Prominent superelastic response induced by Ni4Ti3 phase in NiTi alloys fabricated via wire-arc directed energy deposition

In this study, a wall-shaped NiTi alloy component was manufactured using the wire-arc directed energy deposition (DED) technique. The superelasticity of the NiTi alloy was enhanced through aging treatment, and the subsequent microstructural evolution during the superelastic response was analyzed. Different aging treatments (at 200 °C, 300 °C, and 400 °C) on the NiTi alloy samples were performed, which significantly enhanced the superelastic recovery rate and stability by adjusting the size of the Ni4Ti3 phase. In detail, the samples aged at 200 °C exhibit significant residual strain during the cyclic loading. The 300 °C aged NiTi alloy displays prominent superelasticity, with a stable recovery rate of over 99.7% throughout 10 cycles. In contrast, the samples aged at 400 °C essentially do not demonstrate superelastic behavior. The significant changes in superelasticity are fundamentally related to dislocations and martensitic phases. The precipitates of Ni4Ti3 coherent with the matrix effectively hinder the generation of dislocations during cyclic loading, thereby enhancing the superelastic recovery rate. The appearance of the martensitic phase, however, leads to a decrease in superelasticity, which is related to the nature of superelasticity. The obtained results provide guidance for the fabrication of NiTi alloys with excellent superelasticity.