Mingzhen Ma

Structural, elastic, and thermal properties of Laves phase ZrV2 under pressure

The structural, electronic, elastic and some thermodynamic properties of the cubic C15 structure ZrV2 compound under pressure are investigated by first-principles calculations. Our results for the equilibrium unit cell volume, bulk modulus and band structure are consistent with the calculated and experimental results. Cubic ZrV2 is mechanically stable according to the elastic stability criteria and shows ductile with the G/B and Cauchy pressure analysis. Moreover the pressure and temperature dependence of the bulk modulus, specific heat, Debye temperature and thermal expansion coefficient are discussed, among them our calculated Debye temperature is in good agreement with experiments.

First principle study of elastic and thermodynamic properties of FeB4 under high pressure

The elastic properties, elastic anisotropy, and thermodynamic properties of the lately synthesized orthorhombic FeB4 at high pressures are investigated using first-principles density functional calculations. The calculated equilibrium parameters are in good agreement with the available experimental and theoretical data. The obtained normalized volume dependence of high pressure is consistent with the previous experimental data investigated using high-pressure synchrotron x-ray diffraction. The complete elastic tensors and crystal anisotropies of the FeB4 are also determined in the pressure range of 0–100 GPa. By the elastic stability criteria and vibrational frequencies, it is predicted that the orthorhombic FeB4 is stable up to 100 GPa. In addition, the calculated B/G ratio reveals that FeB4 possesses brittle nature in the range of pressure from 0 to 100 GPa. The calculated elastic anisotropic factors suggest that FeB4 is elastically anisotropic. By using quasi-harmonic Debye model, the compressibility, bulk modulus, the coefficient of thermal expansion, the heat capacity, and the Gruneisen parameter of FeB4 are successfully obtained in the present work.

Potential superhard cubic spinel CSi2N4: First-principles investigations

The structural and mechanical properties of the cubic spinel CSi2N4 (c-CSi2N4) are studied by first-principles total energy calculations based on the density-functional theory. It is found that the calculated lattice constants and bulk modulus are in good agreement with previous calculations. The elastic constants show that c-CSi2N4 is mechanically stable. Based on the microscopic hardness model, c-CSi2N4 is predicted to be a superhard material with the calculated Vickers hardness of 52.07GPa, which is 57.79% higher than that of the cubic spinel Si3N4. The origin of the hardness enhancement is discussed in terms of the density of states and the chemical bonding analysis. It is also found that the elastic anisotropy and the ductility are improved in comparison with those of the cubic spinel Si3N4.

Bulk moduli of wurtzite, zinc-blende, and rocksalt phases of ZnO from chemical bond method and density functional theory

The density functional theory based first-principles calculations and a recently proposed chemical bond method are used to investigate the elastic and compressibility properties of the rocksalt, wurtzite, and zinc-blende phases of ZnO. The calculated bulk moduli from these two means explain the discrepancy between the theoretical and experimental values of the bulk modulus of wurtzite ZnO as well as the scattered experimental data. The results suggest that further experimental efforts are needed in order to obtain the intrinsic bulk moduli of the three phases of ZnO.

Nanoindentation Creep Behavior of an Al0.3CoCrFeNi High-Entropy Alloy

Abstract Nanoindentation creep behavior was studied on a coarse-grained Al0.3CoCrFeNi high-entropy alloy with a single face-centered cubic structure. The effects of the indentation size and loading rate on creep behavior were investigated. The experimental results show that the hardness, creep depth, creep strain rate, and stress exponent are all dependent on the holding load and loading rate. The creep behavior shows a remarkable indentation size effect at different maximum indentation loads. The dominant creep mechanism is dislocation creep at high indentation loads and self-diffusion at low indentation loads. An obvious loading rate sensitivity of creep behavior is found under different loading rates for the alloy. A high loading rate can lead to a high strain gradient, and numerous dislocations emerge and entangle together. Then during the holding time, a large creep deformation characteristic with a high stress exponent will happen.

First principle study of elastic and thermodynamic properties of ZrZn2 and HfZn2 under high pressure

A comprehensive investigation of the structural, elastic, and thermodynamic properties for Laves-phases ZrZn2 and HfZn2 are conducted using density functional total energy calculations combined with the quasi-harmonic Debye model. The optimized lattice parameters of ZrZn2 and HfZn2 compare well with available experimental values. We estimated the mechanical behaviors of both compounds under compression, including mechanical stability, Youngs modulus, Poissons ratio, ductility, and anisotropy. Additionally, the thermodynamic properties as a function of pressure and temperature are analyzed and found to be in good agreement with the corresponding experimental data.

Co-electrodeposition of hard Ni-W/diamond nanocomposite coatings.

Electroplated hard chrome coating is widely used as a wear resistant coating to prolong the life of mechanical components. However, the electroplating process generates hexavalent chromium ion which is known carcinogen. Hence, there is a major effort throughout the electroplating industry to replace hard chrome coating. Composite coating has been identified as suitable materials for replacement of hard chrome coating, while deposition coating prepared using traditional co-deposition techniques have relatively low particles content, but the content of particles incorporated into a coating may fundamentally affect its properties. In the present work, Ni-W/diamond composite coatings were prepared by sediment co-electrodeposition from Ni-W plating bath, containing suspended diamond particles. This study indicates that higher diamond contents could be successfully co-deposited and uniformly distributed in the Ni-W alloy matrix. The maximum hardness of Ni-W/diamond composite coatings is found to be 2249 ± 23 Hv due to the highest diamond content of 64 wt.%. The hardness could be further enhanced up to 2647 ± 25 Hv with heat treatment at 873 K for 1 h in Ar gas, which is comparable to hard chrome coatings. Moreover, the addition of diamond particles could significantly enhance the wear resistance of the coatings.

Effects of Aspect Ratio on the Shear Band Arrangements of Zr-Based Metallic Glasses

Effects of aspect ratios of Zr-based metallic glasses on the shear band arrangements are investigated through molecular dynamics simulations and experiments. It is found that as the aspect ratio decreases, the dense multiple shear bands form, effectively depressing the formation of the penetrating shear bands, which improves plasticity and strength of metallic glass. Simulation reproduces the images of the evolution of the shear bands in metallic glass, explaining the experimental observation. It is found that as the aspect ratio decreases, shear transformation zones disperse evenly in the entire model, restraining connecting into penetrating shear bands.

Pressure-induced pseudoatom bonding collapse and isosymmetric phase transition in Zr2Cu: First-principles predictions

The structural evolution of tetragonal Zr2Cu has been investigated under high pressures up to 70 GPa by means of density functional theory. Our calculations predict a pressure-induced isosymmetric transition where the tetragonal symmetry (I4/mmm) is retained during the entire compression as well as decompression process while its axial ratio (c/a) undergoes a transition from ~3.5 to ~4.2 at around 35 GPa with a hysteresis width of about 4 GPa accompanied by an obvious volume collapse of 1.8% and anomalous elastic properties such as weak mechanical stability, dramatically high elastic anisotropy, and low Youngs modulus. Crystallographically, the tetragonal axial ratio shift renders this transition analogous to a simple bcc-to-fcc structural transition, which implies it might be densification-driven. Electronically, the ambient Zr2Cu is uncovered with an intriguing pseudo BaFe2As2-type structure, which upon the phase transition undergoes an electron density topological change and collapses to an atomic-sandwich-like structure. The pseudo BaFe2As2-type structure is demonstrated to be shaped by hybridized dxz + yz electronic states below Fermi level, while the high pressure straight Zr-Zr bonding is accommodated by electronic states near Fermi level with dx(2) - y(2) dominant features.

Polymorphism in glassy silicon: inherited from liquid-liquid phase transition in supercooled liquid.

Combining molecular dynamics (MD) simulation and Voronoi polyhedral analyses, we discussed the microstructure evolution in liquid and glassy silicon during cooling by focusing on the fraction of various clusters. Liquid-liquid phase transition (LLPT) is detected in supercooled liquid silicon However, freezing the high-density liquid (HDL) to the glassy state is not achieved as the quenching rate goes up to 1014 K/s. The polyamorphism in glassy silicon is found to be mainly associated with low-density liquid (LDL).