Superior superelasticity of laser powder bed fusion fabricated NiTi alloys achieved by post heat treatment

Abstract

In this work, we found that the long strip-like Ti₄Ni₂O_x phase, concentrated at the grain boundaries (GBs), is the primary reason for the inferior superelasticity of laser powder bed fusion (L-PBF) fabricated NiTi alloys. In order to improve the superelasticity of L-PBF produced NiTi alloy, we proposed a simple yet efficient heat treatment protocol, which includes solution treatment at 1273 K for 3 h and subsequent aging treatment at 623 K for 0.5 h. The solution treatment leads to the spheroidization and coarsening of the Ti₄Ni₂O_x phase, as well as the growth of NiTi grains. The grain growth captures Ti₄Ni₂O_x phase at GBs into the grain interior, which improves largely the integrity of the matrix. Aging treatment introduces Ni₄Ti₃ nanoprecipiates, which could improve the stability of superelasiticity. As a result, NiTi alloy with excellent superelasticity is obtained, which shows a recovery rate of >80 % (pre-strian of 5 %) after 317 cycles.