Interfacial stabilization for electron beam welding of molybdenum and Kovar alloy with Permalloy addition

Abstract

No continuous brittle phase precipitates at the fracture location of the molybdenum/Kovar alloy electron beam welded joint, indicating that unstable phase interfaces resulting from solid-state phase transitions constitute the fundamental cause of joint brittleness. To inhibit solid-state phase transitions and optimize interfacial stability, Permalloy was added to the weld. The original solid-state eutectoid reaction (R(Fe₅Mo₃) → α-Fe + μ(Fe₃Mo₂)) was successfully replaced by a liquid-phase eutectic reaction (L → γ-Ni + MoNi). Since the eutectic γ-Ni + MoNi in contact with α-Mo precipitates directly from the liquid phase, no habitus growth occurs between the products and the parent phase during solid-state phase transitions. The eutectic nuclei grow along lattice planes exhibiting minimal distortion with α-Mo to minimize interface energy with adjacent phases. After adding Permalloy, the lattice misfit at the α-Mo/MoNi and α-Mo/γ-Ni interfaces is reduced to 11.2 % and 6.3 %, respectively, significantly lower than the excessive misfits of 22.1 % and 30.6 % observed at the original α-Mo/eutectoid α-Fe + μ(Fe₃Mo₂) interfaces without Permalloy addition. The transformation of the primary phase interface within the weak zone from incoherent to nearly coherent significantly enhances the mechanical properties of the welded joint, resulting in a tensile strength of 312 MPa, a 23 % increase compared to joints welded without Permalloy addition.