Microstructure and mechanical properties of the vacuum diffusion bonding joints of 4J29 kovar alloy and 316L stainless steel using pure cobalt interlayer

The joining of 4J29 kovar alloy and 316L stainless steel with a Co interlayer was successfully accomplished by vacuum diffusion bonding in a temperature range of 855–945 °C, and an axial load of 10 MPa was employed to acquire diffusion-bonded joints. Interfacial microstructures of vacuum diffusion-bonded joints were analyzed using scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The mechanical properties of bonded joints were evaluated by tensile strength and micro-hardness tests. The results showed that bonding temperature had significant effects on microstructures and mechanical properties of diffusion-bonded joints. The microstructures of diffusion layer at kovar alloy side and stainless steel side were comprised of Co solid solution of Fe–Co–Ni and solid solution + FeCo, respectively. In addition, a Kirkendall effect was found near Co interlayer. The diffusion behavior of various atoms was evaluated by Fick's law and Arrhenius relationship, and growth kinetics and activation energy of diffusion layer were also investigated. If bonding temperature was 945 °C, bonded joints would fracture at kovar alloy side, and maximum tensile strength of 516 MPa was acquired. In addition, the bonding mechanism of 4J29/Co/316L vacuum diffusion-bonded joints was also analyzed.