Grain refinement of TiAl-based alloys: The role of TiB2 crystallography and growth

A crystallographic model is used to predict the nucleation potencies of TiB₂ particles during solidification of TiAl-based alloys. Two nucleation scenarios are investigated. In scenario 1, primary TiB₂ grows in the melt before formation of the body-centred cubic β phase. In scenario 2, secondary TiB₂ precipitates after the first β phase but before formation of the hexagonal close-packed α phase. The model predicts high α and β nucleation potencies of TiB₂ in both scenarios. However, pre-existing β grains in scenario 2 are predicted to be preferred α nucleation sites. The experimentally observed β refinement by primary TiB₂ agrees with the model predictions. Grain refinement in scenario 2 is attributed to α nucleation on β, which is interleaved with secondary TiB₂.