Ageing effects in Cu-Zn-Al shape memory alloys

In the literature concerned with the martensitic 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: Firstly, the presence of quenched-in vacancies and secondly, the degree of chemical order present within the parent phase prior to transformation. The recent upsurge of scientific and commercial interest in the shape memory associated with such martensitic transformations has highlighted the importance of understanding these factors. By the use of differential thermal analysis and other standard metallurgical techniques after carefully controlled post quench heat treatments, it has been found possible to separate the influences of retained disorder from the effects of excess quenched-in vacancies in in Cu-Zn-Al beta phase alloys. In particular, a low temperature secondary ordering reaction of the type B2-DO[3] has been discovered at low temperatures (273K-423K) the presence of which increases both the forward and reverse martensitic transformation temperatures. This allows a qualitative relationship to be proposed between the stability of B2 and DO[3] ordered beta phases and their respective martensites. A correlation has also been established between the compositional dependence of the experimentally determined B2-DO[3] ordering temperatures and the predicted ordering behaviour. The presence of non-equilibrium quenched-in defects after quenching from the solution treatment temperature has been demonstrated by variations in low temperature ordering kinetics and changes in the mechanical properties of the martensite after low temperature annealing treatments. Such considerations have made it possible to characterise the spectrum of thermally activated processes associated with ageing phenomena in Cu-Zn-Al shape memory alloys. A fuller appreciation of these ageing 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 (NiTinol) alloys currently in use.