A multiscale approach to understand the role of dislocation motion on carbon mobility and the breakup of carbides in martensitic bearing steels.

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Plasticity in metals is mainly due to dislocation motion. The presence of carbon in steels hinders dislocation motion, leading to a stronger material and this strengthening effect depends on the carbon staying where it is. The decay of martensite under rolling contact fatigue (RCF) is thought to be due to carbon migration - it is the goal of this project to understand the role that dislocation motion plays in the redistribution of carbon under RCF.Results from atomistic simulations such as carbon dislocation binding energies will be used in the next stage of a multiscale approach featuring kinetic Monte Carlo (kMC) simulations of dislocations. From these simulations dislocation mobility rules can be extracted; i.e. functions that relate dislocation velocity to applied stress, temperature and solute concentration. The kMC simulations will also incorporate solute diffusion to ascertain how carbon moves with dislocations in different stress, temperature and concentration regimes. The results of the kMC simulations can be used in a more course-grained approach called discrete dislocation dynamics (DDD) where multiple dislocations and their interactions are simulated. From this approach it is possible to calculate the stress produced in a crystal under a variety of straining conditions. However, it may not be necessary to use DDD if the kMC code developed in this project is successful in simulating multiple dislocations.