Aging effects in copper-based shape-memory alloys

Abstract

In the literature concerned with the martensite transformation in binary and ternary brasses there have been many reports of both variations in the transformation temperatures and anomalously poor martensite reversibility. Of the many factors which may influence the martensitic transformation behaviour in noble metal beta phases, there are two which seem particularly important: in the first place, the presence of excess quenched-in vacancies, and secondly, the degreee of chemical order present within the parent phase before transformation. The recent upsurge of scientific and commercial interest in the shape memory associated with such martensitic transformations bas highlighted the importance of understanding these factors. By the use of differential thermal analysis and metallographic techniques after carefully controlled post-quench heat treatments it has been found possible to separate the effects of retained disorder from the effects of excess quenched-in vacancies in Cu-Zn-Al beta-phase alloys. This makes it possible to characterize the spectrum of thermally activated processes associated with both these phenomena. A fuller appreciation of these aging effects assists in explaining some of the previously reported anomalies, and is of paramount importance if copper-based alloys are to be used as an alternative to the costly nickel-titanium alloys currently in use.