Microstructure and texture evolution under strain-path changes in low-carbon interstitial-free steel
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I. INTRODUCTION
DURING sheet metal forming, the material undergoes complex strain paths involving large plastic strains. The combination of several simple loading test sequences is known to be an effective way to investigate the plastic behavior of sheet metals under such real-forming conditions. The mechanical behavior of low-carbon steels under strain-path changes has been repeatedly studied in the literature (e.g., References 1 through 3). A temporary stagnation of the work-hardening rate was observed under Bauschinger sequences,[1,2] whereas a strong hardening followed by softening and again hardening was found to be typical of orthogonal tensile/shear and shear/shear deformation sequences.[2,3] The TEM observations[4,5] showed the differences in dislocation structure evolution taking place during Bauschinger and orthogonal strain-path changes. Thus, the work-hardening stagnation was shown to be associated with the partial disintegration of dislocation sheets, whereas the stress decrease at the beginning of the second path during cross-loading has been associated with the intragranular localization of the deformation in microbands. However, these microstructure investigations were focused mainly upon morphological peculiarities, dislocation density and arrangement, whereas the influence of the initial or deformation-induced grain orientation on the microstructure evolution under strain-path changes has scarcely been considered. On the other hand, in view of the already proved difference in microstructure evolution displayed by grains of different orientations after monotonic loading,[6,7] this kind of information is considered essential for a better understanding of the deformation mechanisms taking place under strain-path changes. A detailed analysis of the texture evolution in low-carbon steel sheets during Bauschinger and orthogonal-shear sequences was performed in References 8 and 9. In particular, it has been shown that the grain orientation (geometrical E.V. NESTEROVA, Senior Researcher, is with CRISM Prometey, St. Petersburg 193015, Russia. B. BACROIX and C. TEODOSIU, CNRS Directors of Research, are with LPMTM-CNRS, University of Paris 13, 93430 Villetaneuse, France. Manuscript submitted June 15, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
factor) may have a significant influence on the experimental work-hardening characteristics. Both microstructural (associated with dislocation arrangement and density) and texture contributions had to be taken into account in order to completely explain the observed shapes of the stress–strain curves after strain-path changes. Notwithstanding, these contributions can hardly be considered as two independent factors because the texture evolution is strongly influenced by the heterogeneity of plastic deformation, whereas microstructural changes are in turn expected to depend on the initial and deformation-induced grain orientations. That is why a simultaneous examination of the microstructure and texture evolution should be performed, in order to achieve a more com
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