The mechanism of grain refinement in TiAl alloys by boron addition — an alternative hypothesis

The as-cast microstructures of a series of TiAl-based alloys with and without boron additions have been studied and the observations have been used to infer the mechanism by which boron induces grain refinement in these alloys. It has been shown that the minimum level of boron required to achieve grain refinement is different for alloys with different ternary or quaternary alloying elements and that alloying with strong boride formers, such as Ta, Nb, etc., can increase the minimum level required. It has also been found that when less than the critical level of boron is used, the dendritic structure becomes more pronounced with reduced dendritic arm spacings. However, when the levels of boron added are higher than the critical level, dendritic structures become less obvious. In one case where the level of boron added was only slightly less than the critical level, the alloy was refined in the region adjacent to the mould but became coarse towards the centre of the ingot. These microstructural observations are not consistent with the mechanisms proposed previously for grain refinement in such alloys. As a result, an alternative hypothesis has been put forward which is based on renucleation in the constitutionally supercooled zone ahead of the solidification front.