Formation mechanism of heterogeneous microstructures and shape memory effect in NiTi shape memory alloy fabricated via laser powder bed fusion

Additive manufacturing involves the process of track-by-track melt pools accompanied by the localized rapid melting/solidification, which can determine unique nonequilibrium microstructures. In this study, we report formation of heterogeneous nonequilibrium microstructures in near-equiatomic NiTi fabricated via laser powder bed fusion (LPBF) additive manufacturing, and further discuss their underlying formation mechanisms and influences on shape memory effect of the LPBF NiTi. Specifically, the heterogeneous microstructures include the core (HCP a-Ti)-shell (Ti2Ni) structural nano-sized precipitation phases in columnar grains, which were resulted from high undercooling and cooling rate during LPBF solidification, the intermediate R phase in cellular and columnar grains, which was stemmed from residual thermal stress during LPBF, the nano-sized cellular substructure in columnar grains with boundaries decorated with Ti2Ni precipitates, which was originated from enriched Ti atoms at the solidification front during directional and orderly solidification of melt pools, together with the abundant dislocations. Interestingly, the two-way shape memory strain of 0.8% in LPBF NiTi was obtained by cycle loading-u nloading-heating-cooling training process. These findings achieved in this work enrich the knowledge on formation mechanism of heterogeneous microstructures in LPBF NiTi SMAs, and further pave the way for engineering applications of two-way shape memory effect of LPBF NiTi shape memory alloys. & COPY; 2023 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http:// creativecommons.org/licenses/by/4.0/).

Automated summary

In this study, the formation mechanisms and influences of heterogeneous nonequilibrium microstructures in NiTi fabricated by laser powder bed fusion (LPBF) additive manufacturing were investigated. The results showed that the heterogeneous microstructures were due to high undercooling, cooling rate, and residual thermal stress during LPBF solidification, as well as the enrichment of Ti atoms at the solidification front. Interestingly, a two-way shape memory strain of 0.8% was achieved in LPBF NiTi through cyclic loading-unloading-heating-cooling training process.