Xiyue Cheng

Extra-electron induced covalent strengthening and generalization of intrinsic ductile-to-brittle criterion

Traditional strengthening ways, such as strain, precipitation, and solid-solution, come into effect by pinning the motion of dislocation. Here, through first-principles calculations we report on an extra-electron induced covalent strengthening mechanism, which alters chemical bonding upon the introduction of extra-valence electrons in the matrix of parent materials. It is responsible for the brittle and high-strength properties of Al12W-type compounds featured by the typical fivefold icosahedral cages, which are common for quasicrystals and bulk metallic glasses (BMGs). In combination with this mechanism, we generalize ductile-to-brittle criterion in a universal hyperbolic form by integrating the classical Pettifors Cauchy pressure with Pughs modulus ratio for a wide variety of materials with cubic lattices. This study provides compelling evidence to correlate Pughs modulus ratio with hardness of materials and may have implication for understanding the intrinsic brittleness of quasicrystals and BMGs.

Interstitial-boron solution strengthened WB3+x

By means of variable-composition evolutionary algorithm coupled with density functional theory and in combination with aberration-corrected high-resolution transmission electron microscopy experiments, we have studied and characterized the composition, structure, and hardness properties of WB3+x (x < 0.5). We provide robust evidence for the occurrence of stoichiometric WB3 and non-stoichiometric WB3+x, both crystallizing in the metastable hP16 (P63/mmc) structure. No signs for the formation of the highly debated WB4 (both hP20 and hP10) phases were found. Our results rationalize the seemingly contradictory high-pressure experimental findings and suggest that the interstitial boron atom is located in the tungsten layer and vertically interconnect with four boron atoms, thus forming a typical three-center boron net with the upper and lower boron layers in a three-dimensional covalent network, which thereby strengthen the hardness.

Computational materials discovery: the case of the W-B system.

By means of variable-compositional evolutionary algorithms, in combination with first-principles calculations, the compositions, structures and mechanical properties of the W-B system have been theoretically investigated. As well as confirming the experimental observations (including their crystal structures) for the four known compounds W2B, WB, WB2 and WB3, the new stable compound W8B7 and two nearly stable compounds, W2B3 and WB4, have also been predicted in the ground state. The elastic properties and estimated Vickers hardnesses of all these borides have been systematically derived. The results show that, among these borides, hP6-WB2 exhibits the largest ultra-incompressibility along the c axis, with the highest C33 value (953 GPa, comparable with that of the most incompressible diamond). hP16-WB3 exhibits the highest hardness of 36.9 GPa, in good agreement with the experimentally measured data from 28.1 to 43.3 GPa, close to the superhard threshold, and oC8-WB shows the highest bulk modulus of about 350 GPa. The new stable compound W8B7 crystallizes in the monoclinic mP15 phase, with infinite zigzag B chains running parallel to the W-atom layers, resulting in a relatively high estimated hardness of 19.6 GPa. The anisotropic Youngs modulus E and torsion shear modulus G(t) have been derived for both oC8-WB and hP16-WB3. The current state of research and the historic inconsistency of the W-B system are briefly summarized, in particular clarifying the fact that the previous experimentally attributed hP20-WB4 is in fact the defect-containing hP16-WB3.

Crystal structure and magnetic properties of LaCo13−xSix compounds

The crystal structure and magnetic properties of LaCo13-xAlx compounds have been investigated by X-ray powder diffraction and magnetization measurement. X-ray powder diffraction shows that the compounds crystallize in cubic NaZn13-type structure with lattice parameter a=11.345(2) to 11.473(4) Angstrom in the range of x less than or equal to 2.7, and in the tetragonal Ce2Ni17Si9 structure at x=3. Magnetization measurements reveal that both cubic and tetragonal LaCo13-xAlx are ferromagnetic with Curie temperature higher than 840 K, and the magnetic moment per Co atom in LaCo13-xAlx compounds decreases with Al content.

Giant magnetoresistance effect in bulk La1/3Nd1/3Ca1/3MnO3 at low field

A magnetoresistance ratio as high as −96% in a low field of 0.67 T is achieved in bulk La1/3Nd1/3Ca1/3MnO3. The magnetization exhibits a transition from a low moment state to a high moment state with decreasing temperature, accompanied by a thermal hysteresis of the magnetization. The magnetic transition might be an order‐order transition and would be responsible for the observed giant magnetoresistance effect in the compound at low field.

Effects of dilute substitutional solutes on interstitial carbon in α-Fe: Interactions and associated carbon diffusion from first-principles calculations

By means of first-principles calculations coupled with the kinetic Monte Carlo simulations, we have systematically investigated the effects of dilute substitutional solutes on the behaviors of carbon in

Crystal structure and magnetism of LaCo13−x−yFexSiy compounds

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Topological Metal of NaBi with Ultralow Lattice Thermal Conductivity and Electron-phonon Superconductivity

-Fe. Our results uncover the following. (i) Without the Fe vacancy the interactions between most solutes and carbon are repulsive due to the strain relief, whereas Mn has a weak attractive interaction with its nearest-neighbor carbon due to the local ferromagnetic coupling effect. (ii) The presence of the Fe vacancy results in attractive interactions of all the solutes with carbon. In particular, the Mn-vacancy pair shows an exceptionally large binding energy of

Combined fast reversible liquidlike elastic deformation with topological phase transition in Na3Bi

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Dirac Node Lines in Pure Alkali Earth Metals.

0.81 eV with carbon. (iii) The alloying addition significantly impacts the atomic-scale concentration distributions and chemical potential of carbon in the Fe matrix. Among them, Mn and Cr increase the carbon chemical potential, whereas Al and Si reduce it. (iv) Within the dilute scale of the alloying solution, the solute concentration- and temperature-dependent carbon diffusivities demonstrate that Mn has a little impact on the carbon diffusion, whereas Cr (Al or Si) remarkably retards the carbon diffusion. Our results provide a certain implication for better understanding the experimental observations related with the carbon solubility limit, carbon microsegregation, and carbide precipitations in the ferritic steels.