[en] A multiscale approach is proposed to predict how the presence of hydrogen influences the onset of homogeneous dislocation nucleation (HDN) and thus of plasticity. The model takes inputs that can be solely obtained from atomistic calculations, such as dislocation core structure, stacking fault energy and hydrogen–hydrogen interaction. The equilibrium hydrogen concentration around the dislocation loop is calculated using a recently developed self-consistent iterative method [1]. The complex nature of the dislocation field, as well as the equilibrium hydrogen concentration around the loops, is taken into account. The onset of HDN as a function of bulk hydrogen concentration and temperature is quantitatively predicted and is consistent with nano-indentation experiments on hydrogen loaded samples. Applying the approach to Ni, we find that even low hydrogen concentrations of about 1 at-% result in largely reduced HDN energy barriers and thus largely reduce the critical shear stress.

[en] A multiscale simulation approach based on atomistic calculations and a discrete diffusion model is developed and applied to carbon-supersaturated ferrite, as experimentally observed in severely deformed pearlitic steel. We employ the embedded atom method and the nudged elastic band technique to determine the energetic profile of a carbon atom around a screw dislocation in bcc iron. The results clearly indicate a special region in the proximity of the dislocation core where C atoms are strongly bound, but where they can nevertheless diffuse easily due to low barriers. Our analysis suggests that the previously proposed pipe mechanism for the case of a screw dislocation is unlikely. Instead, our atomistic as well as the diffusion model results support the so-called drag mechanism, by which a mobile screw dislocation is able to transport C atoms along its glide plane. Combining the C-dislocation interaction energies with density-functional-theory calculations of the strain dependent C formation energy allows us to investigate the C supersaturation of the ferrite phase under wire drawing conditions. Corresponding results for local and total C concentrations agree well with previous atom probe tomography measurements indicating that a significant contribution to the supersaturation during wire drawing is due to dislocations.

[en] Ni₂MnGa is a typical example of a Heusler alloy that undergoes a martensitic transformation. In the high temperature austenitic phase it has a cubic L2₁ structure, whereas below 200 K the symmetry is reduced by an orthorhombic distortion. Despite lattice deformations of more than 6% and large strains connected to this change, it is completely reversible. Therefore, Ni₂MnGa serves as a shape memory compound. The fact that Ni₂MnGa additionally orders ferromagnetically below 360 K makes the material particularly attractive for applications in actuators and sensors. Nevertheless, its structural details in the martensitic phase are still a subject of much debate. Several shuffling structures have been observed experimentally. The temperature and magnetic field dependent transformations between these structures need to be understood for improvement of the magnetic switching (e.g. operation with higher reliability and smaller magnetic fields). Our tool for identifying the stable structures and the low energy transition paths is the calculation of free energy surfaces as functions of key reaction coordinates (e.g. the ratio c/a) in density functional theory. (The generalized gradient approximation to the exchange-correlation functional and the projector augmented wave approach implemented in VASP (Vienna Ab initio Simulation Package) were used in these investigations.) The different variants of the low symmetry orthorhombic structures lead to characteristic minima on this surface. However, the ab initio determination of the experimentally observed shuffling structures is challenging, due to the large phase space of possible atomic positions and the small shuffling formation energies of only a few meV per unit cell. Hence, we used the quasiharmonic approximation in order to compute and analyze phonon spectra. Starting with the symmetric structure of the austenite, the TA₂ (TA standing for transverse acoustic) phonon dispersion shows a phonon softening along the [110] direction. We were able to extract detailed information about the type of this lattice instability from the eigenvectors of the unstable phonon modes. By setting up the corresponding modulated harmonics in supercell calculations, we systematically and efficiently identified stable shuffling structures. The resulting structural phases (austenite, martensite, pre-martensite) allow us to assign and to interpret the experimental observations

[en] HfO₂ is a prominent candidate for applications necessitating the use of high-k dielectrics. It could e.g. replace SiO₂ layers in small-sized (DE) metal-oxide semiconductor devices (MOS). Thus the electrical and structural properties of HfO₂ have to be investigated. Using first principles methods, the properties of charged oxygen vacancies in cubic HfO₂ have been analysed, because they are probably one of the major traps contributing to charge trapping in HfO₂. Although the cubic phase is the high temperature phase, it is frequently found in HfO₂ thin films. With the help of supercell calculations the formation energy of differently charged oxygen vacancies in this phase are determined. This will help to understand the nature of defects in this material and consequently to understand the electrical properties. Differently charged states (-2, -1, 0, +1 and +2) of the defect systems have been relaxed with an optimisation algorithm and a bandstructure calculation of HfO₂ bulk has been performed. With these calculations we intend to study the nature of the defect levels in the band gap, e.g. whether they show negative-U behaviour. This can help to understand the electronic transport mechanism in HfO₂

[en] Two different mechanisms have been reported in previous ab initio studies to describe basal slip in complex intermetallic Laves phases: synchroshear and undulating slip. To date, no clear answer has been given on which is the energetically favourable mechanism and whether either of them could effectively propagate as a dislocation. Using classical atomistic simulations supported by ab initio calculations, the present work removes the ambiguity and shows that the two mechanisms are, in fact, identical. Furthermore, we establish synchroshear as the mechanism for propagating dislocations within the basal plane in Laves phases.

[en] Partitioning of Cr and Si between cementite and ferrite was investigated by first-principles thermodynamics taking into account vibrational, electronic, and magnetic Gibbs energy contributions. At finite temperatures, these contributions lower the partitioning Gibbs energy and compete with the configurational entropy, which favors impurity segregation to ferrite due to its larger volume fraction compared to cementite. Due to the large positive partitioning enthalpy contribution of Si at T = 0 K, partitioning of Si to cementite is virtually absent in agreement with experiment. The situation is drastically different for Cr impurities. Incorporation of finite-temperature effects resolves the discrepancy between experimental observations and previous T = 0 K first-principles calculations. Cr strongly segregates to cementite due to the enhanced magnetic entropy of cementite above 400 K, i.e., near the Curie temperature of cementite. The increasing magnetic fluctuations in ferrite cause a strong reduction of the partitioning coefficient in the temperature range from 773 to 973 K in qualitative agreement with experiment. Quantitative agreement with CALPHAD data and experimental data for equilibrium Cr concentrations in a wide range of alloy compositions is achieved by renormalizing the theoretical magnetic partitioning Gibbs energy by a constant scaling factor.

[en] Density-functional formalism is applied to study the ground state properties of γ-U-Zr and γ-U-Mo solid solutions. Calculated heats of formation are compared with CALPHAD assessments. We discuss how the heat of formation in both alloys correlates with the charge transfer between the alloy components. The decomposition curves for γ-based U-Zr and U-Mo solid solutions are derived from Ising-type Monte Carlo simulations. We explore the idea of stabilization of the δ-UZr₂ compound against the α-Zr (hcp) structure due to increase of Zr d-band occupancy by the addition of U to Zr. We discuss how the specific behavior of the electronic density of states in the vicinity of the Fermi level promotes the stabilization of the U2Mo compound. The mechanism of possible Am redistribution in the U-Zr and U-Mo fuels is also discussed. (author)

[en] Field ion microscopy allows for direct imaging of surfaces with true atomic resolution. The high charge density distribution on the surface generates an intense electric field that can induce ionization of gas atoms. We investigate the dynamic nature of the charge and the consequent electrostatic field redistribution following the departure of atoms initially constituting the surface in the form of an ion, a process known as field evaporation. We report on a new algorithm for image processing and tracking of individual atoms on the specimen surface enabling quantitative assessment of shifts in the imaged atomic positions. By combining experimental investigations with molecular dynamics simulations, which include the full electric charge, we confirm that change is directly associated with the rearrangement of the electrostatic field that modifies the imaging gas ionization zone. We derive important considerations for future developments of data reconstruction in 3D field ion microscopy, in particular for precise quantification of lattice strains and characterization of crystalline defects at the atomic scale. (paper)

[en] Graphical abstract: Temperature dependence of the vibrational entropy S"v"i"b (left), the electronic entropy S"e"l (middle), and the magnetic entropy S"m"a"g (right) in the hcp (dash-dotted lines), fcc (solid lines), and bcc (dashed lines) structure for the NM (black, non-magnetic), FM (red, ferromagnetic), and DLM (blue, disordered local moments) state. Gray horizontal lines indicate the configurational entropy S"c"o"n"f. Display Omitted - Abstract: We investigate the thermodynamic properties of the prototype equi-atomic high entropy alloy (HEA) CoCrFeMnNi by using finite-temperature ab initio methods. All relevant free energy contributions are considered for the hcp, fcc, and bcc structures, including electronic, vibrational, and magnetic excitations. We predict the paramagnetic fcc phase to be most stable above room temperature in agreement with experiment. The corresponding thermal expansion and bulk modulus agree likewise well with experimental measurements. A careful analysis of the underlying entropy contributions allows us to identify that the originally postulated dominance of the configurational entropy is questionable. We show that vibrational, electronic, and magnetic entropy contributions must be considered on an equal footing to reliably predict phase stabilities in HEA systems.

[en] Multicomponent alloys with multiple principal elements including high entropy alloys (HEAs) and compositionally complex alloys (CCAs) are attracting rapidly growing attention. The endless possibilities to explore new alloys and the hope for better combinations of materials properties have stimulated a remarkable number of research works in the last years. Most of these works have been based on experimental approaches, but ab initio calculations have emerged as a powerful approach that complements experiment and serves as a predictive tool for the identification and characterization of promising alloys. The theoretical ab initio modeling of phase stabilities and mechanical properties of multi-principal element alloys by means of density functional theory (DFT) is reviewed. A general thermodynamic framework is laid down that provides a bridge between the quantities accessible with DFT and the targeted thermodynamic and mechanical properties. It is shown how chemical disorder and various finite-temperature excitations can be modeled with DFT. Different concepts to study crystal and alloy phase stabilities, the impact of lattice distortions (a core effect of HEAs), magnetic transitions, and chemical short-range order are discussed along with specific examples. Strategies to study elastic properties, stacking fault energies, and their dependence on, e.g., temperature or alloy composition are illustrated. Finally, we provide an extensive compilation of multi-principal element alloys and various material properties studied with DFT so far (a set of over 500 alloy-property combinations).