Tailoring phase transformation behavior, microstructure, and superelasticity of NiTi shape memory alloys by specific change of laser power in selective laser melting

Abstract

In this study, NiTi shape memory alloys (SMAs) were fabricated by selective laser melting (SLM), and it is found that specific change of laser power (from 55 W to 70 W) was effective to tailor their phase transformation behavior, microstructure, and superelasticity. Results indicated that a minor increase of laser power made transformation temperatures increase significantly owing to the depletion of Ni and the formation of precipitates. The microstructure of SLM NiTi SMAs underwent the change from coarse grains to coarse together with ultrafine grains when laser power increased from 55 to 70 W. The superelastic responses demonstrated that SLM NiTi SMAs fabricated by specific laser power exhibited significantly different recovery strains. Specifically, SLM NiTi fabricated by 65 W showed stable phase transformation recovery strain, and obtained recovery strain of 3.91% at 1st cycle and only about 0.03% degeneration after 10 times cyclic compression. These outstanding stable recovery strains were ascribed to the coexistence of high-density dislocations formed in ultrafine grains and abundant nanoprecipitates interior of the grains, which impeded the formation of plastic deformation during loading effectively. These results make it possible to obtain the desired functional properties of SLM NiTi SMAs by specific change of laser power.