Gas tungsten arc welding of NiTi shape memory alloy and AlCoCrFeNi$_{2.1}$ eutectic high entropy alloy using a niobium interlayer

Dissimilar fusion welding of a NiTi shape memory alloy and an AlCoCrFeNii$_{2.1}$ eutectic high entropy alloy wasperformed using a niobium interlayer. The unique properties of these materials complement each other,potentially enabling hybrid structures for advanced applications and smart systems. Gas tungsten arc weldingwith an arc offset technique was employed to create a weld-braze joint at the NiTi-Nb interface while controllingthe heat input. Comprehensive microstructural and phase analysis was carried out using optical and electronmicroscopy, synchrotron X-ray diffraction, and was further supplemented by thermodynamic simulations.Dissolution of the Nb interlayer altered the solidification pathways in the fusion zone, leading to the formation oftopologically close-packed phases (including C14 Laves and σ) and Ti$_2$Ni. Multiple interfacial reactions at theNiTi interface introduced significant strain, which increased hardness but also acted as stress concentratorsduring tensile loading. The addition of the niobium interlayer enabled the formation of a stable, crack-free joint,whereas welding without an interlayer resulted in catastrophic cracking.