In chapter 1, chemically induced liquid film migration and formation of core-shell grain are studied by heat-treating sintered TiC-20 wt%Ni in contact with WC-20 wt%Ni at 1400℃ for relatively short period (2 min to 4 h) in vacuum. Compacts of TiC and Ni powder mixture were first liquid-phase-sintered for 8 h in order to obtain large grains with well-defined shapes by Ostwald ripening. Next the TiC-20 wt%Ni specimens are combined with WC-20 wt%Ni specimens and heat-treated at 1400℃ in vacuum. During the second heat-treatment, TiC and WC in the combined specimen are inter-diffused through Ni-rich liquid matrix. In 2 minutes, TiC grains at border area had shell structure and many intergranular liquid films are observed to migrate to form TiC-WC alloy layer. As second heat-treatment period increases, the thickness of shell increases and the migration of liquid film is obvious.
In chapter 2, the phenomenon in chapter 1 and grain growth are studied in TiC-20 wt%WC-20 wt%Ni system. However, it is difficult to observe clear trace of liquid film migration because the initial powder shapes of TiC and WC are so irregular and the size is so small. TiC-20 wt%WC-20 wt%Ni specimens are sintered for 1 min, 8 min, 1.5 h, and 8 h in vacuum. In the specimen sintered for 1 min, some TiC grains in contact with liquid has thin shell and some liquid films migrate. Grain growth in TiC-20 wt%WC-20 wt%Ni system is slower than that in TiC-20 wt%Ni system. After every grain have shell, grain growth is rarely occurred.
In chapter 3, TiC-20 wt%Ni specimen sintered 1400℃ for 8 h and TaC-20 wt%Ni specimen sintered at 1400℃ for 8 h are combined and heat-treated at 1400℃ for 2 min to 4 h. TaC is a little soluble in TiC while WC are not soluble. Therefore, TaC grains can have shell like TiC grains, but many liquid pockets are trapped and liquid film migration are not clearly observed because of the shell composition and grain shape with sharp corners.
In chapter 4, variation of grain growth rate in TiC-Ni system is studied for various liquid contents. In sintered TiC-20 wt%Ni specimens, cubic average grain size is proportional to sintering time. Grain size distribution of TiC-20 wt%Ni specimens is close to normal distribution and does not change for various sintering periods. When liquid content increases from 10 wt% to 30 wt%, grain growth rate decreases. It means that mechanism of grain growth TiC-Ni system is diffusion controlled process.
