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The Metallurgy of Some Carat Gold Jewellery Alloys

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The Metallurgy of Some Carat Gold Jewellery Alloys The Metallurgy of Some Carat Gold Jewellery Alloys PART I — COLOURED GOLD ALLOYS Allen S. McDonald and George H. Sistare Handy & Harman, Fairfield, Connecticut, U.S.A. The classification of gold alloys in terms of caratage and colour provides no guidance as to the properties and working characteristics of the many carat gold alloys used in jewellery fabrication. Logical relationships do exist, however, between the composition, metallurgical structure and properties of such alloys, and these emerge from a study of the phase diagrams of the alloy systems involved. Part I of this review deals with coloured gold alloys. Part II will discuss their nickel containing counter- parts and will appear in the next issue of Gold Bulletin. Most carat gold for jewellery falls into one or other Three quasi-binary vertical sections of the ternary of two major categories, the coloured gold alloys diagram at constant gold contents characteristic of based on the gold-silver-copper or the gold-silver- jewellery alloys (10, 14 and 18 carat) are schematical- copper-zinc system and the white gold alloys. ly represented in Figure 4 where the parameter used Although many coloured gold alloys contain zinc on the abscissa is defined as: (from less than 1 to about 15 weight per cent), it is Ag wt.% valid to introduce this group in terms of the gold- Ag' = x 100 (per cent) silver-copper ternary system, from which all of the Ag wt.°Io + Cu wt.% alloys inherit certain fundamental metallurgical characteristics. After establishing these, the effects of Gold-silver-copper and gold-silver-copper-zinc- zinc will be introduced, and it will then be seen that alloys can be logically classified and discussed in in the amounts used this element does not change terms of two parameters, namely the caratage or gold these characteristics in kind but rather interacts to content and Ag'. modify them in a regular and symmetrical way. Given constant gold content sections of the ternary Figure I presents the gold-copper, copper-silver diagram, for example the 14 carat section in Figure 4, and silver-gold binary phase diagrams (1). The ter- the concept of three `types' of alloy based on the im- nary phase diagram can be established by progressive- miscibility gap and defined by values of Ag' can be ly adding gold to the copper-silver system. One might introduced. expect a liquidus valley to extend from the copper- Type I where Ag' ranges from 0 to about 10 per silver eutectic point at 780°C to the gold-copper cent or from about 90 to 100 per cent. These alloys liquidus minimum at 911°C. This valley exists and is exist primarily as homogeneous solid solutions at all shown in Figure 2. One might also expect the solid temperatures below the melting point. Alloys of this state immiscibility field in the copper-silver system to type are soft in the annealed condition and are not intrude into the ternary diagram to a considerable ex- age-hardenable. tent. Indeed it does, shrinking laterally and vertically, Type II where Ag' is in the range from about 10 to as compositions approach the gold corner. This is 25 per cent or 75 to 90 per cent. These alloys will illustrated in Figure 3, where the solid state isother- exist as homogeneous solid solutions at temperatures mal boundaries of the immiscibility field have been above the immiscibility gap, but if slowly cooled to projected onto the room temperature plane. The ex- equilibrium at room temperature _.0 (Ag Au) will _ istence of this immiscibility field is by far the most A in silver- precipitate in copper-rich alloys and cc (c„_ „) important factor in determining the basic rich alloys. Type II alloys are moderately soft in the metallurgical characteristics of the coloured jewellery annealed condition but are age-hardenable by heat gold alloys. treatment. r7.7 GOLD-COPPER COPPER-SILVER GOLD ATOMIC PER CENT °C COPPER ATOMIC PER CENT °C 9080 70 60 50 40 30 20 10 90 80 70 60 50 40 30 20 10 1100 1100 1084° WOO 1064 ° 1050 1000 TI1LLI 962° LIQUID LIQUID 1000 900 (Cu) 950 800 780° 281 88 (Ag) SOLID SOLUTION 921 a(AuCu) 1 900 911°,80.1°4 700 IMMISCIBILITY a (Cu)` a (Ag) 450 600 410°,75.6% 400 390°, 50.8°I 500 350 r - 4 00 Cu 90 80 70 60 50 40 30EH' 20 10 Ag AuCu3 X^ COPPER WEIGHT PER CENT AuCu 300 SILVER-GOLD 250 240° ii ° SILVER ATOMIC PER CENT jI^ ( 70 60 50 40 30 20 10 200 90 80 Au 90 80 70 60 50 40 30 20 10 Cu 10 70 ' 1064 GOLD WEIGHT PER CENT 1050 Fig. 1 Gold-copper, copper-silver and silver-gold 1030 _..._.- -- — LIQUID ---- --- 'r-- ----_..- binary phase diagrams. On the gold-copper diagram, i note the minimum liquidus temperature and the ex- 1010 istence of two ordered phases, AuCu and AuCu 3 . The SOLID SOLUTION copper-silver diagram shows that due to the eutectic 990 a (Ag,Au) decomposition of the homogeneous liquid phase into copper-rich and silver-rich phases, binary alloys are two-phase in the solid state except when either of cop- 970 per or silver is present in a very small quantity only. 962 ° 950 TIILt Silver-gold binary alloys are single phase at all concen- Ag 90 80 70 60 50 40 30 20 10 Au trations. After (1) SILVER WEIGHT PER CENT Au Au 10 -- 90 010 20 80 889° ° /30 --- 70 ° 40 60 ci 50 -- 843 ° 50 - /60 40 80 Cu 10 20 30 40 50 60 70\ 80 90 Ag Cu 10 20 30 40 50 60 70 80 90 Ag SILVER WEIGHT PER CENT SILVER WEIGHT PER CENT Fig. 2 Projection on the room temperature plane of the Fig. 3 Projection on the room temperature plane of the gold-silver-copper ternary phase diagram of some gold-silver- copper ternary phase diagram of some isotherms on the liquidus surface. A liquidus valley ex- is6thermal solid state boundaries of the immiscibility tends frone the silvel`edpper eutectic point to the gold- field. After 11) copper liquidus minimum. After (1) 67 IS CARAT 14 CARAT 10 CARAT Ag, Wt. °/° Ag,Wt, % Ag, Wt. % 0 10 20 0 10 20 30 40 0 10 20 30 40 50 1100 1000 OJ5 v^OJ\ 900 SOLI DU S SOLID \ 0LUTION 600 DUS a(Au,Ag,Cu 700 SOLID SOLUTION SOLID SOLUTION W a(Au,Ag,Cu) a (Au,AgCu) a IMMISCIBILITY 600 a (Cu-Au)'(Ag-Au) Ui a 500 Ui I- IMMISCIBILITY 400 a(Cu-Au)`a(Ag-Au) 300 ' (IMMISCI- BILITY 200 AuCu^ a(Cu-Au) (Ag-Au) ‚vu - 0 20 40 60 80 100 0 20 40 60 80 100 0 20 40 60 80 100 Ag' 'I, Ag %° Ag', °% Fig. 4 Schematic quasi-binary sections at constant caratage of the gold-silver-copper ternary phase diagram. The parameter Ag' is defined in the text Au RY_REDYELLOW copper alloys could be expected to range from 10 Q 90 DY-GREEN `copper-red' near the copper-rich corner, to 'gold- YELLOW 20 80 18Ct. o yellow' near the gold-rich corner, to `silver-white' 30 > 0V 70 near the silver-rich corner. A more precise delinea- e46 i tion (2) of the actual colour fields is illustrated in 40 14Ct. 60 = x cti^ Figure 5. 3 3 50 .p^ It will be noted in Figure 5 that `copper-red' _ 1 oct. — J W°— 40 dominates not only the copper-rich corner but most of W /70RED 30 the copper- and gold-rich side of the diagram. Alloys in these red and reddish areas can be, and are, made uj, 20 yellowish by an addition of zinc. WHITISH 90 W 10 The effect of zinc on the concept of alloy types as defined above, will be understood if it is borne in Cu 10 20 30 40 50 60 70 80 90 Ag mind that the addition of this element tends to reduce SILVER WEIGHT PER CENT (3) the volume of solid state immiscibility in the ter- Fig. 5 Relationship between the colour and com- nary phase diagram. position of gold-silver-copper alloys. After (2) The small amounts of zinc (about 0.5 per cent or less) added as a deoxidizer in 10 and 14 carat alloys have no significant effect on the metallurgical Type III where Ag' is in the range from about 25 to characteristics under discussion. Larger additions (up 75 per cent. These alloys are homogeneous solid solu- to about 15 weight per cent) are, however, used to tions at temperatures above the immiscibility gap, but change the red colour of low and middle caratage if slowly cooled to equilibrium at room temperature copper-rich alloys to reddish yellow or dark yellow. they decompose into both '(Ag-Au) and -C(Cu.Au) . Alloys These soften the alloys in both the annealed and of this type are hard in the annealed condition. They precipitation hardened state. This is a consequence of harden markedly during air cooling and are difficult the narrowing and reduction in height of the two to quench. Type III alloys are age-hardenable. phase area on each of the constant gold content sec- In the jewellery industry, alloys are usually tions of the ternary diagram shown in Figure 4. classified in terms of caratage and colour rather than These general concepts will aid a survey of some of metallurgical properties. The hue of gold-silver- commercial alloys. For this purpose, the reader will 68 find it useful to imagine the metallurgical concept of ing schedule best suited to each particular alloy.
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