Evolution of Relationships Between Dislocation Microstructures and Internal Stresses of AISI 316L During Cyclic Loading
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POLYCRYSTALLINE materials usually exhibit complex cyclic deformation responses due to their changing dislocation condition, in particular those whose dislocations first tend to move in a more planar manner during fatigue loading because the movement character can change into a more wavy form upon further loading.[1–4] For example, significant changes in the density and the configuration of dislocations during fatigue loading are responsible for the complex cyclic deformation response of AISI 316L, e.g., cyclic hardening, softening in general,[5–8] or even serrated flow stress and secondary cyclic hardening after a high number of cycles at elevated temperatures.[9–11] Moreover, changes in imposed strain amplitude also lead to variations in dislocation evolution characteristics, resulting in different material responses. These cause difficulties to underMINH-SON PHAM, Research Associate, formerly with the EMPA: High Temperature Integrity Group, Mechanics for Modelling and Simulation, Swiss Federal Laboratories for Materials Science & Technology, 8600 Du¨bendorf, Switzerland, and also with the Center of Mechanics, Department of Mechanical Engineering and Processing, Swiss Federal Institute of Technology Zurich (ETHZ), Zurich, Switzerland, is now with the Materials Science and Engineering Department, Carnegie Mellon University, Pittsburgh PA. Contact e-mail: minhson@ andrew.cmu.edu STUART R. HOLDSWORTH, Group Leader, is with the EMPA: High Temperature Integrity Group, Mechanics for Modelling and Simulation, Swiss Federal Laboratories for Materials Science & Technology. Manuscript submitted April 15, 2013. Article published online September 6, 2013 738—VOLUME 45A, FEBRUARY 2014
stand and ultimately to model the cyclic deformation response of polycrystalline materials.[12] Since effective and back stresses, respectively, are associated with the short-range and long-range order of dislocation interactions,[13–17] the variation of microstructural condition causes the change in associated internal stresses (i.e., effective stress and back stresses). For example, individual dislocations can interact with themselves and with point defects thanks to their associated short-range stress fields, resulting in a change in the local resistance to dislocation movement, in particular for face-centered cubic materials.[18] The change in local resistance to dislocation movement correlates with the effective stress (rE) which is responsible for the expansion (or the contraction) of the yield surface in principal stress coordinates.[15] In contrast, back stress (X) is associated with long-range interactions of collective dislocations, which arise due to inhomogeneous plastic deformation between microstructural heterogeneities of materials on different scales, e.g., grains, sub-grains, cell blocks, and families of dislocation walls/channels.[5,18–23] Consequently, investigation of the evolution of the microstructural condition and associated internal stresses as well as their relationships during cyclic loading provides a thorough understanding of
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