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Date of Award

1992

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Materials Science and Engineering

Supervisor

D.S. Wilkinson

Abstract

The primary purpose of this investigation was to examine the strengthening that arises when reinforcing an aluminum alloy with approximately equiaxed shaped SiC particles. The results indicate that the majority of strengthening that is derived from the particles develops at low strains. For a variety of test conditions explored, about 90% of the total strengthening derived is developed by a plastic strain of about.5%. This is primarily a result of the high strain hardening rate which is exhibit by the composite materials during the onset of plastic flow. An examination of the Bauschinger effect in the composites indicates that an internal stress, resulting from inhomogeneous plastic flow, develops rapidly in the low strain regime. Microstructural observations suggest that the nature of this inhomogeneous plastic flow is influenced by the distribution of the SiC particles. Initial plasticity appears to be associated with regions of the microstructure which are relatively particle free. A self-consistent model, based on Eshelby's equivalent inclusion method, has been developed which is capable of calculating the stress partitioning which occurs between the Al matrix and SiC particles during the elastic/plastic transition and beyond. Excellent quantitative agreement between the experimentally measured elastic modulus and that predicted by the model was achieved. When considering a uniform distribution of second phase particles, the model predicts an increase in the initial strain hardening rate. However, it underestimates the actual strain hardening rate which was measured experimentally. The influence of particle distribution was examined by representing the composite microstructure by a bimodal mixture consisting of a particle rich and particle free phase. The model predicts an increase in the initial strain hardening rate as a result of clustering the microstructure in this way. The magnitude of this increased hardening is comparable to that measured experimentally.

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