Age-hardening characteristics in an AuCu-14at.%Ag alloy

Age-hardening characteristics and associated phase transformations were studied by electrical resistivity, hardness tests, X-ray and electron diffraction, transmission electron microscopy and scanning electron microscopy observations in an AuCu-14at.%Ag pseudobinary alloy. The critical temperature of ordering was determined to be 650 K and the spinodal temperature was estimated to be 640 K. Age hardening at 623 K was attributed to nucleation and growth processes of the AuCu II' metastable ordered phase even though the aging temperature was just below the spinodal temperature. While hardening in the lower temperature range took place at two stages, the former was due to spinodal decomposition and the latter was brought about by ordering of AuCu I' and/ or AuCu II' metastable phases depending on aging temperature. Overaging was induced by grain boundary precipitation of AuCu I and/or AuCu II ordered and silver-rich α₂ stable phases.