Effect of composition on grain refinement in TiAl-based alloys

Grain refinement through boron addition has been investigated in a range of cast TiAl-based alloys. The alloys have 44–50 at.% Al, up to 8 at.% alloying elements, and 1 at.% boron. The observed grain size ranges from 40 to 300 μm. The grain size in as-cast TiAl-based alloys is strongly composition dependent with aluminium having the strongest effect. Decreasing Al concentration from 50 to 44% the grain size can be reduced to 50 from 300 μm with similar casting conditions. Alloying element species and concentration also have strong effect on grain size. The strong boride formers like W, Ta and Nb increase the grain size and change the prevalent titanium boride form from TiB2 to TiB. It is elucidated that the compositional effect on the grain size could be through affecting the local boron concentration in the solidification front during casting.