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 witho ut boron additions have been studied and the observations have been used to infer the mechanism by which boron induces grain refinement in these alloy s. It has been shown that the minimum level of boron required to achieve gr ain 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 str ucture becomes more pronounced with reduced dendritic arm spacings. However , when the levels of boron added are higher than the critical level, dendri tic structures become less obvious. In one case where the level of boron ad ded was only slightly less than the critical level, the alloy was refined i n the region adjacent to the mould but became coarse towards the centre of the ingot. These microstructural observations are not consistent with the m echanisms proposed previously for grain refinement in such alloys. As a res ult, an alternative hypothesis has been put forward which is based on renuc leation in the constitutionally supercooled zone ahead of the solidificatio n front. (C) 1999 Elsevier Science Ltd. All rights reserved.