Gold and Alloys of Noble Metals	65

Type 4 alloys are used in high stress areas and for constructing components of partial dentures and for this reason are normally referred to as partial denture casting alloys. Partial dentures normally have clasps or other devices for retaining the denture. These must be flexible enough to engage undercuts in standing teeth but have sufficiently high values of proportional limit such that they do not become distorted. The type 4 gold alloys possess this useful combination of properties. In addition, the alloys have sufficient ductility in the softened state to allow slight adjustments to be made. Attempts to carry out adjustments on a heat hardened alloy may lead to fracture due to the decrease in ductility which accompanies hardening.

The connectors of partial dentures, be they bars or plates, should be rigid and resist distortion. Whereas the proportional limit of type 4 gold alloys is sufficiently high to resist distortion, the connectors must be constructed in fairly thick section in order to produce sufficient rigidity due to the relatively low value of modulus of elasticity of these alloys. Some base metal denture casting alloys have higher values of modulus and from this point of view are more satisfactory for producing connectors (see Table 8.1).

Biocompatibility

It is often claimed that gold, alloys of gold and other precious metals can be considered well tolerated by soft tissues and by the body in general. However, contact allergy to gold is not as rare as may be imagined and can be readily demonstrated using patch testing using gold salts. Blood levels of gold are correlated with amounts of dental gold and with the incidence of gold allergy, but little is known about the possible effects of gold circulating in the blood.

The effect of exposure to palladium on human health has begun to receive some attention. The use of palladium in both dental and non-dental applications has more than doubled over recent years. A source of concern is the risk of sensitization to palladium, as very low doses are sufficient to cause allergic reactions in susceptible individuals. Dental technicians are thought to comprise a group which is known to be occupationally exposed to palladium and therefore at risk. Individuals with known nickel allergy are particularly thought to be at risk to palladium allergy. Protec-

tion from adverse reactions is best achieved through the use of alloys having good corrosion resistance or, in severe cases, to the use of palla- dium-free alloys. Furthermore, it has been suggested that patients who have an allergy to nickel should be informed that exposure to palladium containing alloys may result in palladium allergy, although the risk is relatively low.

7.4 Hardening heat treatments (theoretical considerations)

The type 3 (hard) and type 4 (extra hard) casting gold alloys can be further hardened by heat treatments. Hardening heat treatments are hardly ever performed for type 3 materials but are occasionally used for type 4 materials which are to be used in very high stress bearing situations. The hardening process can be explained by consideration of the phase diagrams for the silver-copper and goldcopper systems. Hardening heat treatments are not beneficial for the types 1 and 2 alloys because they contain insufficient quantities of copper and silver.

Silver-copper system

The silver-copper system is a good example of an alloy in which the component metals are only partially soluble in the solid state. The solidified alloy consists of a mixture of two solid solutions, one in which small quantities of copper are dissolved in silver (called the α solid solution) and one in which small quantities of silver are dissolved in copper (the β solid solution).

The phase diagram for the silver-copper system is shown in Fig. 7.1. In some respects it resembles the eutectic phase diagram given in Fig. 6.10. The solidus is defined by the line A B E C D, whilst the liquidus is given by A E D. The compositional limits of the α and β solid solutions are denoted by the areas marked α and β on the phase diagram. The lines B F and C G are termed solvus lines and indicate the decreasing solubility of copper in silver and silver in copper as the temperature decreases. Thus the solubility of copper in silver is around 9% at 780ºC (the eutectic temperature) but only around 2% at 400ºC. This relationship between solubility and temperature is normal for any system of limited solubility.

When an alloy of the eutectic composition (72% Ag/28% Cu) is cooled from the molten state it

66 Chapter 7

Fig. 7.1 Phase diagram of the silver–copper system.

undergoes solidification at a constant temperature, equivalent to point E on the phase diagram. The solid formed is a mixture of α and β solid solutions in which the α has composition equivalent to point B (approximately 9% Cu/91% Ag) and the β has a composition equivalent to point C (approximately 8% Ag/92% Cu). An alloy with slightly more copper than the eutectic composition (a hypereutectic alloy) solidifies to give the eutectic mixture plus additional β solid solution. An alloy with slightly more silver (a hypoeutectic alloy) than the eutectic composition solidifies to give the eutectic mixture and additional α solid solution. Using tie lines it can be shown that during crystallization the excess solid solution crystallizes first and that the eutectic mixture is always the last to crystallize.

If the alloy has either less than 9% or greater than 92% copper then the solid formed is either α or β solid solution – no eutectic mixture is formed.

When casting, generally alloys are cooled rapidly to encourage the formation of a fine grain structure. At low temperatures the alloys become rigid and atomic diffusions become difficult if not impossible. The structure of the alloy which was formed during crystallization becomes ‘frozen’ into the alloy at room temperature. Despite the

Fig. 7.2 Disordered, face-centred cubic, gold–copper alloy. Circles represent either gold or copper.

fact that the solubilities of silver in copper and copper in silver are negligible at room temperature, there exist, within the eutectic mixture, α and β solid solutions in which 9% copper remains dissolved in silver and 8% silver remains dissolved in copper. If diffusion were possible, there would be a tendency for copper to precipitate from the α solid solution and silver to precipitate from the

βsolid solution.

It is possible to heat the alloy to a temperature

at which the solubility is exceeded but at which atomic diffusions are possible. In the temperature range 300–600ºC slow diffusion of copper atoms can occur. Given sufficient time in this temperature range copper would begin to precipitate from the α solid solution. Well before any precipitated phase can be observed however, a significant hardening of the alloy takes place, presumably because the diffusing atoms have effectively prevented movement of slip planes. This forms the basis of the precipitation hardening procedure used for type 3 and type 4 casting gold alloys.

The same process may occur to a lesser extent and at a much slower rate at room temperature. Hardening which occurs in this way is referred to as age hardening.

Gold-copper system

Gold and copper form a continuous series of solid solutions over the whole range of compositions. The solid solutions are random-substitutional solid solutions with face-centred cubic lattices (Fig. 7.2).

The phase diagram for the gold-copper system is shown in Fig. 7.3. It can be seen that the solidus and liquidus are close together and almost coincide at point M. Two other areas on the phase

Gold and Alloys of Noble Metals	67

diagram, at compositions between 40% gold and 90% gold, indicate regions in which the alloys are capable of undergoing a solid–solid transition to form an ordered rather than disordered lattice. The ordered lattice, in which gold and copper take up specific lattice sites, is often referred to as a superlattice.

Formation of the superlattice requires that an atomic rearrangement must take place by diffusion of atoms. This cannot occur at normal temperatures since there is insufficient energy in the system to allow diffusion to occur. Consider, for example, an alloy containing 75% gold which lies within one of the areas of interest. If this alloy is heated to a temperature in the region 200–400ºC, sufficient energy is imparted to the system to allow atomic diffusions to occur, atoms take up their preferred sites and the superlattice is formed. In this case the superlattice has the formula CuAu and the superlattice has an ordered tetragonal structure as shown in Fig. 7.4. In alloys containing 40–60% gold an ordered superlattice, based on the formula Cu3Au, is formed. The change in size and shape of the gold–copper lattice sets up a resistance to the movement of slip planes within the multi-component casting gold alloy which results in a significant increase in hardness and strength and a reduction in ductility. Heat treatments which are used to carry out this hardening

Fig. 7.3 Simplified phase diagram of the gold–copper system.

Fig. 7.4 Ordered, tetragonal structure of a heat-treated gold–copper alloy containing 75% gold.

procedure are termed order hardening heat treatments.

7.5 Heat treatments (practical considerations)

On melting the alloy prior to casting, any metallographic structure which it possessed as an ingot is lost and a new crystal structure is created as the metal ‘freezes’ inside the mould.

It is important to quench the gold alloy castings before they cool to the range of temperatures within which heat hardening takes place. An alloy which is allowed to cool slowly to room tempera-

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ture will undergo both precipitation and order hardening prematurely. Before any alteration in shape of such a casting is attempted, it must be softened by cooling rapidly from above 600ºC. Normal casting procedure is to leave the mould until the gold, visible in the sprues of the casting, is no longer at red heat. This indicates that the internal metal temperature is about 600ºC. The mould is then plunged into cold water in order to chill the metal quickly and cause disintegration of the mould. Rapid cooling also helps to ensure a fine grain structure.

The casting is then cleaned and when platinum or palladium are present an homogenization heat treatment may be carried out to remove coring. This involves heating to 700ºC for 10 minutes, then quenching. The casting is then polished and its fit in the mouth is checked. Any minor adjustments, such as bending of clasps, etc., are made at this stage whilst the alloy is still in the softened state. If adjustments are made, a low temperature stress relief anneal should be carried out.

The hardening heat treatment may then be carried out with type 3 and type 4 alloys by heating the casting to above 450ºC and allowing it to cool slowly until its temperature has dropped to about 200ºC, then quenching. This takes about 20 minutes for an average-sized casting. The precise details of the heat treatment vary from one alloy to another and the manufacturer’s instructions should be followed. The procedures are designed such that hardening by both precipitation and ordering can occur. In practice, hardening heat treatments are hardly ever performed on type 3 alloys and only infrequently for type 4 materials, indicating that the properties of the ascast material are adequate for most purposes.

Following hardening the casting is repolished and, in the case of a denture, the teeth are added in order to complete the job.

Ideally, all hardening and softening heat treatments should be carried out in a pyrometrically controlled furnace. The castings should be supported by sand or another refractory material in order to prevent ‘sag’ at elevated temperatures.

7.6 Alloys with noble metal content of at least 25% but less than 75%

The two major groups of alloys covered by this composition range are the low-gold content alloys and the silver palladium alloys. The composition

and properties of the alloys are specified in ISO 8891:2000. The standard recognises the need to specify maximum allowed concentrations of toxic metals such as cadmium (0.02% max.), beryllium (0.02% max.) and nickel (0.1% max.). There is no beneficial effect of having these metals present and the limits are designed only to ensure that proper checks on impurities are performed.

Low gold-content alloys

Large increases in the price of gold have led to the development and increased use of alloys with lower gold content than those described previously. Some alloys contain as little as 10% gold but more normally a gold content of around 45– 50% is used. They have a high palladium content which imparts a characteristic ‘whitish’ colour to the alloys.

The increased popularity and use of these ‘low gold’ alloys has led to the development of an ISO Standard for dental casting alloys with a noble metal content of 25% up to but not including 75%. There are four types of alloy equivalent to those previously described for the high gold materials. The mechanical properties for both high gold and low gold alloys are characterised in the standards using measurements of proof stress and elongation at break (a measure of ductility). The specification limits are shown in Table 7.4, where it can be seen that the requirements for the two groups of alloys are identical.

The casting techniques and equipment used for low-gold alloys are similar to those used for conventional gold alloys. This, coupled with their acceptable properties, good clinical performance and lower cost compared to conventional gold alloys, has led to their widespread use.

Table 7.4 Specification limits for both traditional dental casting gold alloys and dental casting alloys with noble metal content of 25% up to but not including 75%.

	0.2% Proof
Type	stress (MPa)	Elongation (%)
1	80–180	18 minimum
2	180–240	12 minimum
3	240 minimum	12 minimum
4	300 minimum	10 minimum
4 (Hardened)	450 minimum	3 minimum

See Table 7.3 for some typical values.