Size-Dependent Solute Segregation at Symmetric Tilt Grain Boundaries in α-Fe: A Quasiparticle Approach Study

In the present work, atomistic modeling based on the quasiparticle approach (QA) was performed to establish general trends in the segregation of solutes with different atomic size at symmetric ⟨100⟩ tilt grain boundaries (GBs) in α-Fe. Three types of solute atoms X₁, X₂ and X₃ were considered, with atomic radii smaller (X₁), similar (X₂) and larger (X₃) than iron atoms, respectively, corresponding to phosphorus (P), antimony (Sb) and tin (Sn). With this, we were able to evidence that segregation is dominated by atomic size and local hydrostatic stress. For low angle GBs, where the elastic field is produced by dislocation walls, X₁ atoms segregate preferentially at the limit between compressed and dilated areas. Contrariwise, the positions of X₂ atoms at GBs reflect the presence of tensile and compressive areal regions, corresponding to extremum values of the σ and σ components of the strain tensor. Regarding high angle GBs Σ5 (310) (θ = 36.95°) and Σ29 (730), it was found that all three types of solute atoms form Fe₉X clusters within B structural units (SUs), albeit being deformed in the case of larger atoms (X₂ and X₃). In the specific case of Σ29 (730) where the GB structure can be described by a sequence of |BC.BC| SUs, it was also envisioned that the C SU can absorb up to four X₁ atoms vs. one X₂ or X₃ atom only. Moreover, a depleted zone was observed in the vicinity of high angle GBs for X₂ or X₃ atoms. The significance of this research is the development of a QA methodology capable of ascertaining the atomic position of solute atoms for a wide range of GBs, as a mean to highlight the impact of the solute atoms' size on their locations at and near GBs.