Microstructure evolution and shape memory function mechanism of NiTi alloy by electron beam 4D printing

Compared to the traditional additive manufacturing techniques, the electron beam freeform fabrication (EBF3), is carried out in a vacuum atmosphere, indicating its superiority in the manufacturing of element content-sensitive alloy, e.g., NiTi shape memory alloy (NiTi SMA), thus deriving promising application prospects. In this study, with the TC4 alloy was utilized as the substrate, we used EBF3 to fabricate equiatomic Ni50Ti50 SMA successfully. Microstructure evolution, phase transition temperature and structure, mechanical properties, and most significantly, shape memory functional ability after multiple deformations were systematically studied. The deposition was composed of four typical regions: Ni diffusion layer, Ni-Ti reaction layer, Ti diffusion region, and NiTi stable deposited region. The mechanical properties of each region varied from each other. A single reversible martensitic transition occurred during heating and cooling (B2↔B19′). The microstructure of the stable deposited region was B2 austenite phase (Space group: Pm3m), while a small amount of NiTi2 and Ti4Ni2Ox phases precipitated at the grain boundaries. Dislocations tend to form in B2 phase deriving from more slip systems. Because of the incomplete phase transition, tangling and piling up occurred both inside B19′ phase and phase interface during the martensitic transition, which affected the integrity of the B19′ phase and hindered the transition (B19′→B2). As the consequence, the incomplete martensitic transition was also detrimental to the shape memory effect (SME).