Sulfidation properties of TiAl–2 at.% X (X=V, Fe, Co, Cu, Nb, Mo, Ag and W) alloys at 1173 K and 1.3 Pa sulfur pressure in an H2S–H2 gas mixture

TiAl–2 at. % X (X=V, Fe, Co, Cu, Nb, Mo, Ag and W) alloys were sulfidized at 1173 K for 86.4 ks at a 1.3 Pa sulfur pressure in an H2–H2S gas mixture. The structure, phases, and compositions of the external sulfide scale and alloy surface layer were measured using EPMA and X-RD. The TiAl–2Ag and –2Cu alloys sulfidized faster than TiAl, and the alloy surface layer was thicker than that of TiAl. Sulfidation amounts of the TiAl–2X (X=V, Co, Fe, Mo, W and Nb) alloys were almost the same as that of TiAl, while the thickness of the alloy surface layer decreased in the order: V>Co>Fe>Mo>[Cr (by Narita T, Izumu T, Yatagai M, Yoshioka T. Intermetallics, 2000;8:371)]>W>Nb. The sulfide scale was composed of multi-layer structures: an outermost (rich in Ti-sulfides), an outer (rich in Al2S3), an inner (a mixture of Ti-sulfides and Al2S3), and an innermost (rich in Ti-sulfides) layer. The alloy surface layer also had a multi-layer structure, and was classified into four groups: group 1 for TiAl–2V and –2Co alloys as well as TiAl binary alloy where the surface layer consists of alloy substrate/TiAl2/TiAl3/sulfide scale, group 2 for TiAl–2Nb, –2Mo, and –2W (and also– 2Cr) alloys with alloy substrate/TiAl2/TiAl3/(Nb, Mo, W or Cr)–Al alloy/sulfide scale, group 3 for TiAl–2Cu and –2Ag alloys with alloy substrate/TiAl2/Ti (Al, Ag or Cu)3 with an L12 structure/TiAl3/sulfide scale, and group 4 for TiAl–2Fe alloy with alloy substrate/TiAl2/Ti(Al,Fe)3 with an L12 structure/TiAl3/FeAl3/sulfide scale. Diffusion paths for these four groups were shown in a tentative Ti–Al–X ternary phase diagram.