Engulfment and distribution of second-phase nanoparticle during dendrite solidification of an Al-Si binary alloy: a simulation study

To achieve optimum strengthening effects of external nanoparticles (NPs), uniform dispersion of NPs in the melt is necessary for the casting manufacturing of metal matrix nanocomposites in which dislocation-based strengthening mechanisms play a significant role. However, the engulfment of nanoparticles within the solidifying grains and the avoidance of pushing them outside the solidification front are always a major challenge. Therefore, the understanding of local interface velocity and interface/particle dynamics during alloy solidification is of significant importance. Existing numerical studies on particle engulfment/pushing do not take into consideration the anisotropy of crystal growth and assume planar solidification interface, and thus they are unable to obtain the nanoparticle distribution in realistic alloy solidification. In this research, we investigate the engulfment/push behavior and the overall distribution of SiO2 nanoparticles in the dendrite solidification of an Al-Si binary alloy. Phase-field method is used to simulate the dendrite growth and to predict local solidification front velocity. In combination with the critical engulfment velocity obtained from a non-steady-state particle/front interaction model, the engulfment/push behavior of the entire solidification domain as well as the final distribution of nanoparticles can be analyzed. It is found that the distribution pattern of NPs obtained from simulation is overall consistent with the limited experimental results in the literature. In addition, the two main dislocation-based strengthening effects, e.g., Orowan bowing and CTE (coefficient of thermal expansion) mismatch strengthening, brought by external nanoparticles are quantitatively predicted. The degree of undercooling (60, 80, and 100K) and nanoparticle size (10, 20, 30, 40, and 50nm) are varied to investigate their influence on the NP engulfment behavior and the resulted strengthening effects.