Strengthening mechanism and micropillar analysis of high-strength NiTi–Nb eutectic-type alloy prepared by laser powder bed fusion

Abstract

Although laser powder bed fusion (LPBF) can produce complex structures with promising properties, there still exists a concern on the diversity in materials, especially composites, for LPBF. This work innovatively used NiTi and Nb powders to prepare NiTi–Nb eutectic-type alloy by LPBF, which provides a new method to prepare NiTi–Nb shape memory alloy and could be an alternative for tailoring microstructure. The semi-molten Nb particle phase was retained during LPBF, so that β-Nb and the eutectic structure display a gradient distribution in the Nb diffusion concentration. The heat treatment process (annealed at 850 °C for 0.5 h) makes NiTi–Nb samples have high yield strength (~1640 MPa), high compressive strength (~2380 MPa), and high compressive strain (~39%), which are superior to the corresponding ones of its as-built and as-cast counterparts. Large Nb phase particles with a size of 10–30 μm are retained and accelerated the formation of eutectic phase and β-Nb precipitate phase. Overall, the presence of β-Nb phase has a beneficial effect on the alloy, and a large number of intertwined dislocations and stacking faults around the β-Nb phase promote the formation of martensite under stress loading. The eutectic phase makes an important contribution to the high strength and plasticity of the NiTi–Nb alloy.