Flow stress and ductility of duplex stainless steel during high-temperature torsion deformation
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ot-deformation behavior of single-phase ferritic and austenitic materials is very well described in the literature. In general, the high stacking-fault energy (SFE) of ferrite results in the relative ease for dislocations to climb or cross slip at high temperatures. Consequently, during hot working of ferritic steels, a very efficient annihilation of dislocations with opposite Burgers vectors occurs, i.e., dynamic recovery (DRV).[1–4] As a result, the buildup of stress concentrations is slowed down and very high ductilities are obtained, especially in hot torsion, where the deformation is shear-like. In a - curve, DRV results typically in a monotonic hardening to a steady-state plateau, where equilibrium is reached between dislocation generation and annihilation (Figure 1). In austenite, which is characterized by a low SFE, dislocations are less mobile and DRV is limited. Consequently, once a critical strain is exceeded, the driving force becomes large enough for dynamic recrystallization (DRX).[4–9] During DRX, dislocations are eliminated through replacement of deformed grains by new, dislocation-poor grains. The stress-strain curve for austenites generally consists of a work-hardening peak followed by a softening to a steadystate level. In general, reasonable ductility arises from DRX, as the migrating grain boundaries slow the nucleation and growth of cracks. However, the ductilities are lower than the ductilities obtained for ferritic grades.[1,4] In duplex stainless steel (DSS), ferrite and austenite constitute a composite material consisting of similar amounts of these ductile phases. The dynamic restoration behavior L. DUPREZ, Graduate Student, and B.C. DE COOMAN, Professor of Materials Science, are with the Laboratory for Iron and Steelmaking, Ghent University, B-9052 Ghent, Belgium. Contact e-mail: bruno.decooman@ rug.ac.be N. AKDUT, Technology Manager, is with Ocas NV, B-9060 Zelzate, Belgium. Manuscript submitted October 5, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
of both phases is found to be rather similar to the restoration behavior in single-phase materials, with the ferrite softening by DRV and the austenite by DRX.[10–14] Cizek and Wynne[10] describe the ferrite softening as an “extended dynamic recovery,” which is similar to the “continuous recrystallization” observed by some authors in single ferritic steels.[15] Through the recovery processes, part of the dislocations are annihilated. The remaining dislocations are stored in low-angle dislocation walls, resulting in the formation of subgrains. The merging of these dislocation walls leads to a gradual buildup of misorientations between neighboring subgrains. This mechanism results in the formation of largeangle boundaries at higher strains. The coexistence of a hard austenite and a soft ferrite at high temperatures is found to result in a strain partitioning in the early stages of deformation, with most of the deformation accommodated by the ferrite.[13,14] Consequently, as strain energy is the driving force for softening to occur, the
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