Understanding the role of carbon atoms on microstructure and phase transformation of high Nb containing TiAl alloys

The microstructure and solidification behavior of high Nb containing TiAl alloys with the composition of Ti-46Al-8Nb-xC (x = 0.1, 0.7, 1.4, 2.5 at.%) prepared by arc-melting method have been investigated in this work. The results give evidence that the addition of carbon changes the solidification behavior from solidification via the β phase to the peritectic solidification. And carbon in solid solution enriches in the α₂ phase and increases the microhardness. As the carbon content increases to 1.4 at.%, plate-shape morphology carbides Ti₂AlC (H phase) precipitate from the TiAl matrix which leads to the refinement microstructure. By aging at 1173 K for 24 h after quenching treatment, fine needle-like and granular shape Ti₃AlC (P phase) carbides are observed in the matrix of Ti-46Al-8Nb-2.5C alloy, which distribute along the lamellar structure or around the plate-shape Ti₂AlC. Transmission electron microscope observation shows that the Ti₃AlC carbides precipitate at dislocations. The phase transformation in-situ observations indicate that the Ti₂AlC carbides partly precipitate during the solid state phase transformation process.