On the mechanism of Si-promoted destabilization of TiC particles in Al alloys

TiCx is an excellent composite strengthening particle and grain refiner for Al alloys. However, the stability of TiCx is poor when solute Si exists in Al alloy melts, which significantly depresses its strengthening and grain refining effects. In this work, the destabilization mechanisms of the TiCx particles in Al-Si alloy melt with a composition of Al-7Si-7.5TiC were explored via experiments, first-principles calculations and thermodynamic calculations. The experimental results show that Si atoms diffuse into TiCx and Ti atoms are released into the Al melt to form a Ti-rich transition zone during the insulation of TiCx in Al-Si melt, and the TiAlySiz and Al4C3 phases are solidified in the Ti-rich zone and at Ti-rich zone/TiCx interface, respectively. The first principles calculations show that the low formation energy of C vacancies facilitates the rapid diffusion of Si atoms in TiCx, while the doping of Si atoms reduces the energy barrier of diffusion of Ti atoms in TiCx and promotes the formation of Ti-rich zones. The thermodynamic calculations show that the wide crystallization temperature range of the destabilized product TiAlySiz phase is the key to continuous decomposition of TiCx particles. In addition, the driving force of the main destabilization reaction of TiCx in the Al-Si alloys is about 44 times higher than that in the Al alloys without Si addition. This indicates that the presence of solute Si remarkably promotes the subsequent decomposition process of TiCx in the Al-Si alloy melts.