Age-hardening characteristics of (AuCu)1−xPtx pseudobinary alloys

To characterize the effects of platinum addition to a stoichiometric AuCu binary alloy on age-hardening mechanisms and related phase transformations, electrical resistivity measurements, hardness tests, X-ray diffraction study, transmission electron microscopy and selected-area electron diffraction examinations were performed on a (AuCu)1 − xPtx (x = 0.0026, 0.0051, 0.0076, 0.0097, 0.0135, 0.0248, 0.0469 and 0.0692) pseudobinary system. Three distinguishable types of hardening behaviour were found depending upon their platinum content, ageing temperatures and periods in the alloys, i.e. (1) the ordering-and-twinning process, (2) the long-period antiphase domain boundary (LPAPB) process, and (3) the two-phase decomposition process. The hardening mechanism and subsequent softening mechanism were explained rationally in relation to the introduction of transformation strain due to AuCu I ordering and relief of strain by twinning, development of the LPAPBs and formation of a chequerboard-like microstructure which was composed of parallelogram-shaped AuCu I ordered regions and cuboidal blocks of a disordered f.c.c. phase.