Stress-state dependence of dynamic strain aging: Thermal hardening and blue brittleness

Abstract

This study aims to discover the stress-state dependence of the dynamic strain aging (DSA) effect on the deformation and fracture behavior of high-strength dual-phase (DP) steel at different deformation temperatures (25–400°C) and reveal the damage mechanisms under these various configurations. To achieve different stress states, predesigned specimens with different geometric features were used. Scanning electron microscopy was applied to analyze the fracture modes (e.g., dimple or shear mode) and underlying damage mechanism of the investigated material. DSA is present in this DP steel, showing the Portevin-Le Chatelier (PLC) effect with serrated flow behavior, thermal hardening, and blue brittleness phenomena. Results show that the stress state contributes distinctly to the DSA effect in terms of the magnitude of thermal hardening and the pattern of blue brittleness. Either low stress triaxiality or Lode angle parameter promotes DSA-induced blue brittleness. Accordingly, the damage mechanisms also show dependence on the stress states in conjunction with the DSA effect.

Edge fracture of the first and third-generation high-strength steels: DP1000 and QP1000

“…Since both phases are relatively fine microstructures, it is difficult to distinguish between microcracks and voids from the phase boundaries. However, as the ferrite phase shows generally higher ductility and deformed more easily compared to martensite, phase boundary debonding is the main failure type under tensile loading at room temperature [27], while for higher temperatures, more complex damage mechanisms take place due to dynamic strain aging effects [28]. The current investigated QP grade contains both tempered martensite and secondary martensite, arising the fraction of the harder and more brittle phases in general.…”

Investigations on the Johnson-Cook Constitutive and Damage-Fracture Model Parameters of a Q345C Steel

“…The reason for blue brittleness is the deformation aging of carbon and nitrogen interstitial atoms. When deformed within the temperature range of 150-350 • C, dislocations that have already activated are quickly anchored by diffusible carbon and nitrogen atoms, forming a Coriolis gas mass [49,50]. In order for further deformation, new dislocations must be activated, resulting in an increase in dislocation density of the steel at given strain levels, leading to an increase in strength and a decrease in toughness.…”

Determination of temperature dependence in Modified-Mohr-Coulomb failure model for process simulation of shear cutting

“…If the steel material contains a certain mass of dissolved foreign atoms such as carbon and nitrogen, they diffuse to the dislocations at particular temperatures and block them. This means the tensile strength increases [17]. In figure 2 The value of 𝛼 = 0.7 was fitted as the weighting factor.…”