Effects of Applied Stress on the Shape Memory Behaviour of Niti Wires

Abstract

Shape Memory Alloys (SMAs), in particular Nickel-Titanium (NiTi) alloys, are being used in an increasing number of engineering applications due to their special functional properties, namely shape memory effect (SME) and superelastic effect (SE). These properties, which allow large stresses and strains recovery, are due to a thermoelastic phase transformation, between martensite (M) and austenite (A), which is characterized by a hysteretic behaviour, In particular the phase transformation can be induced by a thermal load between the phase transformation temperatures (SME) or by an applied stress (SE). Two-way shape memory effect (TWSME) is another particular property since the material is able to remember a geometrical shape at high temperature, above austenite finish temperature (Af), and another shape at low temperature, below martensite finish temperature (Mf) [1]. This functional property can be developed in NiTi alloys by various thermomechanical treatments, the so-called training.