Plastic deformation mechanisms in pearlite structures with different cementite morphologies in S38C axle steel: a molecular dynamics study

Abstract

Pearlite structure is a key microstructural constituent of carbon steels, including the S38C axle steel for high-speed train, and the morphology of cementite in pearlite structure plays a critical role in governing its plastic deformation behavior. In this work, three atomistic models of pearlite structures are constructed based on the results of transmission electron microscopy observations, which identify the lamellar, broken lamellar, and spheroidized morphologies of cementite within the pearlite structures of S38C axle steel. Molecular dynamics simulations under uniaxial tension and compression are, respectively, conducted on these models to investigate the microstructural evolution and underlying mechanisms of plastic deformation in pearlite structure. The results indicate that dislocations nucleate at the ferrite-cementite interfaces in all three pearlite models, with the nucleation sites strongly influenced by the interfacial stress distribution governed by cementite morphology. In the lamellar pearlite, the dislocations nucleated in the ferrite and piled-up at the ferrite-cementite interfaces can activate the dislocation slipping on some specific slip systems within the cementite, leading to an interfacial slip transfer from the ferrite to the cementite. In the spheroidized pearlite, the influence of dislocations piled-up at the ferrite-cementite interfaces on the dislocation slipping within the cementite is relatively limited, and the plastic deformation of the spheroidized pearlite is primarily governed by the dislocation motion in the ferrite. Moreover, under a compressive loading, the ferrite in all three models initially tends to deform plastically through a single-slip mechanism, which facilitates the dislocation accumulation at the interfaces and enhances the interactions between the dislocations and the interfaces. As a result, more strain hardening is observed under the compression than under the tension. This study offers essential insights into the influence of cementite morphology on the plastic deformation of pearlite microstructure.