Pinning of dislocations in disordered alloys: effects of dislocation orientation

The current interest in compositionally complex alloys including so called high entropy alloys has caused renewed interest in the general problem of solute hardening. It has been suggested that this problem can be addressed by treating the alloy as an effective medium containing a random distribution of dilatation and compression centers representing the volumetric misfit of atoms of different species. The mean square stresses arising from such a random distribution can be calculated analytically, their spatial correlations are strongly anisotropic and exhibit long-range tails with third-order power law decay (Geslin and Rodney 2021; Geslin et al. 2021). Here we discuss implications of the anisotropic and long-range nature of the correlation functions for the pinning of dislocations of arbitrary orientation. While edge dislocations are found to follow the standard pinning paradigm, for dislocations of near screw orientation we demonstrate the co-existence of two types of pinning energy minima.

Chemically induced local lattice distortions versus structural phase transformations in compositionally complex alloys

Recent experiments show that the chemical composition of body-centered cubic (bcc) refractory high entropy alloys (HEAs) can be tuned to enable transformation-induced plasticity (TRIP), which significantly improves the ductility of these alloys. This calls for an accurate and efficient method to map the structural stability as a function of composition. A key challenge for atomistic simulations is to separate the structural transformation between the bcc and the ω phases from the intrinsic local lattice distortions in such chemically disordered alloys. To solve this issue, we develop a method that utilizes a symmetry analysis to detect differences in the crystal structures. Utilizing this method in combination with ab initio calculations, we demonstrate that local lattice distortions largely affect the phase stability of Ti–Zr–Hf–Ta and Ti–Zr–Nb–Hf–Ta HEAs. If relaxation effects are properly taken into account, the predicted compositions near the bcc–hcp energetic equilibrium are close to the experimental compositions, for which good strength and ductility due to the TRIP effect are observed.

Free Energy of Metals from Quasi-Harmonic Models of Thermal Disorder

A simple modeling method to extend first-principles electronic structure calculations to finite temperatures is presented. The method is applicable to crystalline solids exhibiting complex thermal disorder and employs quasi-harmonic models to represent the vibrational and magnetic free energy contributions. The main outcome is the Helmholtz free energy, calculated as a function of volume and temperature, from which the other related thermophysical properties (such as temperature-dependent lattice and elastic constants) can be derived. Our test calculations for Fe, Ni, Ti, and W metals in the paramagnetic state at temperatures of up to 1600 K show that the predictive capability of the quasi-harmonic modeling approach is mainly limited by the electron density functional approximation used and, in the second place, by the neglect of higher-order anharmonic effects. The developed methodology is equally applicable to disordered alloys and ordered compounds and can therefore be useful in modeling realistically complex materials.

Evolution of dislocation substructure under deformation of ordered and disordered Pd3Fe alloy in the region of low stable structural-phase states

The results of an electron microscopy study of the evolution of the dislocation structure of a polycrystalline ordered and disordered Pd3Fe alloy in the region of weakly stable structural-phase States are presented. The scheme of rearrangement of dislocation substructures during the transition from stage to stage, which are highlighted on the deformation curves of the Pd3Fe alloy, is constructed. It is established that in the case of both disordered and ordered alloys, each stage of deformation is characterized by its own special types of dislocation substructure (DSS), which are the main carriers of deformation for this stage. Transitions from some types of DSS to other types occur in certain ranges of values of the degree of deformation ε. The appearance of the DSS type characteristic of this stage of deformation occurs at the previous stage, and as the degree of deformation increases, the proportion of this type of DSS increases. At the stage under consideration, their share is the largest, and when moving to the next stage, it gradually decreases until it disappears. In the case of ordered alloys, the types of dislocation substructure-the main carrier of deformation for this stage differ from the types of DSS that are implemented in disordered alloys at the same stage of deformation. It is shown that each stage of deformation has its own DSS - strain carriers. When moving to a new stage, the transition to new structural carriers of deformation occurs. During the transition, these carriers co-exist, which is a characteristic feature of weakly stable States of the system.

Investigation of thermodynamic properties of single-layer two-component coatings

The results of studying the dynamics of disordered alloys by the method of collective variables are presented. The densities of phonon states of disordered alloys with BCC structure of the kxrb1-x system are calculated. The contribution of phonons to the free energy and entropy of the alloy at different temperatures is determined. It is shown that the contribution of phonons to the total free energy of the alloy decreases with decreasing temperature. The contribution of phonons to the entropy of alloys at 300 K is crucial.

Research of Mechanical Properties of Random Disordered CuNi Alloys

Preparation technique and structural analysis of random CuNi disordered alloys have been discussed. The arcmelting method is used to prepare different compositions of substitutional random disordered Cu1-x Nix (0.1, 0.3, 0.5, 0.7, 0.9) alloys. The stoichiometric amounts of highly purity constituents copper and nickel metals 5 N (99.999%) have been melted under argon atmosphere in vacuum chamber of 10−3 torr. The substitutional random disordered alloys free from carbon and oxygen traces are confirmed from XPS data. A lattice strain is produced in CuNi alloys as the environment of Ni atoms change from sites to sites. Lattice parameters, unit cell volume, structure and inter-planar spacing were calculated from XRD analysis. The average crystallite size of different compositions of random disordered CuNi alloys is calibrated by using Scherer’s method and Williamson-Hall (W-H) method. The roles of crystallite size and lattice strain on the XRD peak broadening of the random disordered CuNi alloys were analyzed. The strain increases with increase in concentration of Ni and exhibits a maximum of 0.00247 at 50% Ni concentration. The CuNi alloys find very wide applications in oil refining and long corrosion free life.

Ordered Intermetallic Nanoparticles with High Catalytic Activity Prepared by an Electrochemically Induced Phase Transformation

<div> <p>The synthesis of alloys with long range atomic scale ordering (ordered intermetallics) is an emerging field of nanochemistry. Ordered intermetallic nanoparticles are useful for a wide variety of applications such as catalysis, superconductors, and magnetic devices. However, the preparation of nanostructured ordered intermetallics is challenging in comparison to disordered alloys, hindering progress in materials development. We report a process for converting colloidally synthesized ordered intermetallic PdBi₂ to ordered intermetallic Pd₃Bi nanoparticles under ambient conditions by an electrochemically induced phase transition. The low melting point of PdBi₂ corresponds to low vacancy formation energies which enables the facile removal of the Bi from the surface, while simultaneously enabling interdiffusion of the constituent atoms via a vacancy diffusion mechanism under ambient conditions. The resulting phase-converted ordered intermetallic Pd₃Bi exhibits 11x and 3.5x higher mass activty and high methanol tolerance for the oxygen reduction reaction compared to Pt/C and Pd/C, respectively,which is the highest reported for a Pd-based catalyst, to the best of our knowledge. These results establish a key development in the synthesis of noble metal rich ordered intermetallic phases with high catalytic activity, and sets forth guidelines for the design of ordered intermetallic compounds under ambient conditions.</p> </div>