Microstructure evolution analysis for the reaction interface between molybdenum and Kovar acquired by electron beam welding-brazing

A brittle reaction interface in molybdenum/Kovar joint acquired by electron beam welding-brazing, which consisted of three zones, was concretely analyzed with respect to microstructure evolution. Zone A was composed of α-Mo and σ (FeMo) intermetallic that was the origin of brittleness. The eutectoid structure in zone B was identified as α-Fe + μ (Fe₃Mo₂), indicating a decent ability of plastic deformation. The columnar phase of α-Fe extending into eutectoid structure showed a great tenacity considering quantities of gliding dislocations. Phase transformation in the reaction interface during solidification process was concretely analyzed, suggesting an amorphous area near α-Fe due to the blocking effect for eutectoid R (Fe₅Mo₃) on further eutectoid reaction between liquid metal and σ (FeMo), and, the subsequently excessive cooling rate. Thermodynamic calculation was conducted to explain the existence of μ (Fe₃Mo₂) with the absence of δ (MoNi) in zone B. The novel stacking faults and their overlapping were found within the body-centered cubic α-Mo due to the decrease in stacking fault energy of Mo with the addition of Fe atoms. Contrast change in diffraction fringes of twinning structure and new stacking fault caused by overlapping was explained.