Stabilisation of martensite due to shear deformation via variant reorientation in polycrystalline NiTi

The effect of deformation via martensite reorientation on the transformatio n behaviour of polycrystalline near-equiatomic NiTi was studied. Differenti al scanning calorimetry measurement showed that both the critical temperatu re and the endothermic heat of the first reverse transformation of the defo rmed martensite were increased as compared to an undeformed specimen, indic ating a stabilisation effect to the reoriented martensite. Thermal dilatati on measurement revealed a two-way memory effect in subsequent transformatio n cycles after the deformation, suggesting that an anisotropic internal str ess field had been established. X-ray diffraction measurement showed a syst ematic line broadening on the major diffraction peaks with increasing amoun t of deformation. These experimental observations are discussed in terms of the changes in the internal elastic and irreversible energies associated w ith the reorientation deformation in a polycrystalline matrix. (C) 2000 Act a Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.