Improved superelastic stability of NiTi shape memory alloys through surface nano-crystallization followed by low temperature aging treatment

In this work, we proposed a process to improve the functional stability of Ni-rich NiTi alloys, which combines surface nano-crystallization by means of ultrasonic shot peening and subsequent low-temperature (523 K) aging treatment. The as-annealed coarse-grained NiTi plates were fist subjected to ultrasonic shot peening, which generates nano-grains at the surface region, and introduces dislocation networks in the interior of the NiTi sample. Both the grain boundaries and the dislocations could serve as the heterogenous nucleation sites for Ni4Ti3 nanoprecipitates during subsequent low-temperature aging treatment. Therefore, with the presence of nano-grains and dislocation networks, a homogeneous distribution of Ni4Ti3 nanoprecipitates in the sample is expected, which will strengthen largely the NiTi matrix and resist the plastic activities during the repetitive martensitic transformation. As a result, the improved stability during cyclic superelastic loading is achieved.

Automated summary

The study proposed a method to improve the functional stability of Ni-rich NiTi alloys through ultrasonic shot peening and low-temperature aging treatment, which can achieve a homogeneous distribution of Ni4Ti3 nanoprecipitates in the sample and strengthen the NiTi matrix.