Influence of nitrogen on the microstructure and solidification behavior of high Nb containing TiAl alloys

The influence of nitrogen interstitial atoms on the microstructure and solidification behavior of high Nb containing TiAl alloys with the composition of Ti-46Al-8Nb-xN (x = 0.1, 0.7, 1.4, 2.5 at.%) is discussed in this work. The α₂ volume fraction increases while the γ volume fraction decreases with the increase of nitrogen content, giving evidences that interstitial nitrogen atoms stabilize α phase and expand the region of α phase. With increasing nitrogen content from x = 0 to x = 2.5, the number of colonies in per square mm increases from ~6 to ~90 and the corresponding average colony size decreases from ~0.17 to ~0.01 mm². The results give the evidences that the nitrogen suppresses the diffusion of solute Al ahead of the solid-liquid interface and leads to the change of primary phase from β to α during solidification. The influence mechanism of nitrogen on the refinement of lamellar structure and colony size of high Nb-containing TiAl alloys is proposed. Meanwhile, the effect of nitrogen on the temperature of state phase transformation of high Nb-containing TiAl alloys is also addressed in this work.