Characterization of the Plasticity Enhancement of MoSi 2 Obtained at Elevated Temperatures by the Addition of TiC
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CHARACTERIZATION OF THE PLASTICITY ENHANCEMENT OF MoSi2 OBTAINED AT ELEVATED TEMPERATURES BY THE ADDITION OF TiC H. CHANG AND) R. GIBALA Department of Materials Science and Engineering University of Michigan, Ann Arbor, M1 48109 ABSTRACT Decremental step strain rate tests in compression were performed on MoSi 2 and MoSi 2 -10 vol % TiC at 1150°C and 1200°C. The composite exhibits a lower stress exponent than MoSi 2 between 10-4 s-1 and 10-5 s-1. TEM observations reveal the deformation of the carbide phase, as well as the efficient generation of dislocations into the matrix from the carbide-matrix interfaces. The substructures of both the matrix of the composite and monolithic MoSi 2 appear to consist of mixed dislocations of predominantly edge character which occur on the same slip systems. INTRODUCTION The brittle-to-ductile transition temperature (BDTT) of MoSi 2 has been' reported by various authors to be between 900°C and 1300°C [1]. Within this temperature range, MoSi 2 exhibits limited deformation capability. The plasticity of MoSi 2 can be enhanced in this region of limited plasticity by the addition of a more deformable phase such as TiC. TiC deforms on fcc slip systems above its BDTT of 600°C to 800°C, and can be deformed to large strains in compression at 900°C and above [2]. Experimental observations [3] and thermodynamic calculations [4] have shown that MoSi 2 and TiC are chemically stable. Previous compression test results have shown that MoSi 2 -TiC composites can be deformed plastically at lower temperatures than MoSi 2 can before brittle fracture occurs [5]. The composites also attain zero strain hardening at much lower strains than in MoSi 2 . These differences in plasticity and in strain hardening behavior between the composites and MoSi 2 are attributed to efficient generation of dislocations into the matrix from sources at the MoSi 2 -TiC interfaces. EXPERIMENTAL PROCEDURE Two materials, monolithic MoSi 2 and a composite of MoSi 2 and 10 vol % TiC in particulate form, were mechanically tested in the current investigation. These materials were hot pressed and subsequently HIPed from commercially available powders. The MoSi2 is nominally 5 to 6 micron powder from CERAC, Inc.; the TiC is 2.5 to 4 micron powder from Johnson Matthey, Inc., with a nominal stoichiometry of TiC0 .9 5 . Hot pressing was performed in argon at 1700°C for one hour at 29.4 MPa. HIPing conditions were at 1800°C for 1.5 hours at 200 MPa. The densities of both materials after HIPing are approximately 98%. The microstructures of these materials have been described previously [5]. Decremental step strain rate tests in compression were performed on the two materials at 1150°C and 1200°C. These are constant crosshead speed tests Mat. Res. Soc. Syrup. Proc. Vol. 288. •1993 Materials Research Society
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with nominal strain rates of 10-4 s- 1 , 5 x 10-5 sO, and 10-5 s-1. Two tests were performed on each material at each temperature. The total plastic strain in these tests ranged from 8% to 13% for the composite, and 13% to 15% fo
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