Сraig. Dental Materials

several outcomes depending on the solubility of the metals in each other. If the metals remain soluble in one another, the result is a solid solution. If the metals are not soluble in the solid state, then a eutectic may form. Sometimes, the elements react to form a specific compound, called an intermetallic compound. The phase diagrams that describe these outcomes will be presented in the following paragraphs.

Solid SolutionAUoy s Figure 6-8 shows a series of binary alloys of Au and Cu. Because there is only a single phase below the solidus (S), this is a solid solution alloy system, common in dental casting alloys.Au and Cu are miscible;that is, they are soluble in any combination. If one were to examine the crystal structure of this alloy, the Au atoms would occupy some the positions of the face-centered cubic structure and the

Cu atoms would occupy others. The relative positions of the Au and Cu atoms would be random, however. The solid solution system (see Fig. 6-8) is also characterized by a series of melting ranges that are more or less a 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.

The Au-Cu system (see Fig. 6-81 also has several dotted lines below the solidus for alloy compositions between 20 and 70 atomic percent Au. These lines indicate the formation of ordered solutions. In ordered solutions, the Au and Cu atoms still occupy the face-centered cubic positions, but in a specific pattern that depends on the composition. Ordered solutions impart higher hardness and strengths to alloys (see Chapter 15).

Eutectic Alloys Fig. 6-9 shows a series of binary Ag-Cu alloys. In this phase diagram, the liquidus and solidus meet at a mid-range composition and the solidus is lower (at 779.4' C) than either pure Ag (960.5' C) or Cu (1083' C). This liquidus-solidus configuration is characteristic of an eutectic alloy system. The Ag-Cu system is especially important in high-Cu dental amalgam, but is also important in the formulation of some dental casting alloys.

The eutectic system shown in Fig. 6-9 contains a pure solid solution below 400' C only at either extreme of composition ( 4 % Ag or 4 % Cu) because the Ag and Cu are essentially insoluble in one another in the solid state. At all other equilibrium conditions below the solidus, a mixture of a solid solution (either a or P) and the eutectic composition occurs. The eutectic composition is 28.1% Cu (and 71.9% Ag) and has a layered appearance under a light microscope

(Fig. 6-10). If the alloy is at the eutectic composition, all of the alloy will be in the eutectic phase at room temperature (see the dotted line in Fig. 6-9). If the composition is other than this, then the alloy will be some combination of the solid solution and eutectic phases. The exact proportions of eutectic and solid solution can also be determined from the phase diagram, but this calculation is beyond the scope of this discussion. Finally, note that a pure eutectic composition has a melting point (vs, a melting range) that is substantially lower than either of the pure components. The eutectic system of Pb-Sn uses the eutectic alloy composition as plumber's solder because of its low melting point.

IntermetallicCompounds If two metals react to form a new compound with a specific composition, the phase diagram reflects an intermetallic compound. Fig. 11-2 shows the for-

Fig. 6-10 A microscopic view of a Ag-Cu eutectic mixture (28.1 wt% Cu and 71.9 wt% Ag) magnified 50x. The eutectic composition is a fine layering between Ag and Cu. This layering is a result of the insolubility of Ag and Cu in the solid condition. This eutectic has a melting point rather than a melting range, as indicated on the phase diagram shown in Fig.

mation of the intermetallic compound AgsSn in the Ag-Sn alloy system. Ag,Sn is a fundamentally important intermetallic compound in dental amalgam. The Ag-Sn phase diagram is extremely complex and its complete description is beyond the scope of this discussion. However, the intermetallic compound is seen in Fig. 11-2 at 26.8 wt% of Sn as a solid vertical line extending to room temperature.

TernaryPhase Diagrams It is possible to construct a phase diagram for a ternary alloy (with three components). These phase diagrams are three-dimensional, as shown in Fig. 6-11.Two dimensional representations in the shape of an equilateral triangle are also used to represent the three-dimensional structure. The ternary phase diagrams are difficult to prepare and interpret;

Fig. 6-11 Ternary phase diagram showing the liquidsolid surface formed between components A, B, and C. The vertical axis represents increasing temperature (T). The AB, AC, and BC axes reflect concentrations of the components. The three-dimensional surface that is visible represents the liquidus for this

system. Other features of the phase diagram below the liquidus require a two-dimensional cross section parallel to the ABC plane. The shape of the liquidus indicates that this is a eutectic system.

their detailed description can be found in engineering and metallurgy texts.

Alloy Microstructure The internal appearance of alloys under light and electron microscopy has been extensively used to describe alloys and interpret alloy behavior. Atomic structure can be determined by x-ray diffraction or high-resolution electron microscopy. Alloy microstructure is viewed by polishing the alloy surface, then etching with an acid to bring out relevant features. The microstructure of any alloy is a consequence of the chemistry and thermodynamics governing the elements involved.

Grains, Grain Boundaries, and Dendrites When a molten alloy 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 highmelting point element such are Ir are added to encourage even nucleation throughout the alloy. These particles, used in this manner, are called grain refiners. 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). At this point, the grains are visible under a light microscope (Fig. 6-12) and are sometimes large enough to be seen with the unaided eye. 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 (see Chapter 15). The junctions between grains are called grain boundam'es. Grain boundaries are important because they often contain impurities such as oxides and are a site of corrosive attack. The grain boundaries are clearly visible in light microscopic views of alloys (see Fig. 6-12).

Dendrites result from grains that grow along major axes of the crystal lattice (Fig. 6-13) 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. The dendritic stmcture is common in dental alloys and can be seen after etching and polishing the alloy (Fig. 6-14). Dendritic structure indicates that the alloy is not at equilibrium and its presence can increase the corrosion of the alloy.

Cast Microstructure Cast alloy microstructures have several distinguishing characteristics. Grains are usually visible and take on the appearance in Fig. 6-12. The size of the grains may be large or small depending on the cooling rate and other factors mentioned previously. 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 (see Chapter 15). Different phases of a multiple-phase alloy may also be seen in cast microstructure.

Other factors may influence cast microstruc-

Fig, 6-13 Sketch of a dentritic structure of a crystal. The crystal grows preferentially in the x, y, or z axis due to the cooling of the alloy mass. If the cooling rate is fast enough, the dendrites will have a composition slightly different from the rest of the alloy because the equilibrium concentration specified by the phase diagram will not have had time to occur.

ture. Insoluble impurities in an alloy may be detected at grain boundaries. Defects such as gas inclusions may cause small pits in the bulk of an alloy or at the surface. In the body of an alloy, pits may concentrate the stress and contribute to restoration failure. At the surface, pits may enhance corrosion, tarnish, or discoloration from the accumulation of organic debris. Voids in an alloy may result from improper cooling or improper investing (see Chapter 17 on Casting).

When a mass of molten metal is cast into a cold mold, the metal at the mold wall freezes first, and

Fig. 6-14 Dendritic structure visible in a low-power light microscopic view of a gold alloy (polished, acidetched). The treelike branching is similar to that sketched in Fig. 6-13.

grains form and grow from the walls of the mold to the center of the mass. This type of grain growth is called colu~nnargrowth and can lead to alloy weakness from interference boundaries between the converging grains (Fig. 6-15). In general, it is more desirable to have the alloy freeze in a less ordered fashion. Sharp corners in any casting mold can enhance columnar grain growth and are generally undesirable.

Cold-Worked (Wrought) Microstructure

Metals and alloys are cast for two quite different purposes. In one instance, the casting serves as the final structure. In the second, it serves as an object that is further manipulated to form wires, sheets, bars, or similar fabricated structures. A typical cast structure in dentistry is an inlay or bridge restoration, which is not given further mechanical treatment except for polishing or marginal adaptation by hand operations. This limited treatment does not significantly modify the microstructure of the casting. Such a casting is designed to form to precision measurements, and the properties of the structure are those displayed by the cast metal or alloy.

Fig. 6-15 Interference grain boundaries developed by an alloy poured into a hexagon-shaped mold (5x1.

The sharp corners of the mold cause the dendrites to "clash"and form the interference boundaries.

When the metal is to be used for wires, bands, bars, or other types of wrought structures, it is first cast into ingots that are then subjected to rolling, swaging, or wire-drawing operations that produce severe mechanical deformation of the metal. Such operations are described as hot or cold working of the metal, depending on the temperature at which the operation is performed. Many dental structures, such as orthodontic wires and bands, are formed by coldworking operations. The finished product is often described as a wrought structure to denote that it has been formed by severe working or shaping operations. The properties of wrought structures are quite different from those of cast structures in both internal appearance and mechanical characteristics.

The microscopic appearance of a cast metal is crystalline and sometimes has dendritic structure, as shown in Fig. 6-16. When this metal is subjected to cold-working operations, such as drawing into a wire, the grains are broken down, entangled in each other, and elongated to de-

Fig. 6-16 Low-power light microscopic view of the typical grain structure of a cast gold alloy. Compare this grain structure to the same alloy after it has been pulled into wire form (see Fig. 6-17).

Fig. 6-17 Low-power light microscopic view of a wrought wire microstructure. The grains visible in

Fig. 6-16 have been broken apart and tangled among one another.The entangled grains are lined up along the axis of the wire. This type of microstructure is called a fibrous structure for obvious reasons.

velop a fibrous structure or appearance that is characteristic of wrought forms, as shown in Fig. 6-17. This change in internal appearance is accompanied by a change in mechanical properties. In general, mechanical properties of the wrought structure are superior to those of a

casting prepared from the same melt or alloy. It should be emphasized that in cast dental structures such as inlays, the mechanical properties of strength and hardness are not modified appreciably by simple polishing or marginal adaptation operations.

Recrystallization and Grain Growth

Metals or alloys that have been cold worked in the process of forming wires or bands change their internal structure and properties when heated or annealed. The characteristic fibrous structure of the wrought mass is gradually lost, and the grain or crystalline structure reappears. The process is known as recystallization or grain growth. The degree of recrystallization is related not only to the alloy composition and mechanical treatment or strain hardening received during fabrication, but also to the temperature and the duration of the heating operation. High temperatures and long heating periods produce the greatest amount of recrystallization. It is

not uncommon to find that during a soldering or annealing operation of a practical appliance, the temperature applied to the wire or band materials was sufficient to cause recrystallization of the wrought structure. Recrystallization of a segment of wire adjacent to a solder joint is shown in Fig. 6-18. Because the strength is usually reduced in recrystallized wrought structure (ductility often increases), it is necessary to guard against excessive heating during the assembly of a wrought metal appliance. Although the tendency for crystallization is more prevalent in some wires than others, it can be kept to a minimum when the time and temperature of heating are kept as low as possible.

Although there is probably some tendency for cast metal structures to recrystallize when heated after casting, grain growth does not become evident when the structure is heated within the range of practical operations.. Under excessive conditions of heating there is some evidence of recrystallization in cast alloys, but the signifi-

178 Chapter 6 NATURE OF METALS AND ALLOYS

cance is not so pronounced as in the case of wrought forms. Within practical limits of operation, therefore, this characteristic of recrystallization and grain growth is limited to wrought structures.

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 atoms. During the formation of the wrought structure, the original grains produced during the crystallization of the original casting were deformed and broken into small units. The deformation of the metal mass occurred by slippage of one portion past another along definite crystallization planes. The deformation and slippage occurred in various directions to distort the grain boundaries. The greater the degree of cold working, the greater 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 the tendency to recrystallize when heated.

Stress relieving, annealing, recrystallization, and grain growth can be illustrated by the series of sketches shown in Fig. 6-19. A wrought wire that has been bent beyond the proportional limit and that contains tensile and compressive stresses at the upper and lower boundaries is shown in Fig. 6-19, A; the wire has the typical fibrous structure and a deformed crystal lattice. Moderate temperatures cause the release of these stresses, as shown in Fig. 6-19, B, without other changes. Higher heating at annealing temperatures (Fig. 6-19, C) causes the disrupted crystal structure to contain sufficient energy to return to its normal crystal structure, but the fibrous wrought structure is still evident; under this condition the corrosion resistance is increased. Further increases in temperature or time, or both, as seen in Fig. 6-19, D, permit recrystallization with grains appearing and the fibrous structure disappearing. Finally, in Fig. 6-19. E, grain growth occurs, and the cast condition again dominates the microstructure.

Fig. 6-19 Sketches depicting the gross view (left column), microstructure (middle column), and crystal view (right column) of wrought wire that has been bent. A, The fibrous microstructure is present and arrows indicate residual stresses. B, Minimal heat leaves the fibrous structure intact but relieves the stresses. However, the lattice remains distorted. C, Annealing with more heat allows the lattice deformation to be relieved. The fibrous microstructure remains. D and

El Further heating causes a loss of the fibrous structure and growth of the grains, which increase in size with increasing application of heat.

Dental alloys are diverse and use a wide variety of metallic elements and some of the metalloids (see Fig. 6-1). The compositions of dental alloys are dependent on the clinical use and environment for an alloy. Thus a carbide bur needs to be very hard to cut tooth structure, but its intraoral

corrosion is of little concern. A dental crown must have excellent corrosion resistance and must not deform permanently. Endodontic files need to have moderate moduli and resist torsional failure. In each case, the alloys comprise elements that optimize the specific properties that are needed most for clinical success. In dentistry, solid solutions, eutectics, and intermetallic compounds and wrought alloys are used. It is the diversity of possible properties that makes alloys suitable for many clinical dental applications.

ALLOY-STRENGTHENING MECHANISMS

Alloys generally require high strength and hardness to be useful in dental applications. Several metallurgical strategies are used to strengthen alloys to appropriate levels. Nearly all of these strategies share the goal of impeding the deformation of the alloy by the dislocation mechanism (see previous discussion, this chapter).

Solid solutions are generally stronger and harder than either component pure metal. The presence of atoms of unequal size makes it more difficult for atomic planes to slide by each other. Even small differences in atomic size can strengthen alloys by the solid solution mechanism. Ordered solutions act to further strengthen a solid solution by providing a pattern of dissimilar sizes throughout the alloy's crystal structure. Solid and ordered-solution strengthening are very common in dental casting alloys.

Precipitation hardening is another strategy used to strengthen dental alloys. By heating some cast alloys carefully, a second phase can be made to appear in the body of the alloy. The new phase blocks the movement of dislocations, thereby increasing strength and hardness. The effectiveness of precipitation hardening is greater if the precipitate is still part of the normal crystal lattice. This type of precipitation is called coherentpreczpitation. Overheating may reduce alloy properties by allowing the second phase to grow outside of the original lattice structure. Fig. 6-20 shows the character of a casting alloy that was strengthened by the addition of as little as

Fig. 6-20 Electron micrograph of FePt, (black, hairlike structures) formed in a gold-platinum alloy containing 0.08 wt% iron.

(From Sims JR Jr., Blumenthal RN, O'BrienWJ: J Biomed Mater

Res 7497, 1973.)

0.08 wt% iron to a gold-platinum alloy. The iron formed an FePt3 precipitate and the hardness of the alloy tripled under appropriate conditions.

Other factors may increase alloy strength and hardness. Grain refiners such as Ir, Rh, and Ru improve the strength of alloys by several times. Moreover, strength and hardness are generally improved without sacrificing ductility. Finegrained alloys have grain sizes below 70 pm in diameter. Cold working an alloy will significantly strengthen it. Cold working works out the dislocations, thereby making further deformation more difficult. However, cold working will also embrittle an alloy by making it less ductile.

PROPERTIES O F CASTING ALLOYS

In general, the properties of the solid solutions resemble those of the metals forming the alloy, with certain exceptions. Solid solution alloys often have higher strength and hardness and lower ductility than either pure metal. This is the basis of solid-solution strengthening. Solid solutions also possess melting ranges rather than melting points and always melt below the melting point of the highest points of both metals. These alloys are commonly used in dentistry because they have higher corrosion resistance than multiplephase alloys. Also, in a few cases, solid solutions have higher corrosion resistance than the pure metals. A notable example is the addition of chromium to iron in solid solution to make the corrosion-resistant alloy "stainless steel." However, in the case of gold, adding other elements reduces the corrosion resistance.

Eutectic mixtures are usually harder and stronger than the metals used to form the alloy and are often quite brittle. They possess a melting point at the eutectic composition, not a melting range, and any other combination of the alloy system has a higher fusion temperature than the melting

point of the eutectic mixture. Eutectic mixtures, along with other multiple-phase microstructures, often have poor corrosion resistance. Galvanic action between the two phases at a microscopic level can accelerate corrosion, as described in Chapter 3.

The intermetallic compounds formed in some alloy systems are usually very hard and brittle. Their properties rarely resemble those of the metals making the alloy. For example, Ag,Hg3 is an intermetallic compound formed in dental amalgam that has properties completely different from those of pure silver or mercury.

PROPERTIES O F WROUGHT ALLOYS

Compared with their cast counterparts, wrought alloys generally have high strengths and hardness. Ductility, on the other hand, decreases with cold work. Clinically,this loss of ductility can be a problem. For example, cast clasps on removable partial dentures may fail if multiple adjustments are made to a clasp. Cast properties can be regained by heating the wrought form sufficiently to allow recrystallization and grain growth.

SELECTED PROBLEMS

Problem 1

In the metallic crystal lattice, the valence electrons are relatively unbound to their atomic centers. What properties of metals result from this configuration?

Solution

The loose valence electrons are mobile and allow metals to readily conduct heat and electricity. The electrons can also accommodate shifts of the nuclear centers that often make the metals malleable and ductile. The high

reflectivity (mirror-like surface) of a polished metallic surface occurs because the valence electrons reflect light that hits the surface.

Problem 2

A person hands you two samples of the same metal. In the first sample (A]>she tells you that there are absolutely no flaws in the crystal structure of the metal. In the second sample (Bj, there are numerous crystal flaws. How do the strengths of A and B compare, and why?