Age-hardening associated with ordering and spinodal decomposition in a AgCu-40 at % Au pseudobinary alloy

The age-hardening mechanism in an AgCu-40 at% Au alloy was studied by means of electrical resistivity measurement, hardness tests, X-ray diffraction and electron microscopy. Two stages of hardening were found by isothermal ageing below 648 K, which was higher than the critical temperature of ordering, Tc=620 K, in the present alloy. The first stage of hardening took place by formation of a modulated structure resulting from spinodal decomposition. Further hardening was brought about by ordering, yielding metastable AuCu I′ and/or AuCu II′ ordered platelets grown from the copper-rich portion of the modulated structure. Transitional ordering which gave rise to a marked hardening of the second stage was found, even though the temperature of below 648 K was higher than the Tc of the present alloy. Drastic softening was also found on disappearance of the transitional ordered phases. Although the modulated structure was observed by ageing at 773 K, there was no age-hardening.