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

Abstract

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.