Effect of low-temperature aging treatment on thermally- and stress-induced phase transformations of nanocrystalline and coarse-grained NiTi wires

Abstract

The phase transformation behaviors of nanocrystalline NiTi alloys coupling with grain size poses a challenge in functional property configuration. To realize this configuration and simultaneously avoid undesirable grain growth, the low-temperature aging (LTA) treatment at 573K for 2h was applied to both the nanocrystalline and coarse-grained NiTi wires in this study and the effect of LTA on both the thermally- and stress-induced phase transformations was respectively investigated. The results show that, after LTA, B2↔R transformation temperature of nanograins was elevated when R→B19′ transformation was maintained suppressed. The stress hysteresis and residual strain of nanograins were increased while those of coarse grains were decreased. Nanograins required higher stress to activate stress-induced R-phase transformation than coarse grains. Aged NiTi coarse grains presented larger thermal hysteresis but smaller stress hysteresis compared with non-aged ones. To have an in-depth understanding of these differences, the microstructures and microhardness were further studied. It turns out that the nanoprecipitation, lattice recovery, as well as the preservation of the preformed grain size are responsible for the differences. This study thus suggests the potential of configuring the functional properties while simultaneously maintaining the constant grain size via LTA treatment, which may facilitate the application of NiTi nanocrystalline.