Effect of thermal cycling on the stabilization of martensite in step-quenched Cu-Zn-Ai alloys

Abstract

The applications of copper based shape memory alloys requiring its prolonged use in the martensitic state have been restricted due to their ageing behavior which results in the increase of the reverse martensitic transformation temperatures with time, effect known as stabilization of martensite (see the reviews 1,2). A similar phenomenon is also observed when quenching the specimens from relatively high temperatures directly to the martensitic state. The shift of reverse transformation temperatures is usually present only in the first retransformation after quench and/or ageing. When the material reverts to the high temperature p phase a rapid recovering process takes place, in such a way that for the following transformation cycles the normal transformation temperatures are restablished. The proposed mechanisms for the martensite stabilisation can be separated in two main groups. In the first one the effect is attributed to configurational changes observed in martensite during ageing (3-7), which modify the martensite free energy. In the second group the stabilization mechanism is related to vacancy concentrations near interfaces and defects, causing atom relaxations which difficult the reversion of the martensite interfaces on heating, referred in the literature as pinning (8,9). On the other hand, the effects of thermal cycling (repetition of the temperature induced martensitic transformation) on the transformation characteristics of Cu-based shape memory alloys have been studied by several authors (10-13). The most remarkable microstructural change caused by thermal cycling is the generation of dislocations in the material (14-18). The evolution of transformation temperatures with cycling has been attributed mainly to two factors: the change in the degree of order and the effects of the dislocations introduced by cycling, To our knowledge, only a recent work (19) has been developed to study the effects of the thermal cycling on the martensite stabilization. Chandrasekaran et ai. (19) reported that single crystalline samples of Cu-Zn-Al alloys previously submitted to 50 thermal cycles suffer none or much less stabilization of martens:ite upon 3 weeks ageing than uncycled or relatively less cycled samples of the same alloy. Following this result, the effect of a more prolonged thermal cycling, up to 250 cycles, on the stabilization induced by ageing in the martensitic state during longer ageing times will be reported in the present paper.