A new model for the solidification of metal matrix nanocomposites: Wet cluster engulfment of nanoparticles by the solidification front

In this article, a new mechanism so-called wet cluster engulfment and the related formula were proposed to predict the critical solidification velocity for nanoparticles engulfment by a solidification front. According to the mechanism, the most important issue in the engulfing of the nanoparticles is the aggregation of nanoparticles, i.e. the formation of wet clusters in front of solidifying phase. In fact, local clustering of nanoparticles created highly viscous zones in the vicinity of the solidification front so that the nanoparticles and the liquid-filled spaces between them move as a unified mass. Thus, the van der Waals and drag forces between a nanoparticles cluster and the solidification front were calculated analytically. The new formulae were derived for the critical solidification velocity and critical diameter of the clusters as a function of the melt viscosity, the nanoparticle size, the cluster size, and the nanoparticle volume concentration inside the cluster. The theoretical results showed that the new model predicted more accurate values for the critical velocity than the earlier models. The proposed model can explain the formation of nanoparticle-rich zones and zones with a low concentration of nanoparticles in the microstructure of the nanocomposites produced via casting.