SHAPE MEMORY BEHAVIOR ASSOCIATED WITH THE R AND MARTENSITIC TRANSFORMATIONS IN A NITI ALLOY

The deformation and transformation behavior 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 yields R transformation is characterized 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.