The aging effect in Au-Cd alloys: A Mössbauer spectroscopy study

Abstract
The martensite aging effect, observed in a number of alloy systems and through which fundamental properties for particular materials are altered by aging in the martensite state, has attracted considerable interest in recent years. It is important to understand and control the effect, particularly in shape-memory alloys for which stability of properties is fundamental to applications. The most likely model, at least for the Au-Cd system in which it bas been most critically tested, is the "symmetry conforming short-range order" (SC-SRO) model first proposed by Ren and Otsuka. ¹⁹⁷Au Mössbauer spectroscopy offers the possibility to observe the nature of defects associated with the gold sites. The spectra for single crystal and polycrystalline Au-Cd in both the "aged martensite" and "quenched austenite" conditions are presented and discussed. While some uncertainty in interpretation of the spectra remains, particularly in relation to any quadruple splitting to be expected from either Au atoms on Cd-nearest-neighbour sites or the martensite lattice distortion, the best single-line fits provide evidence for two defect types.