1998 Volume 47 Issue 5 Pages 333-340
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., N2(Ti) and N2(Ti+AlN) gases. Structural sequences of products on a Ti-50Al alloy are nitrides (Ti2AlN+TiN)-TiAl2 in the N2(Ti+AlN) gas, nitrides (Ti2AlN+TiN+AlN)-TiAl3-TiAl2 in the N2(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 N2(Ti) and N2(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 N2(Ti+AlN) gas, while it tended to increase with time in an as-received gas and the N2(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.