Atomic-scale investigation of triple junction role on defects binding energetics and structural stability in α-Fe

Nanocrystalline (NC) metals (mean grain sizes d ≤ 100 nm) have enhanced mechanical strength as compared to coarse-grained metals (d ≥ 1 μm), and thus, are a promising alternative as structural materials for future high energy nuclear reactors. However, during extreme conditions, the NC microstructure has been found to be thermodynamically unstable, thereby limiting its applicability. Further, for materials with average grain size <10 nm, the triple junctions (TJs) have been observed to have a significant contribution on the mechanical behavior and microstructural stability. Moreover, at the atomic-scale, the region surrounding the TJ demonstrates unique physical properties, such as rapid diffusion, non-equilibrium segregation and increased dislocation activity. Therefore, in this work, we systematically assess the role of TJs on the structural stability and the solute binding behavior in α-Fe. Using atomistic simulations, we show that the TJ resolved line tension is strongly correlated (inversely) with the mean activation energy for self-diffusion along the TJ, i.e., the thermodynamically unstable TJs demonstrated a lower activation energy barrier for self-diffusion along TJs with higher line tension. Next, we demonstrated that the strain energy evolution around the TJ can provide insights into the distinct binding behavior of point defects and solute atoms. In other words, the examination of solute binding behavior revealed a localized region of stable sites around the TJs which aids in accommodation of high solute concentration at high temperatures. In summary, our findings quantify the distinct role of TJs on the defect (vacancy, self-interstitial and solute atom) binding and migration behavior and these findings are necessary for designing future structural materials for extreme environments, including those needed in aerospace, naval, civilian and energy sector infrastructures.