“…High activation energies are known to be associated with the formation of immobile vacancy-impurity clusters involving carbon or nitrogen (Fu et al, 2008;Terentyev et al, 2014;Theodorou et al, 2022). In bcc iron, the effective migration energy of vacancies is defined by the energy of dissociation of a cluster involving a vacancy and a carbon dimer (Paxton, 2014), and this dissociation energy can be as high as 2.22 eV (Kabir et al, 2010), far higher than the activation energy of 0.55 eV characterising vacancy migration in pure elemental Fe (Fu et al, 2005).…”