Microstructure and formation mechanism in a surface carburized tungsten heavy alloy

A 95W–3.5Ni–1.0Fe–0.5Co heavy alloy was surface modified by pack carburization. A new carburized layer consisting of outside surface layer and inner surface layer was formed. The double layered microstructure showed porous WC aggregates in the outside surface layer and W grains coated by shell-like (Ni, Co, Fe)₆W₆C carbides in the inner surface layer. There were two paths for C atoms diffusion and reaction through W grains and matrix phase. Co changed the formation orders of carbides and acted as a medium for promoting C atoms reaction with Ni, Fe, Co and W in the matrix to form shell-like (Ni, Co, Fe)₆W₆C carbides in the inner surface layer. Meanwhile, C atoms diffused in the W grains and reacted with W in an order of W→W₂C→WC. WC nucleated and grew in carburized W grains and micropores produced in the growing WC crystals due to the lattice expansion from W→W₂C→WC. As the carburization process continued, the shell-like (Ni, Co, Fe)₆W₆C carbide was also carburized to WC, and finally the whole W grain was carburized into porous WC aggregates in the outside surface layer.