Characteristics and mechanism of martensite ageing effect in Au−Cd alloys

Characteristics of the martensite ageing effect was extensively studied for the ζ′2 (trigonal) martensite in Au-49.5 and 50.0at.%Cd alloys. Considerable changes in various physical properties were observed with ageing the ζ′2 martensite, such as the increase of the critical stress for the rearrangement of martensite variants and of the reverse transformation temperature etc., in addition to the appearance of the rubber-like behavior, which were shown to originate from the same origin. The ageing effect was further investigated with respect to the temperature dependence, composition dependence and the influence by heat-treatment, from which a close relation with a diffusion mechanism in the martensite was derived. The activation energy for the martensite ageing effect was determined to be 0.44 eV for the ζ′2 martensite in a furnace-cooled Au-49.5at.%Cd alloy, which was found to correspond to activation energy for migration of a vacancy in the martensite. The structural change associated with the ageing effect was examined both for the ζ′2 and γ′2 (orthorhombic) martensites in Au−Cd alloys by careful X-ray diffraction measurements. Only a small and local structural change, which was achieved by some short range diffusion, was found to be responsible for the ageing effect.