Shape Memory Characteristics of Time Gradient Annealing Treated Ti-Ni Alloys

In this study, the shape memory behavior of Ti-Ni alloys with time gradient annealing (TGA) was investigated via differential scanning calorimetry (DSC) and thermal cycling tests under constant load. The as-cast ingot was hot forged and extruded, followed by cold-drawing and intermediate annealing to produce wires of 0.5mm ~ 1.0 mm in diameter, with a final cold-drawing of 10%, 20%, 30% and 45% reduction in cross-section. For the TGA treatment, a new type of radiant furnace was designed to maintain a constant temperature and create a time gradient along the length of the specimen. According to DSC measurements, the martensitic and R-phase transformation intervals along the length of the specimens (80 mm) increase with increasing cold working rate. The results of thermal cycling tests under constant load revealed that the temperature dependence of transformation elongation (dε/dT) of the TGA specimen is smaller than that of the isochronously annealed specimen at every heat-treatment temperature. The lowest dε/dT of R-phase transformation (0.0031%/K) was obtained for the specimen that was TGA-treated at 673 K. The difference in dε/dT of R-phase and the martensitic transformation of the TGA-treated and isochronously annealed specimen was largest at 773 K and increased with increasing stress. Such behavior provides superior controllability for actuation applications