Mechanism of Grain Refinement of Aluminum By Al-5Ti-1B Master Alloy

Grain refinement is a vital technique in aluminum (Al) casting industry, in which Al-5Ti-1B (5 wt. pct Ti and 1 wt. pct B) master alloy is often used as a grain refiner. Castablity and processability of aluminum can be enhanced substantially. Different theories of grain refinement were proposed in the past few decades, however, there is still no consensus on the mechanism of Al grain refinement. In this study, Al-5Ti-1B master alloy was diluted at different ratios. Scanning electron microscopes (SEM) are used for microstructure characterization. Based on microstructural analysis, it is speculated that a dynamic Ti-rich layer is formed on the surface of TiB2 particles to nucleate aluminum (α-Al) grains. But the Ti-rich layer will subsequently be dissolved through a peritectic reaction and release Ti atoms in to the aluminum matrix. A small amount of dissolution of TiB2 releases Ti atoms making a Al3Ti layer of a few atoms thick remain, contributing to the nucleation of α-Al grains. The rates of Al3Ti layer and TiB2 dissolution are different, so the appearance of Al 3Ti layer will be sporadic, forming a dynamic layer. Therefore, the nucleation and growth of Al3Ti phase is a dynamic process. Based on microstructure analysis, a dynamic nucleation theory is proposed in this investigation