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INTERNAL STRESSES IN TiAI BASED LAMELLAR COMPOSITES P. M. HAZZLEDINE, B. K. KAD*, H. L. FRASER* and D. M. DIMIDUK** UES Inc. 4401 Dayton-Xenia Road, Dayton, OH 45432, USA. *Ohio State University, 2041 College Road, Columbus, OH 43210, "**Wright Laboratory, Materials Directorate, Wright-Patterson AFB, OH 45433. ABSTRACT

In the in situ lamellar y-TiAI based composites, very large elastic stresses (-I1GPa) would be generated at coherent interfaces between close packed planes for three reasons: the tetragonality of TiAl, the larger atomic spacing in a2-Ti3Al than in ' and the differences between thermal expansion coefficients in (X2 and y. These stresses appear to be partially relaxed by the creation of van der Merwe dislocations, diffusion across ct2/y interfaces and cracking along interfaces. Measurements of lattice parameters by Convergent Beam Electron Diffraction (CBED) reveal stresses of the order of 1OOMPa. The presence of these stresses and the very specific form of the resulting stress tensor are used to discuss the hard/soft mode deformation of TiAl composites. INTRODUCTION

The mechanical properties of TiAl-based alloys are sensitive to composition and microstructure. A promising two phase microstructure with good strength and ductility consists of laminates of flat. slabs of y/y and/or a2/'y depending upon the exact composition [1]. This microstructure is derived from solid state transformations a-->y and a02->Y, with the orientation relationship of (000)c,cX2//(l 110Y, cc,0E2//