Oxidation and Diffusion in Ti-Al-(Mo, Nb) Intermetallics
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AMONG the various materials and intermetallic compounds that are being considered for high-temperature applications, Ti-Al intermetallic compounds are very promising candidates. This is due to their extraordinary properties such as low density, high-temperature creep resistance, and specific modulus.[1–3] High-temperature oxidation and sulfidation characteristics of TiAl based intermetallics with and without ternary alloying additions were studied. It is well known that TiAl3 favors the formation of a continuous a-Al2O3 protective scale, while TiAl has lower activation energy for the oxidation and Ti3Al has very poor oxidation resistance at high temperatures.[4–6] Although TiAl3 forms an alumina layer on oxidation, the brittle nature of the oxide layer owing to the mismatch of thermal expansion coefficients between the base alloy and the oxide has led to the research on oxidation resistance of Ti3Al intermetallic phase for high-temperature applications. In addition, Ti3Al has the highest solubility of oxygen,[7] which also stabilizes the a2 phase. Improvement of hightemperature oxidation resistance of Ti3Al intermetallic was studied by the addition of elements such as niobium and molybdenum. Oxidation studies of Ti3Al with various compositions of the microalloying elements M. RAMACHANDRAN and C. WILLIAMS, Graduate Students, D. MANTHA, Research Engineer, and R.G. REDDY, ACIPCO Professor, are with the Department of Metallurgical and Materials Engineering, The University of Alabama, Tuscaloosa, AL 35487. Contact e-mail: [email protected] Manuscript submitted July 1, 2009. Article published online October 19, 2010 202—VOLUME 42A, JANUARY 2011
and in various environments such as air, pure oxygen, and SO2 have been investigated.[4–27] High-temperature oxidation studies on microalloyed Ti3Al intermetallic with alloying elements such as Mo, Nb, etc. were carried out in pure oxygen environment at different temperatures. Although macroalloying was reported to stabilize the Ti3Al structure, no significant improvement in mechanical properties was observed.[28] In addition, higher amounts of ternary alloy addition lead to formation of oxides of the ternary alloy causing spallation of the oxide layers.[8] Hence, the effect of microalloying of Ti3Al in its D019 structure with Mo and Nb on the high-temperature oxidation resistance was studied in this research. Studies on the effect of alloying additions such as Nb and Mo to the Ti3Al-based alloys on their microstructure and creep resistance were reported in the literature.[29–32] Alloys containing Nb less than 12 at. pct have a two-phase microstructure a2 (hcp structure based on Ti3Al) and the high-temperature phase b/B2 (bcc structure). Creep behavior of the Ti3Al alloys containing 17 at. pct of niobium and at least 1 at. pct of molybdenum were reported. These studies showed that higher Nb content (17 at. pct Nb) in the Ti3Al alloy demonstrated superior mechanical properties to those containing lower amounts of niobium. A reaction model to determine the diffusion of oxygen through th
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