Strain hardening regimes and microstructural evolution during large strain compression of low stacking fault energy fcc
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INTRODUCTION
STRAIN hardening is one of the most widely used techniques for providing high strength to metallic components. At the microscale, this aspect of plastic deformation is intrinsically coupled with all other aspects of plastic deformation, such as development of preferred lattice orientations, formation of localized shear bands, formation of subgrains, and development of grain-scale residual stresses. Consequently, it plays a vital role in plasticity models. An accurate characterization of the strain hardening behavior of a material is a necessary ingredient for developing models with good predictive capabilities regarding the stress-strain behavior or the load-displacement relationship in deformation processing operations. In addition, characterization of the strain hardening behavior of a material is an important first step in microstructural investigation of plastic deformation in the material, since it often provides clear indications of the onset or curtailment of many of the microstructural phenomena occurring during deformation. SIROUS ASGARI, Graduate Student, EHAB EL-DANAF, Graduate Student, SURYA R. KALIDINDI, Associate Professor, and ROGER D. DOHERTY, Professor, are with the Department of Materials Engineering, Drexel University, Philadelphia, PA 19104. Manuscript submitted December 2, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
Strain hardening response of a material is usually characterized indirectly from the stress-strain curves documented in tension tests.[1] Typically, the strain hardening rate is computed numerically from the stress-strain curve and plotted against stress. Such plots for single crystals of medium to high stacking fault energy (SFE) fcc metals (such as copper or aluminum) usually reveal four distinct stages[1] of hardening, which are labeled as stage I, stage II, stage III, and stage IV, respectively. Stage I exhibits an almost zero hardening rate, and has been associated with single slip. Stage II exhibits an almost constant hardening rate (about G/100, G being the shear modulus of the material), and is associated with early stages of multiple slip. Stage III exhibits a steadily decreasing strain hardening rate, which is associated with dynamic recovery processes. Stage IV exhibits an almost constant strain hardening rate, whose magnitude is very low, but finite. The physical origin of stage IV is not clear at the present time, but is assumed to be associated with dislocation debris such as dipoles.[2,3] Investigations of strain hardening in medium to high SFE, fcc, polycrystalline metals[4] revealed that stage I is essentially eliminated, and stage II is greatly diminished. In other words, the strain hardening rate curves for these materials mainly exhibits only stage III and stage IV hardening. In plasticity models, the previously described phenomenology of strain hardening has been incorporated by saturation-type hardening laws[5] (essentially accounting for stage III hardVOLUME 28A, SEPTEMBER 1997—1781
ening), and in some situations a constant term has been
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