Defect-driven crack formation mechanism and crack suppression in electron beam welded Nb/Fe-based alloy dissimilar joints

Crack formation remains a major challenge in welding Nb to Fe-based alloys. This is mainly due to the rapid formation of brittle intermetallic compounds during non-equilibrium solidification. However, existing studies usually attribute crack initiation simply to the intrinsic brittleness of these intermetallic compounds. The detailed crack initiation process, defect evolution within the reaction layer, and their role in interfacial fracture remain unclear. In this study, a defect-driven cracking mechanism was proposed based on experiment and simulation. The weld zone was mainly composed of Laves intermetallic compounds, with a small amount of γ phase and γ/Laves eutectic structures. Continuous μ and Laves reaction layers formed on the Nb side. The Nb content in the Laves reaction layer reached approximately 38 at%, corresponding to a relative increase of about 15% over the stoichiometric value of 33 at%, which caused severe lattice distortion and the formation of high-density dislocations and stacking faults. TEM observations showed that these defects were preferentially accumulated near the μ/Laves interface. This accumulation is considered to be associated with cooperative shear-driven defect evolution in the TCP reaction layer. Such defect accumulation resulted in local stress concentration and promoted crack initiation under welding residual stress. By combining electron beam offset and beam inclination control, the melting amount of Nb was reduced, the interfacial reaction layer was refined, and visible defect accumulation was weakened. Based on this control method, crack-free joints were obtained, and the tensile strength increased from 120 ± 4 MPa to 203 ± 7 MPa. This work clarifies the role of defect evolution in topologically close-packed (TCP) reaction layers during crack formation and provides an energy-control route for improving refractory metal/Fe-based alloy dissimilar joints.