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

In this study, the shape memory behavior of Ti–50.9 at.% 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 1.0 mm 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 34 K variation in the R-phase transformation interval (i.e., Rs − Rf) was obtained along the length of the specimen (80 mm) that was time-gradient annealed from 3 min to 20 min at 773 K. 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 (673–773 K). The lowest dɛ/dT of R-phase transformation (0.0031%/K) was obtained for the specimen that was TGA-treated at 673 K. The difference in dɛ/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.

► 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 773 K in case of R-phase and martensitic transformation.
► TGA effect increased with increasing applied stress in case of R-phase and martensitic transformation.