X.D. Hui

Ordered clusters and free volume in a Zr–Ni metallic glass

The atomic arrangement of a model metallic glass Zr2Ni was studied by extended x-ray absorption fine structure and x-ray scattering experiments combined with reverse Monte Carlo simulation imposed an additional potential constraint. By an approach to calculating the free volume (FV) on atomic level, we have found a connection between the coordination number and FV, and then revealed that the atomic structure of Zr2Ni metallic glass is essentially an association of the ordered clusters and FV. The ordered clusters about 1.5nm consist of a densely packed core (i.e., icosahedral or fcc-type packing) and the surrounding loosely packed clusters with large FV.

Lattice dynamics, thermodynamics, and bonding strength of lithium-ion battery materials LiMPO4 (M = Mn, Fe, Co, and Ni): a comparative first-principles study

Gaps in our knowledge of phonon and thermodynamics still remain despite significant research efforts on cathode materials LiMPO4 (M = Mn, Fe, Co, and Ni) for rechargeable Li-ion batteries. Here, we employ a mixed-space approach of first-principles phonon calculations to probe the lattice dynamics including LO–TO splitting (longitudinal and transverse optical phonon splitting), quantitative bonding strength between atoms, and finite-temperature thermodynamic properties of LiMPO4. In order to take into account the strong on-site Coulomb interaction (U) presented in transition metals, the GGA + U calculations are used for LiMPO4. It is found that the oxygen–phosphorus (O–P) bond with the minimal bond length is extremely strong, which is roughly five times larger than the second strongest O–O bond. The atom P-containing bonds are apparently stronger than the corresponding atom O-containing bonds, indicating the stability of LiMPO4 is mainly due to atom P. It is observed that the equilibrium volume of LiMPO4 decreases from Mn, Fe, Co, to Ni, and the bulk modulus, zero-point vibrational energy, and Debye temperature increase. Phonon results indicate that the largest vibrational contribution to Gibbs energy is for LiMnPO4, followed by LiFePO4, LiCoPO4, and then LiNiPO4, due to the decreasing trend of phonon densities of state at the low frequency region of LiMPO4. Computed phonon and thermodynamic properties of LiMPO4 are in close accord with available experiments, and provide knowledge to be validated experimentally.

Microstructure and ductile–brittle transition of as-cast Zr-based bulk glass alloys under compressive testing

Abstract This paper investigates mechanical properties and fracture mechanisms of Zr 52.5 Cu 17.9 Ni 14.6 Al 10 Ti 5 alloys with various volume fractions of quenched-in crystalline. The alloys with various volume fractions of quenched-in crystalline were prepared by controlled oxygen content of alloys and overheating of the pouring. The phase structure, particle size and volume fraction of all samples were identified by X-ray diffraction, differential scanning calorimeter (DSC) curves and scanning electron microscopy (SEM) photographs. The mean sizes of crystalline increased from 0.3 to 1.3 μm with increasing volume fraction of crystalline from 4 to 13%. The compressive mechanical tests show a ductile–brittle transition with significant decrease in the fracture stress and ductility. Detailed observations in the flow deformation and fracture surface illustrate the relationship between the quenching-in crystalline and the mechanical behavior. The full bulk amorphous Zr-based alloy exhibits typical ductile deformation and fracture behavior. The torn shear bands form the typical vein patterns on the fracture surface. The effects of quenching-in crystalline on the flow deformation and fracture behavior depend on the nature, size, volume fraction and distribution. The particle size of the crystalline in the sense of the width of shear bands is critical. When the size is larger than the width of the shear bands the particles induce an obvious inhomogeneity of the flow deformation and more microcracks by the separation of the interfaces. Nano-scale particles, on the other hand, may increase the viscosity of the flow but do not form microcracks, resulting in particle strengthening of the metallic glass. Increasing the volume fraction of large-scale particles is favorable to leaking the microcracks and brittle fracture. With increasing particle size and volume fraction up to two times the width of the shear band and 10% vol., respectively, the ductile fracture of bulk amorphous alloy completely transforms to brittle fracture under compressive testing.

Al-centered icosahedral ordering in Cu46Zr46Al8 bulk metallic glass

Icosahedral short-range order, of which Al atoms are caged in the center of icosahedra with Cu and Zr atoms being the vertices, has been evidenced in the Cu46Zr46Al8 glassy structure by ab initio molecular dynamics simulation. These Al-centered clusters distribute irregularly in the three-dimensional space and form a “backbone” structure of the Cu46Zr46Al8 glass alloy. It is suggested that this kind of local structural feature is attributed to the requirement of efficient dense packing and the chemical affinity between Zr–Zr, Zr–Al, and Cu–Zr atoms. Our calculated results are found to be in good agreement with the experimental data.

Primary crystallization in rapidly solidified Zr70Cu20Ni10 alloy from a supercooled liquid region

Abstract The primary crystallization of amorphous Zr70Cu20Ni10 alloy has been investigated by employing the differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). It is interesting to note that the face-centered cubic Zr2Ni (fcc-Zr2Ni) phase precipitates first from the amorphous matrix when the amorphous Zr70Cu20Ni10 alloy is annealed at the supercooled liquid region. At the late crystallization stage, the Zr2Cu particles begins to precipitate. It is thought that at the initial stage the amorphous Zr70Cu20Ni10 alloy undergoes a phase decomposition process, accompanying with a compositional change, which is regarded as a preparation for the precipitation of the fcc-Zr2Ni particles. The whole crystallization process of amorphous Zr70Cu20Ni10 alloy can be divided into two parts, i.e., the interface-controlled transient nucleation and growth process to a critical size at the early stage, and the diffusion-controlled grain growth process at the late stage. This behavior has been analyzed from the viewpoints of thermodynamics and kinetics.

Icosahedral ordering in Zr41Ti14Cu12.5Ni10Be22.5 bulk metallic glass

This paper presents a computational evidence of icosahedral short and medium range ordering in Zr41Ti14Cu12.5Ni10Be22.5 bulk metallic glass using ab initio molecular dynamics simulation. It is found that 1551, 1541, and 1431 types of bond pairs are pronounced in both the liquid and glass states, resulting in icosahedral coordinate polyhedra at low temperatures. By linking the individual icosahedra through vertex-, edge-, face-, and intercross-shared atoms, icosahedral medium range ordering is formed. The predicted homogenized structure factor and pair correlation function of the glass structure have been confirmed to be in agreement with the experimental results.

Preparation, microstructure and mechanical properties of Zr-based bulk amorphous alloys containing tungsten

Abstract A new Zr-based bulky amorphous alloys containing W are designed and prepared by using the combined technique of jet and water-cooled copper mold casting. XRD, DSC and SEM analysis are conducted to investigate the microstructure, the supercooled liquid region Δ T x (= T x − T g ), and thermal stability of the new bulk metallic glass system. The effects of the addition of W and the increase of Cu content on the glass forming ability (GFA) and thermal stability are discussed. Compressive experiments were also carried out to study the Youngs modulus, compressive fracture strength and elongation of the new alloys. The fracture characterization and mechanism of the new bulk amorphous alloys are discussed in detail.

Effect of Ni content on crystallization of metallic Zr–Cu–Ni glass

Abstract Employing the X-ray diffraction (XRD) and differential scanning calorimetry (DSC) techniques, the effect of Ni content on crystallization behavior of metallic Zr 70 Cu 30− x Ni x ( x =5, 10 and 15) glasses has been investigated. It is found that all the DSC traces of metallic Zr 70 Cu 30− x Ni x ( x =5, 10 and 15) glasses display two exothermic peaks, indicating that the crystallization processes of metallic Zr 70 Cu 30− x Ni x ( x =5, 10 and 15) glasses proceed via a double stage mode. It is observed that the two exothermic peaks in the DSC scans of metallic Zr 70 Cu 25 Ni 5 and Zr 70 Cu 15 Ni 15 glasses overlap, implying that the crystallization processes of these two metallic glasses proceed through the almost simultaneous precipitation of crystalline phases. As for the metallic Zr 70 Cu 20 Ni 10 glass, the exothermic peaks in the DSC curves are more separated than those of the metallic Zr 70 Cu 25 Ni 5 and Zr 70 Cu 15 Ni 15 glasses, indicating that the crystallization products of metallic Zr 70 Cu 20 Ni 10 glass precipitate in sequence. To explain these experimental results, a micromechanism for crystallization of metallic Zr 70 Cu 30− x Ni x ( x =5, 10 and 15) glasses is suggested. In addition, the origin of the high stability of the f.c.c.-Zr 2 Ni phase is also interpreted from the viewpoint of local activation energy for crystallization.

Formation of nanocrystals in metallic Zr-Cu-Ni glass

Abstract Differential scanning calorimetry (DSC) and transmission electron microscopy (TEM) are employed to investigate the crystallization behavior in metallic Zr70Cu20Ni10 glass during continuous heating. It is found that the DSC curves of metallic Zr70Cu20Ni10 glass exhibit two exothermic peaks, indicating that the crystallization process proceeds via a double stage mode. TEM microstructural analysis confirms that the first exothermic reaction in the DSC traces of metallic Zr70Cu20Ni10 glass mainly corresponds to the precipitation of the Zr2Cu phase, while the second one corresponds to the formation of nano-scale Zr2Ni particles. Ni plays an important role in the stability of the nanocrystals, and its effect is also discussed.

HRTEM study of crystallization of Zr-Cu-Ni metallic glass

Abstract Employing differential scanning calorimetry (DSC) and high-resolution transmission electron microscopy (HRTEM), the micromechanism for crystallization of Zr 70 Cu 20 Ni 10 metallic glass under isothermal annealing conditions has been investigated. It is found that the relationship between the annealing temperature and the peak position, incubation time and ending time in the isothermal annealing DSC traces of Zr 70 Cu 20 Ni 10 metallic glass obeys a first-order exponential function. However, the time–temperature transformation curves of Zr 70 Cu 20 Ni 10 metallic glass at different crystallized volume fractions can be well fitted by a second-order exponential function. It is observed that at the initial crystallization stage some ordered atomic clusters precipitate first, acting as nucleation sites and facilitating the subsequent crystallization process, and the crystal growth process mainly proceeds through the atomic depositing on the previously formed crystals. This behavior confirms that the new micromechanism for crystallization of amorphous alloys proposed by Lu and Wang can also be applied to the new series of zirconium based amorphous alloys.