Rational design of composite interlayer for diffusion bonding of tungsten-steel joints

Large residual stresses and brittle intermetallic compounds are two detrimental factors in the mechanical reliability of tungsten-steel joints. In this work, we conduct a rational design of interlayer structure to tackle these two pitfalls by combining finite element model (FEM) and Hume-Rothery rules. The FEM results show that the selection of interlayer materials for tungsten-steel joining depends not only on thermal expansion mismatch, but also on the plastic deformation of interlayer. In practice, it is better to relax the residual stress by a soft interlayer with high ductility than to transfer the stress from W by a hard interlayer with a low coefficient of thermal expansion. However, the Hume-Rothery rules indicate adding a single interlayer cannot prevent the formation of brittle intermetallic compounds. A composite interlayer design is then proposed in this study to minimize the possibility of intermetallic compounds formation in the meantime reduce the residual stress. Using V/Cu and V/Ni as typical examples, we demonstrate that composite interlayer can greatly improve joint performance because of the lower residual stresses and less detrimental intermetallic compounds.