Carbon diffusion in bcc- and bct-Fe: Influence of short-range C–C pair interactions studied from first-principles calculations

Abstract

Identifying the mechanisms of interstitial diffusion in iron is important to understanding the low-temperature ageing of Fe–C ferritic and martensitic alloys. In spite of the low solubility of carbon in ferrite at equilibrium, carbon-rich areas are often found at segregated grain boundaries and in Cottrell atmospheres around dislocations. Carbon-rich areas also form by spinodal decomposition in martensite. In those cases, carbon atoms experience short-range interactions, susceptible to modify their migration behaviour. We performed first-principles calculations to study the influence of these C–C interactions on the migration of interstitial carbon in body-centered iron. The ab initio energy database was introduced in kinetic Monte Carlo simulations to compute the thermodynamic and kinetic parameters. We found that the migration energies of carbon are largely affected by the presence of a neighbouring carbon atom. We explain the evolution of these energies by the relative stability of the C–C configurations corresponding to stable and transition-state positions. The C–C pair interactions slightly modify the ferrite/martensite transition conditions and significantly change the carbon atomic migration path. The latter leads to an increase of the diffusivity up to 10 times and an important kinetic correlation at low temperature (<300 K) and high carbon contents (>1 at.%).