Sulfidation processing and Cr addition to improve oxidation resistance of TiAl intermetallics in air at 1173 K

High temperature oxidation properties of TiAl- (1,2,4 and 10) Cr and 40Ti-56Al–4Cr alloys, which were sulfidized at 1173 K for 86.4 ks at 1.3 Pa sulfur partial pressure in a H2–H2S gas mixture, were investigated at 1173 K in air for up to 2.7 Ms. The sulfidation processing formed a (Cr,Ti)Al2 layer between a TiAl3 (TiAl2 included) layer and a Ti-rich sulfide scale by selective sulfidation of Ti. Oxidation of the sulfidation-processed alloys was examined for up to 2.7 Ms in air under isothermal and room temperature to 1173 K heat cycle conditions. In both oxidation experiments the sulfidation processed TiAl–10Cr alloy showed very good oxidation resistance up to 2.7 Ms, due to the formation of a continuous Ti(CrAl)2 Laves layer, which was changed from (Cr,Ti)Al2 and has a composition of 28.7Cr–36.2Al–35.1Ti, between the layers of protective Al2O3 (TiO2 included) and TiAl2, which was changed from TiAl3. The sulfidation processed TiAl, TiAl–4Cr, and 40Ti–56Al–4Cr alloys showed better oxidation resistance than conventional TiAl based alloys, but displayed localized oxidation. The Ti(Cr,Al)2 Laves on the sulfidation processed TiAl–4Cr alloy was discontinuous, leading to a localized oxidation after long oxidation. The sulfidation processed 40Ti–56Al–4Cr alloy oxidized faster than the sulfidation processed TiAl–10Cr alloy due to the formation of an Al2O3 and TiO2 mixture, although the TiAl2 layer remains. It was concluded that the Ti(Cr,Al)2 Laves layer between the oxide scale and alloy substrate caused the good oxidation resistance.