An in situ HVEM study of dislocation generation at Al/SiC interfaces in metal matrix composites
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INTRODUCTION
T H E incorporation of 20 vol pct discontinuous SiC whiskers into a 6061 A1 matrix increases the yield strength of annealed powder compacted 6061 AI alloy by more than a factor of two. This increase in strength cannot be explained directly by continuum mechanics theories. Continuum mechanics formulations developed by Piggott ~ and applied to the case of discontinuous A1/SiC composites by Arsenault 2 predict an ultimate strength of only 186 MPa for 20 vol pct SiC composite, whereas the measured value of ultimate strength for this material is 448 MPa. Arsenault and Fisher 3 proposed that the increased strength could be accounted for by a high dislocation density in the Al matrix which is observed in bulk composite material annealed for as long as 12 hours at 810 K. The dislocation generation mechanism proposed by Arsenault and Fisher to account for this high dislocation density is based on the large difference ( 10: I) in coefficients of thermal expansion (CTE) of AI and SiC. 4 When the composite is cooled from elevated temperatures of annealing or processing, misfit strains occur due to differential thermal contraction at the A1/SiC interface which are sufficient to generate dislocations. Chawla and Metzger, in an elegant investigation of Cu/W composites using etch-pitting techniques, observed a high dislocation density at the Cu/W interface which decreased with increasing distance from the interface, s They observed that if the volume fraction of W was 15 pct, the minimum dislocation density in the matrix was 7 x 10 ~J m -2 inMARY VOGELSANG and R. J. ARSENAULT, Director, are with the Metallurgical Materials Laboratory, University of Maryland, College Park. MD 20742. R.M. FISHER is with Center for Advanced Materials, Lawrence Berkeley Laboratory. University of California, Berkeley, CA 94720. Manuscript submitted April 8, 1985.
METALLURGICAL TRANSACTIONS A
creasing to 4 x 10t2 m -2 at the interface of W and Cu, and concluded that the dislocations were caused by the differences (4: 1) in CTE of Cu and W. Recalling that the CTE difference between AI and Si is 10: 1, i.e., more than twice as great as the Cu/W system, one would expect thermal stresses in AI/SiC to be sufficient to generate dislocations in this composite. Other causes may also contribute to the high dislocation density observed in annealed AI/SiC material. Dislocations are introduced into this material during the plastic deformation processes of manufacturing, such as extrusion. During annealing, the dislocations introduced during processing may not be annihilated; they could be trapped by the SiC, resulting in a high dislocation density after annealing. It is important to determine the origins of the high dislocation density in the composite since the strength of the composite depends on the high density. If the differential thermal contraction is the cause of the dislocations, as Arsenault and Fisher 3 suggest, then dislocations should be observed being generated in a composite thin foil sample on cooling from annealing temperatures
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