Constructing function domains in NiTi shape memory alloys by additive manufacturing

Additive manufacturing (AM) is an orderly construction process involving track-by-track melt pools, which can modulate three characteristic zones between two adjacent melt pools: the heat-affected zone, transition zone, and cellular grain zone. These three zones constitute a function domain that determines the properties of AM metallic materials. In this study, we constructed a novel ordered function domain by AM of modified Ni50.6Ti49.4 shape memory alloy powder, which was produced by adding uniform Ni nanoparticles into atomised Ni49.6Ti50.4 powder. Interestingly, the three zones of the function domain had the same B2 austenite matrix but inhomogeneous Ni4Ti3 clusters and Ti2Ni nanoprecipitates, which caused different compressive recovery and residual strains in the three zones. The synergetic effect of the three zones generated a stable recovery strain and small hysteresis area in the AMed Ni50.6Ti49.4. Our results provide a novel strategy for modulating the microstructure of NiTi by AM to yield superior properties.