Stabilisation of martensite in CuZnSi

Abstract

In many copper based shape memory alloys, the retransformation of martensite to the parent occurs at a higher temperature, immediately after quenching through the transformation and/or aging in the martensitic condition when compared to retransformation in continued cycles performed without any delay. This effect termed stabilization, has been used to raise, temporarily, the retransformation temperature of the martensite in Cu-Zn-Si alloys, notably with compositions which transform to martensite well below ambient temperature. This has been achieved by heating the sample, with the martensite microstructure, very slowly, at a rate of about 0.25[degree]/min. There are few reports to date on the mechanism of stabilization of Cu-Zn-Si martensite. These reports cite all the reasons mentioned above as possible causes for the stabilization. Moreover, martensite boundary pinning by vacancies and narrow lamellae of 3R phase within the 9R martensite have been claimed as fairly important contributors to stabilization. The present work was undertaken to review these earlier conclusions and has been carried out in alloys that transform nominally at sub-zero temperatures. The studies are restricted to the first 10[degree] or 15[degree] of stabilization.