Large strain work hardening of aluminum alloys and the effect of mg in solid solution
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MATERIAL properties at large strains (e . 0.7) are of significant importance for a number of applications, such as in cold rolling of sheets and the subsequent forming or annealing operations. Studies of the material behavior through all stages of work hardening are therefore of interest. When polycrystals are considered, it is common to distinguish between stages II, III, and IV (reviewed by Rollett and Kocks[1]). Stage II has, under ideal conditions, a high and constant work-hardening rate of the order G/200, where G is the shear modulus. The stage is associated with an accumulation of dislocations, but eventually dynamic recovery reactions start taking place. This leads us into stage III, where the hardening rate decreases and the stress–strain curve becomes parabolic. In most commercial alloys, distinguishing stage II from III is difficult because the distribution of grain sizes and the presence of particles contribute to a parabolic hardening also in stage II. A stage of sustained hardening following stage III is commonly described in treatments of large strain work hardening. One of the first well-documented observations of a stage IV is the oft-cited work by Langford and Cohen,[2] where the strength of a drawn iron wire was seen to increase linearly up to strains of e 5 7. Torsion tests of aluminum and copper also reveal a stage IV,[3,4] and so does ECAP processing.[5] Perhaps the most common experimental method used to obtain stage IV is rolling (see Gil Sevillano[6] for references). The stage IV work-hardening rate is often found to be of the order 2 " 10#4 G, considerably lower than the rate of stage II. While the behavior of AlMg alloys in stage II and III is well described in the literature (e.g., in the early work by Sherby et al.[7]), the effect on stage IV of adding Mg to the alloy is not clear. Cold rolling followed by tensile testing is a wellestablished method for obtaining large strain data. One problem that inevitably arises when stage IV is discussed is that there exists no method by which a material can be tested continuously or without some shape change, path ØYVIND RYEN, Project Engineer, is with Reinertsen Engineering AS. HANS IVAR LAUKLI, Research Scientist, is with Hydro Aluminum, Sunndalsøra, Norway. BJØRN HOLMEDAL, Research Scientist, and ERIK NES, Professor, are with the Norwegian University of Science and Technology, Troudheim, Norway. Contact e-mail: [email protected] Manuscript submitted August 12, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
change, texture change, or friction. All these aspects must be kept in mind and the results should be treated carefully. A number of models for the work-hardening behavior of metals have been proposed during the past 30 years. One of the earliest, and perhaps the most widely known, is the model initially presented by Kocks[8] and further developed in collaboration with Mecking (for a recent update see Reference 9). This modeling concept, commonly referred to as the MTS model (mechanical threshold strength[10]), is a oneparameter descrip
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