Tension and compression testing of single-crystalline gamma Ti-55.5 pct Al
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INTRODUCTION
THE stiffness, low density, and oxidation resistance of gamma-based titanium aluminides make them attractive candidates for use as high-temperature structural materials.[1,2,3] Although commercial alloys of TiAl will most likely have a two-phase, fully lamellar structure, the present study has concentrated on the single g phase because a fundamental understanding of its deformation mechanisms will be needed to develop the microstructure-mechanical property relations that govern the more advanced alloys. Like many intermetallic alloys, the mechanical behavior of TiAl is related to its ordered structure (L10). The tetragonal structure is nearly cubic, but it is chemically anisotropic, and a relatively large number of deformation modes are possible. Transmission electron micrograph (TEM) observations of faulted dipoles, superdislocations (b 5 ^101]), ordinary dislocations (b 5 1/2 ^110]), stacking faults, and microtwins have all been reported.[4–8] Orientation, temperature, and sense of the applied stress are all known to play an important role in governing the deformation processes. To date, fundamental single-crystal studies on the temperature and orientation dependence of flow stress and the tension-compression asymmetry of g-TiAl, similar to those previously performed on Ni3Al,[9] have been limited by the fact that high-quality single crystals, sufficiently large to facilitate tensile testing, are very hard to obtain. The first study on single crystals of g-TiAl, by Kawabata and coworkers,[10,11] showed that a compressive flow stress anomaly occurs at a variety of orientations. More recent studies by Li and Whang,[12] Stucke et al.,[13] Wang et al.,[14] Stucke et al.,[15] Mahapatra et al.,[16] and Bird et al.[17] have reported a compressive yield strength anomaly for [7 11 2] oriented Ti-54Al-2V, [010] oriented Ti-56Al, [1 6 12] oriented Ti-56Al, [001] oriented Ti-56Al, [001] and [011] oriMARC ZUPAN, Graduate Research Assistant, and K.J. HEMKER, Associate Professor, are with the Department of Mechanical Engineering, The Johns Hopkins University, Baltimore, MD 21218-2686. This article is based on a presentation made in the symposium ‘‘Fundamentals of Gamma Titanium Aluminides,’’ presented at the TMS Annual Meeting, February 10–12, 1997, Orlando, Florida, under the auspices of the ASM/MSD Flow & Fracture and Phase Transformations Committees. METALLURGICAL AND MATERIALS TRANSACTIONS A
ented Ti-56Al, and [3 16 15] oriented Ti-54Al, respectively. For these orientations, the Schmid factor for superdislocation motion is equal to or greater than that for ordinary dislocation motion, and the TEM observations that were reported in these studies[11,12,14,15,17] all indicated the prevalence of superdislocation activity. In the most complete study to date, Inui et al.[18] measured the compressive strength of single-crystalline Ti-56Al at a wide variety of orientations and temperatures and used TEM observations to identify the active deformation mechanisms. They reported that both ordinary and superdislocatio
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