Mechanisms of Primary Creep in Lamellar TiAl

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9709029 Mills The primary creep response of polycrystalline TiAl is examined as a function of the lamellar spacing and grain size under a GOALI grant at Ohio State with interaction of Universal Energy Systems and Wright Laboratories. Single lamellar colony crystals are prepared and creep tested in several orientations to determine the effects of lamellar orientation on the primary creep behavior. The effect of carbon additions on the creep response of both polycrystalline samples and single crystal lamellar colony crystals is determined. The form of primary creep transient is modeled quantitatively and compared to the hardening behavior under constant strain rate conditions. Stress change, strain transient experiments are conducted to provide new insight into the mechanisms controlling deformation during creep. Activation energies for transient flow are determined by employing rapid temperature change experiments. Transmission electron microscopy is used to study the deformation microstructures in both the major gamma phase and the alpha 2 phase, slip transmission across lamellar interfaces, and deformation mechanisms within the interfaces themselves. Observations are conducted as a function of creep strain in order to develop an understanding of the hardening and recovery processes during creep. The graduate student will have the opportunity to interact extensively with the technical staff at both Universal Energy Systems and Wright Laboratories. This project is jointly funded by the MPS Office of Multidisciplinary Activities and the Division of Materials Research. %%% Titanium aluminide alloys of the gamma and alpha 2 phase structure are considered for a variety of high temperature applications where primary creep could be a critical factor in their use. This program is aimed at understanding the effect of microstructural scale, lamellar orientation, and composition on the primary creep transients in fully lamellar TiAl. ***