Spangold Alloys

Most gold alloys in current commercial use contain additions of copper, silver and zinc. Spangold is a family of specially shaped memory-effect alloys in 18 and 23 carat gold that, on heat treatment, give rise to a multicolored spangled surface which is shown to produce the best aesthetic effect on large flat or curved surfaces.

This spangle arises from a rumpling of the surface caused by a change in crystal structure, but the effect is due to more than a simple surface reaction. It goes throughout the bulk alloy and is a reversible effect. Available in yellow and pink shades in both wrought and cast forms, it offers an exciting new effect that cannot be simulated by conventional treatments to the jewelry surface.

Figure 1 shows a highly magnified picture of the effect—an array of needle-shaped surface rumples that give the spangle effect. As a relatively new finding, this kind of material is only now being taken up by designers and manufacturers, albeit in a limited way.

Spangold is a β-phase alloy with a nominal stoichiometry of Au7Cu5Al4 and a nominal composition of 76% Au, 18% Cu, 6% Al by mass. Au7Cu5Al4 alloy is a decoration alloy, which shows fine surface corrugation at room temperature, hence its name, "Spangold".

Figure 1: Microstructure of Au7Cu5Al4 alloy

The texture is caused by inducing a martensitic-type phase transformation to take place within a sample with a polished surface. Transformations of this type are characterized by considerable crystallographic shear and deformation, which causes a previously flat surface to become faceted. The facets then disperse incident light to cause a pleasing aesthetic effect (Figure 2).

Figure 2: The textured effect produced by a martensitic transformation on the planar surface of a Spangold ‘tension bangle’ is evident in the reflected image of the ring

Spangold is offset against conventional 18 carat gold, as well as a variety of other finishes such as red-ivory wood, rhodium-plating and precious stones. The vivid and stark contrast of the Spangold finish is an eloquent testimony to the attractiveness of these gold alloys.

The characterization of suitable transformation features led to the successful development of prototype Spangold alloys. In addition to exploiting their novel finish, the use of the prototype Spangold alloys in jewelry also realizes the practical application of structural gold base compounds.

Apart from their decorative qualities, the Spangold alloys based on intermetallic com- pounds can exhibit a number of other salient features pertinent to their application in jewelry. These include density, luster, castabilty, and wear-resistance. These gold compounds possess a wide and novel range of properties and are likely to enjoy continued interest.