Effect of Ternary Additions on the Microstructural Stability and Oxidation Characteristics of Ti-48 Al
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EFFECT OF TERNARY ADDITIONS ON THE MICROSTRUCTURAL STABILITY AND OXIDATION CHARACTERISTICS OF Ti-48 Al S. A. Kekare, D. K. Shelton & P. B. Aswath, Department of Mechanical & Aerospace Engineering and Materials Science & Engineering Program, University of Texas at Arlington ABSTRACT Oxidation behavior of binary Ti-48AI and ternary alloys with 1.5 at.% Cr, 1.4 at.%Mn, 00.2 at.% W, 2.2 at% V, 20 vol.% TiNb particles were examined in an air atmosphere at 704 C, 815*C and 982'C. Results indicate that the addition of Cr is detrimental at all temperatures. behavior at 815*C while the Mn The Cr and V containing alloys exhibited a linear oxidation containing alloy showed a linear behavior at 982 0C. At 982 OC the alloys with W and V exhibit the best oxidation behavior. The mechanism of oxidation has been explained with the help of a simple vacancy model for oxygen diffusion. INTRODUCTION Titanium aluminide intermetallics have been a topic of special interest for the past few years in the stream of aerospace materials. These alloys are prominent candidate materials for applications in the hypersonic structures and advanced turbo-jet engines. The high specific strength even at high operating temperatures has made these alloys especially lucrative for these applications. However components like these are expected to face extreme oxidizing environments in combination with high temperatures. This necessitates a careful investigation of the oxidation response of these alloys. Oxidation of any metal proceeds as a combination of two processes, oxygen dissolution and oxide scale formation. The total oxidation rate is thus, controlled by the mass transport characteristics of the oxide film. In most common metallic materials, the oxidation proceeds in a parabolic manner, although in a few isolated cases one comes across a linear oxidation behavior. Wagner's [1] analysis relates the rate of oxidation to the diffusivities of the metal & oxygen ions in the oxide film and the partial pressures of oxygen at the oxide-gas & oxide-metal interfaces. In the case of multilayered oxides, this analysis has been modified to accommodate the partial pressures at oxide-oxide interfaces. Since the oxide formed are ionic crystals, their defect structure monitors the diffusion behavior of the metallic and oxygen ions. Through a detailed analysis of marker experiments on titania crystals, Kofstad [2] and coworkers determined that oxygen ion vacancies are highly mobile in comparison with titanium ions. This result indicates that doubly ionized oxygen ion vacancies are the dominant defects in this oxide. Thus diffusivity of oxygen ion vacancy governs the rate of oxidation. Sankaran et.al. [3] provided an analytical model explaining the importance of the concentration of the doubly ionized oxygen ion vacancies and their dependence on the .valence of the associated cation in the oxide film. MECHANISM OF OXIDATION IN y TiAl ALLOYS Static oxidation of binary two phase 'y-TiAl alloys can occur leading to the formation of oxide of the kind, TiO, Ti2O 3, Ti 2 and
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