Hyperstabilisation of martensite in Cu-Al-Be alloys

Thermoelastic Cu-Al-Be is unusual in that during the first heating cycle after quenching the martensite is strongly stabilised and reverts to parent phase over two distinct temperature ranges. The higher temperature transformation range has been termed "hyperstabilisation" and the volume fraction of parent formed from hyperstabilised martensite has been found to depend sensitively on the quenching conditions. It has been suggested that the lower temperature ("normal") stabilisation is due mainly to vacancy pinning, whereas hyperstabilisation arises from Be-induced suppression of beta phase ordering during rapid cooling. Both stabilisation effects are eliminated by thermal cycling to produce reverse transformation to beta phase, indicating that they arise from transient structural features of the initial quench. However, results presented herein indicate that no significant changes in the stabilisation phenomena occur following long-term room temperature ageing of the quenched martensite, indicating that the quenched-in defect structure is stable under ambient conditions. It was also found that hyperstabilisation can be accentuated by higher temperature/longer time heat treatments through loss of Be and a resultant increase in the martensitic transformation temperatures. The observations are consistent with the hypothesis that suppressed beta phase order plays a dominant role in hyperstabilisation. (c) 2006 Elsevier B.V. All rights reserved.