Effect of post-heat treatments on the microstructure, martensitic transformation and functional performance of EBF3-fabricated NiTi shape memory alloy.

In the present work, a single solution treatment as well as solution/aging treatments were employed to regulate the precipitation behavior, martensitic transformation, mechanical and functional properties of EBF3-fabricated NiTi shape memory alloys. Under the optimized post-process heat treatment conditions, i.e., solution treatment at 1000 oC for 10 h followed by subsequent aging treatment at 500 oC for 4 h, the previously existing elongated Ti 4 Ni 2 O x oxides were partially dissolved and interrupted into granular form and relatively uniformly distributed in the NiTi matrix. Meanwhile, the preexisting nanoscale Ni 4 Ti 3 precipitates gradually transformed from clustered or segregated granular to a lenticular shape with the increase of the aging time. The martensite transformation routes also underwent an accompanying alteration, which changed from a one-step (B2-A ↔ B19'-M) to two-step (B2-A → R → B19'-M) transformation. Moreover, Ni 4 Ti 3 precipitated with a coherent relationship of {123} <111> B2-A //{11–20} <0001> Ni4Ti3 which increase the material strength by hindering dislocation movement and improve plasticity by providing a transition domain for accommodating deformation at the interface. The superelastic stability and narrow response hysteresis promoted by the presence of the R-phase are affected by residual (100) compound twins martensite in the NiTi matrix and the permanent plastic deformation activated in (011) [001] slip system of the B2-A phase. This deteriorates the superelastic recovery ratio from 83.6 to 57.4%, as observed after 10 loading-unloading cycles. The post-heat treatment regime presented in the current investigation can be used to modulate the material precipitation, phase transformation, as well as the mechanical and functional properties of EBF3-fabricated NiTi alloys. These results allow to extrapolate the potential for carefully selected post-heat treatment scheduled for additively manufacturing NiTi shape memory alloys. [Display omitted] • Post-heat treatment can regulate precipitation, martensitic transformation and properties of EBF3-fabricated NiTi SMAs. • Nano-scale Ni 4 Ti 3 alters the transformation routes and improves the strength-plasticity trade-off. • Superelastic stability and narrow response hysteresis are promoted by the presence of R-phase. • Residual (100) compound twins and dislocation activated in (011)[001] system of B2-A deteriorates the superelasticity. [ABSTRACT FROM AUTHOR]