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

Abstract

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.

Statistical analysis of the interaction between irradiation-induced defects and triple junctions

“…However, the resulting vacancy-dominated grain interiors for the two crystals (with distinct stacking fault energies) then evolved into distinct defects, with fcc forming stacking fault tetrahedra and bcc forming large vacancy clusters. Similarly, our conclusions and observations are in agreement with those from Adlakha and Solanki [11] on the role of the TJ strain energy for the aggregation of point defects.…”

“…From the perspective of the microstructural defect management, GB networks, in terms of the character of triple junctions, represent unique features that serve as paths for diffusion/transport and regulate damage accumulation [8][9][10]. For example, Adlakha and Solanki [11] used atomistic simulations to illustrate the distinct role of TJs structures on defect binding and migration behavior in α-Fe for a couple of TJ configurations, demonstrating that interstitials preferably segregate at/near TJs over vacancy defects.…”

Theoretical Prediction of Grain Boundary Segregation Using Nano-Polycrystalline Grain Boundary Model

“…The effect of GB triple junctions is overestimated due to the use of the nano-polycrystalline GB model. It has been reported that the amount of segregation of GB triple junctions differs from that of GBs, 36) which may lead to errors in the prediction of the segregation amount using the nanopolycrystalline GB model. According to the equation proposed by Palumbo et al, 37) the ratio of the triple junctions to GBs was approximately 20% when the GB width was defined as 0.5 nm and the average grain size was 5.3 nm.…”

Revealing the role of nitrogen on hydride nucleation and stability in pure niobium using first-principles calculations

“…a concentration of 1/16. The binding energy (∆E) for hydrogen at different interstitial sites in niobium with a nitrogen atom located at an octahedral site was calculated using the following equation (see [35][36][37]):…”