In chapter 1, when compacts of TiC-WC (or TaC)-Ni powder mixtures are sintered at 1400℃, grains with TiC cores and TiC-WC (or TiC-TaC) alloy shells form. The Ni-rich liquid films between the neighboring grains also migrate, forming the same alloy regions behind them. The thickness of the shells increases with the amount of WC (or TaC) added. The dissolution of some TiC grains and reprecipitation of the alloy phase in the shell formation and liquid film migration process is driven by the coherency strain energy produced by WC (or TaC) diffusion into the surfaces of dissolving TiC grains. The WC diffusion into TiC grains is expected to produce tensile stress, and when TaC is also added, it produces a compressive stress, reducing the net coherency strain and, therefore, the rate of shell formation. Thus by simultaneously adding WC and TaC, it is demonstrated that the shell formation process is driven by the diffusional coherency strain energy.
In chapter 2, when compacts of TiC-20Ni powder mixtures are sintered at 1400℃, relatively large TiC grains form with near equilibrium shapes. When these specimens are heat treated again at 1400℃ in contact with WC (or TaC)-20Ni pieces, the liquid films between the TiC grains in contact region migrate against their increasing curvatures, forming TiC-WC (or TiC-TaC) alloy behind them. These migrating liquid films reverse their directions upon further heat treatment. Many TiC grains in the contact region also form core-shell structures with irregular and often concave interfaces. New TiC-WC (or TiC-TaC) grains also precipitate at some contact regions. All of these processes must occur by dissolution of the TiC and WC (or TaC) grains and reprecipitation of the TiC-WC (or TiC-TaC) alloy phase. The dominant driving force for these processes must be the coherency strain energy in the WC (or TaC) diffusion zone at the surfaces of the TiC grains.
