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 1400°C, 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 1600°C, the edges become round, indicating the surface roughening transition. The grains still grow abnormally, but their number density is larger than that at 1400°C because of the smaller surface step free energy. When sintered at 1820°C, 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 1820°C is heat‐treated again at 1400°C, but some grains show transition shapes with nearly flat edges and slope discontinuities (shocks).

Microstructural changes in (K0.5Na0.5)NbO3 ceramics sintered in various atmospheres

“…7). [31][32][33] In the samples sintered in O 2 and N 2 , the microstructure is coarse with many abnormal grains ( Fig. 1a and b) and the grain size distributions are broad with a main peak and several minor peaks (Fig.…”

“…2b and 6c). [30][31][32][33] Therefore, the grain growth behaviour of KNN in different atmospheres can be explained by considering the change in ε and G C .…”

Effect of TiO2 addition on grain shape and grain coarsening behavior in 95Na1/2Bi1/2TiO3–5BaTiO3

“…Many investigations [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] have shown a close relationship between the coarsening behavior of grains in polycrystals and the equilibrium shape of grains. Abnormal grain coarsening (AGC) is frequently observed in faceted systems with smooth (atomically ordered) interfaces [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] whereas normal grain coarsening (NGC) occurs in systems with rough (atomically disordered) interfaces.…”

“…Abnormal grain coarsening (AGC) is frequently observed in faceted systems with smooth (atomically ordered) interfaces [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] whereas normal grain coarsening (NGC) occurs in systems with rough (atomically disordered) interfaces. [1][2][3] The NGC observed in systems with rounded grains has been attributed to diffusioncontrolled growth due to an unlimited number of nucleation sites for atom attachment at the rough interface.…”

Effect of step free energy on delayed abnormal grain growth in a liquid phase-sintered BaTiO3 model system

“…6,8,10,[14][15][16][17][18][19][20][21] Grain boundaries and solid/liquid interfaces can be rough or faceted on an atomic scale. If the boundaries/interfaces are rough, then normal grain growth takes place.…”