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Reconciling experimental and theoretical stacking fault energies in face-centered cubic materials with the experimental twinning stress
Full text linkStacking fault energy and twinning stress are thought to be closely correlated. All currently available models predict a monotonous decrease in twinning stress with decreasing stacking fault energy and depart from the assumption that the intrinsic stacking fault energy has a positive value. Opposite to this prediction, for medium- and high-entropy alloys the twinning stress was shown to increase with decreasing SFE. Additionally, for metastable materials, first principles methods predict negative intrinsic stacking fault energy values, whilst experimentally determined values are always positive. In the present communication, it is postulated that the twinning stress scaled by the Burgers vector bridges the difference between intrinsic and experimentally measured stacking fault energy. The assumption is tested for Cu-Al alloys, for pure metals and for medium- and high-entropy alloys and, for the first time, provides a consistent quantitative interpretation of data for both alloys with positive and negative stacking fault energy.</p
Ab initio study of the effect of interstitial alloying on the intrinsic stacking fault energy of paramagnetic γ-Fe and austenitic stainless steel
Full text linkIntrinsic stacking fault energy (SFE) values of γ-Fe and AISI 304 austenitic stainless steels were determined as a function of carbon and nitrogen content using ab initio calculations. In contrast to previous investigations, the analysis was conducted incorporating the paramagnetic state to account for the magnetic constitution of real austenitic stainless steels. The effect of finite temperature was partially accounted for by performing ab initio calculations at the experimental volumes at room temperature. Including paramagnetism in γ-Fe increases the SFE of non-magnetic γ-Fe by ∼385 mJ.m−2. Interstitial alloying of non-magnetic γ-Fe causes a linear increase in intrinsic stacking fault energy wγith interstitial content. In comparison, interstitial alloying of paramagnetic γ-Fe increases the SFE at only about half the rate. The SFE of paramagnetic interstitial-free AISI 304 is within the range of -12 to 0 mJ.m−2 and only deviates slightly from the SFE of paramagnetic γ-Fe. It follows a similar, albeit flatter linear dependency on the interstitial content compared to γ-Fe. Both γ-Fe and γ-AISI 304 were found to be metastable in their interstitial-free condition and are stabilized by interstitial alloying. The possible effect of short range ordering between interstitials and Cr on the SFE was discussed. The calculated threshold nitrogen content necessary to stabilize austenite in AISI 304 is in good agreement with experimental investigations of deformation microstructures in dependence of the nitrogen content. Finally, the calculated negative SFE values of AISI 304 were reconciled with experimentally determined positive SFE values using a recent method that accounts for the kinetics of stacking fault formation
Efficient ab initio stacking fault energy mapping for dilute interstitial alloys
Full text linkDensity Functional Theory (DFT) is the prevalent first principles computational method for determining the stacking fault energy (SFE) of face centered cubic (fcc) metals and alloys. Due to several theoretical and computational challenges, SFE determination for interstitial alloys with alloying elements such as carbon, nitrogen, and hydrogen, has so far been limited to few studies at relatively high interstitial content. We propose a new method, rooted in the axial interaction model, that allows rapid and robust mapping of SFE for virtually arbitrary interstitial contents. Instead of computing the total energy of a very large supercell to represent dilute interstitial solutions, representative interstitial-affected and bulk regions are treated separately at the equivalent volume. The SFE is obtained by balancing the SFE values of the regions with a lever rule approach. The method matches SFE values from the axial interaction model within ≤4 mJ.m−2 error, as validated for non-magnetic fcc Fe-N and paramagnetic fcc Fe-N and AISI 304 alloys. The significantly reduced computational workload and equidistant SFE mapping vs. interstitial content down to extremely low values allows accurate fitting of the SFE vs. interstitial content with only few datapoints. This further improves the computational efficiency. So far DFT-based SFE mapping was limited to purely substitutional alloys; we demonstrate the first-time DFT-based SFE mapping in fcc AISI 304 vs. N and Ni, revealing a non-additive contribution of N and Ni to the SFE. Finally, the remaining challenges and future application for high-throughput DFT SFE computation in interstitial alloys is discussed
Total-energy method based on the exact muffin-tin orbitals theory
No full textI present a total-energy method based on the exact muffin-tin orbitals (EMTO) theory and the full charge density (FCD) technique. The FCD-EMTO method combines the accuracy of the full-potential method and the efficiency of the muffin-tin potential method. The one-electron Kohn-Sham equations are solved exactly for the overlapping muffin-tin potential and from the self-consistent solutions the full charge density is constructed. The EMTO kinetic energy, combined with the Coulomb and exchange-correlation terms calculated from the total density, yields the FCD-EMTO total energy. The accuracy of the FCD-EMTO method is demonstrated through several test calculations.</p
Computational Quantum Mechanics for Materials Engineers The EMTO Method and Applications
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