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Quantification of GNDs in DP steels (with three developed criteria)

Ali Ramazani's picture

The current work aims to predict the work-hardening behavior of dual-phase (DP) steel, focusing on the effect of transformation-induced geometrically necessary dislocations (GNDs). Equiaxed and banded microstructures were produced through suitable heat treatment cycles in a laboratory. Electron backscatter diffraction measurements were performed to characterize GNDs. The flow behavior was modeled within the micro-scale finite element method, considering the effect of the microstructures using the representative volume element (RVE) approach. 2-D RVEs were created based on real microstructures. The flow behavior of single phases was modeled using the dislocation based work-hardening approach. The volume change during the austenite-to-martensite transformation was also modeled, and the resulting prestrained areas in ferrite were considered to be the storage place of GNDs. The thickness of the GND layer around martensite islands was quantified experimentally and numerically. Subsequently, three criteria were developed to describe the strength, thickness, and amount of prestrain in the GND zone as a function of microstructural features in DP steel. Then, numerical uniaxial loading in the rolling direction was applied on the RVEs to study the effect of GNDs on the stress and strain distribution in the microstructures, flow curve, and hardening behavior of DP steel. A computational first-order homogenization strategy was employed to obtain the true stress–true strain curves from the RVE calculations. The flow curves of simulations that took the GNDs into account were in better agreement with the experimental flow curves, compared with those of simulations that did not consider the GNDs.

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