Effect of Cr Addition on the Oxidation Behaviour of Ti-48Al-2Ag Alloys

The oxidation behavior of Ti-48Al-2Ag alloys with addition of 0-7% Cr (Cr substituting Ti) was investigated in air at 800 and 900degreesC. It was found that alloys Ti-48Al-2Ag-1Cr and Ti-48Al-2Ag-3Cr exhibited increased oxidation rates compared to the Cr-free ternary alloy Ti-48Al-2Ag. If the Cr content was increased to 5% (alloy Ti-48Al-2Ag-5Cr), a similar oxidation resistance was found as for the ternary alloy Ti-48Al-2Ag, i.e. excellent oxidation resistance at 800degreesC, but poor oxidation properties at 900degreesC. The best oxidation properties were obtained for the quaternary alloy Ti-48AI-2Ag-7Cr, which exhibits excellent oxidation resistance at 800degreesC as well as at 900degreesC. The surface and cross-section microstructure of the oxidized samples were characterized with XRD, SEM/EDX, OM. It was found that optimum oxidation resistance was achieved if a continuous Ti(Cr,Al)(2) Laves or Z phase layer was formed in the depletion layer beneath the external Al2O3 scale. Alloys exhibiting higher oxidation rates appeared to form mixed oxide scales containing Al2O3 and TiO2, Whereby (Al,Cr,Ag)(3)Ti or alpha(2)-Ti3Al phase was formed in the depletion layer beneath the oxide scales. (C) 2003 Elsevier Science Ltd. All rights reserved