Peng Zhang

First-principles calculations of phase transition, elastic modulus, and superconductivity under pressure for zirconium

The elasticity, dynamic properties, and superconductivity of α, ω, and β Zr upon compression are investigated by using first-principles methods. Our calculated elastic constants, elastic moduli, and Debye temperatures of α and ω phases are in excellent agreement with experiments. Electron-phonon coupling constant λ and electronic density of states at the Fermi level N (EF) are found to increase with pressure for these two hexagonal structures. For cubic β phase, the critical pressure for mechanical stability is predicted to be 3.13 GPa and at Pu2009=u20094 GPa the low elastic modulus (E =31.97 GPa) can be obtained. Besides, the critical pressure for dynamic stability of β phase is achieved by phonon dispersion calculations to be ∼26 GPa. Over this pressure, λ and N (EF) of β phase decrease upon further compression. Our calculations show that the large value of superconducting transition temperature Tc at 30 GPa for β Zr is mainly due to the TA1 soft mode. Under further compression, the soft vibrational mode will gradually fade away.The elasticity, dynamic properties, and superconductivity of α, ω, and β Zr upon compression are investigated by using first-principles methods. Our calculated elastic constants, elastic moduli, and Debye temperatures of α and ω phases are in excellent agreement with experiments. Electron-phonon coupling constant λ and electronic density of states at the Fermi level N (EF) are found to increase with pressure for these two hexagonal structures. For cubic β phase, the critical pressure for mechanical stability is predicted to be 3.13 GPa and at Pu2009=u20094 GPa the low elastic modulus (E =31.97 GPa) can be obtained. Besides, the critical pressure for dynamic stability of β phase is achieved by phonon dispersion calculations to be ∼26 GPa. Over this pressure, λ and N (EF) of β phase decrease upon further compression. Our calculations show that the large value of superconducting transition temperature Tc at 30 GPa for β Zr is mainly due to the TA1 soft mode. Under further compression, the soft vibrational mode will ...

Failure Analysis of High Strength Steel Bar Used in a Wind Turbine Foundation

Screw-thread steel bars made of high strength steel fractured after exposure to a constant pre-stress of 650xa0MPa for more than 10xa0h. Visual examination, chemical analysis, mechanical tests, metallographic analysis, and fractographic examination were carried out in order to determine the causes of this failure. The fracture surfaces consisted of three regions: the crack source or fracture origin region, steady-state crack growth, and unstable or final fracture region. The crack initiated at the center of the cross section and then propagated to the periphery. The fracture exhibited a brittle appearance, accompanied with secondary cracks, tear ridges, and micropores on the grain boundary. The results revealed that this failure was due to hydrogen embrittlement of the steel.

Structure and electrochemical properties of low-cobalt La1−x LixNi3.2 Co0.3Al0.3(0≤x≤0.2) hydrogen storage electrode alloys

Abstract The structure and electrochemical properties of a new low cobalt hydrogen storage electrode alloys La1−xLixNi3.2Co0.3Al0.3(0 ≤ x ≤ 0.2) were investigated with a different additions of Li in replacement of La. With the increase of Li contents the maximum discharge capacity increases from 240 mAh·g−1(x = 0) to 328.4 mAh·g−1(x = 0.1) and the cycle stability is improved correspondingly. The capacity decay can remain 28.6% (x = 0.2) after 230 charge/discharge cycles. The high rate discharge (HRD) ability of the alloys (x ≤ 0.1) is improved and the best HRD is 34.1% (x = 0.1) under the discharge current density 1200 mA·g−1. It is found that the prepared alloys are basically composed of LaNi5 as matrix phase and La-Ni3 as second phase (x ≤ 0.1). But the abundance of LaNi3 phase dramatically decreases with increasing x. When x = 0.2, a new phase Al(NiCo)3 is formed.

Investigation of tensile deformation mechanisms at room temperature in a new Ni-based single crystal superalloy

Abstract The deformation microstructures of a new Ni-based single crystal superalloy, M 4706, have been characterized by transmission electron microscopy after interrupted tensile tests at room temperature. It is found that besides shearing of γ′ precipitates by strongly coupled dislocations, another unusual shearing process involving a single a/2 〈1u20090u20091〉 matrix dislocation as well as the formation of the isolated superlattice stacking fault and Shockley loop also operates actively during initial yielding. Based on experimental observations, occurrence of these different shearing processes is discussed.

First-principles study of atomic hydrogen adsorption and initial hydrogenation of Zr(0001) surface

The atomic hydrogen adsorption on Zr(0001) surface is systematically investigated by using density functional theory within the generalized gradient approximation and a supercell approach. The coverage dependence of the adsorption structures and energetics is studied in detail for a wide range from 0.11 to 2.0 monolayer. At low coverage of 0<Θ≤1.0, the most stable adsorption site is identified as the on-surface hcp site followed by the fcc site, and the adsorption energy gradually increases with the coverage, thus, indicating the higher stability of on-surface adsorption and the tendency to form H clusters. The origin of this stability is carefully analyzed by the projected density of states and the charge distribution showing the Zr-H chemical bonding with a mixed ionic/covalent feature during the surface hydrogenation. In addition, the minimum energy paths as well as the activation barriers of the on-surface diffusion and the penetration from on-surface sites to subsurface sites are also calculated. At ...

Adsorption and diffusion of H2O molecule on the Be(0001) surface: A density-functional theory study

Abstract Using first-principles calculations, we systematically study the adsorption behavior of a single molecular H 2 O on the Be(0001) surface. We find that the favored molecular adsorption site is the top site with an adsorption energy of about 0.3 eV, together with the detailed electronic structure analysis, suggesting a weak binding strength of the H 2 O/Be(0001) surface. The adsorption interaction is mainly contributed by the overlapping between the s and p z states of the top-layer Be atom and the molecular orbitals 1 b 1 and 3 a 1 of H 2 O. The activation energy for H 2 O diffusion on the surface is about 0.3 eV. Meanwhile, our study indicates that no dissociation state exists for the H 2 O/Be(0001) surface.

Structural characterization of individual graphene sheets formed by arc discharge and their growth mechanisms

Graphene sheets formed by arc discharge are hard to characterize in detail due to their complex pristine states in raw soot, which always exhibit an overall morphology of overlapping aggregation, together with other carbonaceous by-products. Here we used an improved arc method and simple separation procedure to obtain a large number of individual graphene sheets with single- to few-layers, and further probed their structural details using optical microscopy, transmission electron microscopy (TEM), atomic force microscopy and Raman spectroscopy. By TEM characterization, two major types of graphene sheets are shown; one is observed with folded fringes and polycrystalline structure, whereas the other is with an even graphene plane and single crystalline structure. In contrast to that of supported graphene, the Raman spectra of these graphene sheets show some different characteristics such as opposite shift of G band frequency as the layers increase. With increasing layers, the frequencies of G and G′ bands and the full width at half maximum (FWHM) of the G′ band totally exhibit layer-dependence. According to the FWHM of G′ bands, the folding within graphene sheets is also discussed. In addition, the defect types for arc graphene are analysed based on the D and D′ bands. Our results suggest that the D bands of such graphene sheets result from edges, rather than topological defects or disorder. Based on the findings, a new growth mechanism of arc graphene is proposed rationally responsible for the difference of two types of graphene sheets.

Evolution of Defects and Defect Clusters in β-SiC Irradiated at High Temperature

Abstract A molecular dynamics study has been performed to investigate the generation and evolution of damage states in irradiated β-SiC at high temperature. It is found that most of the C antisites (SiC) are created during the early collisional phase, while the Si antisites (CSi) are significantly produced during the thermal spike phase. A modified near-neighbor point defect density (NPDD) is introduced to study the spatial aggregation of different defects during the displacement cascades, and feature of defect clusters evolution is analyzed in details. The dominated types of vacancy clusters after the displacement cascades are two- and three-size chainlike ones. And the vacancy NPDD (V-NPDD) decreases as the recoil energy increases. Furthermore, after the thermal spike phase, there is an additional annealing process during which the interstitials and antisites turn into defect clusters, respectively.

Extraordinary plastic behaviour of the γ′ precipitate in a directionally solidified nickel-based superalloy

Abstract The deformation behaviour of the γ′ precipitate in a directionally solidified nickel-based superalloy is investigated using microscopic observations after tensile testing at room temperature. It is found that coarse γ′ precipitates (604 nm) are sheared by strongly coupled dislocations, and some γ′ precipitates are elongated to approximately 3–6 times of their original lengths. It reveals that, at room temperature, the γ′ precipitate within the experimental superalloy has a significant plastic deformation capacity in comparison with Ni3Al bulk alloys. Based on the experimental observations, the extraordinary plastic behaviour of the γ′ precipitate is analysed.

Nonlinear elastic behavior and failure mechanism of polyhedral graphite particles undergoing uniaxial compression

Load-displacement responses and ultimate strength of polyhedral graphite particles (PGPs) undergoing in situ nano-compression at ambient temperature have been studied. The dynamic responses of PGPs to uniaxial loads exhibit a typical nonlinear elastic behavior for graphitic nanomaterials. Based on the analysis of stress-strain relationship, the intrinsic strength is slightly larger than actual ultimate strength, indicating the mechanical properties influenced by the initial defects in PGPs. For a given case, compressive Youngs modulus E and third-order elastic modulus D achieve to 12.8u2009GPa and −13.9u2009GPa, respectively. Weibull probability analysis confirmed its broad range of structural defects inside PGPs and mechanical properties are sensitive to initial defects. The values of ultimate strength of tested PGPs with diameter of 150–400u2009nm fall within 2–4.5u2009GPa, which are in the range between shear elastic modulus C44 of turbo-g (minimum) and C44 of hex-g (maximum) in the literature. The deformation and fa...