Tailoring of microstructure and shape memory properties of hot-forged NiTiNb alloy by multi-pass ECAP at high temperatures

NiTiNb is a wide-hysteresis shape memory alloy, and the microstructure directly affects the shape memory properties. In this study, the microstructures and shape memory properties of hot-forged NiTiNb alloys were tailored by multi-pass equal-channel angular pressing (ECAP) at high temperatures. First, grain refinement structures with severely deformed grains and fully dynamic recrystallized equiaxed grains were prepared by ECAP at 650 °C and 800 °C (denoted as ECAP-650 and ECAP-800, respectively). The correlation between the two types of ECAP-based microstructures and shape memory properties of the NiTiNb alloy was established. The results indicated that there was no direct relationship between the grain size and thermally induced martensite (TIM) transformation. However, the dislocations in the deformed grains hindered the TIM transformation, resulting in a lower martensite transformation temperature. The greater β-Nb deformation during the stress-induced martensite (SIM) increased the martensite transformation hysteresis of the ECAP-processed samples. Simultaneously, the recrystallized grains were more beneficial to the stability of the SIM. Compared with the initial sample, the transformation hysteresis temperatures of the ECAP-650 and ECAP-800 samples increased by 29 °C and 34 °C, respectively. The refined grains were not conducive to the recovery of transformation strain, but they were beneficial for increasing the recovery stress. Compared to the initial sample, the recoverable transformation strain of the ECAP-650 and ECAP-800 samples decreased by 18.3% and 6.6%, respectively, and the corresponding maximum recovery stresses increased by 23.7% and 12.2%. Similar to the recovery stress, the thermomechanical cyclic stability of the ECAP-processed samples was also improved because the dislocation content introduced by lattice mismatch was reduced, and the sample with deformed grains had better stability than the sample with recrystallized grains.