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Measurement and statistical interpretation of slip line length and microstrain in copper single crystals Garner, Andrew

Abstract

In order to test the apparently conflicting predictions of some current theories of strain hardening, slip line length measurements were made on a series of oriented copper single crystals, identically prestrained at 673°K, polished and incrementally strained at temperatures between 573°K and 4.2°K; slip lines formed during low temperature increments were found to be longer than those formed during strain increments at higher temperature (Garner and Alden, 1974). The result is shown to be in conflict with any theory of strain hardening in which slip lines are blocked by specific obstacle configurations, such as Lomer-Cottrell barriers, ribbons of converted pile-ups or dislocation cell walls. In contrast, the result is shown to be consistent with theories of strain hardening in which slip lines are blocked by statistical interaction between expanding glide loops and forest dislocations, on the condition that, within the framework of such a theory, the glide loops are able to expand athermally over a newly available free area of slip plane, after a thermally activated process. Two possible thermally activated processes are discussed. A unified view of slip lines properties is presented which is shown to provide a self-consistent explanation of the temperature variation of slip line length, slip band formation, the existence of multipole carpets and the variation of flow stress with temperature. The statistical aspects of this interpretation were investigated further by obtaining 77°K microstrain curves from a series of oriented copper single crystals, prestrained at temperatures between 1000°K and 77°K, to produce dislocation microstructures with differing degrees of regularity, yet with approximately the same overall density. The forest dislocation microstructures of an identically prepared series of crystals were examined using a dislocation etch on the primary slip plane. A statistical sampling technique was devised, which was used to measure local dislocation densities. In addition, new parameter is introduced, namely the ratio of the sampled standard deviation, to mean local dislocation density, which quantifies the degree of regularity of a dislocation micro-structure. All microstructures were found to have a smaller degree of regularity than a random distribution. For crystals prestrained at temperatures above 293°K, at any given fraction of the 77°K yield stress, the amount of microstrain was found to increase as the microstructures became less regular. Crystals prestrained at and below 293°K exhibited the Haasen-Kelly effect, which was attributed to restricted source operation. However, once sources begin to operate, the amount of microstrain anticipated from the degree of regularity was indeed detected.

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