Effect of heat treatment on microstructure, phase transformation behavior and shape memory effect of NiTi shape memory alloy manufactured by additive manufacturing

Abstract

NiTi shape memory alloy (SMA) components fabricated via additive manufacturing exhibit broad application prospects in biomedical and aerospace fields. However, directly additively manufactured NiTi SMAs often suffer from inhomogeneous phase distribution and unstable shape memory properties. Heat treatment can effectively homogenize phase distribution and enhance the stability of shape memory performance. In this study, aging treatments at varying durations and temperatures were conducted on NiTi SMAs produced by laser-directed energy deposition (L-DED) to achieve more stable and superior mechanical performance. The effects of different heat treatment regimes on microstructure evolution, elemental/phase distribution, and shape memory behavior were systematically investigated through experimental characterization and molecular dynamics (MD) simulations. Key findings include: Compared to as manufactured samples, aging at 400 °C to 600 °C minimally influenced NiTi₂ distribution but universally promoted an increase in phase transformation temperatures and transformation enthalpy, meanwhile, prolonged aging durations further elevated phase transformation temperatures. Aging at 500 °C and 600 °C facilitated a reduction of high angle grain boundaries in L-DED NiTi SMAs and enhanced shape memory effects, which MD simulations attributed to diminished grain boundary resistance to phase transformation. Aging at 500 °C and 600 °C also stimulated the precipitation of Ni₄Ti₃ phases, enabling matrix strengthening and improved shape memory performance, with extended aging times promoting precipitate coarsening. This work provides insights into optimizing aging treatments to stabilize and enhance the properties of L-DED NiTi SMAs, thereby advancing their broader industrial adoption.