Step-wise martensite to austenite reversible transformation stimulated by temperature or stress: a comparison in NiTi alloys

In shape memory alloys the thermoelastic martensitic transformation (TMT) can be either thermally induced or stress-induced. The TMT is intrinsically step-wise: moreover it has already been shown that an extrinsic fragmentation of thermally induced transformations can be stimulated (`step-wise martensite to austenite reversible transformationʹ, SMART) by means of incomplete cycles on heating (ICH): the SMART transformation shows `micromemoryʹ features which are reversible. The SMART phenomenon induced by stress had not yet received great attention and has stimulated the present investigations. Several stress-induced SMART key features, similar to the ones found in thermally induced transformations, are reported for a NiTi alloy. In the stress-induced transformation SMART, some evidence is given for micromemory effects related to the direct transformation that have no thermal analogue.