Thermal Stability of Fully Lamellar Microstructures in TiAl Alloy

The effects of Al content,continuous cooling rate and boron addition on the thermal stability of fully lamellar (FL) microstructure in TiAl alloys at 1 150 ℃ were investigated.For the binary TiAl alloys with 46%~48%Al,the richer the Al content is,the heavier the γ dendritic segregation is and the more instable the as-cast FL microstructure is.The thermal stability of coarse lamellar microstructure in Ti-48Al alloy formed after solution treatment in α single phase field by furnace cooling is superior to that of fine lamellar microstructure formed by air cooling.For the fine lamellar microstructure,the lamellae discontinuous coarsening is apt to occur at the grain boundaries and the interfaces between the Widmnstatten lamellar colonies in the grains and the lamellar matrix.Addition 0.8% boron into the Ti-48Al alloy results in that the TiB2 particles at grain boundaries restrain the lamellae discontinue coarsening effectively.Meanwhile,the new γ grains can nucleate at the TiB2/matrix interfaces and grain boundaries,as a result,the fine lamellar microstructure transforms into the homogeneous and fine near γ microstructure after a long time anneal.