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The role of internal stress in strengthening heavily codeformed two-phase materials Killick, Matthew J.

Abstract

Heavily codeformed two-phase materials can show anomalous increases in strength as a function of the level of deformation. The strength of these materials can vary considerably from what is predicted by the rule of mixtures. Understanding the mechanisms responsible for the strengthening of these materials can lead to the better understanding of the strengthening of the individual phases and aid in the development of other materials that exhibit similar properties. This thesis examines the role of internal stresses on the strengthening of heavily codeformed two-phase materials. The magnitude of these stresses and the effects of annealing on them are studied in various types of copper-niobium composites: bundle drawn and in-situ drawn wire produced at the National High Field Magnet Laboratory in Los Alamos, USA; and a layered composite produced at the University of British Columbia as part of this work. Tensile testing was done to determine the mechanical properties of the copper-niobium composites and load-unload or tension-compression tests were done to quantify the levels of internal stresses in the material. The wire drawn composites were found to possess flow stresses much higher than predicted by the rule of mixtures and to contain considerable levels of internal stress. Internal stresses, measured by investigating the Bauschinger effect, and the flow stresses of all the drawn materials were found to decreases as the annealing temperature increased; however, the in-situ composite was found to be more resistant to annealing than the bundle drawn wire. The strength of the laminate composite, produced via multiple roll bonding, was found to be adequately described by the rule of mixtures. Although the rolling strain was not sufficient to produce anomalous strengthening, the internals stresses in the laminate where found to be more than twice that predicted by the rule of mixtures.

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