Age-hardening and the associated phase transformation in an Au-55.2 at% Cu-17.4 at% Ag ternary alloy

To develop a low gold content dental alloy, age-hardening characteristics in an Au-55.2 at% Cu-17.4 at% Ag alloy were studied by means of hardness, electron microscopy and X-ray diffraction examination. Three distinct age-hardening behaviours depending on temperature were found in the alloy, i.e. (i) a dual mechanism of spinodal decomposition and Cu₃Au ordering below 673 K, (ii) a single mechanism of spinodal decomposition at 693 K, and (iii) a single mechanism of nucleation and growth of silver-rich precipitate at 773 K. A marked over-ageing was observed by lengthy ageing over the whole range of temperature. The long-period superstructure of Cu₃Au(II) was found only in the grain boundary product at temperatures between 623 and 633 K.