Effect of proportional control treatment on transformation behavior of Ti–50.9at.% Ni shape memory alloys

► We designed TGA furnace for gradient annealing. ► R-phase and martensitic transformation temperature were changed in the specimen. ► The dɛ/dT of TGA specimens was lower than that of isochronously annealed specimens. ► TGA treatment was most effective at 773K in case of R-phase and martensitic transformation. ► TGA effect increased with increasing applied stress in case of R-phase and martensitic transformation. In this study, the shape memory behavior of Ti–50.9at.% Ni alloys with time gradient annealing (TGA) was investigated via differential scanning calorimetry (DSC) and thermal cycling tests under constant load. The ingot was prepared using a high-frequency induction vacuum furnace. The as-cast ingot was hot forged and extruded, followed by cold-drawing and intermediate annealing to produce wires of 1.0mm in diameter, with a final cold-drawing of 30% 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, a 34K variation in the R-phase transformation interval (i.e., Rs−Rf) was obtained along the length of the specimen (80mm) that was time-gradient annealed from 3min to 20min at 773K. The results of thermal cycling tests under constant load revealed that the temperature dependence of transformation elongation (dɛ/dT) of the TGA specimen is smaller than that of the isochronously annealed specimen at every heat-treatment temperature (673–773K). The lowest dɛ/dT of R-phase transformation (0.0031%/K) was obtained for the specimen that was TGA-treated at 673K. The difference in dɛ/dT of R-phase and the martensitic transformation of the TGA-treated and isochronously annealed specimen was largest at 773K and increased with increasing stress. Such behavior provides superior controllability for actuation applications.