the alloy via a solid-solution or ordered-solution mechanism. The presence of copper in gold-based alloys in quantities between approximately 40% and 88% by weight allows the formation of an ordered phase. Copper is also commonly used in palla- dium-based alloys, where it can be used to reduce the melting point and strengthen the alloy through solid-solution hardening and formation of ordered phases when Cu is between 15 and 55 wt%. The ratio of silver and copper must be carefully balanced in gold- and palladium-based alloys, because silver and copper are not miscible. Copper is also a common component of most hard dental solders.
ZINC (Zn)
Zinc is a blue-white metal with a tendency to tar- nish in moist air. In its pure form, it is a soft, brittle metal with low strength. When heated in air, zinc oxidizes readily to form a white oxide of relatively low density. This oxidizing property is exploited in dental alloys. Although zinc may be present in quantities of only 1% to 2% by weight, it acts as a scavenger of oxygen when the alloy is melted. Thus zinc is referred to as a deoxidizing agent. Because of its low density, the resulting zinc oxide lags behind the denser molten mass during casting and is there- fore excluded from the casting. If too much zinc is present, it will markedly increase the brittleness of the alloy.
INDIUM (In)
Indium is a soft, gray-white metal with a low melt- ing point of 156.6°C. Indium is not tarnished by air or water. It is used in some gold-based alloys as a replace- ment for zinc and is a common minor component of some noble ceramic dental alloys. Recently, indium has been used in greater amounts (up to 30% by weight) to impart a yellow color to palladium-silver alloys.
TIN (Sn)
Tin is a lustrous, soft, white metal that is not subject to tarnish in normal air. Some gold-based alloys con- tain limited quantities of tin, usually less than 5% by weight. Tin is also an ingredient in gold-based dental solders. It combines with platinum and palladium to produce a hardening effect, but also increases brittleness.
GALLIUM (Ga)
Gallium is a grayish metal that is stable in dry air but tarnishes in moist air. It has a very low melting point of 29.8°C and a density of only 5.91 g/cm3. Gallium is not used in its pure form in dentistry, but is used as a component of some gold- and palladium-based dental alloys, especially ceramic alloys. The oxides of gallium are important to the bonding of the ceramic to the metal.
NICKEL (Ni)
Nickel has limited application in gold- and palladium- based dental alloys, but is a common component in base-metal dental alloys. Nickel has a melting point of 1453°C and a density of 8.91 g/cm3. When used in small quantities in gold-based alloys, nickel whitens the alloy and increases its strength and hardness.
alloy, the percentage elongation will decrease from 30% to about 12%.
The formation of ordered solutions has been commonly used to strengthen cast dental restora- tions, particularly in gold-based alloys. The process requires precise control of the cooling rate. For the correct composition capable of forming the ordered state, slow cooling will result in a harder, stronger alloy due to the formation of the ordered crystals.
However, rapid cooling of the cast metal, such as by plunging it into cold water soon after the casting pro- cess, leaves the metal in the softer, disordered state because there is simply not enough time during cool- ing for the atoms to arrange correctly on the crystal lattice. The conversion between the ordered solu- tion and solid solution is reversible in the solid state.
Thus it is possible to heat an alloy that exists in a dis- ordered state under appropriate conditions and tem- peratures to provide time for the ordered structure to form. An example of how this process can be utilized for dental castings is as follows. An alloy is cooled rapidly after casting, leaving the alloy in the softer, disordered state, and thus allowing it to be more eas- ily ground and burnished at the margins to fit the die, and ultimately the prepared tooth. However, because it is beneficial for the alloy to be in a hard- ened state once placed in the mouth, the casting that has been worked in the laboratory is reheated to cause the formation of the harder, ordered structure, and this is the material that is ultimately cemented on the patient’s prepared tooth.
Formulation of Noble Alloys
The desired qualities of noble dental casting alloys determine the selection of elements that will be used to formulate the alloys. The ideal noble casting
alloy should have (1) a low melting range; (2) ade- quate strength, hardness, and elongation; (3) a low tendency to corrode in the oral environment; and (4) low cost, among other properties. Traditionally, the noble elements gold and palladium have gener- ally been the foundation to which other elements are added to formulate dental casting alloys. Gold and palladium are preferable to other noble ele- ments because they have relatively low melting points, low corrosion, and form solid solutions with other alloy elements, such as copper or silver. Solid- solution systems are desirable for the formulation of alloys because they are generally easier to man- ufacture and manipulate, have a lower tendency to corrode than multiple phase systems, and pro- vide increased strength through solid-solution or ordered-solution hardening. Thus it is not surpris- ing that combinations of these elements have been extensively used in the formulation of noble dental casting alloys.
GRAIN SIZE
Studies have shown that minute quantities of various elements can influence the grain size of dental casting alloys. With the addition of small amounts (0.005% or 50 ppm) of elements such as iridium and ruthenium, fine-grained castings are produced. Adding one of these elements to the alloy is believed to develop centers for nucleating grains throughout the alloy. Most alloy manufac- turers use grain refinement in present-day prod- ucts. The mechanical properties of tensile strength and elongation are improved significantly (30%) by the fine grain structure in castings, which con- tributes to uniformity of properties from one cast- ing to another. Other properties, however, such as A Magn1000x Deguplus #1,Trip/Rouge Polish Magn
1000x Rex #6, lac acid, 4 h, pH 4
20 µm B 20 µm
FIG. 10.6 Electron micrographs of single-phase (A) and multiple-phase (B) alloys. (A) Few distinguishing microstructure characteristics are seen because the alloy is nearly homogeneous. Only a few scratches from polishing and some debris on the surface are visible. (B) A rich microstructure is evident, reflecting the several phases present. Each phase has a different composition.
hardness and yield strength, show less effect from the grain refinement.
Properties
MELTING RANGE
Dental casting alloys do not have melting points, but rather melting ranges, because they are combinations of elements rather than pure elements. It is desirable for the dental casting alloy to have a relatively nar- row melting range, because if the alloy spends a long time in the partially molten state during cast- ing, there is increased opportunity for the formation of oxides and contamination. Therefore alloys with wide melting ranges are more difficult to cast with- out problems.
The melting range of the alloys determines the burnout temperature, type of investment, and type of heat source that must be used during casting. In general, the burnout temperature must be about 500°C below the bottom temperature of the melting range. For the Au-Cu-Ag-Pd-I alloys, therefore, a burnout temperature of about 450°C to 475°C should be used. If the burnout temperature approaches 700°C, a gypsum-bonded investment cannot be used because the calcium sulfate will decompose and embrittle the alloys. At temperatures near 700°C or greater, a phosphate-bonded investment is used. As shown in Table 10.9, a gypsum-bonded investment may be used with the Au-Cu-Ag-Pd-I, II, and III and the Au-Ag-Pd-In alloys, but a phosphate-bonded investment is advisable for the other alloys. The gas- air torch will adequately heat alloys with liquidus temperatures below 1100°C. Above this temperature, a gas-oxygen torch or electrical induction method must be used. Again, from Table 10.9, a gas-air torch
would be acceptable only for the Au-Cu-Ag-Pd-I, II, and III and the Au-Ag-Pd-In alloys.
The maximum temperature of the melting range is important to soldering and formation of ordered phases, because during both of these operations, the shape of the alloys is to be retained. Therefore dur- ing soldering or hardening-softening, the alloy may be heated only to 50°C below the maximum of the melting range to avoid local melting or distortion of the casting.
DENSITY
Density is important during the acceleration of the molten alloy into the mold during casting. Alloys with high densities will generally accelerate faster and tend to form complete castings more easily.
Among the alloys shown in Table 10.9, all have densities sufficient for convenient casting. Lower densities (7 to 8 g/cm3) seen in the predominantly base-metal alloys are more difficult to cast. Alloys in Table 10.6 with high densities generally contain higher amounts of denser elements such as gold or platinum. Thus the Au-Ag-Pt alloys and Au-Cu- Ag-Pd-I alloys are among the densest of the casting alloys.
STRENGTH
Strength of alloys can be measured by either the yield strength or tensile strength. Although tensile strength represents the maximum strength of the alloy, the yield strength is more useful in dental applications because it is the stress at which permanent defor- mation of the alloys occurs (see Chapter 4). Because permanent deformation of dental castings is gener- ally undesirable, the yield strength is a reasonable TABLE 10.9 Physical and Mechanical Properties of several Types of Noble Dental Casting Alloys
Alloy
Property
Solidus (°C) Liquidus (°C) Color
Density (g/cm3)
Yield Strength at 0.2% Offset (Soft/Hard) (MPa)
Elongation (Soft/Hard) (%)
Vickers Hardness (Soft/
Hard) (kg/mm2) HIGH NOBLE
Au-Ag-Pt 1045 1140 Yellow 18.4 420/470 15/9 175/195
Au-Cu-Ag-Pd-I 910 965 Yellow 15.6 270/400 30/12 135/195
Au-Cu-Ag-Pd-II 870 920 Yellow 13.8 350/600 30/10 175/260
NOBLE
Au-Cu-Ag-Pd-III 865 925 Yellow 12.4 325/520 27.5/10 125/215
Au-Ag-Pd-In 875 1035 Light
yellow 11.4 300/370 12/8 135/190
Pd-Cu-Ga 1100 1190 White 10.6 1145 8 425
Ag-Pd 1020 1100 White 10.6 260/320 10/8 140/155