Diffusion Behavior of Secondary Carbides and Subsequent Influence on Microstructure of NbC-Ni-Based Cermets

The diffusion behavior of secondary carbides is essential for understanding the microstructure evolution during sintering process of the cermets, which was rarely studied in the literature. Herein, the diffusion behavior of secondary carbides Mo2C, WC, VC, and Cr3C2 in NbC-Ni-based cermets was investigated by using NbC-Ni/MeC-Ni (Me = Mo, W, V, Cr) diffusion couples annealed at 1150 to 1350 degrees C. The diffusion coefficients of Mo, W, V, and Cr were initially calculated based on the experimentally measured concentration profiles at 1150 and 1250 degrees C. Then their subsequent influence on grain growth of NbC grains was explored with the measured concentration profiles at 1350 degrees C and the calculated (NbC-Ni)-(MeC-Ni) vertical sections. The results show a diffusion rate sequence of Cr > V > Mo > W. The fast diffusion of Cr in the Ni binder changes the morphology of NbC grains from irregular to spherical shape. Moreover, compared with W, the larger diffusion coefficient of V exhibits a stronger inhibition to grain growth of NbC grains. The gradient distribution of the binder phase through Mo diffusion causes an uneven distribution of NbC grains near the interface. The distinct diffusion behavior of Mo, W, V, and Cr in NbC-Ni-based cermets further reveals the evolution of morphology, grain size, and distribution of NbC grains.