Effects of Impurity Oxygen and Si Addition on High-Temperature Nitridation of Ti-Al Alloys

Abstract

Nitridation behaviour of Ti-50Al, Ti-50Al-3Si, and Ti-47Al-3Si alloys was investigated at 1173K for up to 518.4ks in a N₂ gas with 99.9995 purity (less than 0.5ppm O₂) and the N₂ gases purified with Ti or Ti+AlN getter, i.e., N_2(Ti) and N_2(Ti+AlN) gases. Structural sequences of products on a Ti-50Al alloy are nitrides (Ti₂AlN+TiN)-TiAl₂ in the N_2(Ti+AlN) gas, nitrides (Ti₂AlN+TiN+AlN)-TiAl₃-TiAl₂ in the N_2(Ti) gas, and oxides (TiO₂+ Al₂O₃)-nitrides (Ti₂AlN+TiN) in an as-received N₂ gas. In cases of TiAl alloys containing Si, oxides (TiO₂+Al₂O₃)-nitrides(Ti₂AlN+TiN+AlN)-TiAl₃-TiAl₂ were formed in an as-received N₂ gas, and nitrides (Ti₂AlN+TiN+AlN)-TiAl₃-TiAl₂ in both the N_2(Ti) and N_2(Ti+AlN) gases. Selective nitridation of Ti yields Al-enriched layers of TiAl₂ and TiAl₃ on the alloy surface and they could be explained from thermodynamical considerations. A periodic formation of Ti₂AlN+TiN and AlN layers, looks like a Liesegang phenomenon, is due to a competitive process of a rapid N₂ gas ingress and a so-called back-diffusion of Al toward the alloy substrate. A part of a Ti-Al-N ternary phase diagram at 1173K was given, showing compositional paths for these nitridation reactions. Nitridation rate was very low for a Ti-50Al alloy in the N_2(Ti+AlN) gas, while it tended to increase with time in an as-received gas and the N_2(Ti) gas. TiAl alloys containing Si showed large nitridation rates, accompanied by a catastrophy in nature in each atmosphere. It was suggested that Si addition and impurity oxygen in N₂ gas gave deteriorative effects on nitridation property of Ti-Al alloys at high temperatures.