Displacive Phase Transformations in Spangold 18 Carat Gold Alloys
Mintek, Physical Metallurgy Division, Private Bag X3015, Randburg 2125, South Africa
2 University of the Witwatersrand, Metallurgy Dep., P.O. Wits 2050, South Africa
The displacive phase transformations in an Au-32 at% Cu-22 at% Al gold alloy are discussed. This alloy, and others similar to it, is used to produce SpangoldTM jewellery pieces. Complex textures of intersecting laths are produced on polished surfaces of the alloys by thermal treatment, giving the material a glittering appearance. Dilatometry of both loaded and unloaded samples indicated that the Ms and Mf temperatures were, depending on prior treatment, in the range of 30 to 15°C and 20 to -10°C respectively. The As and Af temperatures also depended on prior treatment, and were in the range of 55 to 80°C and about 85 to 95°C, respectively. The unloaded specimen exhibited a closed hysteresis loop on a graph of change of length with temperature. Martensite laths of the loaded specimen became aligned along the tensile axis during thermal cycling. X-ray diffraction revealed that both the parent phase and the martensitic phase possessed orthogonal lattices. Although a face centred cubic disordered phase is presumed to be present close to the melting point of the alloy, it cannot be preserved down to room temperature by quenching.
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