Analysis of densification kinetics during the sintering of reduced chromium powder

Abstract

1. Chromium powder in a deformed condition contains no substantial type II distortions; this points to a low ductility of the strating reduced chromium powder. 2. Thermodynamic calculations and experiments demonstrate that chromium oxides may be reduced under the usual sintering conditions in hydrogen with a dew point of 253–243°K (between −20 and −30°C) at 1473°K. 3. The experimentally determined energy of activation for the densification process of chromium powder at the initial instant of sintering in hydrogen in the 1373–1473°K is 134±8.3 kj/mole (32±2 kcal/mole), which is equal to 0.43 of the energy of activation for volume self-diffusion (Evol). 4. The fact that sintering durations obtained by calculation on the basis of phenomenological theory are longer than the experimental values and that the energy of activation for the shrinkage of chromium is lower than Evol indicates that the densification of active reduced chromium powder at the initial stages of sintering is not governed by the volume-viscous flow mechanism. 5. Without diminishing the role played by boundary and surface diffusion, it is necessary to note that the mechanism of activated dislocation slip under the influence of capillary forces may play an important part in the densification process of reduced chromium powder at the early stages of sintering; this is indicated by the circumstance that the value of the energy of activation for dislocation intersection is similar to that for the shrinkage of chromium powder.