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THE INFLUENCE OF PRESTRAIN ON THE FLOW STRESS ANOMALY IN TiAl SINGLE CRYSTALS M. A. Stucke*, D. M. Dimiduk*, and P. M. Hazzledine** *Wright Laboratories, WL/MLLM, Wright-Patterson AFB, OH 45433-6533 **UES, Inc., Materials Research Division, 4401 Dayton Xenia Rd., Dayton, OH 45432 ABSTRACT Single crystal specimens of Ti4 4AI 5 6 , grown by the optical float zone method, were compression tested at room and elevated temperatures. The compression axis was close to [010] but the slip lines show essentially single slip. The material exhibits the anomalous yield behavior and a high work hardening rate. The work hardening rate at 1% strain is =3.5 GPa and is higher at 773K than at 296K. Specimens tested first at 773K and then at 296K retained their high temperature yield stress at the lower temperature, unlike L1 2 Ni3 AI, but the work hardening rate was thermally reversible. The experiments suggest that the dislocations which first move at the yield point are 'permanently' locked at high temperature but that the dislocation interactions responsible for work hardening are characteristic of the deformation temperature. INTRODUCTION Gamma titanium-aluminide alloys have been the subject of interest for the last several years because of their high strength to density ratio [1]. Much of the past work on these alloys has included processing, heat treatment and microstructural studies [2,3]. The deformation behavior in binary gamma alloys has been studied extensively by several authors [4-6]. However, all of this work is on polycrystalline materials or polysynthetically twinned materials. Hug et al. reported an extensive study of dislocation behavior in polycrystalline Ti-56AI at room and elevated temperatures [7,8]. In 1985, Kawabata et al. tested single crystal Ti-56A1 and found a yield stress anomaly, similar to that in Ni 3 AI alloys [9]. Dislocation behavior and mechanical properties have been observed in single crystals from 41273K [9,10]. Whang and co-workers [11-14] found that the yield stress anomaly also occurs in single crystal Ti-55A1 with ternary additions of V and Nb. In the deformed crystals, both perfect 1/2