Phase identification and thermodynamic modeling of a quaternary system in dissimilar laser-welded NiTi/ASS

Defect-free joints of NiTi/austenitic stainless steel wires have extraordinary combined properties of shape memory, superelasticity, and biocompatibility. However, problems arise from the dissimilar welding of NiTi to stainless steel, including the formation of brittle intermetallic compounds and the loss of shape memory and superelastic properties. To overcome the welding challenges of these alloys, phases formed in the weld zone must be identified. Therefore, we used CALPHAD quaternary phase analysis to predict the formation of intermetallic compounds and other probable phases in the weld zone. The calculated results were compared with existing ternary phase diagrams containing Fe, Ni, Ti, and Cr. According to these calculations and experimental observations using electron backscatter diffraction, energy-dispersive X-ray spectroscopy, transmission electron microscopy, and micro X-ray diffraction techniques, brittle Fe2Ti is the predominant phase in the final solidified structure, even in the equilibrium state at the NiTi-weld interface. In addition, a ductile Ni3Ti phase, together with the γ-(Fe,Ni) and B2-(Fe,Ni)Ti phases, is feasible in the weld zone. Moreover, Cr appears as a solid solution with Fe and Ti (BCC_A2 and β(Cr,Ti)) at low Cr concentrations. At higher concentrations, Cr can potentially appear as a σ phase and a C14 Laves phase, which can cause a stress concentration at the NiTi-weld interface, degrading the mechanical properties of dissimilar joints. Therefore, a suitable modification process is required to improve the mechanical properties of NiTi to stainless steel joints. Thus, any adopted strategy should preclude brittle Fe2Ti and facilitate the formation of more ductile phases such as γ-(Fe,Ni).