Investigation of damage mechanisms related to microstructural features of ferrite-cementite steels via experiments and multiscale simulations

Abstract

Spheroidized ferrite-cementite steel (SFC) is susceptible to micro-defects owing to the mismatched deformation between the ferrite and cementite during cold forming, which seriously deteriorates the fatigue life. However, the damage mechanism related to microstructural features in the SFC steel during cold deformation has not been fully investigated, hindering control of the microstructure and forming process. This study presents a method coupled with experiments and multiscale simulations to research the damage mechanisms and the dependence of microstructure features under uniaxial tension in SFC steel. In situ tensile test revealed three damage mechanisms in SFC steel under uniaxial tension: cementite cracking, ferrite/cementite interface debonding, and ferrite cracking. In particular, the final fracture was mainly dominated by the ferrite/cementite interface debonding. In multiscale simulations, the macroscale tensile and nanoindentation simulations were carried out to obtain the strain history and mechanical properties of the mesoscale simulations, respectively. The mesoscale IP-based RVE and CP-based unit cells simulations were performed to capture the driving forces from the three damage mechanisms and the dependence of microstructural features, respectively. The nanoscale molecular dynamics simulation was conducted to identify the model parameters of cohesive zone model (CZM) in the mesoscale simulations. The numerical results indicated that particle-cracking void occurred when the internal von Mises stress exceeded the ultimate strength of the cementite particles. Interface-debonding void nucleated when the load on the particle was insufficient to resist its strength. Matrix-cracking should be attributed to the stress concentration caused by the dislocation pile-up or the crossing of multiple slip bands. In addition, interfacial debonding were more likely to occur in the stable orientations and high-angle grain boundaries of ferrite matrix. Compared with a circular cementite particle, an elliptical particle was more prone to cause interfacial debonding. Also, the ferrite/ferrite grain boundary showed earlier interfacial debonding perpendicular to the loading direction than parallel to the loading direction.