1,720,955 research outputs found
First-principles calculations on the stabilization of iron carbides (Fe3C, Fe5C2 and ?-Fe2C) in steels
No full textFirst-principles Density Functional Theory (DFT) based calculations are used to quantitatively study the effect of alloying elements on the stability of iron carbides (Fe3C, Fe5C2 and ?-Fe2C) that form in steels.Materials Science & EngineeringMechanical, Maritime and Materials Engineerin
Interstitials in tetrahedrally close-packed phases: C, N, O, and F in ?-tungsten from first principles
No full textSeveral tetrahedrally close-packed (tcp) phases, such as ?-W, ?-Ta, and ?-U, are believed to be stabilized by impurities. Here we analyze the various ways in which impurities can be dissolved in tcp structures paying special attention to interstitial configurations in the ?-W (A15) structure. We find that at most there are only seven interstitial positions possible. Through ab initio calculations we show that common impurities such as C, N, O, and F, dissolve interstitially in ?-W and that N, O, and F interstitials prefer the same position in the ?-W (A15) structure.Materials Science and EngineeringMechanical, Maritime and Materials Engineerin
Site occupation in the Cr-Ru and Cr-Os ? phases
No full textThe site occupation in the Cr-Ru and Cr-Os ? phases is computed as a function of temperature. Generally, in ? phases the larger atoms occupy the sites with larger coordinations numbers, as can be explained on the basis of atomic-size and electronic structure. However, for Cr2Ru and Cr2Os the atomic-size argument predicts that Ru and Os occupy the sites with larger coordination numbers, whereas the reasoning based on the approximate degeneracies of electronic levels predicts that Cr occupies those sites. By comparing these predictions with the theoretically computed and the experimentally measured site occupations, the atomic-size and electronic arguments can be judged on their predictive merits.Materials Science and EngineeringMechanical, Maritime and Materials Engineerin
Ab initio prediction of vacancy properties in concentrated alloys: The case of fcc Cu-Ni
No full textVacancy properties in concentrated alloys continue to be of great interest because nowadays ab initio supercell simulations reach a scale where even defect properties in disordered alloys appear to be within reach. We show that vacancy properties cannot generally be extracted from supercell total energies in a consistent manner without a statistical model. Essential features of such a model are knowledge of the chemical potential and imposition of invariants. In the present work, we derive the simplest model that satisfies these requirements and we compare it with models in the literature. As illustration we compute ab initio vacancy properties of fcc Cu-Ni alloys as a function of composition and temperature. Ab initio density functional calculations were performed for SQS supercells at various compositions with and without vacancies. Various methods of extracting alloy vacancy properties were examined. A ternary cluster expansion yielded effective cluster interactions (ECIs) for the Cu-Ni-Vac system. Composition and temperature dependent alloy vacancy concentrations were obtained using statistical thermodynamic models with the ab initio ECIs. An Arrhenius analysis showed that the heat of vacancy formation was well represented by a linear function of temperature. The positive slope of the temperature dependence implies a negative configurational entropy contribution to the vacancy formation free energy in the alloy. These findings can be understood by considering local coordination effects.Materials Science and EngineeringMechanical, Maritime and Materials Engineerin
Cluster Expansions for Thermodynamics and Kinetics of Multicomponent Alloys
No full textCluster expansions have proven a very useful tool to model thermodynamics and kinetics of substitutional alloys in metallic, ionic, and even covalently bonded systems. Cluster expansions are usually obtained with the structure inversion method in which the energies, or other relevant property, of a set of structures are used to obtain expansion coefficients. The expansion coefficients are multipliers of correlation functions which pertain to clusters of sites on the parent lattice. There are significant practical issues associated with obtaining a cluster expansion, such as selecting which structures and especially which correlation functions are required for an adequate description of the energy. While these issues are significant for binary alloys, they become much more daunting when dealing with multicomponent alloys. Moreover, oftentimes interest is not limited to the energetics of the thermodynamic equilibrium state, but the evolution of quenched alloys with time is just as important. The treatment of diffusion within the context of cluster expansions is then another challenge. The article describes a formal method for utilizing cluster expansions for transition states as occur during vacancy mediated diffusion in substitutional alloys. The methods are illustrated with some applications to the prediction of initial coherent precipitates in Al-Cu and Al-Mg-Si alloys.Materials Science and EngineeringMechanical, Maritime and Materials Engineerin
Molecular dynamics simulation of graphene on Cu (1 0 0) and (1 1 1) surfaces
Full text linkAccepted Author Manuscript(OLD) MSE-7Micro and Nano Engineerin
Kinetically driven ordering in phase separating alloys
No full textIt is shown that in substitutional alloys, peculiar ordered patterns can result from neighborhood-dependent diffusion activation barriers even when there are no metastable ordered phases. Lattice gases with pure phase separation character are shown to exhibit transient ordered structures that can be retained almost indefinitely, although these structures are not at thermodynamic equilibrium. It is shown that such structures can come about relatively easily by quenching from the high-temperature configurationally random solid solution.(OLD) MSE-
Early stage precipitation in aluminum alloys: An ab initio study
No full textMultiscale computational materials science has reached a stage where many complicated phenomena or properties that are of great importance to manufacturing can be predicted or explained. The word “ab initio study” becomes commonplace as the development of density functional theory has enabled the predictions to be independent of experimental data or empirical parameters. For some crucial phenomena, e.g., precipitation processes in multicomponent alloys, however, challenges exist due to the requirement of an accurate and efficient description of both energetics and kinetics of a complex system. In the present thesis, a systematic methodology has been established for predicting the morphology and realistic formation kinetics of precipitates in multicomponent alloys. Aluminum alloys are chosen as prototype applications of the present methodology, because of the well-known strengthening mechanism—age or precipitation hardening which is a typical and important precipitation process utilized in industrial materials. As one of the main computational approaches, cluster expansion technique is applied to study vacancy properties in concentrated Cu-Ni alloys. Diffusion kinetics in dilute Al-Cu alloys including the role of multiple diffusion barriers has been investigated by kinetic Monte Carlo simulations. At finite temperature, electronic entropy contribution to the free energies of the transition metals is also discussed.(OLD) MSE-
Modelling self healing creep steel
No full textAdvances in self-healing creep steels are driven by the understanding of its mechanism. Previous work on self-healing creep metals has shown selective precipitation inside the creep cavities, which has a beneficial effect on the creep lifetime. This effect occurswhen a solute supersaturation exists at the creep temperature. In the case of self-healing creep steels, the precipitation of gold was found to have an exceptional effect. Gold precipitation was found to be efficient because of the atomic size mismatch of gold and iron, which induces a high selectivity for precipitates to form inside creep cavities and not in the bulk. However, gold is not the most widely used alloying element in steel, to put it mildly. Ideally the self-healing effect could be achieved by another solute element dissolved in steel. In order to identify some other possible solute element for self healing creep steels, the mechanism must be understood. In this thesis the mechanism of self healing creep steels, andmetals in general, is investigated.RST/Fundamental Aspects of Materials and EnergyMaterials Science and Engineerin
First principles phase diagram calculation for the 2D TMD system WS<sub>2</sub>−WTe<sub>2</sub>
No full textFirst principles phase diagram calculations, that included van der Waals interactions, were performed for the bulk transition metal dichalcogenide system (1−X)·WS2−(X)·WTe2. To obtain a converged phase diagram, a series of cluster expansion calculations were performed with increasing numbers of structural energies, (Nstr) up to Nstr=435, used to fit the cluster expansion Hamiltonian. All calculated formation energies are positive and all ground-state analyses predict that formation energies for supercells with 16 or fewer anion sites are positive; but when 150⪅Nstr⪅376, false ordered ground-states are predicted. With Nstr≥399, only a miscibility gap is predicted, but one with dramatic asymmetry opposite to what one expects from size-effect considerations; i.e. the calculations predict more solubility on the small-ion S-rich side of the diagram and less on the large-ion Te-rich side. This occurs because S-rich low-energy metastable ordered configurations have lower energies than their Te-rich counterparts which suggests that elastic relaxation effects are not dominant for the shape of the miscibility gap.Accepted Author Manuscript(OLD) MSE-
- …
