Magnesium-yttrium alloys show significantly improved room temperature ductility when compared with pure Mg. We study this interesting phenomenon theoretically at the atomic scale employing quantum-mechanical (so-called ab initio) and atomistic modeling methods. Specifically, we have calculated generalized stacking fault energies for five slip systems in both elemental magnesium (Mg) and Mg-Y alloys using (i) density functional theory and (ii) a set of embedded-atom-method (EAM) potentials. These calculations predict that the addition of yttrium results in a reduction in the unstable stacking fault energy of basal slip systems. Specifically in the case of an I 2 stacking fault, the predicted reduction of the stacking fault energy due to Y atoms was verified by experimental measurements. We find a similar reduction for the stable stacking fault energy of the {11 22} 11 23 non-basal slip system.

We present a scale-bridging approach for modeling the integral elasticresponse of polycrystalline composite that is based on a multi-disciplinary combination of(i) parameter-free first-principles calculations of thermodynamic phase stability andsingle-crystal elastic stiffness; and (ii) homogenization schemes developed forpolycrystalline aggregates and composites. The modeling is used as a theory-guidedbottom-up materials design strategy and applied to Ti-Nb alloys as promising candidatesfor biomedical implant applications. The theoretical results (i) show an excellent agreementwith experimental data and (ii) reveal a decisive influence of the multi-phase character ofthe polycrystalline composites on their integral elastic properties. The study shows thatthe results based on the density functional theory calculations at the atomistic level canbe directly used for predictions at the macroscopic scale, effectively scale-jumping severalorders of magnitude without using any empirical parameters.

Using density-functional theory, we investigate the stability of the half-metallic ground state of magnetite under different strain conditions. The effects of volume relaxation and internal degrees of freedom are fully taken into account. For hydrostatic compression, planar strain in the (001) plane and uniaxial strain along the (001) direction, we derive quantitative limits beyond which magnetite becomes metallic. As a major new result, we identify the bond length between the octahedrally coordinated iron atoms and their neighbouring oxygen atoms as the main characteristic parameter, and we show that the transition occurs if external strain reduces this interatomic distance from 2.06 Å in equilibrium to below a critical value of 1.99 Å. Based on this criterion, we also argue that planar strain due to epitaxial growth does not lead to a metallic state for magnetite films grown on (111)-oriented substrates.

We present a first-principles study of four magnetic phases of iron along a transformation path connecting the bcc and hcp structures at various atomic volumes using the generalized gradient approximation. The total energies and magnetic moments are calculated using the full-potential method and are displayed in contour plots as functions of the parameter of the path and volume; borderlines between various magnetic phases are shown. A low-symmetry antiferromagnetic state with a total energy lower than that of the equilibrium nonmagnetic hcp state is found. A comprehensive picture of the martensitic transition from cubic to orthorhombic iron including both the magnetic and the structural degrees of freedom is presented, and an alternative three-stage mechanism is proposed.

Changes in stoichiometric NiTi allotropes induced by hydrostatic pressure have been studied employing density functional theory. By modeling the pressure-induced transitions in a way that imitates quasistatic pressure changes, we show that the experimentally observed B19$'${} phase is (in its bulk form) unstable with respect to another monoclinic phase, B19$'${}$'${}. The lower symmetry of the B19$'${}$'${} phase leads to unique atomic trajectories of Ti and Ni atoms (that do not share a single crystallographic plane) during the pressure-induced phase transition. This uniqueness of atomic trajectories is considered a necessary condition for the shape memory ability. The forward and reverse pressure-induced transition B19$'${}\,$\leftrightarrow${}\,B19$'${}$'${} exhibits a hysteresis that is shown to originate from hitherto unexpected complexity of the Born-Oppenheimer energy surface.