Shape memory behaviour associated with the R and martensitic transformations in a NiTi alloy

Abstract

The deformation and transformation behaviour associated with both the R and martensitic transformations in a near equiatomic NiTi alloy has been investigated using thermal cycling tests under constant applied load. Strain measurements exhibit separate stages of yielding associated with the R and martensitic transformations. The transformation sequence during cooling is found to depend on the applied stress, resulting from the differing stress dependencies of the R and martensitic transformation temperatures. Strain, resistivity, and DSC measurements indicate that the B2 → R transformation is characterised by a highly reversible single stage, first order reaction. Repeated thermal cycling resulted in irreversible changes to both the transformation temperatures and strains associated with the martensitic transformation.