Plastic relaxation of thermoelastic stress in aluminum/ceramic composites
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INTRODUCTION
P L A S T I C relaxation of the elastic stress and strain energy associated with both thermal misfitting inclusions and lattice mismatching precipitates has been long recognized. Examples of dislocation generation due to a thermal misfit in the vicinity of an inclusion include the observations by Vogelsang et al. t~l and Arsenault and Fished 2] on dislocation generation in the aluminum alloy matrix surrounding SiC whiskers or platelets and the finding by Chawla and Metzger ]3~ of higher dislocation densities at the C u / W composite interface. The mechanical properties of the composite, therefore, should be affected by the magnitude and extent of the dislocation generation that takes place in the soft matrix around hard particles as a result of the relaxation of thermal misfit stress in the interface region. The plastic deformation of a misfitting inclusion in an infinite matrix has been analyzed via a continuum mechanics model t4,5,61 as well as a dislocation looppunching model. 17,sl Lee et al. t41 were able to deduce a particle size-dependent yield stress from the AshbyJohnson model is] for the nucleation of a dislocation from a particle/matrix interface. They showed two effective yield stresses which depend upon the coherency of the particle/matrix interface. One is essentially independent of the particle size and equal to the theoretical yield strength, /z/2zr, where /x is the matrix shear modulus. The other effective yield stress applies to incoherent particles and is a strong function of particle size. When the particle is very small, say of the order of 10 to 20 nm, this stress approaches the theoretical yield strength. When the particle size becomes of the order of a micron, the effective yield stress approaches an average (lowtemperature) macroscopic yield stress. Since in metal/matrix ceramic composites (MMC's) the particle/matrix interface is considered to be incoC.T. KIM, Graduate Student, J.K. LEE, Professor, and M.R. PLICHTA, Associate Professor, are with the Department of Metallurgical Engineering, Michigan Technological University, Houghton, MI 49931. Manuscript submitted June 7, 1989. METALLURGICALTRANSACTIONSA
herent, it is expected that the dislocation generation due to a thermal expansion mismatch would depend strongly on the size of the ceramic particles in MMC's. Although the continuum mechanics and dislocation loop-punching models appear to yield fairly reasonable predictions for the experimentally observed dislocation generation, to date there have been no systematic experimental studies on the particle size-dependent dislocation generation. Additionally, the previous theoretical models were based on a single inclusion embedded in an infinite matrix, thus neglecting the effect of a multiparticle distribution. Therefore, a theoretical model for the plastic relaxation is desired to account for the volume fraction of ceramic particles in composites. The purpose of this work was to examine experimentally the dislocation generation due to the different thermal contractions in A1/c
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