Noble Metal Alloys

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INTRODUCTIONAlthough 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 metalceramic restorations, the most common crown and bridge prosthesis and most durable of all esthetic restoration, especially when used to restore posterior teeth.

CHEMICAL AND ATOMIC STRUCTURE OF METALSy The Metals Hand Book (1992) defines a metal as an opaque lusturous

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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 threedimensional,branched network structures emanating from the central nucleus.

y Crystal growth continues until all the material has solidified and all the

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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.


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.



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.y DEFINITION:-A crystalline substance with metallic properties

that is composed of two or more chemical elements , at least one of which is metal.y Alloys may be classified as binary (2 constituents) , ternary (3

constituents ) , quaternary(4 constituents) etc.y Usually, the properties of an alloy relate more directly to the

atomic percentage rather than weight percentage of each element.


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 alloys 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.

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 .

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.

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 cellsmay be set up on the surface of the alloy between areas of different alloy composition.

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 .

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.

The internal stresses of a cold worked metal can be removed by a heat treatment at a temperature w