First-principles local stress in crystalline and amorphous metals

Medium-range atomic correlation in simple liquids. II. Theory of temperature dependence

The spatial atomic correlations in liquids and glasses extend often significantly beyond the nearest neighbors. Such correlations, called the medium-range order (MRO), affect many physical properties, but their nature is not well understood. In this article the variation of the MRO with temperature is calculated based upon the concept of the atomic-level pressure, focusing on simple liquids, such as metallic liquids. It is shown that the structural coherence length that characterizes MRO follows the Curie-Weiss law with a negative Curie temperature as observed by experiment and simulation. It is also shown that the glass transition is induced by freezing of the MRO, rather than the freezing of the nearest-neighbor shell. The implications of these results are discussed.

Precise Characterization of the Rich Structural Landscape Induced by Pressure in Multifunctional FeVO4

We have studied the high-pressure behavior of FeVO₄ by means of single-crystal X-ray diffraction (XRD) and density functional theory (DFT) calculations. We have found that the structural sequence of FeVO₄ is different from that previously assumed. In particular, we have discovered a new high-pressure phase at 2.11(4) GPa (FeVO₄-I'), which was not detected by previous powder XRD studies. We have determined that FeVO₄, under compression (at room temperature), first transforms at 2.11(4) GPa from the ambient-pressure triclinic structure (FeVO₄-I) to a second previously unknown triclinic structure (FeVO₄-I'), which experiences a subsequent phase transition at 4.80(4) GPa to a monoclinic structure (FeVO₄-II'), which was also previously detected in powder XRD experiments. Single-crystal XRD has enabled these novel findings as well as an accurate determination of the crystal structure of FeVO₄ polymorphs under high-pressure conditions. The crystal structure of all polymorphs has been accurately solved at all measured pressures. The pressure dependence of the unit-cell parameters and polyhedral coordination have been obtained and are discussed. The room-temperature equation of state and the principal axes of the isothermal compressibility tensor of FeVO₄-I and FeVO₄-I' have also been determined. The structural phase transition observed here between these two triclinic structures at 2.11(4) GPa implies abrupt coordination polyhedra modifications, including coordination number changes. DFT calculations support the conclusions extracted from our experiments.

Shear Softening in a Metallic Glass: First-Principles Local-Stress Analysis

Metallic glasses deform elastically under stress. However, the atomic-level origin of elastic properties of metallic glasses remain unclear. In this Letter using ab initio molecular dynamics simulations of the Cu_{50}Zr_{50} metallic glass under shear strain, we show that the heterogeneous stress relaxation results in the increased charge transfer from Zr to Cu atoms, enhancing the softening of the shear modulus. Changes in compositional short-range order and atomic position shifts due to the nonaffine deformation are discussed. It is shown that the Zr subsystem exhibits a stiff behavior, whereas the displacements of Cu atoms from their initial positions, induced by the strain, provide the stress drop and softening.

Engineering atomic-level complexity in high-entropy and complex concentrated alloys

Quantitative and well-targeted design of modern alloys is extremely challenging due to their immense compositional space. When considering only 50 elements for compositional blending the number of possible alloys is practically infinite, as is the associated unexplored property realm. In this paper, we present a simple property-targeted quantitative design approach for atomic-level complexity in complex concentrated and high-entropy alloys, based on quantum-mechanically derived atomic-level pressure approximation. It allows identification of the best suited element mix for high solid-solution strengthening using the simple electronegativity difference among the constituent elements. This approach can be used for designing alloys with customized properties, such as a simple binary NiV solid solution whose yield strength exceeds that of the Cantor high-entropy alloy by nearly a factor of two. This study provides general design rules that enable effective utilization of atomic level information to reduce the immense degrees of freedom in compositional space without sacrificing physics-related plausibility.

A simple numerical approach for reconstructing the atomic stresses at grain boundaries from quantum-mechanical calculations

The atomistic stress state at a metal grain boundary is an intrinsic attribute which affects many physical and mechanical properties of the metal. While the virial stress is an accepted measure of the atomistic stress in molecular dynamics simulations, an equivalent definition is not well-established for quantum-mechanical density functional theory (DFT) calculations. Here, we introduce a numerical technique, termed the sequential atom removal (SAR) approach, to reconstruct the atomic stresses near a symmetrical-tilt Σ5(310)[001] Cu grain boundary. In the SAR approach, individual atoms near the boundary are sequentially removed to compute the pair (reaction) force between atoms, while correcting for changes to the local electron density caused by atom removal. We show that this SAR approach accurately reproduces the spatially-varying virial stresses at a grain boundary governed by an embedded atom method potential. The SAR approach is subsequently used to extract the atomistic stresses of the grain boundary from DFT calculations, from which we reconstruct a continuum-equivalent grain boundary traction distribution as a quantitative descriptor of the grain boundary atomic structure.