Cemented carbide alloys of TaC, with 30 wt.% nickel as a binder phase, were sintered at 1400℃ under the fully saturated carbon condition (excess carbon addition of 0.05 wt.%) and the depleted carbon condition (without carbon addition), respectively. The average size of TaC powders was 1㎛, but 6 percent of the particles was larger than 20㎛. The total content of carbon in TaC was 6.26 wt.%. This amount was more than the nominal carbon content of TaC by 0.03 wt.% owing to free carbon of 0.06 wt.% in TaC. In this study, the shape change of the carbide grain and grain growth behaviors in TaC-Ni have been investigated according to the total content of carbon in TaC.
Under the fully saturated carbon condition, the equilibrium shape of TaC grain was observed to be uniformly cubic during the sintering and abnormal grain growth occurred. Under the depleted carbon condition, by decarburization, the microstructure was initially the same as that of the fully saturated carbon condition. After sintering for eight hours, the shape of TaC grain was gradually to be spherical and grain growth behavior was being varied from abnormal grain growth to normal grain growth.
The fact that the faceted grains in liquid matrices undergo abnormal grain growth, while the spherical grains undergo normal grain growth, shows that the conditions for abnormal and normal grain growth depend on the solid-liquid interface structure. If the interface energy is highly anisotropic, the solid-liquid interface has the faceted shape and the atomically singular structure. Without the ledge-generating sources such as screw dislocation, these grains with such singular interfaces can grow only by the 2-D nucleation. If the growth of the faceted grains occurs by 2-D nucleation, the growth rate is expected to increase abruptly at a critical value of the driving force for growth. Thus, only the large grains enough to have driving force larger than the critical value for the 2-D nucleation can grow rapidly and the growth of other grains is almost suppressed. The abnormal grain growth is expected to occur when the solid-liquid interfaces have the singular structure. If the interface energy is isotropic, the solid-liquid interface has the spherical shape and the atomically rough structure. The rough interface allows the continuous attachment of atoms from the liquid and the interface mobility remains constant. Then all the grains having the driving force for growth can grow. Under this condition, the normal grain growth is expected to occur.
Under the depleted carbon condition, the shape of TaC grain gradually changed with increasing the sintering time. This shape change has a close relation with the total content of carbon in TaC. Such cubic carbides as TaC have the wide range of homogeneity and carbon deficiency does not produce an extra phase in cemented carbide. On the other hand, WC has the very small range of homogeneity and carbon deficiency produces η phase in cemented carbide. At the beginning of the sintering, free carbon in TaC powder was dissolved into the binder phase in order to satisfy the solubility of carbon in binder phase and the shape change of TaC grain did not occur. With increasing the sintering time, decarburization made the combined carbon in TaC dissolved, which caused the shape of TaC grain to be spherical. In order to examine the effect of the content of combined carbon on the shape of TaC grain, the specimen which had been sintered at 1400℃ for 40 hours in the depleted carbon condition was annealed again at 1400℃ for 5 hours under packing it with carbon powder in carbon crucible. The spherical shape of TaC grain changed into the cubic shape of TaC grain according to the content of combined carbon in TaC reversibly. It is certain that the increase of the content of combined carbon in TaC makes the structure of the solid-liquid interface singular.
