Coarsening of cubic TiC grains with round edges in Ni-rich liquid matrix

Abstract

When compacts of TiC powder mixed with various amount of Ni powder are sintered at 1400℃ where TiC grains coexist with Ni-rich matrix after packing in carbon powder in a graphite crucible, the grains and the liquid pockets in the grains have {100} faces and rounded edges. The equilibrium shape must thus be a cube with {100} faces and round edges. Such a grain shape indicates partial roughening of the singular {100} surfaces. The TiC grains undergo further roughening transition with temperature increase indicated by the increase of area of rounded edges. Since the grains with such an equilibrium shape grow by step nucleation process and the diffusion of atoms in the liquid matrix is definitely needed for the grain growth, the coarsening behavior of TiC-Ni system at temperature ranging from 1400℃ to 1600℃ can be explained by mixed control of diffusion and step nucleation process. In the TiC-8wt%Ni specimens sintered at 1400℃, the coarsening goes through three stages in order; the stages of normal grain growth, abnormal grain growth, and stagnant growth. In the stage of normal grain growth, the cube of the average grain size increases linearly with the sintering time and the grain size distributions normalized to the average grain radius are fairly narrow and almost time-invariant, but little bit broader. In the stage of abnormal grain growth, only a few large grains grow continuously and a large number of small grains do not grow, so the cube of the average grain size with the sintering time deviates downward from the linear relationship and the normalized grain size distributions are broader and broader. However, when a lot of small grains dissolve and disappear, the cube of the average grain size increases abruptly with the sintering time and the normalized grain size distributions are narrower again. In the stage of stagnant growth, only large grains remain and impinge each other, and the microstructure and the average grain size rarely change. In the TiC-40w...