Tuning the microstructure, martensitic transformation and superelastic properties of EBF3-fabricated NiTi shape memory alloy using interlayer remelting

Abstract

In this work, different interlayer remelting strategies are applied to regulate the microstructure evolution, martensitic transformation, and superelastic features of NiTi shape memory alloys prepared using the EBF3 additive manufacturing technique. The NiTi deposits prepared under different remelting beam currents are all composed of the B2 austenite, residual B19′ martensite, and submicron-scale Ti4Ni2Ox precipitates, and exhibit a one-step phase transformation (B2 ↔ B19′). Meanwhile, the crystallographic orientation, grain boundaries, and residual strain of these alloys present a distinct variation with the application of different remelting beam currents. During mechanical testing, the critical stress (σMs) of the EBF3-fabricated NiTi alloys was seen to possess a significant dependence on the martensitic transformation behavior, namely with the amount of B19′ martensite and the corresponding Ms. However, the broadening and stabilization of lamellar martensite created by dislocation pile-ups and plastic deformation in the cyclic loading–unloading procedure is the key reason for the deterioration of the superelastic response of the NiTi deposits. This work confirms that the mechanical and functional performances of NiTi alloys produced via EBF3-technique can be modified upon the application of proper interlayer remelting strategies, which can be extrapolated to the directed energy deposition of shape memory alloys or other metal components.