Microstructures, micro-segregation and solidification path of directionally solidified Ti-45Al-5Nb alloy

To investigate the effect of solidification parameters on the solidification path and microstructure evolution of Ti-45Al-5Nb（at.%） alloy, Bridgman-type directional solidification and thermodynamics calculations were performed on the alloy. The microstructures, micro-segregation and solidification path were investigated.The results show that the β phase is the primary phase of the alloy at growth rates of 5-20 μm·s~（-1） under the temperature gradients of 15-20 K·mm~（-1）, and the primary phase is transformed into an α phase at relatively higher growth rates（V 〉20 μm·s~（-1））. The mainly S-segregation and β-segregation can be observed in Ti-45Al-5Nb alloy at a growth rate of 10 μm·s~（-1） under a temperature gradient of 15 K·mm~（-1）. The increase of temperature gradient to 20 K·mm~（-1） can eliminate β-segregation, but has no obvious effect on S-segregation. The results also show that 5 at.% Nb addition can expand the β phase region, increase the melting point of the alloy and induce the solidification path to become complicated. The equilibrium solidification path of Ti-45Al-5Nb alloy can be described as L L→β L＋β L＋β→αα＋β_R β→ααα→γα＋γα→α_2＋γγ_R＋（α_2＋γ）, in which β_R and γ_R mean the residual β and γ