Toward additive manufactured crack-free NiTiCu shape memory alloys with low-hysteresis and tailorable phase transition behavior supported by theoretical design

In this study, a novel NiTiCu alloy composition (i.e., Ni41.5Ti49.5Cu9.0 in at%) with low hysteresis and two-step martensitic transformation was first designed for laser powder bed fusion (LPBF) technique through a combination of CALPHAD and machine learning techniques. The LPBF-fabricated NiTiCu alloy displayed a cellular grain structure devoid of discernible cracks. Following solution and aging treatments, the NiTiCu samples exhibited the desired two-step martensitic transformation with a low thermal hysteresis of 8.9 K, aligning closely with the design specifications. Interestingly, the heat-treated samples demonstrated enduring stability in recovery strain, achieving a remarkable 5.76 % over the course of 50 cyclic loading tests. Fundamentally, the sustained recovery strain observed and the nuanced variations in superelastic behaviors can be ascribed to the interaction between dislocations and Ti2(Ni, Cu)/Ni4Ti3 precipitates. The saturation of dislocations around precipitates stabilized the partial martensite and hindered the slip deformation, thereby ensuring stable superelastic behavior. Finally, an effective roadmap toward crack-free LPBF-fabricated NiTiCu shape memory alloys with desired phase transition behavior was proposed. It is envisaged that such a roadmap holds promise for broader applicability across other NiTi-based shape memory alloys.