Investigation of the interfaces between boride particles and aluminium in the as-received and re-melted Ti-B-Al grain refiner master alloy

In the Al casting industry, TiB2 grain refiner particles are added to the melt in order to promote heterogeneous nucleation of α-Al crystallites. It has been suggested that thin Al3Ti DO22 layers, adsorbed on the boride particles, might be necessary for successful α-Al nucleation via the peritectic reaction. In order to elucidate this suggestion, the boride/α-Al interfaces were studied in the as-received Ti-B-Al grain refiner rod, as well as after remelting and rapid quenching of the same rod. The investigation was performed using the TEM, EDS, STEM and HREM techniques. For the as- received rod, the EDS study failed to show conclusively enrichment of the interfaces in Ti and indicated clearly only a small enrichment in Si and Fe. The HREM investigation found no indication of a possible crystalline layer coating the boride particles. The borides found in the re-melted rod were observed to readily nucleate α-Al on their basal faces with a well-defined low-index orientation relationship but there was again no indication of any layer on the boride particles. Thus, the borides themselves appeared to be potent sites for the heterogeneous nucleation of α-Al crystals during rapid quenching without a necessary presence of the aluminide layers.