic, to 232°C for pure tin.  Below these lines, the excess metal will start to crystallize out when an alloy of any composition is cooled, and the mass will entirely crystallized below the temperature of 183°C.  The composition of 62% tin and 38% lead represents the lowest melting mixture of tin and lead and is described as the eutectic composition.  At this composition no excess lead or tin separates, but instead a homogenous mixture of lead and tin crystallizes simultaneously from the liquid state.

To the right of the eutectic composition, at 80% tin for example, the excess in separates during the cooling from 200°C to the eutectic melting temperature, at which time the eutectic mixture crystallizes to surround the separated tin.  In the lower right portion the solid alloy is described as solid eutectic and tin.

In the left portion, on cooling, the excess lead in a composition of 60% lead and 40% tin will separate before reaching 183°C after which the eutectic will surround the lead crystals.

The phase diagram of the eutectic composition of Ag-Cu is given below :

          This has a composition of 28.1% Cu and 71.9% Ag.   It can be seen that a small amount of solid solution exists at each end of the diagram, indicating, that silver is slightly soluble in copper and that copper is slightly soluble in silver.   The eutectic structure does not appear in alloys of less than 8.8% copper.  Only the α solid exists with varying amount of ß solid solution depending on the temperature.

          A photomicrograph of this alloy is interesting, since it indicates that silver and copper have separated as mixtures rather than as homogeneous solutions of silver and copper.  Such an appearance is typical of eutectic alloys.

PROPERTIES OF EUTECTIC ALLOYS

          Alloys with composition less than that of the eutectic are called as hypoeutectic and those with a composition greater than that of the eutectic are known as hyper eutectic alloys.  The primary crystals of hypoeutectic are composed of α – solid solution and those of hyper eutectic are composed of ß solid solution.

Therefore :

1.              A linear variation between the composition and the physical properties cannot be expected.

2.              Since there is a heterogeneous composition, they are susceptible to electrolytic corrosion.

3.              They are brittle, because the present of insoluble phases inhibits slip.

4.              They have a low melting point and therefore are important as solders.

PERITECTIC ALLOYS

          Peritectic is a phase where there is limited solid solubility.  They are not of much use in dentistry except for silver tin system.  Like the eutectic, this is also an invariant transformation since this occurs at a particular temperature and composition.  The reaction is written as :

Liquid + ß = α

          This type of reaction occurs when there is a big differences in the melting points of the components.  The peritectic phase diagram is given below.

          The α phase is a silver rich phase, the ß phase, a platinum rich, and α + ß, a two phase region resulting from limited solid solubility.  The peritectic transformation occurs at the point P at which the liquid, plus the platinum rich ß phase transforms into the silver rich α phase.  The substantial composition change involved can lead to large amounts of coring if rapid cooling occurs.  If the alloy has a hypoperitectic composition, as does alloy 1 in the figure, cooling of the alloy through the peritectic temperature results in the transformation.

LIQUID + ß = LIQUID +α

          Rapid cooling results in precipitation of α phase around the ß grains before diffusion can occur.  The solid α phase inhibits diffusion, and substantial coring occurs.  The cored structure is more brittle and has corrosion resistance inferior to that of the homogenous α phase.

          These alloys undergo phase reactions and transformations upon solidification because of partial solubility of the constituent metals.

INTERMETALLIC COMPOUNDS

          These are compounds that are soluble in the liquid state but unite and form a chemical compound on solidification E.g.  Ag3 – Sn,

-         Ag2 – Hg3

-         Sn7 – Hg8

These are called as intermetallic compounds because ; the alloy is formed by a chemical reaction between a metal and a metal.  At space lattice level, the atoms of one metal, instead of appearing randomly in the space lattice of another metal, occupy a definite position in every space lattice.

The phase diagram of an intermetallic compounds is :

         

The most important feature in this diagram, from the stand point of silver tin amalgam alloy, is the fact that when an alloy containing 26.85% tin is slowly cooled with a temperature of 480°C, there is produced an inter metallic compound, (Ag₃-Sn) known also as gamma phase (g).  This silver tin compound is formed only at the lower temperatures over a narrow composition range from about 25 to 27%.  The silver content for such an alloy would be 73.15% on the basis of the presence of 26.85% tin.

          These diagrams are generally more complex than those for eutectic and solid solution alloys. Few general effects can be predicted from alloys forming chemical compounds.

PROPERTIES OF INTER METALLIC COMPOUND

1.     Very hard

2.     Brittle

The properties do not resemble that of the pure metal.

LAYER TYPE SYSTEM

          In this, the two metals are completely insoluble in both the liquid as well as the solid state.  The two metals appear to solidify at their individual freezing points into two separate distinct layers.  The phase diagram of this is shown below :

          All this while, the discussion was on binary alloys and their phase diagrams.  But the same can be obtained for  ternary alloys.  The three pure metals may be represented as the vertices of an equilateral triangle, with the temperature indicated by the length of the vertical line perpendicular to the plane of the triangle. Ternary diagrams have not been developed to the extent of binary diagrams because of the difficulty in their preparation and interpretation

DENTAL CASTING ALLOYS

          Metal restorations can be made by a number of methods like direct compaction as in the case of pure gold, swaging of metal foils,  CAD-CAM process for pure titanium or titanium alloys, electroforming and copy milling.

          Thus, although a variety of methods are available, the best and the most popular method in use is casting.  In this, the impression of the prepared tooth is replicated in a refractory die, and a required pattern is done using wax.  This is then invested in an investment material and burned out.  Now in the mold available, the molten metal or alloy is casted under pressure using centrifugal force.

          The major events in the history of dental casting alloys are given below :

                                                Event                                                Year

Introduction of lost wax technique                                                1907

Replacement of Co-Cr for Au in removable partial dentures    1933

Development of resin veneers for Au alloys                                1950

Introduction of the porcelain fused to metal technique               1959

Palladium based alloys as alternatives to Au alloys                            1968

Ni based alloys as alternatives to Au alloys                                1971

Introduction of all ceramic technologies                                                1980

Au alloys as alternative to palladium based alloys                              1999

         

          The history of the dental casting alloys have been influenced by quite a number of factors which involve the following :

1.     The technological changes of dental prosthesis

2.     Metallurgic advancements

3.     Price changes of the noble metals

The fabrication of the cast inlay restoration which was presented  by TAGGART in 1907 to the New York Odontological group has been acknowledged as the first reported application of the lost wax technique.

DESIRABLE PROPERTIES OF THE CASTING ALLOYS

The metals must exhibit

1.     Biocompatibility

2.     Ease of melting

3.     Ease of casting, brazing, soldering, and polishing

4.     Minimal reactivity with the mold material

5.     Good wear resistance

6.     High strength, stiffness and rigidity

7.     Sag resistance

8.     Excellent tarnish and corrosion resistance

9.     Should be inert in the oral conditions

10.            Should have fatigue resistance

11.            Should be amenable to heat treatment

12.            Little solidification shrinkage

CASTING SHRINKAGE

          This includes both the solidification shrinkage and the thermal contraction from the solidification temperature to room temperature.   The shrinkage occurs in three stages :

1.              The thermal contraction of the liquid metal between the temperature to which it is heated and the liquidus temperature.

2.              The contraction of the metal inherent in its change from the liquid to the solid state

3.              The thermal contraction of the solid metal that occurs on further cooling to room temperature.

The first mentioned one is not of much consequence, because this is compensated by the molten metal that flows into the mold.

          In order to obtain accurately fitting prosthesis, it is necessary to obtain compensation for this casting shrinkage.  This can be achieved by either generating computer aided over sized dies or through controlled expansion techniques, which include both setting or hygroscopic expansion and thermal expansion.

Linear solidification shrinkage of casting alloys :

          Alloy type                              Casting shrinkage (%)

          Type I (Au based)                  1.56

          Type II (Au based)                1.37

          Type III (Au based)               1.42

          Type IV (Ni-Cr based)          2.30

          Type V ( Co –Cr based)        2.30

          Generally type 2 and type 3 gold alloys represent the standards against which the performance of other casting alloys are judged.

The classification of alloys for all metal, metal ceramic and frameworks for removable partial denture are given below.

Classification of casting metals for full metal and metal ceramic prosthesis and partial dentures

Metal Type	All-metal prostheses	Metal ceramic prostheses	Partial denture frameworks
High Noble (HN)	Au-Ag-Pd	Pure Au (99.7%)	Au-Ag-Cu-Pd
	Au-Pd-Cu-Ag	Au-Pt-Pd
	HN metal ceramic alloys	Au-Pd-Ag       (5-12 wt % Ag) Au-Pd-Ag    (>12 wt% Ag) Au-Pd
Noble (N)	Ag-Pd-Au-Cu	Pd-Au
	Ag-Pd	Pd-Au-Ag
	Noble metal ceramic alloys	Pd-Ag Pd-Cu-Ga Pd-Ga-Ag
Predominantly Base metal (PB)	CP Ti Ti-Al-V	CP Ti Ti-Al-V	CP Ti Ti-Al-V
	Ni-Cr-Mo-Be	Ni-Cr-Mo-Be	Ni-Cr-Mo-Be
	Ni-Cr-Mo	Ni-Cr-Mo	Ni-Cr-Mo
	Co-Cr-Mo	Co-Cr-Mo	Co-Cr-Mo
	Co-Cr-W	Co-Cr-W	Co-Cr-W
	Cu-Al

Solidus and liquidus temperature of the commonly used classes of alloys  :

Alloy type	ADA classification	Solidus temperature (°C)	Liquidus temperature (°C)
Au-Pt	High Noble	1060	1140
Au-Pd	High noble	1160	1260
Au-Cu-Ag-Pd	High noble	905	960
Au-Cu-Ag-Pd	Noble	880	1270
Pd-Cu	Noble	1145	1230
Pd-Ag	Noble	1185	1045
Ag-Pd	Noble	990	1270
Ni-Cr-Be (Cr<20 wt %)	base metal	1160	1270
Ni-Cr (Cr<20 wt %)	base metal	1330	1390
Ni-Cr-Be (Cr<20 wt %)	base metal	1250	1310
Co-Cr	base metal	1215	1300

Different Metals Used In Dentistry

Gold (Au)

n  Gold provides a high level of corrosion and tarnish resistance

n   increases an alloy's melting range slightly.

n   Gold improves workability, burnish ability, and raises the density .

n  However, gold imparts a very pleasing yellow color to an alloy (if present in sufficient quantity).

n   Unfortunately, that yellow color is readily offset by the addition of "white" metals, such as palladium and silver. Gold is a noble metal.

Palladium

n  Palladium is added to increase the strength, hard­ness (with copper), corrosion and tarnish resistance of gold-based alloys.

n   Palladium will also elevate an alloy's melting range and improve its sag resistance.

n  It has a very strong whitening effect, so an alloy with 90% gold and only 10% palladium will appear platinum-colored.

n  Palladium possesses a high affinity for hydrogen, oxygen, and carbon.

n  It lowers the den­sity of the gold-based alloys slightly but has little similar effect on silver-based metals. Palladium, a member of the platinum group, is a noble metal

Platinum

n  Platinum increases the strength, melting range, and hardness of gold-based alloys while improving their corrosion, tarnish, and sag resistance.

n   It whitens an alloy and increases the density of non gold-based metals because of its high density.

n   Platinum is a member of the platinum group and is a noble metal

Iridium

n  serves as a grain refiner for gold- and palladium-based alloys to improve the mechanical properties as well as the tarnish resistance.

n  Iridium is a member of the platinum group and is a noble metal.

Ruthenium (Ru)

n  Ruthenium acts as a grain refiner for gold- and palladium- based alloys to improve their mechanical properties and tarnish resistance (like iridium).

n   Ruthenium is a member of the palladium group and is a noble metal.

Silver

n  Silver lowers the melting range, improves fluidity, and helps to control the coefficient of thermal expansion in gold- and palladium-based alloys

n   Silver-containing porcelain alloys have been known to induce discolor­ation (yellow, brown, or green) with some porcelains.

n  Silver possesses a high affinity for oxygen absorp­tion, which can lead to casting porosity and/or gas­sing.

n  However, small amounts of zinc or indium added to gold- and silver-based alloys help to control silver's absorption of oxygen.

n  Silver will also corrode and tarnish in the presence of sulfur. Although silver is a precious element, it is not universally regarded as noble in the oral cavity .

Aluminium

n  Aluminum is added to lower the melting range of nickel-based alloys.

n  Aluminum is a hardening agent and influences oxide formation.

n  With the cobalt - chromium alloys used for metal ceramic restorations, aluminum is one of the elements that is "etched" from the alloy's surface to create micromechanical reten­tion for resin-bonded retainers (Maryland Bridges).

Beryllium

n  Like aluminum, beryllium lowers the melting range of nickel-based alloys, improves castability, improves polishability, is a hardener, and helps to control oxide formation.

n   The etching of nickel-chromium-beryllium alloys removes a Ni-Be phase to create the micro re­tention so important to the etched metal resin-bonded retainer.

n  Questions have been raised as to potential health risks to both technicians and patients associ­ated with beryllium-containing alloys .

Boron

n  Boron is a deoxidizer.

n  For nickel-based alloys, it is a hardening agent and an element that reduces the surface tension of the molten alloy to improve castability.

n   The nickel-chromium beryllium-free alloys that contain boron will pool on melting, as opposed to the Ni-Cr-Be alloys that do not pool.

n  Boron also acts to reduce ductility and to increase hardness.

Chromium (Cr)

Chromium is a solid solution hardening agent that contributes to corrosion resistance by its passivating nature in nickel- and cobalt-based alloys

 Cobalt (Co)

n  Cobalt is an alternative to the nickel-based alloys, but the cobalt-based metals are more difficult to process.

n   Cobalt is included in some high-palladium alloys to increase the alloy's coefficient of thermal expansion and to act as a strengthener

Copper (Cu)

n  Copper serves as a hardening and strengthening agent, can lower the melting range of an alloy, and interacts with platinum, palladium, silver, and gold to provide a heat-treating capability in gold-, silver-, and palladium-based alloys.

n   Copper helps to form an oxide for porcelain bonding, lowers the density slightly, and can enhance passivity in the high palladium-copper alloys.

Gallium (Ga)

n  Gallium is added to silver-free porcelain alloys to compensate for the decreased coefficient of thermal expansion created by the removal of silver. (Concerns over silver's potential to discolor dental porcelain have greatly limited its use in systems other than palladium-silver )

Indium

n  Indium serves many functions in gold-based metal ceramic alloys.

n  It is a less volatile oxide-scavenging agent (to protect molten alloy);

n  lowers the alloy's melting range and density; improves fluidity;

n  Has a strengthening effect. Indium is added to non gold­based alloy systems to form an oxide layer for porce­lain bonding.

n  Alloys with a high silver content (eg, palladium-silver) rely on indium to enhance tarnish resistance.

Iron (Fe)

n  Iron is added to some gold-based porcelain systems for hardening and oxide production.

n  Iron is included in a few base metal alloys as well.

Manganese (Mn)

n  Manganese is an oxide scavenger and a hardening agent in nickel- and cobalt-based alloys.

Molybdenum (Mo)

n  Molybdenum improves corrosion resistance, influ­ences oxide production, and is helpful in adjusting the coefficient of thermal expansion of nickel-based alloys.

Nickel (Ni)

n  Nickel has been selected as a base for porcelain alloys because its coefficient of thermal expansion approximates that of gold and it provides resistance to corrosion.

n  Unfortunately, nickel is a sensitizer and a known carcinogen.

n   Estimates of nickel sensitivity among women in the United States range from 9% to 31.9% and from 0.8% to 20.7% among men .

Tin (Sn)

n  Tin is a hardening agent that acts to lower the melting range of an alloy. It also assists in oxide production for porcelain bonding in gold- and palladium-based al­loys. Tin is one of the key trace elements for oxidation of the palladium-silver alloys.

Titanium (Ti)

n  Like aluminum and beryllium, titanium is added to lower the melting range and improve castability.

n   Tita­nium also acts as a hardener and influences oxide formation at high temperatures.

Zinc (Zn)

n  Zinc helps lower the melting range of an alloy and acts as a deoxidizer or scavenger to combine with other oxides.

n   Zinc improves the castability of an alloy and contributes to hardness when combined with palladium.

ALLOYS FOR ALL METAL RESTORATION

          As it can been seen from the table, the metals that can be used for all metal restoration can be classified as highly noble, noble and base metal alloys.

          Among the highly noble metals are Au-Ag-Cu-Pd and metal ceramic alloys.  The metal ceramic alloys are dealt under a separate section.

          In the noble group are the Ag-Pd-Au-Pd and metal ceramics.

          The base metal alloys that can be used for all metal restorations are those that are used for metal ceramics and removable partial denture frameworks.  Since they are used for the latter two purposes, they are discussed under that.

          It can be seen that all of the metal ceramics can be used for all metal restorations but it is not the same vice versa.  The principle  reasons for this may be because the alloys of all metal restoration may not be able to form metal oxides that is required for bonding to porcelain, their melting temperature may be too low to resist sag deformation at porcelain firing temperatures, and their thermal co-efficient of contraction may not be close enough to match that of porcelain.

Typical compositions of Casting Alloys for Full-Metal, Resin-Veneered and Metal- Ceramic Prostheses

Alloy type	Classification	Elemental composition (wt%)
		Au	Pd	Ag	Cu	Ga, In, and Zn
I	High Noble          (Au-based)	83	0.5	10	6	Balance
II	High Noble          (Au-based)	77	1	14	7	Balance
III	High Noble          (Au-based)	75	3.5	11	9	Balance
III	Noble               (Ag-based)	46	6	39	8	Balance
III	Noble         (Ag-based)	-	25	70	-	Balance
IV	High Noble          (Au-based)	56	4	25	14	Balance
IV	Noble          (Ag-based)	15	25	45	14	Balance
Metal Ceramic	High Noble          (Au-based)	52	38	-	-	Balance
Metal Ceramic	Noble          (Ag-based)	-	60	30	-	Balance
Metal Ceramic	High Noble          (Au-based)	88	7	1	-	Balance
Metal Ceramic	Noble          (Ag-based)	0-6	74-88	0-10	0-15	Balance

The alloys used for all metal restoration are described below :

GOLD AND GOLD BASED ALLOYS

          Gold in the as cast condition is very soft and can be easily cold worked.  The gold in the pure form is used for direct restorations whereas the alloys of gold are used for casting purposes.  The alloys of gold are classified as :

Type                              Au%           Ag%           Cu%           Pt/Pd%     Zn%

I (soft)                  85               11               3                 -                  1

II (Medium)        75               12               10               2                 1

III (Hard)             70               14               10               5                 1

IV (Extra hard)   65               13               15               6                 1

It can be seen that the gold content or nobility of the alloys decreases on going from type I to type IV. nobility of gold alloys is often indicated by either carat value of fineness.  Carat value represents the number of parts by weight of gold per 24 parts of gold.  Fineness indicate the number of part per thousand parts of gold.  Thus the fineness rating is 10 times the gold percentage of the alloy.   Fineness is considered a more practical term than the carat value.

Their comparative properties, also are shown below

Type	Hardness	Proportional limit	Strength	Ductility	Corrosion resistance
I
II	Increases	Increases	Increases	Decreases	Decreases
III	Downwards	Downwards	Downwards	Downwards	Downwards
IV

Mechanical Property Requirements in ANSI/ADA Specification No.5 for Dental Casting Alloys (1997)

Alloy type	Yield strength (0.2% offset)			Elongation
	Annealed		Hardened	Annealed	Hardened
	Minimum  (MPa)	Minimum  (MPa)	Minimum  (MPa)	Minimum  (%)	Minimum (%)
Type 1	80	180	-	18	-
Type 2	180	240	-	12	-
Type 3	240	-	-	12	-
Type 4	300	-	450	10	3

hardness, strength and the proportional limit increases from type I to type IV whereas the ductility and the corrosion resistance decreases from type I to type IV.   This is due to the property of forming solid solution by the alloying elements.  The last two types can be further hardened be hardening heat treatments.  The corrosion resistance is due to the effects of platinum and palladium which form a cored structure on solidification de to their high melting points.  There is a consequent increase in the separation of the liquidus and the solidus lines in the phase diagram.

USES

1.  Type I   :  are sure for inlays which are well suppose and do not have to resist high masticatory forces.  The high ductility values allow them to be burnished thus improving the marginal fit.

2.  Type 2 : are the most widely used metals for inlays.  They have superior mechanical properties than type I.

3.  Type 3  : are used when there is less support from tooth structure and when the opposing stress are high like for crowns, bridges.

4. Type 4 : are used exclusively for construction of components of partial dentures and for this reason are referred to as partial denture casting alloys.

The functions of each of the ingredient metals in the casting alloy are :

1.       Gold                    -        Yellow color, ductility, resistance to tarnish

and corrosion.

2.       Silver         -        Hardness and strength.  Whiten the alloy thus

reducing the reddening effect of copper, but tarnishes the alloy.

3.       Copper       -        Hardness and strength.  Reddish color but

lowers tarnish resistance.  Lowers fusion temperature.  Reduces the density of the alloy.

4.       Palladium           -        Increases resistance to tarnish and corrosion. 

Whitens the alloy Cheap.  Absorbs gases formed during casting, and thus reduces porosity.  Increases hardness.

5.       Zinc            -        Acts as a scavenger and removes the oxides. 

Makes the alloy more castable.

The classification based on the color of the allow :

1.                 Yellow gold – Those with more than 60% Au

2.                 Low gold or economy gold – With 42-55% Au, also has yellow color

3.                 White gold – are those with gold more than 50%, but palladium gives the white color

4.                 Silver palladium  with or without gold but mainly silver – Has white color

5.                 Palladium silver with mainly palladium gives white color.

6.                 Japanese gold – Also known as technique alloy used for training students in casting technology - has yellow color.  It has the composition of

Cu     -        53%

Zn     -        37%

Al     -        7%

Others -      3%

          The grain refined alloys are those that contain iridium or ruthenium in 100-150 parts per million.  By this the grain size is decreased to 150-50 microns.  Therefore better physical properties can be obtained since they depend on the smaller grain size for better properties.

The advantages of the refined alloys are :

          High yield strength

          High elongation

          Homogenous casting

          More resistance to corrosion

HEAT TREATMENT OF GOLD ALLOYS

The heat treatments are  :

1.     Softening heat treatment :

In this, the alloy is heated in an electric furnace at a temperature of above 700°C for 10 min and then quenched rapidly in water.  The normal procedure is to leave the mould until the gold is no longer at red heat which is visible in the sprues of the casting.  This ensures that the internal metal temperature is about 600° C after which it is quenched.  This causes a fine grain structure.  The ductility and the corrosion resistance increase whereas the strength, hardness and the proportional limit decrease.

2.     Homogenization heat treatment :

This is done when platinum and palladium are present, to remove coring.  This involves heating to 700°C for ten minutes, followed by quenching.

3.     Stress relief anneal :       

This is done when any adjustments are done to the appliance to remove the stresses. This involves heating in a low temperature to remove the stresses for a given period of time.

4.     Hardening heat treatment :

This is done for type III and type IV alloys which contain sufficient amount of copper.  This is due to solid state transformations.  The casting is heated to above 450° C and allowed to cool slowly until 200°C, then quenching.  This takes about 20 min.  This causes an increase in the strength, hardness and proportional limit with a decrease in corrosion resistance and ductility.

Hardening heat treatment (theoretical considerations):

          The hardening process can be explained by the consideration of phase diagrams for silver copper and gold copper systems.

          Silver and copper are immiscible in each other.  They form eutectic phase at a composition of 71.9% Ag and 28.1% Cu.   Although they are not soluble in each other, they tend to form little amount of solid solution at room temperature in the eutectic mixture.  When the alloys are heated, the diffusion of atoms become possible and copper tends to precipitate from the α solid solution.  This occurs of the precipitation hardening  procedure used for type III and type IV alloys.

Gold and copper form a continuous series of solid solution with face centered cubic lattices.  The copper is randomly substituted in the gold lattices.  From the phase diagram it can be seen that the solidus and the liquidus are close together and almost coincide at point M.  Two other areas on the phase diagram, at composition between 40% and 90% gold, indicate regions in which the alloys are capable of forming an ordered state from a random one.  This ordered lattice is known as super lattice. 

This occurs by the rearrangement of atoms when their energy is increased to allow diffusion as when heating to 200°C – 400°C.  The super lattice has a formula of Cu₃-Au.  This heat treatment is known as ordered heat treatment.  Similarly, when an alloy containing 75% gold is heated, an ordered tetragonal structure of the formula Cu-Au is formed.

LOW GOLD CONTENT ALLOYS

          These contain about 45% - 50% gold and was introduced due to rise in the price of gold.  They have a high palladium content which imparts a whitish color to them.  The properties are similar to that of type III and IV alloys, but the ductility is considerably lower.  They have an elongation percent of only 2% whereas, type III alloy has 20%.

SILVER PALLADIUM ALLOYS

          These alloys, as the name suggest, contain predominantly silver in composition but have substantial amounts of palladium (25%) that provide nobility and promote the Silver tarnish resistance.  They may or may not contain Copper or Gold.  These contain small amounts of Zinc and Indium.  They are whitish in color.

          These have casting temperatures in the range of yellow gold alloys.  They have lower density than the gold alloys and therefore, present difficulties in casting.  Care must be paid to the casting temperature and the mold temperature if no defects are to be expected.  Alloys containing palladium have a propensity to dissolve oxygen in the molten state which may lead to a porous casting.

          The copper free Ag-Pd alloys contain 70% - 72% Ag and 25% Pd.  These have properties of type III Gold alloys.  Other silver based alloys contain 60% Ag, 25% Pd and as much as 15% or more of Cu.  These have properties of type IV gold alloy.  The major limitation of Ag-Pd alloys in general and Ag-Pd Cu in particular is their greater potential for tarnish and corrosion.

ALUMINIUM BRONZE ALLOYS

          This is the only alloy that is based on Copper as its main component and approved by the ADA.  Although, Bronze is defined as Copper rich Copper – Tin phase, Bronze alloys containing no Tin like Aluminium bronze (Cu-Al), Silicon bronze (Cu-Si), and Beryllium bronze the surface.

          The aluminium bronze alloys contain 81-88wt% Cu, 7-11 wt% Ni and 1-4 wt% Fe.  This has the potential to react with Silver and form copper sulphide which tarnishes the surface.

ALLOYS FOR METAL CERAMIC RESTORATIONS

          Because of the poor tensile and shear bond strength, All porcelain restorations are weak and brittle.  Therefore, they break easily.  But porcelain is necessary for aesthetics.  This problem is solved by, making the restoration in metal and applying porcelain to labial and buccal areas of the appliance in thin layers (veneers) for esthetics.  Thus both strength and appearance are met.

          Metal ceramic alloys are also referred to as porcelain-fused-to-metal or ceramometal alloys. But the preferred term is metal-ceramic.  Likewise the preferred acryonym is PFM rather than PBM (porcelain bonded to metals) and PTM (porcelain to metal).

          These are classified as high noble, noble and base metal, like in All Metal Restoration Alloys.

Properties of metal ceramic alloys :

1.              High fusing temperature of the alloys.  This should be 100C greater than the fusion temperature of porcelain.

2.              The contact angle between the ceramic and metal should be less than 60°

3.              They should form oxides on the surface for bonding to porcelain.  For this purpose, base metals like Tin, Indium and Iron are added.

4.              They should have compatible co-efficient of thermal expansion.  For, this is added Palladium which tends to lower the co-efficient of thermal expansion.  Although they should be equal theoretically, it is ideal to have the co-efficient of thermal expansion of metal greater than that of the porcelain by 0.5 x 10^-6/°C.  Most metals have a coefficient of thermal expansion of 13.5 x 10^-6 /°C and porcelains of about 13-144 x 10^-6/°C.

5.              Adequate stiffness and strength

6.              High sag resistance

7.              Accurate casting of the alloy even under high temperature.  The bond between metals and porcelain is that of chemisorption and the most common failure that occurs in metal ceramic is due to debonding of the metal.

The high noble alloys used for PFM are :

1.       Au-Pt-Pd alloys :

These have a gold cement ranging up to 88% with varying amounts of palladium, platinum, and small amounts of base metals.  These are yellow in color.  They have a high melting range.  The base metals are tin, indium and iron, and these are added to form metal oxides for bonding to porcelain.  Rhenium is added as a grain refiner.  The hardening that occurs in this is due to the precipitate of Fe-Pt3.  The heat treatment consists of heating the alloy for 30min at 550°C.  They have high stiffness, hardness, strength and reasonable elongation but low sag resistance.  Because of the yellow color, producing esthetics is easier.    

2.       Au-Pd alloys :

They have a Pd content of 35% - 45% and Au content of 44% - 45%.  These have remained popular as metal ceramic alloys in spite of their relatively high cost.  Due to the absence of silver, there is freedom from porcelain greening and also decreased thermal coefficient of contraction.  Since they do not contain Pt or Fe there is no possibility for precipitation hardening to occur.  Only solution hardening occurs.  Indium is added for bonding purposes and Gallium, for lowering of the fusion temperature.  Rhenium is the grain refiner and Ruthenium is added for improving castability.  This difficulty in casting is due to their low density.  These are white in color and therefore esthetics is difficult to obtain.  They are harder, stiffer and stronger than Au-Pt-Pd.  They are more ductile and easier to solder but have higher casting temperatures.

3.       Au-Pd-Ag Alloys :

These contain between 39% and 77% Au, up to 35% Pd and Silver levels as high as 22%.  Silver increases the thermal coefficient of contraction, but it has the tendency to discoloration porcelain.  Indium and Tin are used for bonding the Rhenium for grain refining.  Ruthenium is used for improving the castability.  The hardening is by means of solution hardening and the properties are similar to that of Au-Pd.

The noble metals used for metal ceramic restorations are :

1.       Pd-Ag Alloys :

This was the first gold free noble metal to be marketed.

This contains about 53% - 61% Pd and 28% - 40% Ag. These have the lowest noble content of the five noble metal alloys. These contain Tin and Indium for oxide formation for porcelain bonding, and to increase the alloy hardness and Ruthenium for improving the castability, since they produce greening effect. This is due to the escaping of the silver vapor to the surface of the alloys during the firing which diffuses into the porcelain as silver ions and is reduced to colloidal metallic silver in the surface layer of porcelain.  This can be minimized by using ceramic coating agents or gold metal conditioners.

In some of these alloys there is the formation of internal oxides rather than an external oxide.  This produces nodules on the surface which causes a mechanical type of bond rather than a chemical one.  Because of the increased Pd content, there is a decrease in the coefficient of thermal expansion but the increased silver content increases this and lowers the melting range.

2.       Pd-Cu-Alloys :

These are recent introduction to the market and the cost is similar to that of the previous alloy type.

These contain 74% to 80% Pd and 9% - 15% Cu.  They may contain 2% Gold.  These tend to form dark brown or black oxides during porcelain firing. This should be eliminated by proper masking of the oxide.  It is necessary that a brown rather than a black oxide is formed.  Otherwise poor adherence to porcelain may occur.  These are susceptible to sag deformation at elevated firing temperatures.  Indium is added for oxide formation and Gallium for improving casting qualities. It has high strength and hardness, moderate modulus of elasticity and elongation.

3.       Pd-Co Alloys :

This is comparable in cost to the above previous groups.  They are often advertised as gold free, nickel free, beryllium free and silver free alloys.

These are the most sag resistant of all noble metal alloys.  These have fine grain size.  These tend to discolor porcelain in spite of the absence of the silver due to the formation of cobalt oxides.  But this is not considered as a significant problem and no metal coating agents are necessary to mask the oxide layer.  Like the above three alloys, they have a high coefficient of thermal expansion and can be used with higher expansion porcelains.

4.       Pd-Ga-Ag and Pd- Ga – Ag-Au Alloys :

These are the most resistant of the noble alloys.  These were introduced because of their tendency to form lighter oxides than Pd-Cu or Pd-Co. They are compatible with lower expansion porcelains like vita porcelain.

PHYSICAL PROPERTIES OF HIGH NOBLE AND NOBLE METAL ALLOYS :

1.     All are biocompatible

2.     Good resistance to tarnish and corrosion

3.     Melting temperature of around 1000°C.  The casting temperature is obtained by adding 75-150°C to the liquidus temperature.

4.     Density of 15gm/cm³.  This gives an idea of how many castings can be done from a unit weight of metal and therefore the cost.

5.     Hardness from soft to hard

6.     Elongation which is a measure of ductility of about 20-39%

7.     Linear coefficient of thermal expansion in the range of 14 – 18 x    10 ^-6/°C.

8.     Yield strength in the range of 103 – 572 MPa.

BASE METAL ALLOYS USED FOR PFM :

          Base metals were introduced to the field of dentistry as an alternative to the noble metals due to rise in the price of gold.  They were found to possess some good qualities which have made them a commonly used material in dentistry.  The two commonly used alloys are Co-Cr and Ni-Cr.

According to the ADA the following combinations are available :

1.     Cobalt – chromium

2.     Nickel – chromium

3.     Nickel – chromium – beryllium

4.     Nickel – cobalt – chromium

5.     Titanium – aluminium – vanadium

Although Ni-Cr is used for PFM, and Co-Cr for partial dentures, these are described here because of the similarity in certain properties.

i.                   Co-Cr Alloys :

These were introduced by the name stellites by Eldwood Haynes an automobile engineer in the early 1900.  They were so named because of their bright, lustrous, mirror like surface resembling starts at night.

The first introduced Co-Cr alloy in dentistry was called as vitallium and it was introduced in 1928.  It was nickel free.  This closely resembled satellites.  This has been in use since 1930’s.  The first dental application of this alloy is recognized as having been made by R.W. Erdle and C.H.P range.

In 1943, a report appeared which described the properties of these alloys including other products under various names like ticonium, niranium and lunorium.

TYPES

          There are two types of alloys and they are :

a.        Type I which is high fusing with fusion temperature greater than 2400F.

b.        Type II which is low fusing with fusion temperature less than 2400F.

COMPOSITION

These alloys generally contain 35% - 65% Co, 20-35% Cr, 0-23 Ni and trace quantities of other elements such as molybdenum, silicon, beryllium, boron and carbon.

Cobalt and nickel are strong metals and the purpose of the chromium is to further strengthen the alloy by solution hardening and to impart corrosion resistance by the passivating effect.  This is because of the chromium oxide that is formed when exposed to air.  The minimum percentage required to provide this protective coat is 12%.  Nickel increases the ductility.

The minor elements are added to improve the casting and handling characteristics and modify the mechanical properties.

Molybdenum decreases the thermal co-efficient of expansion and strengthens the alloy while Tungsten, when present also acts to strengthen it.  Beryllium causes grain refinement and uniformity of the properties.  It also lowers the melting point and strengthens and harden the alloy. Ruthenium improves the castability, since the alloys are low is density.

Carbon acts as a major strengthener and also affects the strength and hardness when present at 0.2%.  When this increases to above 0.25% it causes brittleness in the alloy.  This is due to the formation of carbide core.  This concentration not only depends on the manufacturer but also on the type of flame used. When oxyacetylene flame is used there is possibility of introducing carbon inadvertently.

These are stronger than Ni-Cr and used mainly for partial denture frameworks rather than PFM.  They are stronger than noble alloys.

PROPERTIES

1.     Melting point :

Is between 1250°.  Therefore, they cannot be melted using gas air torch.  Only induction method and oxyacetylene flame should be used.  While using care should be taken not to incorporate carbon in excess.

2.     Yield Strength :

This is between 470-710  MPa which for gold is 320mpa.  As a result high stresses are required to deform the appliance.  This is important in constructing clasps.

3.     Modulus of elasticity :

          This is greater than the gold alloys and determines the thickness and the thinness of the various parts of the denture framework.  High stiffness is an advantage since less undercut is involved but this can also be damaging to the abutment tooth because of the excessive stresses introduced when the clasp is taken out and inserted into the mouth.

4.     Tensile strength :

This is 685 to 870 MPa

5.     Hardness :

This is in between 264 – 432 VHN.  For gold it is around 264 VHN.  Because of this it is difficult to grind, cut or polish.  The polishing of these is carried out by sand blasting using aluminium oxide of size 50 microns or by electrolytic deposition.  This is in contrast to electroplating.

6.     Ductility :

This denotes the elongation percentage which is less than gold which has around the ductility of these is around 1.6 and 3.8 this is related to the fracture of the clasp and how it occurs.

7.      Specific gravity :

This is a measure of the weight of the appliance and is half of that of gold.

8.     High resistance to tarnish and corrosion

9.     Solidification shrinkage :

This is greater than that of gold and is about 2.3.  Therefore, it is necessary that the investment compensates this shrinkage.  For the alloys, either silica or phosphate bonded investments are used.

10.            Castability :

This does not produce very accurate castings because of the low density which decreases the thrust of the molten metal during casting.  This is improved by alloying beryllium with it.

11.            Cost :

This is less than that of gold and therefore economical.

OTHER USES :

i.                   As part of the implant denture

ii.                For making surgical screws and plates

iii.             Orthopedic surgery

ii. Ni-Cr Alloys :

          These are base metal alloys with a composition of nickel -70 to 90% and chromium of about 13-20%.  Other elements added are iron, aluminum, molybdenum, beryllium, silicon and copper.  These are mainly used for PFM.  These were developed after Co-Cr gained wide spread popularity.

PROPERTIES

a.     Higher modulus of elasticity

b.    Increased hardness

c.      High yield strength

d.    Less density

e.     Less costly

f.       Superior sag resistance which is about 25 microns as compared to 225 microns for gold.

g.    Ductility greater than that of Co-Cr.

MANIPULATION OF BASE METAL CASTINGS

          Since the fusion temperatures of these are high, they cannot be casted as for gold alloys in gypsum bonded investments.  Instead they should be casted in silica or phosphate bonded investments.  Melting of these should be done only by electrical induction or by acetylene/oxygen flame.

          These alloys have low density and therefore do not develop the necessary thrust required for filling the mould.  Therefore the casting machines should be capable of producing this extra thrust.  Because of the increased hardness, these materials should be polished by electrolytic method.

COMPARISON OF PROPERTIES OF THE VARIOUS TYPES OF BASE METAL ALLOYS

Property	High noble alloy	Co-Cr	Ni-Cr-Be	CPTi
Biocompatibility	Excellent	Excellent	Fair	Excellent
Density (g/cm3)	14	7.5	8.7	4.5
Elastic Modulus (GPa)	90	145-220	207	103
Sag resistance	Poor-excellent	Excellent	Excellent	Good
Technique sensitivity	Minimal	Moderately High	Moderately	Extremely
Bond to porcelain	Excellent	Fair	High	High
Metal	High	Low	Good Excellent Low	Fair Low

COMPARISON WITH CASTING GOLD ALLOYS

The two main components of cast partial denture frameworks are the connectors and clasps.  The connectors should be rigid and should not be permanently deformed.  Thus it can be inferred from the above comparison that Co-Cr alloys meet the requirements.

For a clasp, a high value of proportional limit is required in order to prevent deformation. A lower value of modulus of elasticity would enable the clasp to engage relatively deep undercuts due to its increased flexibility.  In addition the alloy used to construct clasps should be ductile so that adjustments can be made to clasps without fracture.  Therefore the gold alloys most closely match the requirements for a clasp.  But in practice, both are cast from Co-Cr alloys.   When designing clasps from this, due regard must be paid to the high modulus of elasticity and low ductility.  Clasps should not be designed to engage deep undercuts and alterations leading to fracture.  A reduction in thickness decreases the force necessary to push the clasp over the bulge of the tooth but leaves it exposed to the dangers of deformation during handling of the denture.  This can be overcome by reducing the undercut area and also the thickness of the clasp.

COMPARISON OF THE PROPERTIES OF TYPE IV AND Co-Cr ALLOY :

Properties	Co-Cr	Type Gold	IV Comments
Tensile Strength (Mpa)	850	750	Both acceptable
Density (gms / cu.cm)	8	15	More difficult to produce defect the castings for CO-Cr but dentures are lighter.
Hardness (Vickers)	420 (Hard than enamel)	250 (Softer than enamel)	More difficult to polish but retains polish during services.
Stiffness	Stiff	More flexible
Ductility	2	15 (as cast) 8 (hardened)	Co-Cr clasps may fracture if adjustments are made.
Modulus of  elasticity (GPa)	220	100	Co-Cr more rigid for the same thickness
Proportional limit (MPa)	700	500	Both resist stresses without deformation.
Melting temperature (oC)	As high as 1500	Lower than 1000	Co-Cr require electrical induction or oxyacetylene
Casting shrinkage	2.3	1.25 – 1.65
Heat treatment Tarnish resistance price	Complicated adequate Reasonable	Simple adequate high

          The success of the crown and bridge alloys depends to a great extent on the accuracy of the restorations.  The gold alloys have a significant advantage from this point of view.  The casting shrinkage is less (approximately 1.5% when for base metal alloys it is around 2.3%).  This is well compensated by the mould whereas, for the base metals it is not so.  But one advantage of the Ni-Cr alloys is that, the margins are not destroyed during finishing and polishing procedure.  These are rarely used for all-metal but widely, for metal – ceramic restorations.

COMPARISON OF PROPERTIES OF TYPE III AND Ni-Cr ALLOY

Density (gm/cu.cm)	8	15	More difficult  to produce to produce defect free castings for Ni-Cr alloys.
Fusion temperature (oC)	as high as 1350	Lower than 1000	Ni-Cr alloys require electrical induction or oxyacetylene flame.  Both adequate
Tensile strength (MPa)	230	290	Both high enough to prevent distortions when used.
Modulus elasticity (GPa)	220	85	Higher modulus of Ni-Cr advantage for larger restorations.
Hardness (Vickers Ductility)	300 upto 30%	20 (as cast) 10 (hardened)	Ni-Cr more difficult to polish but retains polish during service.  Burnishing is possible but high forces are required.

BIOCOMPATIBILITY OF BASE METALS

          The main disadvantage of base metal alloys in from the beryllium vapor.  This is greatest for the dental technicians who are exposed to the dust and vapor during the various processes of casting and finishing.  According to OSHA,  the exposure to beryllium dust in air should be limited to particulate beryllium concentration of 2mg/cu.m determined from 8 hour time weighted coverage.  The allowable ceiling concentration is 5 mg/cu.m not to be exceeded for a 15 minutes period.  For a minimum duration of 30 minutes a maximum ceiling concentration of 2 mg/cu.m is allowed.  This vapor can be reduced effectively by the use of exhaust fans.

          Exposure to beryllium may result in acute or chronic forms of beryllium disease.  The symptoms may vary from contact dermatitis to severe chronic pneumonitis which can be fatal.  The chronic disease is characterized by symptoms of severe coughing, chest pain and general weakness to pulmonary dysfunction.

          To other disadvantage of these base metal alloys is the allergy of patients to nickel.  This allergy can be tested by a patch test using 25% nickel sulfate.  Positive reactions were reported by 9.4% women and. 79% of men.

          The effects of nickel exposure to humans have included dermatitis, cancer of the lungs, cancer of the nasal sinus and larynx, irritation and perforation of the nasal septum loss of smell, asthma like lung disease, pulmonary irritation, pneumoconiosis, a decrease in lung function and death.

          NIOSH has recommended OSHA to adopt a standard to limit employee exposure to inorganic nickel in the laboratory office to 15µg/cu.m of air determined as a time weighted average (TWA) concentration for upto a 10 hr work shift (40 hr work week) the existing OSHA standard specifies an 8 hr TWA concentration limit of 1000µg/cu.m of air.

          Thus it is better to follow certain methods like using high speed evacuation systems when procedures are performed intra orally and using exhaust fans in the laboratory.

DISADVANTAGE OF THE BASE METALS

i.                   Difficult to grind and polish because of their hardness.

ii.                They are technique sensitive

iii.             Checking or delayed failure of porcelain due to difference in the thermal co efficient of contraction.

iv.             The greatest disadvantage lies in the variability in the strength and quality of the brazed or pre soldered connectors.  These are susceptible to brittle fracture and this is due to the fact that the pre soldered parts contain voids, flux inclusions and localized shrinkage porosity.  This can be avoided using the cast joining process.

iii.  Titanium

          Commercially pure titanium is an element rather than an alloy.  But since it is also used, it is discussed here.

          It is a slight weight metal with a density of 4.51g/cm³.  It has a low elastic modulus of 110 GPa, which is about half that of the other base metal alloys.  IT has a relatively high melting point of 1668°C and a low coefficient of thermal expansion of 8.4 x 10^-6/°C.  This value is far below that of porcelains.  Therefore, low fusing porcelains should be used.  IT has a good passivating property.  IT has a poor oxidation resistance above 650°C.  At room temperature, it exists as a low strength but a ductile metal while heating to above 883°C, it forms a hard, more brittle ß phase.

          This is non toxic and found to be the most bio compatible of all metals.

          This is being used for crowns and removable partial dentures.  It is an excellent choice to patients with known allergy to nickel.

Titanium alloys

The most common alloy used is Ti-Al-Va.   This contains 90% Ti, 6% Al, 4% Va.  The major benefits of alloying are strengthening and stabilization of the alloy against the formation of α and ß phases seen in the pure metal.  The former is formed by the addition of Aluminium and the latter due to Copper, Palladium or Vanadium.

COBALT CHROMIUM NICKEL ALLOYS

          These alloys which were first marketed for use in 1950s, were originally developed as watch springs.  They were known as elgiloy.

Composition

          40% cobalt, 20% chromium, 15% nickel, 15.8% iron, 7% molybdenum, 2% manganese, 0.16% carbon and 0.04% beryllium.

These exhibit excellent tarnish and corrosion resistance in the oral environment.

Types

          It is available in four tempers (soft, ductile, semi resilient and resilient) which are color coded.  The soft variety is color blue and the most widely used.  All can be heat treated.

Heat treatment

          The softening heat treatment is at 1100°C to 1200°C followed by a rapid quench.

          The age hardening temperature is 260°C C to 650°C for elgiloy it should be kept at 482°C for 5 hours.

          Heat treatment is 482C for 7 to 12 minutes.

          These stress relief heat treatment is at 370°C for 11 minutes.  This treatment not only improves the elastic properties but also decreases the corrosion.

Properties

          These alloys should not be annealed, since the softening effect cannot be reversed by heat treatment.  The hardness, yield strength and the tensile strength are the same as the stainless steel alloys.  Ductility is greater than the stainless steels in the softened state whereas less in the hardened state.

NICKEL-TITANIUM ALLOYS

          It was introduced commercially during the 1970s following research by Andreason and his colleagues.  They were called as NITINOL and this name came from the two elements nickel and titanium and the Naval Ordinance Laboratory where these alloys were developed first by duehler and associates.

Composition

          These contain 54% nickel, 44%.  Titanium and generally 2% or les of cobalt.  This result in the 1:1 atomic ratio of the two major components.

          As with the other systems this alloys can exist in various crystallographic forms.  At high temperature a BCC lattice referred to as austenitic phase is table.  Whereas appropriate cooling can induce the transformation HCP martenistic phase.  This transformation can also be induced by the application of stress.  There is a volumetric change associated with the transition and an orientation relation is developed between the phases.  This phase transition results in two unique features. Shape memory and super elasticity (Psuedoelasticity).

          The cobalt is used to control the lower transition temperature which can be near mouth temperature. The memory effect is achieved by establishing a shape at temperature near 482°C and cooling it followed by forming it into another shape.  When this is heated through the lower transition temperature the wire will return to its original shape.

          Inducing the phase transition by stress can produce super elasticity.  The strain developed due to the stress is caused by a phase change that results from a change in the crystal structure.  These alloys have large working radius.  They are difficult to form and have to be joined by mechanical crimps as they can not be soldered or welded.

ß-TITANIUM ALLOYS

          Pure titanium is polymorphic or allotrophic.  At temperature above 880°C, the HCP or the α crystal lattice is stable whereas at high temperatures the metal rearranged into a BCC or ß crystal lattice.  Certain elements like Al, C, O and N stabilize the HCP structure whereas other such as V, Mo and Ta stabilize the BCC structure.

The Ti-Al-V alloy contains both these crystal structures.

The Ti60% Al 40% alloy is based on the HCP lattice.  An alloy to the composition of Ti 79% Mo -115 and Sn 4% is produced as TMA and is used for orthodontic purposes.  These contain the ß crystal structure.  This can be cold worked and heat treated.

It can be joined by electrical resistance welding which need not be reinforced with solder.  This is the only orthodontic alloy which is considered to possess true weld ability.

Both the forms of Ti have excellent corrosion resistance and environmental stability.  This is because of the oxide.  B Ti is the only major orthodontic alloy that is Ni free.  These properties of Ti stimulated its use in heart valves, hip implants and orthodontic wires.

RECENT ADVANCEMENTS

          The recent advancement in the metal field is the development of SINTERED COMPOSITE

          These composites consist of sintered high noble alloy sponge infiltrated with an almost pure gold alloy.  The result is a composite between the two gold alloys that is not cast, but fired onto a refractory die.  The porcelain does not bond through an oxide layer in these systems, but it bonds mechanically to a micro rough surface.

          The advantages of this that any stress concentration on the ceramic is relieved by the excellent ductility of the metal.

          It has been claimed that these systems support few periodontal pathogens around the restoration have yet to be substantiated.

CONCLUSION

          Thus a variety of metals and alloys are available.  These possess the main advantage over resins in that, they are able to transfer heat which is due to the thermal conductivity.  This is gives a more acceptable appliance.  But the main disadvantages as we all know is the esthetics because of which the metal free dentistry is gaining wide spread popularity.

          But the use of all ceramic is not favored, since they require extensive tooth preparation.  More over they are susceptible to fracture because their brittleness.  Therefore the vast majority of restorations are metal ceramic.

          Finally the guidelines for the selection of an alloy for a restoration should be based on :

1.       A thorough understanding of the alloy

2.       Avoid selecting an alloy based on its color unless all other factors are equal

3.       Know the complete composition of alloys, and avoid elements that are allergic to the patient

4.       Whenever possible use single phase alloys

5.       Using clinically proven products from quality manufacturers

6.       Use alloy that have been tested for elemental release and corrosion and have the lowest possible release of elements.

7.       Focus on long term clinical performance

8.       Finally it is important for the dentist to remember and take up the responsibility of being responsible for the safety and efficacy of any restoration.

REFERENCES

1.       Anderson’s Applied Dental Materials – John F.Mc. Cabe

2.       Dental Materials – Craig. O’Brien – Powers

3.       Essentials of Dental Materials – S.H. Soratur

4.       Material and Metallurgical Science – S.R.J. Shantha Kumar

5.       Materials Science and Engineering – V. Ragahavan

6.       Phillips Science of Dental Materials  (Eleventh Edition) – Anusavice

7.       Restorative Dental Materials (Eleventh Edition) – Robert G. Craig and John. M. Powers

8.       Restorative Dental Materials – Floyd. A. Peyton

9.       J.P.D. April 2002 Volume 87 No.4 Page 351 – 363.