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

The effect of deformation via martensite reorientation on the transformation behaviour of polycrystalline near-equiatomic NiTi was studied. Differential scanning calorimetry measurement showed that both the critical temperature and the endothermic heat of the first reverse transformation of the deformed martensite were increased as compared to an undeformed specimen, indicating a stabilisation effect to the reoriented martensite. Thermal dilatation measurement revealed a two-way memory effect in subsequent transformation cycles after the deformation, suggesting that an anisotropic internal stress field had been established. X-ray diffraction measurement showed a systematic line broadening on the major diffraction peaks with increasing amount of deformation. These experimental observations are discussed in terms of the changes in the internal elastic and irreversible energies associated with the reorientation deformation in a polycrystalline matrix. (C) 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.