Surface roughening transition and coarsening of NbC grains in liquid cobalt-rich matrix

Abstract

When NbC-30 wt% Co powder compact is sintered at various temperatures where NbC grains (with small amounts of Co) coexist with a liquid Co-NbC matrix, the NbC grains undergo a surface roughening transition with temperature increase and the grain growth changes from abnormal to normal growth. When sintered at 1400degreesC, the grains are polyhedral with sharp edges (and corners) and grow abnormally because their singular surfaces move by nucleation of surface steps. When sintered at 1600degreesC, the edges become round, indicating the surface roughening transition. The grains still grow abnormally, but their number density is larger than that at 1400degreesC because of the smaller surface step free energy. When sintered at 1820degreesC, the grains are nearly spherical, but the flat-surface segments still remain. The grain growth at this temperature is nearly normal because of very small surface step free energy. The surface roughening transition is reversed when a specimen initially sintered at 1820degreesC is heat-treated again at 1400degreesC, but some grains show transition shapes with nearly flat edges and slope discontinuities (shocks).