An investigation of microstructure formation during solidification of gamma titanium aluminide alloys

The microstructure formation during solidification of arc-melted gamma(TiAl) binary and multi-component alloys has been studied by scanning electron microscopy, micro-analytical and texture investigations. It was found that alloys based on Ti-45Al (at.%) exhibit an isotropic, equiaxed and texture-free microstructure with modest micro-segregation, whereas peritectic alloys solidify in the lower part of the buttons in a cellular or cellular-dendritic mode resulting in anisotropic microstructures as well as significant texture and segregation. This difference is attributed to the columnar/equiaxed transition, which for the beta phase seems to be favoured over the a phase due to a kinetic preference of heterogeneous nucleation and a larger freezing interval. Among the investigated ternary or further alloying additions, only B was found to have a significant effect on the solidification microstructure, disregarding microstructural changes in the solidified material due to solid-state transformations.