A review of shape memory stability in NiTi alloys

Stability and fatigue are terms often referred to when discussing the durability of the shape memory effect. This paper considers the meaning of these terms and how they relate to the macroscopic memory response and the microstructural accommodation response of NiTi alloys. The results of previous workers are analysed and compared to establish how the three parameters that apparently control the macroscopic memory effect - temperature, stress and strain, are controlled themselves by the alloy's microstructure. It will be shown that repeated phase changes will alter the microstructure and hysteresis of the transformation and in turn this will lead to changes in transformation temperatures, transformation stresses and transformation strains. Both superelastic and memory effects are considered simultaneously so as to emphasise the similarities between the respective stress/strain and strain/temperature degradation effects. Although the mechanisms of both are often regarded as physically separate it is shown that in fact the macroscopic changes observed during cycling are a result of similar microstructural changes. It is also shown how as a result of the inherently large transformation strains internal cracking may occur and that even apparently stable microstructures can fail catastrophically via conventional crack growth mechanisms.