Formation mechanism of inherent spatial heterogeneity of microstructure and mechanical properties of NiTi SMA prepared by laser directed energy deposition

Ni₅₁Ti₄₉ at.% bulk was additively manufactured by laser-directed energy deposition (DED) to reveal the microstructure evolution, phase distribution, and mechanical properties. It is found that the localized remelting, reheating, and heat accumulation during DED leads to the spatial heterogeneous distribution of columnar crystal and equiaxed crystal, a gradient distribution of Ni₄Ti₃ precipitates along the building direction, and preferential formation of Ni₄Ti₃ precipitates in the columnar zone. The austenite transformation finish temperature (A_f) varies from −12.65 ℃ (Z = 33 mm) to 60.35 ℃ (Z = 10 mm), corresponding to tensile yield strength (σ_0.2) changed from 120 ± 30 MPa to 570 ± 20 MPa, and functional properties changed from shape memory effect to superelasticity at room temperature. The sample in the Z = 20.4 mm height has the best plasticity of 9.6% and the best recoverable strain of 4.2%. This work provided insights and guidelines for the spatial characterization of DEDed NiTi.