Origin of Inhomogeneous Microstructure in As-Cast Ti-45Al-8.5Nb-(W, B, Y) Alloy at Different Cooling Rates

In this work, the microstructure evolution of Ti–45Al–8.5Nb–(W, B, Y) alloy at different cooling rates in mushy zone is investigated. The microstructures including segregation behavior, solidification route, and lamellar colony size as well as microstructural homogeneity are analyzed. The results show that with the decrease of cooling rate, the solidification routes change from β solidification to partial peritectic solidification due to Al enrichment in residual liquid. The variation of solidification paths results in a non‐uniform microstructure that coarse lamellar colonies form in interdendritic regions and fine lamellar colonies form in dendrite cores. The average lamellar colony size increases significantly with decreasing cooling rate. The microstructure is the most inhomogeneous at the cooling rate of 10 °C min–1. The average lamellar colony size and microstructure homogeneity are controlled by the volume fraction of peritectic α phase. This work analyses the effect of cooling rate in mushy zone on the microstructure features of a high Nb containing TiAl alloy. It shows that microstructure characteristics including segregation behavior, lamellar colony size and microstructural homogeneity change with the variation of cooling rates. The variation of these microstructure characteristics is discussed based on the element distribution and phase transformation sequences in local regions, as shown in the TOC figure.