The effects of combined strain-path and strain-rate changes in aluminum
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INTRODUCTION
THE effect of a change in the proportions of the straining components during a deformation upon the development of flow stress in metals has been the subject of much research. The effect of such a change in strain path is important, because it not only represents a significantly more common situation in practical forming operations than proportional straining but also, as Kocks Eli noted, provides an effective way of "poking" the dislocation structure. The usual technique has been to deform or prestrain a specimen to some level of total plastic strain, to halt the test, and to reload in order to produce a significantly different proportion of straining components referred to the specimen axes but not dominated by strain reversal. After strains of the order 0.1, testing in the second stage shows a characteristic regime of low strain hardening. Early work by Basinski and Jackson i~,3j using single crystals showed not only the characteristic form of the path change effect but also the relevance of the geometry of the dislocation structure produced during the first stage of the test. Subsequent work, primarily with polycrystals, has revealed essentially the same type of behavior. Studies using transmission electron microscopy (TEM) with low-carbon ferritic steel have shown that substructural dissolution ~41 and microscopic strain localization in the form of microbands I5'61 are associated with deformation involving a path change. Schmitt et al. ~7~ have investigated the effect in copper. They found no evidence of microbanding and proposed that the lack of a suitable population of slip dislocations for the second stage of deformation was at least partially responsible for the effect. They observed a general dissolution of the cell structure after the path change, and this was thought to be associated with the strain-hardening rate reduction due to an increase in the rate of dynamic recovery. Juul Jensen and Hansen t8] and Li and Bate, tgl both working with aluminum, proposed that the orientation of dense P.S. BATE, Senior Research Fellow, is with the IRC in Materials for High Performance Applications, The University of Birmingham, Birmingham, United Kingdom B15 2TT. Manuscript submitted April 5, 1993. METALLURGICAL TRANSACTIONS A
dislocation structures had the dominant effect, and an analogy between these structures and shearable secondphase planar precipitates was proposed. [9j Clearly, the formation of these structures and their subsequent modification following a path change would be of prime importance. One possibly significant variable in the behavior is the effect of strain rate. Information about the effect of rate both prior to and after the path change is useful not only because of its potential significance in strain localization failure when changes of both strain path and rate occur together but may also help in the microstructural understanding of path change effects. Jackson and Basinski El~ reported that a path change had no significant effect upon the strain-rate sensitivity of flow st
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