The solidification path related columnar-to-equiaxed transition in Ti-Al alloys

Columnar-to-Equiaxed Transition (CET) of binary Ti-Al alloys and multi-component Ti-48Al-2Cr-2Nb alloys is studied using Bridgman solidification technique. The effect of aluminum concentration and growth rate on CET is determined. It is found in Ti-46Al and Ti-50Al alloy ingots equiaxed grains develop ahead of the moving solid-liquid interface with a growth rate of 500 μm/s; microstructures in Ti-49Al alloy stay columnar dendrites with the same growth rate. CET in Ti-Al alloys are not only influenced by growth rate, but also by the solidification path that is related to alloying composition. CET in Ti-Al alloys is predicted using the dendritic growth model based on the criterion of growth at marginal stability. According to the calculated results and directionally solidified microstructures, values of the nucleation undercooling for α and β phases are given. The growth rates to avoid CET in Ti-48Al-2Cr-2Nb alloy are suggested.