Martensite stabilisation effect in Ni-rich NiTi shape memory alloy with different structure and martensitic transformations

Abstract

The aim of the present work was to study the martensite stabilisation effect in Ni51Ti49 alloy with different structures and martensitic transformations. The quenched Ni51Ti49 alloy underwent the B2→B19’ transformation, whereas the B2→R→B19’ transformation occurred in the annealed sample. The martensite stabilisation effect was studied after deformation in the B19’ state in both samples, and in the R state in the annealed sample. It was found that despite the sample structure, no plastic strain appeared during deformation up to 7%, however, the martensite stabilisation effect was observed in both the quenched and annealed samples. If deformation occurred due to the reorientation of the R phase, the martensite stabilisation effect was negligible and did not exceed 1 °C. If the stress induced B19’ phase appeared during deformation in the R state, the martensite stabilisation effect occurred. It was found that the shift in the reverse transformation temperatures on heating of the pre-deformed sample, depended on the structure of the B19’ phase. It was assumed that the damage to the interface coherency resulted in the appearance of an additional energy barrier that influenced the finish temperature of the reverse transformation. Furthermore, it was believed that the martensite reorientation during deformation changed the stored elastic energy, and the shift in the start temperature of the reverse transformation was due to the change in the stored elastic energy and loss in the interface coherency.