Papers by Author: Shao Qing Wang

Abstract: Periodic chemically homogenized high-entropy alloy structures are constructed according to maximum entropy principle. The method can efficiently generate equimolar and non-equimolar high-entropy alloy atomic structures. Nine high-entropy alloys are simulated based on the constructed models using density functional theory techniques. The calculated lattice parameters are consistent with the available experimental data. The calculated enthalpies of mixing are more negative than the values estimated by using Miedema model, due to severe lattice distortion. The lattice distortion parameters were calculated. The results showed that fcc structure tend to stable with smaller and bcc structure with larger. The bulk modulus of Al_1.5CoCrNiFe high-entropy alloys was fitted and the value is consistent with the available experimental data.

Abstract: The local structures of Zn and Y in the long period stacking order (LPSO) phase in Mg-Zn-Y system were investigated by first principles calculations in details. The clustering of Zn and Y atoms ranging from single stacking fault layer to four consecutive layers was explicitly demonstrated. The calculations indicate that Zn and Y atoms prefer clustering in the form of Zn₆Y₉ embedding in ABCA-type building block to the random or ordered arrangements of Zn and Y atoms being enriched in two stacking fault layers. The cluster of Zn₆Y₉ can be regarded as the ideal stoichiometric component of LPSO and it plays a predominant role in the LPSO phases. The formation of LPSO phases is highly associated with the Zn₆Y₉ cluster and its derivatives.

Abstract: Atomic structure model of bulk AlCoCrCuFeNi multi-principal-element alloy was build by following the principle of maximum entropy. Morse pair-potentials to describe the inter-atomic interaction among neighboring atoms in the alloy were generated directly from first-principles calculations within density-functional theory. Molecular statics simulation was carried out to achieve the optimized atomic configuration of AlCoCrCuFeNi alloy. The results show that the crystallographic behavior in lattice structure observed experimentally is just caused by the average of the disordered atomic position and composition in wide range since there is neither short-range nor long-range order in the local atomic arrangement of this kind of materials.

Abstract: An elabrate study on the structrural and mechanical properties of the five-element FeNiCrCuCo high-entropy alloys is carried out by first-principles calculation within the density-functional theory. The combination application of plane-wave pseudopotentials and alchemical pseudoatom methods is realized to imitate the random elemental lattice occupation in the alloys. The dependence of composition variation to the crystallographic and thermodynamic properties of FeNiCrCuCo alloys in simple BCC and FCC lattices are investigated. The key role of chromium in strengthening the inter-atomic cohesion and stabilizing the lattice structure of HEAs is suggested.

Abstract: TiAl alloys have great potential because of its low density and the outstanding performance at high temperature. However, the brittleness influences its industrialization process. It is known that the macroscopic nature is greatly influenced by its microscopic structure, and the fault development plays a vital role during the material working process. The paper performed the molecular dynamics (MD) study of the thermodynamic shear deformation in TiAl/Ti₃Al system to promote the understanding in this aspect. Above all, we adopt a special shear deformation model based on the experimental consideration, and conduct the optimal calculation of the related parameters. Then, a series of thermodynamic deformation simulation were carried out using the previous optimized model. The analysis of the potential variation and the structural snapshots showed that the shear deformation is related with the “stick-slip” behavior. The Ti₃Al (TiAl) shows obvious (little) covariant deformation stage before the initiation of the fault transition. For Ti₃Al region near the interface, the final structure is the continued FCC stacking. For TiAl, twin and SISF are observed and the block of twin is the main remnant. The atomic diffusion is locally observed in Ti3Al phase. The interface transits the energy and counterpoises the deformation between the hetero-phases.

Abstract: The lattice dynamics and thermodynamic properties of MgS and related II-VI compounds are studied by the first-principles linear-response function calculation in the framework of densityfunctional perturbation theory. The ab initio structural, mechanic and dielectric parameters of these phases are presented. From the theoretical phonon dispersion relations, the linear thermal expansion coefficient and its temperature dependence are calculated. The differences in structural and thermodynamic behaviors of these compounds are explained from their phonon dispersion characters.

Abstract: Perhydropolysilazane was synthesized by the ammonolysis of dichlorosilane-pyridine adduct, and used as precursor to prepare three-dimensional silica fiber reinforced silicon nitride matrix composites via preceramic polymer infiltration and pyrolysis at 1073K
in anhydrous ammonia atmosphere. The polymer-derived ceramic matrix was amorphous near-stoichiometric silicon nitride with an empirical formula of SiN1.34O0.02C0.01H1.21. The as-received composites were amorphous with a high density of 1.96g/cm3 after five infiltration-pyrolysis cycles. The process was successful to fabricate dense silicon nitride matrix composites reinforced by three-dimensional silica fiber due to the low viscosity, good wettability and high ceramic yield of perhydropolysilazane.

Abstract: The dielectric properties and sintering activity of nano-BaTiO3 powders synthesized by HGRP method were investigated. The starting BaTiO3 powders were calcined at different temperatures from 700 to 900°C to improve their crystallinity and the mean particle size of BaTiO3 powders obtained increased from 40nm to 80nm. After being formed by conventional dry pressing the green bodies were sintered at 1100°C and 1200°C for 2hr in air. The effects of both calcinating and sintering temperatures on the sinterability, dielectric property and microstructure of BaTiO3 ceramics were discussed. The experimental results showed that the BaTiO3 powders have high sintering activity and the highest dielectric constant of this material at room temperature may reach 2880.

Abstract: We employ density functional calculations to investigate the doped Al/TiC interfaces. The effects of different segregation atoms are discussed. The results show that the different transition metal atoms have different effects on the adhesion. Results of analysis of atom size and electronic structure have shown that both atom size and activity of the doped atom influence on the adhesion. Our results are consistent with other results of doped metal-oxide interface.