Dissimilar fusion welding of NiTi shape memory alloy and AlCoCrFeNi$_{2.1}$ eutectic high entropy alloy using multi-interlayer strategy

Dissimilar fusion welding between NiTi shape memory alloy and AlCoCrFeNi$_{2.1}$, eutectic high entropy alloy wasachieved using a combination of Nb and Cu interlayers. Both base metals exhibit unique properties that complementeach other making them attractive for integration in smart systems. However, direct joining of thesematerials is not feasible due to the formation of brittle intermetallic compounds that compromise the jointintegrity. Thus, to avoid this metallurgical issue dissolution of the Nb and Cu interlayers promoted elementalintermixing and altered solidification pathways, leading to the formation and intensification of topologicallyclose-packed phases. Phase evolution was assessed through thermodynamic CALPHAD simulations and validatedusing synchrotron X-ray diffraction, while further microstructural characterization employed backscatteredelectron imaging and energy dispersive spectroscopy. The weld exhibited pronounced spatial heterogeneity, witha peak hardness of 856 HV0.3 near the NiTi-side interfacial region, consistent with enrichment of TCP phases andTi-rich intermetallics. Tensile testing showed fracture at an ultimate tensile strength of 372 MPa and a fracturestrain of 1.11%, with crack initiation in the hardened interfacial region followed by mixed-mode fracture towardthe fusion zone. Although tensile ductility remains limited, the multi-interlayer strategy enabled defect-free jointformation and confined embrittlement to a narrow region compared with direct joining.