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DEFORMATION MECHANISMS IN THE INTERMETALLIC COMPOUND MoSi 2 D.J. EVANS" 2 , S.A. COURT3 , P.M. HAZZLEDINE 4 , H.L. FRASER' 1 : Department of Materials Science and Engineering, The Ohio State University, 2041 College Road Columbus,OH, 43210 2: Wright Laboratory, Materials Directorate, Wright-Patterson AFB, Ohio, 45433 3: Alcan Banbury Laboratories, Banbury, United Kingdom 4 : UES, Inc., 4401 Dayton - Xenia Road, Dayton, Ohio 45432 ABSTRACT The nature of dislocations which have been activated during plastic deformation of MoSi2 at 1400*C in compression have been identified using transmission electron microscopy. Particular attention has been paid to the possible dissociation of dislocations, and it has been confirmed that dislocations with Burgers vectors lying parallel to are dissociated. The dissociation is represented by: 1/2 --+ 1/4 + SISF + 1/4, where SISF stands for superlattice intrinsic stacking fault. The SISF energy has been estimated from the separation of the partial dislocations to be = 255 mJ.m- 2 . A simple explanation to account for the occurrence of dissociation for particular dislocations is presented. INTRODUCTION MoSi 2, an intermetallic with the C11 b-type ordered structure (i.e. structure derived by stacking three b.c.c. lattices and compressing along the c-axis) has been the subject of study in recent years with a view to its use as a high temperature material in aerospace applications. Its high melting point, approximately 2020'C, and excellent oxidation resistance are among its most attractive properties. However, like most ordered compounds, it also lacks ductility or plasticity except at very high temperatures, above 1000'C. It is of interest to determine the factors that influence this brittle behavior and to that end, the deformation mechanisms, in this compound slip systems and nature of dislocations, have been studied. The deformation behavior of single crystals of MoSi 2 has been studied recently by Umakoshi, Sakagami, Yamane, and Hirano (1). Single crystals compressed at a nominal strain rate of 1.4 x 10- 4s- 1 along a number of orientations between 900'C and 1200'C were found to slip predominately by the