Microsample Testing of Single Crystalline Ti-52 at%Al and Ti-55.5 at%Al
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dislocation activity appears to be confined to a limited range of orientations and temperatures [6, 9, 11, 12, 13, 14]. Serious questions have been raised as to whether the controlling deformation mechanisms in the Al rich material are the same for stoichiometric Ti-50 at%Al. Much of this concern stems from the observation that in contrast to the deformation of Al rich crystals, deformation of polycrystalline and two-phase microstructures of near stoichiometric alloys is almost always dominated by the motion of ordinary dislocations, see for example Viguier, Hemker, Bonneville, Louchet and Martin [15]. Mahapatra, Chou, Girshick, Pope and Vitek [16], have studied the deformation mechanisms of nearstoichiometric single-phase TiAl single crystals, and used atomistic simulations to suggest that changes in the Al content may lead to decreased twinning and increased ordinary dislocation activity. This work incorporates the development of high-temperature microsample testing and the preparation of "single-crystalline" microsamples from polycrystalline Ti-52 at%Al material. Development of the microsample testing technique has allowed for the tensile testing of specimens on the microscale [17, 18]. Microsamples with a gage section of approximately 250gm X 3001tm and an overall size of 1mm x 3mm are now routinely prepared and tested in tension. Addition of a resistive heating system and an optical temperature sensor allows for microsamples to be tested in excess of 1273K. Testing on the microscale alleviates many of the restrictions imposed by the size of the single crystals that can be grown. The reduced size of microsamples also allows for single crystalline samples to be cut from individual grains in ingots of near stoichiometric compositions that have been heattreated, to produce large grain sizes. Reasons for the development of these techniques are two-fold: to investigate the effect of Al content on the mechanical behavior of y-TiAl, and to provide high-temperature tensile data that can be compared to the published compressive data. EXPERIMENTAL TECHNIQUES Single Crystal Microsample Preparation
Single crystals of y-Ti-55.5 at%Al have been grown for this study using the optical floatzone furnace at the University of Pennsylvania Laboratory for Research on the structure of Matter (Philadelphia, PA). The floatzone crystal growing method produced high quality single crystal rods approximately 6-10mm in diameter and 50mm long. The single crystal rods were quartz encapsulated and homogenized at 1573 K for 24 hours, furnace cooled to 1273K, held for 100 hours and finally furnace cooled to room temperature in order to remove excess point defects and compositional inhomogeneity [12]. The single crystal rods were mounted in a two axis- 3600 goniometer, sliced perpendicular to the growth direction and TEM foils were made to orient the crystal and resolve the chemical anisotropy of the [001] axis. 400g.tm slices were cut from the crystal rod and microsamples were punched from these disks at orientations within 40 of [001] and [0
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