Sulfidation properties of TiAl–2 at.% X (X=Si, Mn, Ni, Ge, Y, Zr, La and Ta) alloys at 1173 K and 1.3 Pa sulfur pressure in an H2S–H2 gas mixture

TiAl–2 at.% X (X=Si, Mn, Ni, Ge, Y, Zr, La and Ta) alloys were sulfidized at 1173 K for 86.4 ks at a 1.3 Pa sulfur pressure in an H2S–H2 gas mixture. The structure, phases, and compositions of the external sulfide scale and alloy surface layer were measured using EPMA and X-RD. The sulfides formed were mainly TiS, Ti3S4 and Al2S3. The alloy surface layer consisted of multi-layered structures that can be classified into three groups: group 1 of Mn, Ni, Y and Zr as well as TiAl binary alloy with alloy/TiAl2/TiAl3/sulfide scales; group 2 of Si and Ge with alloy/TiAl2/Ti(Al,X)3/sulfide scales, and group 3 of La and Ta with alloy/TiAl2/TiAl3/X–Al alloy/sulfide scales. The TiAl–2La and –2Ni alloys sulfidized faster than the TiAl, while the sulfidation amount for the TiAl-2X (X=Si, Mn, Ge, Y, Zr and Ta) alloys was almost the same as that of TiAl. The thickness of the alloy surface layer increased in the order: Ta <Si <Ni <Mn <Zr <(TiAl binary alloy) <Ge <La <Y.