The effect of thermal exposure on microstructural stability and creep resistance of a two-Phase TiAl/Ti 3 Al lamellar al
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I.
INTRODUCTION
THE low density and good oxidation resistance of TiA1- and Ti3Al-ordered intermetallic alloys make them potential candidates for high-temperature engineering applications, t~,21 Two-phase TiA1/Ti3AI lamellar alloys possess a number of properties that distinguish them from the TiA1 and Ti3A1 constituent single-phase alloys; namely, appreciably improved toughness, fatigue resistance, and creep resistance, t~-4~ Bartholomeusz and co-workers [5,6,71 demonstrated that the minimum creep rate of a two-phase TiA1/Ti3A1 lamellar alloy (referred to as the larnellar alloy in the present article) is up to an order of magnitude lower than the creep rates of the constituent single-phase alloys. The lower creep rate of the lamellar alloy is attributed to the enhanced strain hardening of the constituent phases within the lamellar microstructure, t6~ These results suggest that one approach to development of titanium aluminides as high-temperature engineering materials involves synthesis of multiphase alloys with specific phase morphologies. One of the challenges involved in developing alloys possessing multiphase microstructures for use at elevated temperatures is understanding and preventing degradation resulting from thermally induced coarsening of the microstructure. Thermal exposure of the two-phase TiA1/Ti3AI lamellar alloy investigated in the present study results in microstructural evolution. The lamellar microstructure undergoes capillarity driven coarsening and ultimately transforms into a coarse, globular microstructure. The MICHAEL F. BARTHOLOMEUSZ, formerly Graduate Student, Department of Materials Science and Engineering, University of Virginia, is Research Scientist, Reynolds Metals Company, Corporate Research and Development, Richmond, VA 23219. JOHN A. WERT, Professor, is with the Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22903-2442. Manuscript submitted January 27, 1994. METALLURGICALAND MATERIALS TRANSACTIONSA
driving force for coarsening is the reduction of surface energy attained by eliminating microstructural features with small principal radii of curvature, tsJ An ideal lamellar microstructure comprised of lamellae possessing flat faces is intrinsically stable; the principle radii of the flat faces are infinite and therefore no driving force exists for coarsening. I8] However, the presence of lamellae terminations (plate edges) possessing small radii of curvature and the presence of internal boundaries ensure microstructural coarsening of real lamellar alloys annealed at elevated temperatures. A survey of the literature yields numerous proposed coarsening mechanisms in lamellar alloys. [9-16] The objective of the present article is to identify the mechanisms governing the microstructural evolution of a two-phase TiAI/Ti3A1 lamellar alloy subject to thermal exposure. Additionally, the effect of thermally induced microstructural evolution on the creep properties of the lamellar alloy is experimentally evaluated and modeled.
II.
EXPERIMENTAL
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