Mechanical stabilisation of martensite due to cold deformation

The transformation behaviour of near-equiatomic NiTi after mild deformation via either martensite reorientation or stress-induced martensitic transformation was studied by thermal analyses. It was found that the critical temperature for the reverse transformation of the deformed martensite was increased as a result of the deformation, indicating a stabilisation of the martensite. Accompanying the increase in the transformation temperature, the heat effect associated with the reverse transformation also increased. This stabilisation effect is attributed to changes in the internal elastic and irreversible energies that are known to accompany a thermoelastic martensitic transformation. These observations are also of practical importance in applications when the critical temperature for shape memory recovery, the critical stress for pseudoelastic recovery and the energy calculation for a transformation cycle are to be considered.