Liu Chang-Song

Segregation of alloying atoms at a tilt symmetric grain boundary in tungsten and their strengthening and embrittling effects

We investigate the segregation behavior of alloying atoms (Sr, Th, In, Cd, Ag, Sc, Au, Zn, Cu, Mn, Cr, and Ti) near Σ3 (111) [10] tilt symmetric grain boundary (GB) in tungsten and their effects on the intergranular embrittlement by performing first-principles calculations. The calculated segregation energies suggest that Ag, Au, Cd, In, Sc, Sr, Th, and Ti prefer to occupy the site in the mirror plane of the GB, while Cu, Cr, Mn, and Zn intend to locate at the first layer nearby the GB core. The calculated strengthening energies predict Sr, Th, In, Cd, Ag, Sc, Au, Ti, and Zn act as embrittlers while Cu, Cr, and Mn act as cohesion enhancers. The correlation of the alloying atoms metal radius with strengthening energy is strong enough to predict the strengthening and embrittling behavior of alloying atoms; that is, the alloying atom with larger metal radius than W acts as an embrittler and the one with smaller metal radius acts as a cohesion enhancer.

Crystal Structure of β-La2Mo2 O9 from First Principles Calculation

Arrangements of O ions in β-La2 Mo2 O9, are studied by first principles calculation with two different calculation schemes. All final structure configurations consist of MoO4-tetrahedra, MoO5-hexahedra, LaOg and LaO7 polyhedra. Molybdenum polyhedra are isolated from each other, lanthanum polyhedra are connected together by sharing O ions. The occupancies of three crystallographic distinct O sites O(1), O(2) and O(3) are 100%, 91.7% and 25%, respectively, consistent with experiments. All configurations are related to each other by one of 12 symmetry operations of P213 space group, suggesting that the structure observed experimentally may be interpreted as a time and spatial average of these local or inherent structures.

Turing Patterns in a Reaction-Diffusion System

We have further investigated Turing patterns in a reaction-diffusion system by theoretical analysis and numerical simulations. Simple Turing patterns and complex superlattice structures are observed. We find that the shape and type of Turing patterns depend on dynamical parameters and external periodic forcing, and is independent of effective diffusivity rate σ in the Lengyel–Epstein model. Our numerical results provide additional insight into understanding the mechanism of development of Turing patterns and predicting new pattern formations.

Low-Frequency Internal Friction Study on the Structural Changes in Polymer Melts

With the help of the low-frequency internal friction method, we investigate the structural properties of polymer melts, such as amorphous polystyrene (PS), poly(methyl methacrylate) (PMMA), and semi-crystalline poly(ethylene oxide) (PEO). An obvious peak of relaxation type is found in each of the internal friction curves. The peak temperature Tp follows the relation Tp ≈ (1.15 – 1.18) Tg for PS and PMMA melts, while it follows Tp ≈ 1.22Tm for PEO melt, with Tg being the glass transition temperature and Tm the melting temperature. Based on the analysis of the features of this peak, it is found that this peak is related to the liquid-liquid transition temperature Tu of polymer melts. Mechanism of the liquid-liquid transition is suggested to be thermally-activated collective relaxation through cooperation. This finding may be helpful to understand the structural changes in polymer melts. In addition, the internal friction technique proves to be effective in studying dynamics in polymer melts.

Correlation of Lithium Ionic Diffusion with Nb Concentration in Li7−xLa3Zr2−xNbxO12 Evaluated by an Internal Friction Method

Solid lithium-ion conductors Li7−xLa3Zr2−xNbxO12 (x = 0.25, 0.5, 1, 1.5) with cubic garnet structure are successfully prepared by a solid state reaction method, and the effects of Nb concentration on lithium ion diffusion are investigated by means of internal friction (IF) technique. A prominent relaxation-type IF peak (actually composed of two components) is observed in each Nb doped Li7La3Zr2O12 compound: with apeak PL at lower temperature and a peak PH at higher temperature. The mechanisms of the two components are suggested to be associated with two diffusion processes of lithium ions via vacancies: 48g ↔ 48g and 48g ↔ 24d. The relaxational strength of the IF peak gradually decreases, which is accompanied by the activation energy increasing from 0.45 eV to 0.64 eV with the increasing Nb doping level. The corresponding mechanism is ascribed to originate from lattice contraction as well as the lower concentration of diffusion ions induced by the substitution of Zr4+ by Nb5+.

UNIVERSAL SCALING LAW FOR ATOMIC DIFFUSION AND VISCOSITY IN LIQUID METALS

The recently proposed scaling law relating the diffusion coefficient and the excess entropy of liquid [Samanta A et al. 2004 Phys. Rev. Lett. 92 145901; Dzugutov M 1996 Nature 381 137], and a quasi-universal relationship between the transport coefficients and excess entropy of dense fluids [Rosenfeld Y 1977 Phys. Rev. A 15 2545], are tested for diverse liquid metals using molecular dynamics simulations. Interatomic potentials derived from the glue potential and second-moment approximation of tight-binding scheme are used to study liquid metals. Our simulation results give sound support to the above-mentioned universal scaling laws. Following Dzugutov, we have also reached a new universal scaling relationship between the viscosity coefficient and excess entropy. The simulation results suggest that the reduced transport coefficients can be expressed approximately in terms of the corresponding packing density.

Molecular Dynamics Simulations of Liquid Phosphorus at High Temperature and Pressure

By performing ab initio molecular dynamics simulations, we have investigated the microstructure, dynamical and electronic properties of liquid phosphorus (P) under high temperature and pressure. In our simulations, the calculated coordination number (CN) changes discontinuously with density, and seems to increase rapidly after liquid P is compressed to 2.5 g/cm3. Under compression, liquid P shows the first-order liquid-liquid phase transition from the molecular liquid composed of the tetrahedral P4 molecules to complex polymeric form with three-dimensional network structure, accompanied by the nonmetal to metal transition of the electronic structure. The order parameters Q6 and Q4 are sensitive to the microstructural change of liquid P. By calculating diffusion coefficients, we show the dynamical anomaly of liquid P by compression. At lower temperatures, a maximum exists at the diffusion coefficients as a function of density; at higher temperatures, the anomalous behavior is weakened. The excess entropy shows the same phenomena as the diffusion coefficients. By analysis of the angle distribution functions and angular limited triplet correlation functions, we can clearly find that the Peierls distortion in polymeric form of liquid P is reduced by further compression.

Final Structures of Crystallization of Liquid Copper Studied by Molecular Dynamics Simulation

Molecular dynamics simulation is performed to study the cooling rate effects on the resulting crystallization microstructure of bulk liquid copper (homogeneous nucleation). Our studies reveal that mixtures of the metastable hcp phase and most-stable fcc phase, in all sorts of proportion and in various forms such as layering and phase separation, can be obtained by controlling the cooling rate. We have also found that, below the glass forming critical cooling rate, the ratio of the number of fcc-type polyhedra to the number of hcp-type polyhedra at 0 K decreases with increase of the cooling rate.

Dynamic Behaviors of Hydrogen in Martensitic T91 Steel Evaluated by Using the Internal Friction Method

Hydrogen atoms are electrochemically introduced into commercial martensitic T91 steel, and the hydrogen dynamic behaviors are investigated by internal friction (IF) technology. A complex spectrum with multicomponent peaks is detected in the hydrogen-charged T91 steel in the temperature range of 135–290 K. Analysis of peak configuration reveals that the multicomponent peaks consist of one relaxational peak and two non-relaxational peaks. The mechanism of the wide relaxational component is ascribed to the combination of a hydrogen Snoek-like diffusion process and the interaction of hydrogen with movable dislocations, while the two non-relaxational peaks are preliminarily suggested to be caused by some kinds of transient processes related with hydrogen redistribution and outgassing. The binding energy of hydrogen to dislocation is determined to be about 0.19 eV.

Structural Characteristics of Liquid Sn

We investigate the structural properties of liquid Sn. With the help of the internal friction (tan Φ) method, it is found that a peak appears in the tan Φ−T curve, suggesting that an anomalous discontinuous temperature-induced structure change may take place in liquid Sn. From the experimental data of pair distribution functions, we calculate the viscosity η and the excess entropy S and it is found that there are a peak of viscosity in the η−T curve and a bend of excess entropy in the S−T curve, which give a positive support to the appearance of the internal-friction peak in the tan Φ−T curve.