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 beta phase. In scenario 2, secondary TiB2 precipitates after the first beta phase but before formation of the hexagonal close-packed alpha phase. The model predicts high alpha and beta nucleation potencies of TiB2 in both scenarios. However, pre-existing beta grains in scenario 2 are predicted to be preferred alpha nucleation sites. The experimentally observed beta refinement by primary TiB2 agrees with the model predictions. Grain refinement in scenario 2 is attributed to alpha nucleation on beta, which is interleaved with secondary TiB2. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.