Defect Structures and Planar Faults in Plasma Spray Deposited MoSi 2
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DEFECT STRUCTURES AND PLANAR FAULTS IN PLASMA SPRAY DEPOSITED MoSi 2
Bimal K. Kad, Kenneth S. Vecchio and Robert J. Asaro Dept. of Applied Mechanics and Engineering Sciences, MC-0411 University of California-SanDiego, LaJolla, CA 92093-0411
ABSTRACT Plasma-sprayed microstructures of MoSi 2 have been studied by electron microscopy. The asdeposited microstructures are metastable and inhomogenous. Two new dissociations of [001] and 1/2[331] dislocations have been observed for the first time. The 1/2[331] is dissociated on the (1ro) plane bounding a superlattice intrinsic stacking fault (SISF). The reaction is given as: 1/2[33T] = 1/4[331"] + SISF + [331]. The [001] is also dissociated on (11-0) plane into two symmetrical components, with the reaction being given as [001] = 1/2[001] + 1/2[001].
INTRODUCTION Intermetallic matrix composites are attractive materials for high temperature structural applications. One of the most promising intermetallic in the silicides is MoSi2 which combines excellent oxidation resistance, a high melting point (20200C), reasonable strength and density and good thermal conductivity making it a promising material for structural applications above 12000C. MoSi2 essentially behaves like a ceramic, possessing little ductility below the brittle to ductile transformation temperature (BDTT) and exhibits plastic behavior above the BDTT (1,2). The brittle behavior of MoSi2 poses two problems: i) limits use as a potential structural material and ii) severe cracking problems in conventional processing/casting routes. While the former affects it end usage, the latter hinders the evaluation of its intrinsic properties in a rapidly evolving research effort. Current research initiatives are the incorporation of a wide variety of ductile and/or hard dispersoids in a MoSi2 matrix to increase fracture toughness. Towards this end, plasma co-spraying is a promising near-net shape processing technology combining melting, blending and consolidation into a single operation. A wide variety of microstructures can thus be created in tetragonal (Cllb) MoSi 2 based alloys (3). Controlled compositions/microstructures of MoSi2, MoSi2+Mo5Si 3 and MoSi2 + ductile/brittle reinforced particulates are being successfully fabricated (4). Of particular interest is the incorporation of fine scale (=100nm) dispersoids with lattice parameters, elastic modulii (E) and coefficient of thermal expansion (CTE) different than MoSi2. These property mismatches create a myriad of matrix precipitate interactions that can be tailored to enhance matrix plasticity. Due to the somewhat rapid nature of processing, non-equilibrium effects and planar faults reminiscent of C1 lb (tetragonal) => C40 (hexagonal) transformations are frozen in these microstructures; these transformation related faults are suggested to enhance ductility (5,6). In this paper we present our preliminary observations of the microstructural modifications obtained by plasma spraying and their possible contributions to enhancing plasticity and fracture toughness. The charac
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