Rheological model for viscous flow densification during supersolidus liquid phase sintering

A model is derived for the viscous flow densification of prealloyed powders heated to just over the solidus temperature, a process termed supersolidus liquid phase sintering. The model builds from the viscous flow concepts first introduced by Frenkel with a new porosity effect. Viscous flow densification starts with the formation liquid on the grain boundaries inside the particles, with subsequent spreading of liquid to form a capillary bond between contacting particles. Pores lower the initial semisolid viscosity, but as densification progresses the viscosity increases. On the other hand, viscosity decreases higher temperatures increase. Densification is induced by the capillary force acting against the semisolid system, but densification is delayed until the particles sufficiently softened from liquid spreading on the grain boundaries. Thus, both viscosity and strength vary with the liquid content and particle microstructure. Distortion in sintering traces to an excess of liquid that lowers the skeletal rigidity, mainly due to grain growth with a concomitant release of grain boundary liquid. This often occurs after full densification is achieved. This modification to the model includes the correction for porosity effects on viscosity. The model is compared with data on a 316L stainless steel doped with boron, subjected to in situ densification and slumping observations.