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Although popular press dental journals have occasionally promoted “metal free” dentistry as desirable , the metals remain the only clinically proven materials for many long term dental applications.

Although the use of cast metals has decreased in recent years because of increased consumer demand for esthetics over durability , a knowledge of the structure and properties of cast metals and alloys is essential to ensure proper handling of this materials in clinical practice.

Furthermore, cast metals are used as copings or substructures for metal-ceramic restorations, the most common crown and bridge prosthesis and most durable of all esthetic restoration, especially when used to restore posterior teeth.

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The Metals Hand Book (1992) defines a metal as “an opaque lusturous chemical substance that is good conductor of heat and electricity ,and when polished , is a good reflector of light.”

Of the 115 elements currently listed in most recent version of periodic tables of elements, about 81 can be classified as metals.

Metals are usually-hard, lusturous,dense, good conductor of heat and electricity,opaque, ductile and malleable.

Metals are electropositive , that is , they give positive ions in solution.Metals usually have crystalline structures in the solid state. When a

molten metal or alloy is cooled, the solidification process is one of crystallization and is initiated at specific sites called nuclei.

Crystals grow as dendrites, which can be described as three-dimensional,branched network structures emanating from the central nucleus.

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Crystal growth continues until all the material has solidified and all the dendritic crystals are in contact.Each crystals is known as a grain and the area between two grains in contact is the grain boundary.

The atoms within each grain are arranged in a three dimensional lattice .

Although there is a tendency towards a perfect crystal structure , occasional defects occur. Such defects are normally called dislocation.

When the material is placed under a sufficiently high stress the dislocation is able to move through the lattice until it reaches a grain boundary.The plane along which the dislocation moves is called a slip plane .

Grain boundaries form a natural barrier to the movement of dislocations.The concentration of grain boundaris increases as the grain size decreases.Metals with finer grain structure are generally harder and higher values of elastic limit than those with coarser grain structure.Hence it can be seen that material properties can be controlled to some extent by controlling the grain size.

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Adjacent grains have different orientations , because the initial nuclei acted independently from each other .Thus at the grain boundary – a narrow region 2 to 3 atomic diameters wide , the atoms take up positions intermediate between those of the atoms in the adjacent lattices.

The good electrical and thermal conductivity of metals occurs because of mobility of valence electrons in the crystal lattice.

The corrosion properties of metals depend on the ability of atomic centres and electrons to be released in exchange of energy.

Like the physical properties , the mechanical properties are also a result of the metallic crystal structure and metallic bonds.

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An alloy is a substance composed of two or more elements at least one of which is a metal, possess metallic properties and holds metallic bonds.

DEFINITION:-A crystalline substance with metallic properties that is composed of two or more chemical elements , at least one of which is metal.

Alloys may be classified as binary (2 constituents) , ternary (3 constituents ) , quaternary(4 constituents) etc.

Usually, the properties of an alloy relate more directly to the atomic percentage rather than weight percentage of each element.

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ALLOY MICROSTRUCTURE When a molten metal is cooled , the first solid alloy particles form as the

temperature reaches the liquidus . This process is called nucleation . In some alloys , fine particles of a high – melting point element such as Ir are

added to encourage nucleation throughout the alloy. As cooling continues , the nuclei grow into crystals called grains , and the grains enlarge until all of the liquid is gone and the grains meet and form boundaries between each other (at the solidus temperature).

The size of the grains depends on the cooling rate , alloy composition , presence of grain refiners , and other factors . Grain size may influence an alloy’s strength , workability , and even susceptibility to corrosion.

Dendrites result from grains that grow along major axes of the crystal lattice early in the freezing process. The dendritic skeleton structure persists to room temperature if the cooling rate of the alloy is too fast to allow equilibrium to occur. Dendritic structure indicates that the alloy is not at equilibrium and its presence can increase the corrosion of the alloy.

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A slow cooling rate and few impurities generally lead to large grains .Faster cooling rates or the presence of grain refiners lead to smaller grains . Grains that are uniform in size and shape throughout the alloy are described as equiaxed .

Fine – grained (equiaxed) alloys are generally more desirable for dental applications because they have more uniform properties.

Different phases of a multi-phase alloy may also be seen in cast microstructure. The microscopic appearance of a cast metal is crystalline and sometimes has

dendritic structure.When the metal is subjected to cold working , such as drawing into a wire , the grains are broken down, entangled in each other , and elongated to develop a fibrous structure .

During the formation of wrought structure , the original grains produced during the crystallization of original casting are deformed and broken into small units .The deformation of the metal mass occurs by slippage of one portion past another along definite crystallization planes. The deformation and slippage occur in various directions to distort the grain boundaries.The greater the cold working the greater is the degree of grain boundary deformation .

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This deformed structure is unstable in nature , with greater internal energy than one that is in the cast condition. Accordingly , it possesses modified physical properties and has the tendency to recrystallize when heated.The characteristic fibrous structure of the wrought mass is gradually lost, and the grains or crystalline structure reappears. This process is known as recrystallization or grain growth. The cause for grain growth in the wrought structure is related to the tendency for metals to maintain a crystalline internal orientation of the component atom.

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On cooling a pure molten metal , a temperature –time curve is obtained.

This graph shows three portion –(1)for the cooling of molten metal (2) a plateau-a horizontal portion – during this time the metal is solidifying , and there is evolution of latent heat of fusion which compensates for the heat loss to the surrounding ; and (3) a portion for the cooling of the completely solidified metal.

The cooling curves for alloys show no such plateau region.Here crystalization takes place over a range of temperatures.

Each alloy grain can be arranged as having a concentration gradient of metals;the higher melting metal being concentrated close to the nucleus and the lower melting metal close to the grain bounderies.The material is said to have a cored structure .Such

coring may influence corrosion resistance since electrolytic cells may be set up on the surface of the alloy between areas of different alloy composition.

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Although dental base metal casting alloys typically solidify with a dendritic microstructure , most noble metal casting alloys solidify with an equiaxed polycrystalline microstructure ; The term equiaxed means that the three dimensions of each grain are similar , in contrast to the elongated morphology of the dendrites .

A given atomic plane is discontinuous at a grain boundary. The dislocations can not cross from one grain into an adjacent grain , and they will subsequently pile up at the grain boundaries. When this occurs , further deformation in these regions will require greater stress.

It is also evident that the grain boundaries are the final sites to undergo freezing for a molten metal that forms an equiaxed grain structure.Consiquently low melting phases ,precipitates, and porosity are typically found at the grain boundaries .

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Hot tears (microcracks) can form at elevated temperatures in thin areas of castings prepared from alloys with dendritic structures, where there is insufficient bulk metal to resist the stresses imposed by the stronger casting investment, and these cracks will degrade the mechanical properties of the restorations.To avoid hot tears ,castings need to have adequate thickness,and an alloy should be selected that has an equiaxed grain structure in the as-cast condition.

Cooling of molten metal should be done rapidly,to get a fine grain structure,if strength and hardness are important.

Cold working increases hardness and strength – this is known as work-hardening. However this reduces percentage elongation ,the material becomes more brittle . It becomes liable to fracture if further cold work is carried out , because the potential for further slip has been lost.

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The internal stresses of a cold worked metal can be removed by a heat treatment at a temperature well below the recrystallisation temperature.

Recrystallisation gives a fine grain structure .Sometimes cold working followed by heat treatment is used to obtain a fine structure , to improve the properties of the material.

Overheating , causing grain growth , must be avoided if properties such as high yield strength and hardness are desired.

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Metals can be mixed together much as liquids are.Alloys may be a mixture of as few as two or as many as nine or more different metals. As with liquids , not all metals will dissolve in one another freely ; some metals will not dissolve at all into other metals.The concept of phases and phase diagrams was developed to help explain the nature of alloys and metal solubility.

Phase diagrams are “maps” of the phases that occur when metals are mixed together.

If a series of cooling curves for alloys of different composition within a given alloy system are available , a phase diagram can be constructed from which many important predictions regarding coring and other structural variation can be made.

When metals are mixed together in the molten state , then cooled in the solid state , there are several outcomes depending on the solubility of the metals in each other. If the metals remain soluble in each other ,the result is a solid solution .If the metals are not soluble in the solid state ,then an eutectic may form . Sometimes the elements react to form a specific compound , called an intermetallic compound.

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SOLID SOLUTION AND ITS PHASE DIAGRAM In a solid solution there is only one phase present. A phase is defined as any

physically distinct , homogeneous , mechanically separable part of a system. Solid solutions may be of two types:

(1)Substitutional solid solution : These types of solid solutions are formed where two different types of atoms occupy different position in crystal lattice.They are formed between two metals if;

a) Their atomic sizes differ by less than about 15%.

b) They have the same type of crystal lattice.

c) They have the same chemical valency .

d) They do not react to form intermetallic compounds.

Substitutional solid solutions may be either disordered with a random distribution of the atoms of two metals , or ordered.Ordered solutions impart higher hardness and strengths to alloys.

(2) Interstitial solid solution : These are formed where very small atoms can be accommodated in the interstices or spaces between larger atoms.Ex-carbon in iron to form steel.

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The solid solution system is characterized by a series of melting ranges that are more or less smooth transition between the two melting points of the pure elements .The temperature distance between the liquidus and solidus determines the melting range; it is characteristic for each alloy system and varies with the composition within a system .

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An indication of the degree of coring is given by the separation of the

solidus and liquidus lines on phase diagram.The potential for coring is greater when there is wide separation of solidus and liquidus lines in the phase diagram.

With slow cooling the crystallization process is accompanied by diffusion and a random distribution of atoms results, with no coring.Rapid cooling quickly denies the alloy the energy and mobility required for diffusion of atoms to occur and the cored structure is ‘locked in’ at low temperatures. Reducing the cooling rate as means of eleminating coring will be self – defeating since it would produce an alloy with large grain size which , of course , would have inferior mechanical properties.

Since coring may markedly reduce the corrosion resistance of some alloys , a heat treatment is sometimes used to eliminate the cored structure.Such a heat treatment is called a homogenization heat treatment.


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This involves heating the alloy to a temperature just below the solidus temperature for a few minutes to allow diffusion of atoms and the establishment of homogeneous structure.The alloy is then quenched in order to prevent grain growth from occuring.

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When two metals are mutually soluble in solid state , the solvent is that metal whose crystal structure persists.If two metals are completely soluble in all proportions and the same type of crystal structure occurs throughout the alloy system , the solvent is defined as the metal whose atoms occupy the majority of the total number of positions in the crystal structure.

Whenever a solute atom substitutes for a solvent atoms in a crystal structure of a metal,the different size of the solute atom resuls in a localised distortion, and the movement of the dislocations become more difficult.The strength, proportional limit and hardness are increased, and the ductility is usually decreaased.Thus solid solution alloying can be a highly efficient means of strengthening a metal.

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EUTECTIC ALLOYEUTECTIC ALLOY Many binary alloy systems do not exhibit complete solubility in both the liquid

and solid states.The eutectic system is an example of an alloy for which the component metals have limited solid solubility.

Such a system of interest to dentistry is the silver – copper system . In the phase diagram for this system 3 phases are found.1)liquid phase (L)

2)a silver –rich substitutional phase (α) containing a small amount of copper atoms , and 3)a copper-rich substitutional solid solution phase (β) containing a small amount of silver atoms.

The α and β phases are sometimes called as terminal solid solutions because of their location at the left and right sides of the phase diagram.

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The solidus can be identified as the boundary line ABEGD,since the liquid phase is not found below this line.

The liquidus can identified as AED since there are no solid phases above this line.

The major portion of the diagram below 780⁰ is composed of a two phase system which represents a mixture of the silver rich and copper rich phases in the alloy microstructure.

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The first difference to be noted when comparing with the solid solution system that the liquidus and solidus meet at a point (E).This composition is known as the eutectic composition or simply the eutectic .

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The temperature at which the eutectic composition melts is lower than the fusion temperature of the parent metals(eutectic literally means ‘lowest melting’ ) and is the lowest temperature at which any alloys of the composition metals is entirely liquid.

There is no solidification range for composition E .The eutectic liquid composition freezes at a constant temperature , similar to a pure metal, but the solid consists of two phases (α and β).Eutectic alloys are often used when a lower fusion temperature is desired, such as for dental solders.

However eutectic alloys are inferior in other important properties compared with solid solution alloys.

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In a peritectic system the terminal solid solution in combination with liquidus give rise to a separate single phase different from its two components. This is again a liquid solid transformation where two phases combine to give rise to a single phase, unlike eutectic, where a single phase give rise to two phases.

The silver-tin system ,which is the basis of the original dental amalgam alloy, is a peritectic system.

(Ref: Philips’ Science of Dental Materials )

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INTERMETALLIC COMPOUNDINTERMETALLIC COMPOUNDMetals with chemical affinity for each other can form intermetallic

compounds .If two metals react to form new compound with a specific composition an intermetallic compounds results.

Ex- the compound Ag3Sn can be formed between silver and tin.

(Ref:Craig’s Restorative Dental Materials & Notes On Dental Materials; E.C. Combe)

In the formation of these chemical compounds normal chemical valencyof the metals does not always apply.

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Removable Partial Denture (RPD)

Metal-ceramic orPorcelain-fused to metal restoration

All metal restoration

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ObjectivesObjectivesUnderstand the alloy classificationsKnow the roles of each element in

dental casting alloysKnow the requirements of porcelain-

fused to metal (or metal-ceramic) alloys

Understand the relation between the TCOE of PFM alloys and that of ceramics

Recognize the importance of some properties of the alloys

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1907 : Lost wax technique by Taggart1932 - 1948 : Standardization of dental

casting alloys1950s -1960s : Development of porcelain-

fused-to-metal (PFM) alloysFound that adding Pd and Pt to gold (Au) would

lower coefficient of thermal expansion sufficiently to ensure physical compatibility between the porcelain veneer and the metal substructure.


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1970s : Placement of gold on the free marketIncreased prices stimulates the search for

alternative low gold and base metal alloys

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Noble metals◦ Elements with good metallic surfaces that

retain their luster in clean dry air◦ Indicate the relative inertness of the element in

relation to the standard EMF series◦ Resist oxidation, tarnish and corrosion during

heating casting and soldering Platinum group (6 metals)

Platinum, Iridium, Osmium (atomic wt 190, density 22 g/cc)

Palladium, Rhodium, Ruthenium (atomic wt 100, density 11-12 g/cc)

Gold (atomic wt 196, density 19.3 g/cc) (Silver?)

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Precious metals◦Indicates how expensive a metal is

based on supply and demand.◦**The descriptors precious and

semiprecious should be avoided because they are imprecise terms.

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Gold content of a dental alloy◦Karat (K)

Parts of pure gold per 24 parts of alloy . e.g. 18K, 24K

◦Fineness Parts of pure gold per 1,000 parts of alloy.

e.g. a 650 fine alloy has a gold content of 65% Primarily used for gold solders

Pennyweight (dwt.)◦ 1 dwt = 1.555 gm = 0.05 oz

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ADA ADA SpecificationSpecification


ADA ADA SpecificationSpecification


ADA’s ADA’s ClassificationClassification

ADA’s ADA’s ClassificationClassification DescriptiveDescriptive



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Referred to G - old based alloys • Alloys can have any composition as long as they

pass the tests for toxicity, tarnish, yield strength, and percent elongation.

Type %Au & Pt VHN Restoration

I (soft) 83 50-90 Inlay

II (medium) 78 90-120 Inlay/onlay

III (hard) 78 120-150Onlay/


IV (extra-hard)

75 150-250Crown&Bridg


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1 . High noble (HN)2. Noble (N)3. (Predominantly) Base metal (PB)

Alloy Type Total Noble Metal Content

High Noble


Predominantly Base Metal

Must Contain >40% Au and >60% of noble metal elements.Must contain >25%of noble metal elements.Contain <25% of noble metal elements.

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Principal ElementsWhen an alloy is identified according to the elements

it contains, the components are listed in declining order of composition, with the largest constituent first followed by the second largest constituent.◦ e.g. Au-Ag-Pt (Au ~ 78%, Ag ~ 12%, Pt ~10%)

Exception: Certain elements that significantly affect physical properties or that represent potential biocompatibility concerns are often designated (regardless of their small amounts).◦ e.g. Au-Cu-Ag-Pd (Au ~40%, Cu ~7.5%, Ag ~47%, Pd~4%)

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Descriptive Classification

Normal-fusing alloys ◦ Medium-gold◦ Low-gold◦ Silver-palladium◦ Silver-indium

High-fusing alloys (mostly for PFM)◦ Gold-platinum-

palladium◦ Gold-palladium-silver◦ Gold-palladium◦ High-palladium◦ Palladium-silver◦ Base-metal

Cr/Co; Cr/N

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All-Metal Restorations

Metal-Ceramic and All-MetalRestorations


High Noble> 40 wt% Au and > 60% of the noble metal elements

Au-Ag-Cu-PdAu-Pt-PdAu-Pd-Ag (5-12 wt% Ag)Au-Pd-Ag (>12 wt% Ag)Au-Pd (no Ag)


Noble> 25 wt% of the noble metal elements


Pd-Au (no Ag)Pd-Au-AgPd-AgPd-CuPd-CoPd-Ga-Ag


Base Metal< 25 wt% of the noble metal elements

Pure TiTi-Al-VNi-Cr-Mo-BeNi-Cr-MoCo-Cr-MoCo-Cr-W

Pure TiTi-Al-VNi-Cr-Mo-BeNi-Cr-MoCo-Cr-MoCo-Cr-W

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• Gold (Au)• Platinum (Pt)• Palladium (Pd)• Silver (Ag)• Minor alloying elements

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Gold (Au) Gold (Au)

Soft, (most) malleable and ductile

Relatively low strength

Tarnish resistant in air and water at any tem

p. Attacked by only a fe

w of the most powerfu l oxidizing agents

Insoluble in sulfuric, ni tric, or hydrochloric ac

ids Soluble in a combinati

on of nitric and sulfuri c acids -(aqua regia)

Small amounts of im purities (ie. lead, mer

cury , base metals) h ave a pronounced an

d usually detrimental effect on its propertie


Fusion temp = 1063°C

Density = 19.3 g/cm3

Thermal coef. of exp. = 14.2x10-6/°C

MOE = 80 GPa

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Platinum (Pt) Platinum (Pt)

Tough, malleable a nd ductile

Ver y hi gh cost ( u sual l y r epl aced b

y Pd i n most moder n al l oys)

Hi gh cor r osi on r esi st ance

Higher melting tem p than porcelain

Fusion temp = 1 7 55 °C >Au

Density = 2 1 .3 7 g/cm3 >Au

Thermal coef. of exp. =89. x10-6/°C <Au

MOE = 1 4 7 Gpa >Au

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Palladium (Pd) Palladium (Pd)

Not used in the pure state dentistry

Has replaced Pt in d ental casting alloys

Decreased cost v.s.Pt

Helps prevent corro sion of silver in the o

ral environment Absorbs H

2 gas whe

n heated improperly

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Silver (Ag) Silver (Ag)

“Noble?” Malleable and ductile Best known conductor

of heat and electricity Harder than gold Unaltered in clean dry

air, however, combine s with sulfur, chlorine

and phosphorus result ing in severe tarnish in the oral environment

Occludes large quantitie s of O

2 in molten state


gas will evolve durin g solidification resulting in pits and porosities.

Fusion temp = 960.5°C Density = 10.4 g/cm3


MOE = 120 GPa

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Minor Alloying Elements Minor Alloying Elements

- Iridium (Ir) grain refining - Ruthenium (Ru) grain refining TTTTT TTTTTTTT

The addition of as little as 50 ppm (0.005%) o 100f Ir and Ru results in a x increase in the n

o. of grains per unit volume. Increases the alloy’s tensile strength and %e

longation by >30% Increases tarnish resistance, slightly increas

es yield strength TTTT TTT TTTTTTTTTTly aff ect har dness

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FUNCTION OF EACH ELEMENT Primarily ternary alloys of Au, Ag and Cu, with

minor amounts of Pt, Pd and Zn. Approx. >90% of the total alloy content is Au, Ag

and Pd Gold (Au)

Tarnish and corrosion resistance○ Tarnish is an inverse function of gold content.

Contributes burnishability, ductility, and ability to heat harden the alloy

Silver (Ag) Helps control the color of the alloy, neutralizing the red color imparted by

Cu Promotes ductility

○ Au/Cu alloys (75% Au) break apart at grain boundaries during heat treatment if no Ag is present.

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Platinum (Pt)Very expensive ingredientContributes strengthWhitens the alloyIncreases the fusion temperature

Platinum (Pt)Very expensive ingredientContributes strengthWhitens the alloyIncreases the fusion temperature

Copper (Cu) ***Principle hardener in gold alloysConc. >12% of Au amount alloy can be heat treatedConc. >18% decrease the melting temp of the alloy

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When alloyed with Ag, Cu increases the alloy’s hardness and decreases melting temp.

Cu imparts a reddish color to the metal and contributes most to the corrosion of gold alloys.

Ag/Cu ratio is important to tarnish resistance (but not as important as the Ag/Pd ratio).

Cu is not found in PFM alloys due to its tendency to discolor the porcelain.

Zinc (Zn) O2 scavenger 1-2% helps to counteract the absorption of O2 by silver. Increases the castability, decreases porosities, and increases the

hardness and brittleness of the alloy

Indium (In), Tin (Sn), Iron (Fe)Hardens the alloy (Provides oxides for ceramic bonding in PFM alloys)

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Iridium (Ir), Ruthenium (Ru), Rhenium (Rh)Grain refining

Gallium (Ga) Added to high Pd alloys or non-silver Au/Pd metal ceramic alloys to

compensate for a decrease in the TCOE caused by the elimination of the Ag.

(Also provides oxides for ceramic bonding)

Type %Au %Cu %Ag %Pd %Pt %In,Sn,Fe,Zn,Ga

I 83 6 10 0.5 - Balance

II 77 7 14 1 - Balance

III 75 9 11 3.5 - Balance

IV 69 10 12.5 3.5 3 Balance

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Heat TreatmentCu/Au system is the basis for heat treatment

Cu:Au ratio > 12:88 the alloy is heat treatable. Above 424°C solid solution

Quenching from above 424°C will result in a softer, more ductile alloy with decreased strength

Below 424°C ordered crystal lattice Alloy has increased strength, hardness and decreased

ductility. The amount of transformation is time and temperature

dependent and the process is reversible

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Softening Heat Treatment (Solution Heat Treatment) Heat alloy to 700°C for 10 min. then quench.

Decreased tensile strength, proportional limit and hardness Increases ductility and %elongation MOE not significantly altered.

Indicated prior to adjusting, burnishing and polishing

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Hardening Heat Treatment (Age Hardening) Heat alloy to 450°C for 2 min., cool slowly to

250°C over 30 mins then quench. Or Heat to 350°C for 10 –15 min. and quench Increases strength, proportional limit and hardness Decreases ductility and %elongation

Indicated for RPD frameworks and long span FPD’s

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Alloys for PFM or Metal Ceramic Alloys for PFM or Metal Ceramic RestorationRestorationAu-Pt-PdAu-Pd-AgAu-PdPd-Ag High Pd

No Copper!No Copper!

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some importantsome important Requirements RequirementsMust h ave t he pot ent i al t o bond t o dent al por c

TTTTT - need oxide forming elements (small amount of base metal



- permit the application of low fusing porcelains>100°C than the firing temp of the ceramic

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Ceramic-Metal BondCeramic-Metal Bond Typically, TCOE of porcelain = 13.0 to 14.0 x

10-6 /°C and the metals = 13.5 to 14.5 x 10-6TT C.

The difference of 0.5 x 10-6 / °C causes t he me tal to contract slightly more than does the cer

amic during cooling after firing the porcelain. This condition TTTT TTT TTTTT TT TTTTT TTTTTT T

esidual compression , which makes it less sens itive to applied tensile forces.

Chemical bonding by oxide layer.Also micromechanical bonding is there.

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Palladium in PFM AlloysPalladium in PFM Alloys Hardens the alloy Whi t ens t he al l oy TTTTTTT TTTTTTT TTTTT’ . Increases the alloy’s MOE Renders silver tarnish resistant Decr eases t he al l oy’ s densi t y Decr eases t he al l oy’ s t her mal coef . of

exp. T T - TTTTTTT , , , met al l i c oxi des for porcel

ain bonding, and harden the alloy. - Ga increases the thermal coef. of exp. to compens

ate for decreased or absence of Ag.

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Considerations on Properties Melting Range

The solidus-liquidus range should be narrow to avoid having the alloy in a molten state for extended times during casting. To decrease oxides and contamination

Liquidus temp determines the burnout temp, type of investment, and type of heat-source. Burnout temp liquidus temp – 500°C Burnout temp >700°C, cannot use gypsum-bonded

investment Liquidus temp: Base-metal 1400°-1500°C vs. cast

gold Type I-IV 800°-1050°C Liquidus temp < 1100°C gas-air torch, >1100°C

gas-oxygen torch or electrical induction Solidus temp is important to soldering and formation of

ordered phases. Limit heating to 50°C below the solidus temp.

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Alloys with high densities will generally accelerate into the mold during casting and tend to form complete castings more easily. Base-metal 7-8 g/cc vs. High Noble 13-18

g/cc Alloys with lower density lighter Yield Strength Can be increased with treatment and

changing the compositions

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Is a good indicator of the ability of an alloy to resist local permanent deformation under occlusal load

Gives some indication of the difficulty in polishing the alloy

Most noble casting alloys < enamel (343 Kg/mm2) and < base-metal alloys

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Noble alloys related to elemental release from the alloys (i.e., from the corrosion process).

Base-metal alloys Be from contact dermatitis to

severe chemical pheumonitis Ni sensitivity

5-10 times higher for females 5%-8% of females

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Low Gold Alloys:- They are crown and bridge alloys having the gold

content below 60%(usually in range of 42%-55%).However gold must be the major element.

The low gold alloys were developed because of the increase in gold prices.However reducing in gold content increase in tarnish and corrosion.This problem was overcome by two discoveries.

1) Palladium made the silver more tarnish resistant.1% palladium is required for every 3% of silver.

2) The silver copper ratio had to be carefully balanced to yield a low silver rich phase in the microstructure.

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SILVER -PALLADIUM ALLOYS These alloys were introduced as an alternative

to gold alloys. It is predominantly silver in composition. Palladium (atleast 25%) is added to provide nobility and resistance against tarnish.They may or may not contain copper and gold.

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The 20 th century generated substantially new changes to dental prosthetic materials.The major factors that are driving new developments are;economy,performance and esthetics.

But we must not forget that it all had been started with gold and gold itself is the GOLD STANDARD in cast alloy.So although gold is now old (material) but ,we shouldn’t forget ,OLD IS GOLD .

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REFERENCES:- Phyllips’ science of dental materials(11 th ed). Craig’s restorative dental materials(12 th ed). Applied dental materials –McCabe(7th ed). Dental materials science – P.K.Basu(4th ed). Basic dental materials- John J.Manappallil(2nd

ed). Notes on dental materials- Coomb(6th ed). Material science in dentistry-

Greener;Harcourt;Lautenschlager. Dental Clinics Of North America –

July ;2007 ;Vol-51,no-3

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