Effect of Planar Fault Energies on Dislocation Core Structures and Mobilities in L1 0 Compounds
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EFFECT OF PLANAR FAULT ENERGIES ON DISLOCATION CORE STRUCTURES AND MOBILITIES INL10 COMPOUNDS J.P. SIMMONS-, S.I. RAO", D.M. DIMIDUK*** *NRC Research Associate, Wright Laboratory, WL/MLLM, Wright-Patterson AFB, OH 45433 **Universal Energy Systems, Inc., Dayton, OH 45432 ***Wright Laboratory, WL/MLLM, Wright-Patterson AFB, OH 45433 ABSTRACT A set of three Embedded Atom Method potentials are presented that produce a stable Lio structure. All three were nominally fitted to experimental parameters of the y-TiAI phase: variations in the fit were allowed in order to give potentials with differing values of planar fault energies. These potentials were evaluated and found to produce stable APB(11l), APB(100), CSF(111), and SISF(111) faults. Dislocation core structures were computed for 1/2-type dislocations in both edge and screw orientation with all three potentials. The screw orientation of the highest fault energy potential was found to have a non-planar configuration, being spread about equally on two {111) close packed planes. All other cores were found to be planar and spread on a single (111) plane. No significant edge components of strain developed for the screw orientation, suggesting that these dislocations would not appear to be spread in Atomic Resolution TEM. Preliminary evaluations showed that dislocation mobilities were low, requiring stresses on the order of 10-3 p.for motion. INTRODUCTION y-Titanium aluminide alloys have attracted much interest in recent years as candidates for new their high temperature strength retention, superior generation high temperature alloys 1because 3 oxidation resistance, and low density. ,2, However, these alloys suffer from low strength overall and efforts have been directed poor ductility and toughness at room temperatures. Many experimental 4 towards modifying alloy parameters in order to increase ductility. Experimental results to date indicate that the dislocations important for deformation processes are of type 1/2. , and 1/2.5 The first of these is an 'ordinary' dislocation of the disordered parent FCC structure, while the other two are superdislocations of the ordered structure. that the alloys exhibiting higher ductility have increased activity of the Experiments have shown 5 orientations, 1/2 dislocations. Microstructures often consist of these type dislocation in screw 5 indicating that the screw orientation has a lower mobility than the other orientations. 9 10 6 7 8 Atomistic modeling studies by Vitek , and Duesbery and theoretical work by Greenberg, , indicate that dislocation mobility in most alloys and intermetallic compounds is controlled by the atomic structure of the dislocation cores. Atomistic simulations have been performed by pair More recently, the Embedded Atom potentials that were fit to give stable crystal structures and1 faults. 1 applied to 1B2 and Method (EAM), originally developed 1by Daw and Baskes, has been successfully 13 4 and et a0. and 1to6 Ni3AI by Foiles, et al., Voter and Chen, Li 2 structures by Pasianot, 15 Parthasarathy, et al. used EA
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