Growth kinetics of primary Si particles in hypereutectic Al-Si alloys under the influence of P inoculation: Experiments and modelling

Abstract A quantitative study on the growth kinetics of primary Si particles (PSPs), especially under the effect of P inoculation, during isothermal melt solidification of hypereutectic Al-Si-Cu alloys has been realized by using a unique in-situ micro-focus X-radiography method. The morphology evolution has been captured by in-situ observation and the growth velocity have been measured. It is found that P inoculation reduces the branching ability and the growth velocity of PSPs. As a result, the morphologies were changed from star-like, interconnecting multi-plate shapes into more compact, block-shaped and individual plate-shaped by P addition. A post-situ crystallographic study of PSPs by electron backscatter diffraction (EBSD) shows that the growth of plate-shaped PSPs in both alloys is through the twin-plane reentrant-edge mechanism, while block-shaped particles in the P inoculated alloy do not show any preferential growth. A simple growth model has been developed to simulate the growth kinetics of plate-shaped Si particles. A comparison between the simulated and measured growth velocity indicates that growth of plate-like particles is close to the diffusion-controlled lengthening kinetics at high undercooling while the effect of P addition in reducing growth velocity of PSP plates is due to the lower nucleation undercooling and stronger solute impingement effect caused by nucleation of high density PSPs.