Stress induced martensitic transformation in NiTi at elevated temperatures: Martensite variant microstructures, recoverable strains and plastic strains

Functional behavior of NiTi shape memory alloys is restricted to temperatures below 150 °C. To shed light on the origin of the loss of functional properties of NiTi with increasing temperature, we have investigated stress induced martensitic transformations in nanocrystalline NiTi shape memory wire by thermomechanical testing supplemented with post-mortem reconstruction of martensite variant microstructures in grains by transmission electron microscopy. It was found that the forward stress induced martensitic transformation proceeding via propagation of macroscopic Luders band fronts in stress plateaus is not completed at the end of the stress plateau and generates unrecoverable plastic strain in addition to recoverable transformation strain. The higher the test temperature, the larger is the plastic strain as well as the volume fraction of retained austenite. The reconstructed martensite variant microstructures in grains of the NiTi wire deformed up to the end of the stress plateau at 120 °C were found to be filled with single domains of (001) compound twin laminate, some grains were nearly detwinned and some grains contained (100) deformation twins. The recoverable transformation strain (~6%) generated by the forward martensitic transformation is nearly independent on the test temperature (plateau stress) because detwinning of (001) compound twins in the microstructure is prohibited by lateral constraint from the surrounding grains in nanocrystalline wire. It is concluded that the gradual loss of functionality of NiTi with increasing temperature does not originate from the plastic deformation of austenite, but that it derives from the lack of resistance of the stress induced martensite to the plastic deformation under increasing stress.