Damage Evolution and Microstructural Fracture Mechanisms Related to Volume Fraction and Martensite Distribution on Dual-Phase Steels

The scope of this work is the analysis of the damage evolution and microstructural fracture mechanisms of dual-phase steels through quasi-static uniaxial and cyclic tensile tests, considering the influence of the volume fraction and distribution of martensite. The samples are intercritically annealed at different temperatures to obtain three different martensite volume fractions (MVF). Damage evolution is evaluated as a function of stiffness loss. The steels show lower ductility and strength under cyclic loading conditions. The work hardening rate is affected by MVF in different stages of the deformation. A higher MVF is linked to a prompter damage evolution rate. The primary void nucleation mechanisms are ferrite-martensite and ferrite-ferrite decohesion. Martensite fracture can be activated depending on MVF and martensite distribution along the ferrite grain boundary. The dislocation density on grain boundaries is high due to the austenite-martensite transformation during the quenching process and subsequent plastic deformations. Dislocation density is related to stored strain energy, stress concentration, and the observed fracture mechanisms, particularly near martensite grains. Nanoindentation is conducted to evaluate the ferrite and martensite hardness, which depends on its carbon content. It is observed that the martensite/ferrite hardness ratio affects the uniform elongation of the material.