Micromechanical behavior of eutectoid steel quantified by an analytical model calibrated by in situ synchrotron-based X-ray diffraction

A eutectoid steel with three types of ferrite (α)+cementite particle (θ) microstructures, i.e., a coarse-grained α+θ structure, a fine-grained α+θ structure and an ultrafine-grained α+θ structure, was fabricated to explore the effects of the microstructural features on the micromechanical behavior of hard particle-strengthened two-phase alloys. An analytical model based on the Kocks–Mecking model was established to elucidate the evolution of the geometrically necessary dislocations (GNDs) and statistically stored dislocations (SSDs) in the hard particle-strengthened alloys and, hence, to predict the stress partitioning for each phase and the enhancement in the work hardening during uniform plastic deformation. In situ synchrotron-based X-ray diffraction was used to verify the stress partitioning and the important material parameters predicted by our analytical model. Our results showed that a decrease in the geometric slip distance leads to an appreciable increase in the GND density, whereas an increase in the grain size of the ferrite causes an increase in the SSD density under uniform plastic deformation for eutectoid steel with an α+θ structure. Both the stresses for the individual phase and the difference in stress between the two phases for eutectoid steel with various α+θ structures were closely related to the change in the GND density near the phase interfaces. The GND density also played an important role in determining the work-hardening rate for eutectoid steel with various α+θ structures.