Nitridation behaviour of Ti-50Al, Ti-50Al-3Si, and Ti-47Al-3Si alloys was investigated at 1173K for up to 518.4ks in a N
2 gas with 99.9995 purity (less than 0.5ppm O
2) and the N
2 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 (Ti
2AlN+TiN)-TiAl
2 in the N
2(Ti+AlN) gas, nitrides (Ti
2AlN+TiN+AlN)-TiAl
3-TiAl
2 in the N
2(Ti) gas, and oxides (TiO
2+ Al
2O
3)-nitrides (Ti
2AlN+TiN) in an as-received N
2 gas. In cases of TiAl alloys containing Si, oxides (TiO
2+Al
2O
3)-nitrides(Ti
2AlN+TiN+AlN)-TiAl
3-TiAl
2 were formed in an as-received N
2 gas, and nitrides (Ti
2AlN+TiN+AlN)-TiAl
3-TiAl
2 in both the N
2(Ti) and N
2(Ti+AlN) gases. Selective nitridation of Ti yields Al-enriched layers of TiAl
2 and TiAl
3 on the alloy surface and they could be explained from thermodynamical considerations. A periodic formation of Ti
2AlN+TiN and AlN layers, looks like a Liesegang phenomenon, is due to a competitive process of a rapid N
2 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 N
2 gas gave deteriorative effects on nitridation property of Ti-Al alloys at high temperatures.
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