In-situ synthesis of NiTi shape memory alloys with tunable chemical composition and thermomechanical response by dual-wire-feed electron beam directed energy deposition

Abstract

In this study, we demonstrate the direct in-situ synthesis of NiTi alloys with tunable chemical composition (Ni/Ti atomic ratio) and corresponding thermomechanical response. This synthesis is achieved by regulating the feeding speed ratio of pure Ni and Ti wires during the additive manufacturing process based on dual-wire-feed electron beam directed energy deposition (EB-DED) technology. Under appropriate process conditions, the resulting NiTi alloys exhibit a controllable evolution around the near-equiatomic composition and display a typical columnar grain morphology characteristic of additively manufactured NiTi alloys. With an increase in Ni content (shifting from Ti-rich to Ni-rich), the second phase particles present in the samples change from Ti-rich phase (Ti2Ni) to Ni-rich phases (such as Ni4Ti3 and Ni3Ti2). The phase transformation temperatures gradually decrease with increasing Ni content, and the predominant matrix phase transitions from martensite to austenite. The as-built NiTi alloy exhibits a typical tensile curve with a good tensile elongation of 11 %, fabricated under suitable composition and microstructure conditions. This result surpasses values reported in current in-situ synthesized NiTi alloys through additive manufacturing methods. Moreover, it almost reaches the levels achieved by additively manufactured NiTi alloys using pre-alloyed raw materials. Furthermore, this study reports, for the first time in the field of in-situ synthesized NiTi alloys, a good tensile shape memory effect, achieving an impressive recovery rate of up to 70 % under a tensile strain of 6 %. This investigation provides a meaningful theoretical perspective and technical strategy for the integrated customization of NiTi alloy components in structure, composition, and function. This low-cost and high-efficiency approach is particularly attractive for the preparation of functional graded, large-scale and disposable NiTi components.