Assessment of solid state and liquid phase sintering models by comparison of isothermal densification kinetics in W and W-Cu systems

The role of liquid phase and grain growth during the intermediate stage of sintering is investigated by comparing the isothermal densification rate of the tungsten skeleton in two systems: solid-state sintered W and liquid-phase sintered W-lOwt%Cu. Densification curves at 1380 degrees C, i.e. well after completion of capillary rearrangement in W-Cu, are confronted to the predictions of a model that considers a single representative grain size with quasi-equilibrium interfaces and a monotonic increase in grain size and coordination with density. In both systems, the densification mechanism is taken to be governed by W diffusion along grain boundaries. For system W-Cu, a phenomenological law is proposed to allow accounting for the decrease in liquid surface curvature during funicular filling of the pores by the liquid phase. The kinetics of grain growth is described via current literature model. Good agreement is demonstrated between experimental measurements and model predictions when relying on material parameter values given in literature. In system W-Cu, the detrimental effect of the decrease in dihedral angle is, at the beginning of the funicular stage, compensated by the contribution to sintering stress brought about by the concave curvature of liquid/vapour interface. The sensitivity of computational results to model hypotheses and material parameters is assessed. The most influential parameters are diffusivities along W grain boundaries and along W/vapour and W/Cu interfaces. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.