Evolution of Microstructure in Semisolid Alloys During Isothermal Holding (Soaking)

Abstract

In a recent review of coarsening, Flemings [19] states that this process refers to the growth “of solid regions of low curvature at the expense of regions of higher curvature,” and this includes “the growth of larger particles or dendrite arms with the simultaneous dissolution of smaller particles or arms (so-called ‘ripening’), the filling of spaces between particles or dendrite arms (‘coalescence’) and the breakup of dendrites (‘dendrite multiplication’).” This is a view adopted in the present chapter. The driving force is of course always the reduction of total solid–liquid interface area and the reduction in the associated interfacial energy, and the general mechanism by which this is achieved is diffusion of solute atoms through the liquid from concentration gradients established between regions of high and low curvature. Owing to the complex geometries that may exist in coarsening semisolid systems with competing fluxes between different particles, which can change with both time and place, simple kinetic equations to describe the overall coarsening process is probably not possible. However, where the geometry of the solid may be clearly described, for instance in terms of dendrite arms formed early in solidification, simple kinetics can be derived. More complex geometries may develop later during coarsening that are more difficult to provide with an adequate geometrical description.