Characterization of grain boundary segregation in an Fe-Si alloy

Atomistic and machine learning studies of solute segregation in metastable grain boundaries

The interaction of alloying elements with grain boundaries (GBs) influences many phenomena, such as microstructural evolution and transport. While GB solute segregation has been the subject of active research in recent years, most studies focus on ground-state GB structures, i.e., lowest energy GBs. The impact of GB metastability on solute segregation remains poorly understood. Herein, we leverage atomistic simulations to generate metastable structures for a series of [001] and [110] symmetric tilt GBs in a model Al–Mg system and quantify Mg segregation to individual sites within these boundaries. Our results show large variations in the atomic Voronoi volume due to GB metastability, which are found to influence the segregation energy. The atomistic data are then used to train a Gaussian Process machine learning model, which provides a probabilistic description of the GB segregation energy in terms of the local atomic environment. In broad terms, our approach extends existing GB segregation models by accounting for variability due to GB metastability, where the segregation energy is treated as a distribution rather than a single-valued quantity.

Aluminum depletion induced by co-segregation of carbon and boron in a bcc-iron grain boundary

The local variation of grain boundary atomic structure and chemistry caused by segregation of impurities influences the macroscopic properties of polycrystalline materials. Here, the effect of co-segregation of carbon and boron on the depletion of aluminum at a Σ5 (3 1 0 )[0 0 1] tilt grain boundary in a α - Fe-4 at%Al bicrystal is studied by combining atomic resolution scanning transmission electron microscopy, atom probe tomography and density functional theory calculations. The atomic grain boundary structural units mostly resemble kite-type motifs and the structure appears disrupted by atomic scale defects. Atom probe tomography reveals that carbon and boron impurities are co-segregating to the grain boundary reaching levels of >1.5 at%, whereas aluminum is locally depleted by approx. 2 at.%. First-principles calculations indicate that carbon and boron exhibit the strongest segregation tendency and their repulsive interaction with aluminum promotes its depletion from the grain boundary. It is also predicted that substitutional segregation of boron atoms may contribute to local distortions of the kite-type structural units. These results suggest that the co-segregation and interaction of interstitial impurities with substitutional solutes strongly influences grain boundary composition and with this the properties of the interface.

Interstitial and substitutional solute segregation at individual grain boundaries of α-iron: data revisited

Theoretical calculations (usually density-functional-theory methods performed at 0 K) confirm the formerly assumed substitutional phosphorus segregation in α-iron. In contrast, the enthalpy-entropy compensation effect predicts that phosphorus should segregate interstitially. To resolve this discrepancy, we recalculated the values of the segregation enthalpy and entropy for the interstitial segregation of phosphorus according to the Guttmann model of segregation in multicomponent systems. This recalculation is based on earlier measured experimental data and shows that only slight changes in the values of the standard enthalpy and entropy of phosphorus, carbon and silicon segregation are obtained. Consequently, all dependences constructed previously remain qualitatively the same. By thermodynamic considerations based on the enthalpy-entropy compensation effect we quantitatively show that there is an alteration of the position of phosphorus at grain boundaries in α-Fe with increasing temperature: while substitutional segregation is preferred at 0 K, interstitial segregation occurs at temperatures of practical interest.

Parameterized electronic description of carbon cohesion in iron grain boundaries

We employ a recently developed iron-carbon orthogonal tight-binding model in calculations of carbon in iron grain boundaries. We use the model to evaluate the properties of carbon near and on the Σ5 (3 1 0)[0 0 1] symmetric tilt grain boundary (GB) in iron, and calculations show that a carbon atom lowers the GB energy by 0.29 eV/atom in accordance with DFT. Carbon segregation to the GB is analyzed, and we find an energy barrier of 0.92 eV for carbon to segregate to the carbon-free interface while segregation to a fully filled interface is disfavored. Local volume (via Voronoi tessellation), magnetic, and electronic effects are correlated with atomic energy changes, and we isolate two different mechanisms governing carbon's behavior in iron: a volumetric strain which increases the energy of carbon in interstitial α iron and a non-strained local bonding which stabilizes carbon at the GB.