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

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 has 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 plateshaped 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 reentrantedge 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. (C) 2020 Elsevier B.V. All rights reserved.