Microstructural development during the liquid-phase sintering of two-phase alloys, with special reference to the NbC/Co system

Abstract

An investigation was made of the grain growth and other microstructural changes occurring during the liquid-phase sintering of NbC alloys with ∼20 wt % cobalt. The effects of sintering time, sintering temperature, and small alloying additions were studied. It was found that the grain growth of NbC in liquid cobalt, at 1420° C, can be described by the equation: $$\bar d^3 - \bar d_0 ^3 = {\text{K}}t$$ where \(\bar d\) is the mean linear intercept of the grains after time t, and \(\bar d_0\) the initial mean intercept, K being a temperature-dependent constant with an “activation energy” of 95±15 kcal/mole. This equation suggests that grain growth occurs by a solution/ precipitation process controlled by diffusion in the liquid phase. Small alloying additions of WC, TiC or NbB₂ inhibit the growth and/or alter the growth process, as well as affecting such properties as the shape and contiguity of the carbide grains. The relative significance of grain coalescence to grain growth in a liquid phase is discussed. By examining theoretically the effect of anisotropy of interface energy on the cube ⇌ sphere grain-shape change, it has been possible to explain the observed sensitivity of grain shape towards sintering conditions.