Strain-field effects on the formation and migration energies of self interstitials in α-Fe from first principles

Ab initio electronic structure calculations are employed to study the stability and mobility of mono-self interstitial atoms (SIA) in alpha-Fe under external deformation. The ab initio results indicate that the volumetric and uniaxial strain dependences of the SIA formation energy are different in the expansion and compression regimes, in contrast to the linear behavior in continuum elasticity theory. We find a < 111 >-> < 100 > SIA reorientation mechanism induced by uniaxial expansion which proceeds via < 11x >vertical bar(x=2.7) configuration. Volumetric and uniaxial deformations are also found to have a considerable influence on the migration paths and activation energy barriers for the < 110 >{110}<->< 100 >{100} transformation and the < 111 ><->< 100 > reorientation. The results reveal that (i) the volumetric expansion (compression) decreases (increases) substantially the migration energy barrier and renders the diffusion process three (one) dimensional, (ii) the uniaxial strain removes (decreases) the migration energy barrier for the < 111 >->< 11x >vertical bar(x=2.7)(< 11x >vertical bar(x=2.7)->< 100 >) transformation, leading to spontaneous reorientation of the < 111 > SIA, and (iii) the uniaxial deformation breaks the cubic symmetry of the system and in turn induces anisotropy of the migration rates along different directions. These calculations demonstrate that changes in the electronic structure induced by global elastic deformation lead to additional contributions to the formation and migration energies, which cannot be adequately accounted for neither by elasticity theory nor by empirical interatomic potentials.