Ab initio electronic structure calculations are employed to study the stability and mobility of mono-self interstitial atoms (SIA) in $\alpha \text{-Fe}$ 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 $⟨111⟩\to ⟨100⟩$ SIA reorientation mechanism induced by uniaxial expansion which proceeds via $⟨11x⟩{\mid }_{x=2.7}$ configuration. Volumetric and uniaxial deformations are also found to have a considerable influence on the migration paths and activation energy barriers for the $⟨110⟩\left\{110\right\}\leftrightarrow ⟨100⟩\left\{100\right\}$ transformation and the $⟨111⟩\leftrightarrow ⟨100⟩$ 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 $⟨111⟩\to ⟨11x⟩{\mid }_{x=2.7}\left(⟨11x⟩{\mid }_{x=2.7}\to ⟨100⟩\right)$ 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.

Atomistic simulations of the interaction of a screw dislocation in $\alpha \text{-Fe}$ with different size bcc Cu precipitates suggest two plausible strengthening mechanisms. For precipitate diameters in the range $1.5\text{nm}\le d\le 3.3\text{nm}$, the dislocation core structure within the Cu precipitate undergoes a polarized to nonpolarized transformation, leading to the dislocation pinning at the precipitate-matrix interface and the bowing out of the dislocation line. The calculated bow-out angle and resolved shear stress required to detach the dislocation from the precipitate are in agreement with recent experiments. The structural transition of larger $\left(d\ge 3.3\text{nm}\right)$ Cu precipitates under high shear stress is responsible for the loss of slip systems and hence for dislocation pinning.