Probabilistic and constitutive models for ductile-to-brittle transition in steels: A competition between cleavage and ductile fracture

Abstract

We propose a probabilistic model coupling with the temperature dependent constitutive relationship to describe the competition between the cleavage and ductile void failure of ferritic/martensitic steels with irradiation effects. It is found that both the material deformation and failure modes exhibit significant temperature dependence. Regarding the material deformation, two regimes have been found for the ductile-to-brittle transition (DBT). At low temperature, the flow stress is controlled by the mobility of screw dislocations. The high-temperature regime is about the thermally activated jog drag of dislocations. The failure modes depend on the carbide precipitate and temperature. Cleavage micro-cracks initiate at the carbide sites at low temperature giving rise to brittle behaviors. At high temperature, void nucleation evolving from the carbide precipitate suppresses the cleavage nucleation and propagation, and leads to good ductility. It is demonstrated that the competition between the cleavage fracture and ductile void failure is the major mechanism for the DBT. Our probabilistic model successfully predicts the temperature-dependent fracture toughness and ductile-to-brittle transition temperature (DBTT). Besides temperature, irradiation has a significant effect on the DBT of steels. Upon irradiation, the steels exhibit both irradiation hardening and irradiation embrittlement, i.e., rise in yield and flow stresses, and increase on DBTT. The main reason for these irradiation effects is that the irradiation-induced hardening increases the flow stress, and the plasticity localization weakens the overall hindering effect from ductile void growth on the cleavage nucleation and propagation, promoting the development of cleavage. Our probabilistic model takes into account the cleavage nucleation-propagation process and the influence of ductile void growth simultaneously, and it gives a sound explanation for the fundamental relation between the DBTT and mechanical properties of metallic materials.