Grain Size of Gold and Gold AlloysA REVIEW AND SOME RECENT DEVELOPMENTS

Dieter Ott and Christoph J. RaubForschungsinstitut fr Edelmetalle und Metallchemie, Schwbisch Gmnd, Federal Republic of Germany

Although the use of grain refining agentsin dental gold casting alloys is welldocumented, little has been written con-cerning the use of such agents in castingand other gold alloys used in jewelleryfabrication. This review of the control ofgrain size in gold alloys includesreference to recent findings regarding theuse of cobalt in carat gold alloys.

The structures of gold and its alloys, like those ofother metals, are determined in many respects by thethermal, mechanical and other treatments to whichthey have been subjected. The structure of an ingot inthe as-cast condition, for example, is typical and dif-ferent from that of the metal after cold working, andthis again differs from that which is formed on coldworking and annealing (Figure 1). Grain size is a par-ticularly important aspect since it can significantly af-fect several important properties of metals and alloys.An understanding of the factors which determine thisvariable is therefore of considerable importance.

Solidification StructuresWhen a metal or alloy solidifies from the molten

state, as in casting for example, its structure is deter-mined by the processes of nucleation and dendriticgrowth (1). The growing dendrites are variouslyoriented in the liquid metal and where they come intocontact with one another grain boundaries are formed.The solid, therefore, is polycrystalline and the bound-aries which separate the grains that are formedaround each of the nuclei are interconnected in ahoneycomb arrangement. The sizes of the grains andtheir shapes and orientations are determined by suchfactors as the rates of nucleation and dendriticgrowth, and by the tendency of crystals to form in-itially on the cold walls of the mould and to grow in-wards towards the centre of the casting. In the case ofalloys which solidify over a range of temperatures,differences in composition may occur both within in-dividual grains and/or from grain to grain. The

former are due to differences in composition of liquidand solid which are in equilibrium at any onetemperature, thus causing the nuclei of the dendritesto be richer in the high-melting component than theouter regions that are formed subsequently. The lat-ter differences in composition arise when solidifica-tion occurs with the formation of two distinct solidphases in equilibrium with one another. Finally, withboth metals and alloys there is a tendency for im-purities to accumulate in the last of the liquid phasewhich solidifies at grain boundaries and in the in-terdendritic spaces. This can have an adverse in-fluence on the properties of alloys, particularly incases where they are coarse-grained.

In precious metals technology a structure is general-ly regarded as coarse if the average grain diameter isgreater than about 0.35 mm and as fine if it is lessthan 0.1 mm (2).

In tune with experience gained from alloys of othermetals, fine-grained as-cast gold alloys show many ad-vantages over coarse-grained ones (3). Their surfacesare more easily polished, they have improved tensileproperties and they maintain their integrity betterwhen cast into sharp-edged or cornered shapes.Moreover, fine-grained castings are far less likely tocrack during rolling and can therefore be more heavi-ly deformed before annealing becomes necessary.Further, they have the important advantage of beingmore consistent in properties from melt to melt (4).Thus, means for controlling the grain size of goldalloy castings are of considerable practical impor-tance. It is surprising therefore, that although grainrefinement has been studied and applied extensivelyin dental gold casting alloys (2) and in electronics (5),it has received much less attention in the productionof cast gold jewellery, in which the problems causedby large grain size have long been recognized (6).

Recrystallization StructuresIn processes such as swaging, hammering, rolling

and drawing which are commonly used in themechanical processing of cast ingots, metals and theiralloys are deformed. The cast structure is brokendown and the material becomes progressively harderand more difficult to work, as lattice defects increasein number. The metal or the alloy must therefore be

Gold Brill., 1981, 14, (2) 69

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in cases where the inoculum is a metal that is solubleand oxidizes readily in the melt to form finely dis-persed oxide particles. Furthermore, inoculated al-loys tend to give rise to problems during operationssuch as polishing or milling and on solidificationafter remelting (even in welding), owing to theagglomeration of the additives at grain boundaries.

Special Casting -Methods

It is known in the trade that cold pouring', mouldvibration and agitation, and gas-melting, producefine-grained castings. By cold pouring, nucleation isenhanced and grain growth retarded. By vibrationand agitation of the melt, nuclei are more evenlydistributed or grains that have formed are broken up.The practical observation that grains in gas-meltedalloys tend to be smaller than, for example, ininduction-melted ones is probably due to the absorp-tion of impurities such as carbon-containing gases.

Homogeneous Nucleation

Iu homogeneous nucleation, nuclei are producedwithin a supercooled liquid by the segregation duringsolidification of additives that were previouslydissolved in the melt. Theoretical free energy con-siderations indicate that elements having highmelting points and low solid solubilities should beespecially suitable for this purpose (2). This is thecase for ruthenium, iridium, rhenium, molybdenum,tungsten and cobalt, for example, which have beenused as grain refining agents in dental gold castingalloys for a number of years (2, 4). For such grainrefiners there is usually a critical, but low concentra-tion above which no further refining effect occurs.This is demonstrated in Figure 2 which shows thegrain size of gold against concentration of dissolvedruthenium and iridium.

It is often difficult to ensure that the distribution ofrefractory metals in the melt is completely uniformand that their concentrations are kept under accuratecontrol. Problems also arise in the remelting of scrapcontaining too high a concentration of the additives,or with incorrect temperature control during castingand solidification leading to the segregation of coarseparticles of the additive.

Although gold alloys in which the formation of afine-grained structure has been promoted by the in-troduction of nuclei into the melt find wide applica-tion in current dental casting technology, they are notused in gold jewellery manufacture. This is largelydue to the problems which are associated with thepresence of segregated materials or inclusions,especially the difficulty in achieving the high qualitysurface finish which is demanded of jewellery. Thus,even the small oxide inclusions which can act asnuclei for grain growth in dental castings cannot be

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Fig. 2 The effect of ruthenium and iridium on thegrain size of gold in the ats-c^) t condiition, after 12).Grain refining occurs as a result of hornogeneouaimeleation

tolerated in most jewellery applications. The same ap-plies if silicon is used as a grain refining agent in com-bination with suicide-forming metals, or if poorlysoluble metals such as molybdenum, tungsten,iridium or rhenium are added and their concentra-tions are not carefully adjusted so as to avoidexcessive segregation. Such additions also lead tocomplications if scrap is re-melted for direct use.Homogeneous nucleation can therefore be used as amethod for grain refining gold jewellery alloys only ifit can be carefully controlled.

Grain Refinement in the Solid StateIn gold, as in aluminium alloys, it has been found

that certain elements in solid solution can stronglyhinder grain growth during recrystallization, therebyensuring a fine-grained structure (3, 5). This may beascribed to an interaction between the dissolved ad-ditives and the grain boundaries of the host matrix, asa consequence of which the added elements ac-cumulate at grain boundaries (3, 9). Thus, the migra-tion velocity of the grain boundaries is determined inconsiderable measure by the rate of migration of theforeign atoms. At very low concentrations and at hightemperatures the interaction is very small, hence themovement of the grain boundaries is rapid andrecrystallization and grain growth are unrestrained.When the concentration of the additives exceeds acertain critical value, however, the interactionbecomes significant, recrystallization rates arediminished and the recrystallization temperature ofthe alloy is raised. It is characteristic of the effect thatthe influence of the foreign atoms on the process of

Gold Bull, 1981, 14, (2) 71

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Fig. 5 Deep drawing, in the Ericlisen cupping test, of the 58.5 gold/30 silver/11.5 copper weight per cent alloy

Top: Microstructure of the undeformed alloy sheets, free of cobalt `left) and with the addition of 0.2 weight per centcobalt (right) x 100

Bottom: Surfaces of the corresponding deformed alloys x 2

Note that the fine structure of the cobalt-containing alloy prevents the occurrence of the 'orange peel' effect

(6) Their concentrations in the alloys should be easi-ly determinable without the use of expensiveequipment.

In the course of investigations by the authors andtheir associates (10, 11, 12), the findings of Masing,Lcke and Nolting (3), and of Lcke and Detert (9)were confirmed in regard to the grain refining effectswhich numerous elements have on fine