The grains dispersed in a liquid matrix in an alloy prepared, for example, by liquid phase sintering, undergo coarsening during the heat-treatment at high temperatures. While those alloys with spherical grains show the normal Ostwald ripening, those with faceted grains often show the abnormal grain growth(AGG). The purpose of this work is to examine the mechanism of the AGG of these faceted grains in liquid matrices as related to the grain surface structure and the growth mechanism.
The spherical and the faceted shapes of the solid grains dispersed in liquid matrices imply rough and singular interfaces, respectively. The ease of the atomic attachment to the rough interface leads to the diffusion-controlled Ostwald ripening, but an appreciable barrier to the atomic attachment exists in the case of the faceted interface, presumably leading to the interface-controlled Ostwald ripening. Without the ledge-generating sources such as screw dislocations, growth of the singular interface is known to proceed by two-dimensional (2-D) nucleation. The grains, which are large enough to have the capillary driving force larger than the critical value for 2-D nucleation, are allowed to grow while growth of other grains is almost suppressed, thus satisfying the condition for AGG.
The critical driving force for 2-D nucleation in the solid-liquid interface is estimated to be ~30 J/mole from the available data on Ga solidification. Estimation based on the typical value of the solid-liquid interfacial energy indicates that the capillary driving force which amounts to ~30 J/mole can be provided by the systems having submicron-sized particles. In agreement with this prediction, the specimens prepared from a sub-micron size WC powder show AGG during sintering at 1500℃, while those prepared from a 5.48 ㎛ size powder do not show any AGG. The frequency of AGG is enhanced if some 5.48 ㎛ size powder is added to the sub-micron size WC powder. Increasing the sintering temperature to 1600℃ also enhances the frequency of AGG. These observations are consistent with the predictions based on the 2-D nucleation model.
The large growth rate of abnormal grains may be explained based on the behavior of the critical radius of systems coarsened via 2-D nucleation. Since the dissolution of a singular interface proceeds without the energy barrier for 2-D nucleation, the critical radius of the system coarsened by 2-D nucleation is estimated to be smaller than that of the system treated by Wagner in LSW theory. And the increasing rate of the critical radius during AGG is smaller than that during normal Ostwald ripening because only restricted grains grow during AGG. Therefore, the large driving force for growth is persistently provided in the system coarsened via 2-D nucleation, which induces the fast growth of abnormal grains.
With the ledge-generating sources such as screw dislocations, the growth of a singular interface dose not need 2-D nucleation. If the WC is ball-milled before compacting and sintering, all grains grow more rapidly while AGG is suppressed. Compared with the AGG via 2-D nucleation of unmilled WC powders, the distinction between matrix grains and abnormal grains is not clear and the growth rate of abnormal grains is small. These results are attributed to defects as screw dislocations produced during the ball-milling. Even matrix grains which cannot be provided the critical driving force for 2-D nucleation from the capillarity can grow via ledge-generating sources. Thus the driving force for growth is not concentrated to a few abnormal grains during the coarsening process.
In polycrystalline materials, there are two types of grain boundaries : special and rough. Special grain boundaries have non-linear mobility characteristics and are significantly more mobile than rough grain boundaries. Thus, if there exist special boundaries. AGG may occur without second phase particles in grain boundaries. Grain boundary/surface roughening transitions from special to random increase the grain boundary/surface diffusivity significantly. Impurity additions as well as increasing temperatures cause grain boundary/surface roughening transitions. The activated W sintering by Ni addition has been analyzed in the point of the grain boundary/surface roughening transitions.
