Development and elemental powder metallurgy of a Y-containing two-phase TiAl alloy

The Y modification of a two-phase (γ+α ₂) TiAl-(Mn,Mo,C) alloy was studied with an aim to improve, mainly, the oxidation resistance and the mechanical properties in a high-temperature air environment. The experimental alloy was prepared by the elemental powder metallurgy (EPM) method. The addition of up to 0.6 at. pct Y resulted in a significant improvement in tensile properties and compressive yield strength and an anomalous yielding phenomenon withal. Two structural characteristics were identified: first, microstructural refinement in terms of the grain size as well as the interlamellar spacing, and second, precipitation of fine oxides that might scavenge harmful oxygen. Deformation was found to be mainly provided by 1/2<110] ordinary dislocations and a much lesser amount of <011] superdislocations as compared to what has been reported in other (γ+α ₂) TiAl alloys. The oxidation resistance of the experimental alloy was evaluated by air-exposure tests at 800 °C, from which the oxidation kinetics and the morphological and phase characteristics of the oxide scales were analyzed. With a Y addition, the constituents of the oxide scale changed from those of the Y-free alloy. In the case of the Y-free alloy, the oxide scale which formed upon extended air exposure (350 hours) at 800 °C consisted of a mixture of TiO₂ and α-Al₂O₃. In the case of the alloy modified with Y (0.6 at. pct), however, the oxide scale formed in an identical environment was considerably different: it consisted of a complex mixture of TiO₂, α-Al₂O₃, Y₂O₃, and Al₅Y₃O₁₂. The formation of the multiphase (Y,Al)O-rich oxide scale impedes the oxygen transport and the thermal-expansion stress in the Al₂O₃ layer. It is also suggested that a Y addition reduces the oxygen solubility and concentration of oxygen vacancices in the TiO₂ layer. This article is based on a presentation made in the symposium entitled “Fundamentals of Structural Intermetallics,” presented at the 2002 TMS Annual Meeting, February 21–27, 2002, in Seattle, Washington, under the auspices of the ASM and TMS Joint Committee on Mechanical Behavior of Materials.