Stable superelasticity with large recoverable strain in NiTi alloy via additive manufacturing

We report a stable superelasticity with large recoverable strain in Ni 51 & sdot;2 Ti 48.8 (at.%) shape memory alloy (SMA) via additive manufacturing of laser powder bed fusion (LPBF). Microstructure analysis indicates that the LPBFed SMA samples have a non-homogeneous microstructure of two different grain zones, i.e., the coarse columnar one and the fine cellular one. Specifically, the coarse columnar grain zone accounts for a predominated content as high as-79 vol% and has a relatively low dislocation density. In contrast, the fine cellular one has a low content of-21 vol% yet a high dislocation density. Especially, all LPBFed non-homogeneous SMA samples exhibit a strong (100) texture with high intensities of 40.2 - 56.1, accompanied by homogeneous coherent Ti 4 Ni 2 O x nanoprecipitates. Constant-stress cyclic compression shows that the LPBFed sample with 79 vol% coarse columnar grain zone and 52.8 (100) texture presents stable superelasticity with large recoverable strains, 5.71 % from 15 to 30 cycles at high loading of 1200 MPa. Such a large recoverable strain herein is superior to those in NiTi SMAs fabricated by various methods. Basically, the stable superelasticity is attributed to the strong (100) texture against plastic deformation and the pinning effect of the homogeneous coherent Ti 4 Ni 2 O x nano- precipitates on dislocation motion during cyclic loading. Meanwhile, the large recoverable strain originates from the high ability to accommodate new dislocation in the high content columnar grain zone. This work can provide significant insights into design of high-performance NiTi SMAs and further accelerate their engineering application by additive manufacturing.