Effects of different thermomechanical treatments on fatigue of NiTi shape memory alloys

Abstract
Fully recrystallized NiTi sheets were deformed in four different temperature ranges : fully transformed martensite (T < M_f), transforming austenite (M_s < T < M_d), low temperature, stable austenite (M_d < T < 1/3 T_s) and austenite at elevated temperatures (T > 1/3 T_m). The types and distributions of defects produced in these materials depend on the deformation temperature. The characteristic defect structures resulting from the low temperature deformation lead to different microstructures even after annealing. The thermomechanically treated as compared to the fully recrystallized NiTi alloys were subjected to thermal, mechanical and thermomechanical fatigue tests. Their microstructural origin was interpreted by TEM investigations. In conclusion it is proposed that a low ratio of pseudo-yield stress to true yield stress is responsible for good fatigue resistance. Regarding fatigue it is better to increase the yield stress by a small grain size than by a high dislocation density (work hardening).