Energy barriers for carbon diffusion in ferrite under heterogeneous stress

​International workshop on multiscale simulation of the prediction of radiation damage in reactor pressure vessel steel and internals up to 60 years of operation / 10-12 décembre 2013, Fontainebleau (France)

The context and objectives are :

• The diffusion of carbon in iron controls the kinetics of many transformations in steels(such as, cementite precipitation, martensite ageing, massive austenite-ferrite transformation and bainite formation).
• Diffusion of carbon in defect-free ferrite (the body-centred cubic structure of iron): a fairly well known mechanism, extensively studied and characterized.
• When other defects are present: both the diffusion mechanism and its kinetics are affected. Case of dislocations: they create very large and non-uniform stresses, inducing important effects on the energy barrier of impurities.
• Diffusion and segregation of interstitial carbon to dislocations introduced by plastic deformation in ferritic iron: leads to the growth of so-called Cottrell atmospheres around the dislocations (responsible for static strain aging in ferritic steels).
• In this work, the effect of an heterogeneous stress field on diffusion energy barriers is adressed: a novel method, called LinCoSS (Linear Combination of Stress States), which is very fast and easy to implement alternative to existing approaches [1].
• Atomistic simulations provide a good alternative for studying the diffusion processes in the presence of stress: 1/Molecular dynamics (MD): could serve as a perfect framework if not so limited in simulation time span (typically, a few ns). 2/ Atomistic Kinetic Monte-Carlo (aKMC) method: very well adapted for studying the diffusion of atomic species, based on knowledge of the different escape pathways and the corresponding escape rates.

• The impact of the stress field induced by the presence of a defect on the energy barriers cannot be neglected, and need to be quantitatively accounted for.

• In this work, the effect of an heterogeneous stress field on diffusion energy barriers is adressed: a novel method, called LinCoSS (Linear Combination of Stress States), which is very fast and easy to implement alternative to existing approaches.