NiTiHf shape memory alloy: effect of aging and thermal cycling

Abstract

Abstract A great effort was performed in order to develop alloys which exhibit transformation temperatures higher than about 150°C. This request has a 2-fold aim: (a) to provide an alloy that can be used in devices in which exercise temperatures as high as 100°C are easily reached (e.g. automotive applications, production plants, etc.); (b) to decrease the reset time by increasing the temperature gap between normal exercise and actuation temperatures. It has been demonstrated that ternary NiTiHf alloys can be a viable option to increase the operating temperature range of SMA. In this work, a NiTiHf alloy has been produced by a plasma arc melting furnace. Plasma technology ensures a good control of alloy composition and contamination, because of its high working pressure (850 mbar) and possibility to melt in a pure gas atmosphere. The produced material has been processed to obtain specimens suitable for calorimetric measurements. The effect of aging and thermal cycling on the thermoelastic martensitic transformation that takes place in the alloy has been evaluated. Specifically, samples taken from plasma melted buttons have been aged at different temperatures. Aging effects and evolution under thermal cycling on martensitic transformation behavior have been investigated by calorimetric measurements. Results obtained allow us to fix the reference framework for the effect of thermal treatments on the transformation properties of the chosen alloy and to compare with similar ones performed on semifinished products, to evaluate the effect of plastic deformation and residual cold work.