Displacive Phase Transformations in Spangold 18 Carat Gold Alloys

¹ Mintek, Physical Metallurgy Division, Private Bag X3015, Randburg 2125, South Africa
² University of the Witwatersrand, Metallurgy Dep., P.O. Wits 2050, South Africa

Abstract

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 Spangold^TM 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 A_s and A_f 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.