http://www.uniroma2.it/didattica/tbs2/deposito/Manufacture,_characterisation_and_application_of_cellular_metals_and_metal_foams.pdf

Manufacture, characterisation and application of cellular metals and metal foams

We investigate the application of embedded atom method (EAM) interatomic potentials in the study of crystallization kinetics from deeply undercooled melts, focusing on the fcc metals Al and Cu. For this application, it is important that the EAM potential accurately reproduces melting properties and liquid structure, in addition to the crystalline properties most commonly fit in its development. To test the accuracy of previously published EAM potentials and to guide the development of new potential in this work, first-principles calculations have been performed and new experimental measurements of the Al and Cu liquid structure factors have been undertaken by X-ray diffraction. We demonstrate that the previously published EAM potentials predict a liquid structure that is too strongly ordered relative to measured diffraction data. We develop new EAM potentials for Al and Cu to improve the agreement with the first-principles and measured liquid diffraction data. Furthermore, we calculate liquid-phase diffus...

Analysis of semi-empirical interatomic potentials appropriate for simulation of crystalline and liquid Al and Cu

A novel experimental approach is employed to understand the mechanisms of laser induced damage. Using an OPO (optical parametric oscillator) laser, we have measured the damage thresholds of deuterated potassium dihydrogen phosphate (DKDP) from the near ultraviolet into the visible. Distinct steps, whose width is of the order of ${k}_{B}T$, are observed in the damage threshold at photon energies associated with the number of photons ($3\ensuremath{\rightarrow}2$ or $4\ensuremath{\rightarrow}3$) needed to promote a ground state electron across the energy gap. The wavelength dependence of the damage threshold suggests that a primary mechanism for damage initiation in DKDP is a multiphoton process in which the order is reduced through excited defect state absorption.

Wavelength Dependence of Laser-Induced Damage: Determining the Damage Initiation Mechanisms

On the basis of density functional theory calculations, we present the energetics, structure, and electronic and mechanical properties of crystalline and amorphous Li−Si alloys. We also discuss the dynamic behavior of the alloys at finite temperatures based on ab initio molecular dynamics. When the Li content is sufficiently high, alloying between Li and Si is energetically favorable as evidenced by the negative mixing enthalpy; the alloy is most stable around 70 atom % Li in the crystalline phase and 70 ± 5 atom % Li in the amorphous phase. Our calculations unequivocally show that the incorporation of Li leads to disintegration of the tetrahedrally bonded Si network into small clusters of various shapes. Bader charge analysis shows that the charge state of Li remains nearly unchanged around +0.8, while that of Si varies approximately from −0.5 to −3.3 depending on the number of Si neighbors as can be understood as Zintl-like phases. Electronic structure analysis highlights that the charge transfer leads ...

Structure and Properties of Li―Si Alloys: A First-Principles Study

We analyse gather-scatter performance bottlenecks in molecular dynamics codes and the challenges that they pose for obtaining benefits from SIMD execution. This analysis informs a number of novel code-level and algorithmic improvements to Sandia's miniMD benchmark, which we demonstrate using three SIMD widths (128-, 256and 512bit). The applicability of these optimisations to wider SIMD is discussed, and we show that the conventional approach of exposing more parallelism through redundant computation is not necessarily best. In single precision, our optimised implementation is up to 5x faster than the original scalar code running on Intel®Xeon®processors with 256-bit SIMD, and adding a single Intel®Xeon Phi™coprocessor provides up to an additional 2x performance increase. These results demonstrate: (i) the importance of effective SIMD utilisation for molecular dynamics codes on current and future hardware; and (ii) the considerable performance increase afforded by the use of Intel®Xeon Phi™coprocessors for highly parallel workloads.

/pdf/exploring-simd-for-molecular-dynamics-using-intel-xeon-1p8tlenlxj.pdf

Exploring SIMD for Molecular Dynamics, Using Intel® Xeon® Processors and Intel® Xeon Phi Coprocessors

We present an ab initio study of the stability and defect reactions of neutral and charged H interstitial (H i ) and H vacancy (H v ) in KH 2 PO 4 (KDP). We find that while there is no interaction between the neutral H i and the host, the addition of an electron leads to the ejection of a H host atom and the subsequent formation of an interstitial H 2 molecule and a H v . In sharp contrast, the addition of a hole results in the formation of a hydroxyl bond. Thus, H i in both charged states severs the H-bonded network. For the H v , the addition of a hole leads to the formation of a peroxyl bridge. The neutral H i and the positively charged H v induce states in the gap. The results elucidate the underlying atomic mechanism for the defect reactions suggested by experiment.

/pdf/electron-or-hole-assisted-reactions-of-h-defects-in-hydrogen-bp75oimbya.pdf

Electron- or hole-assisted reactions of H defects in hydrogen-bonded KDP.

A first-order liquid-liquid (LL) phase transition as a function of temperature in Pb-Sn melts is suggested by internal friction experiments and verified by differential thermal analysis with an entropy-driven order-disorder mechanism. It differs remarkably with other already found LL phase transitions in some aspects. The findings may throw light on the nature of viscous liquids in condensed state physics and be of significance in materials science. The present work makes a beneficial attempt at exploring internal friction as an experimental method for studying liquids.

Liquid-liquid phase transition in Pb-Sn melts

Employing realistic many-body potentials for a series of simple melts, including Ag, Al, Au, Co, Cu, Mg, Ni, Pb, Pd, Pt, Rh, and Si, we tested by molecular-dynamics simulation the scaling laws of diffusion coefficients with different expressions of the reduction parameters. Our simulation results give sound support to the universal excess entropy scaling laws proposed by Rosenfeld [Phys. Rev. A 15, 2545 (1977)] and Dzugutov [Nature (London) 381, 137 (1996)] for transport coefficients in liquid metals. In particular, we find that excess entropy $({S}_{\mathit{ex}})$ universally scales with temperature as ${S}_{\mathit{ex}}=\ensuremath{-}{E}_{S}∕T$. When the diffusion coefficient is scaled as Dzugutov suggested, ${E}_{S}$ is essentially identical to the Arrhenius activation energy, indicating that the entropic component in the Arrhenius activation energy is solely responsible for controlling the diffusion rate. Thus, there exists a link between the scaling law and the Arrhenius law, i.e., the excess entropy scaling law for the diffusion coefficient can be interpreted as a straightforward extension of the Arrhenius law.

Scaling law for diffusion coefficients in simple melts

The constant-pressure molecular dynamics simulations based on the second-moment approximation of tight-binding scheme have been performed to study the relationship between the resulting crystallization microstructure of the liquid copper and the cooling rate. Below the glass-forming critical cooling rate, the metastable hcp phase and the stable fcc phase can coexist in the resulting configuration with all sorts of proportion and various forms such as layering and phase separation. The sizes and the distributions of the two crystalline phases depend on the cooling rate: the faster the cooling rate the larger percents of the metastable hcp phase and the more easily the layering take place. From the split of peaks of the angular distribution function, for both hcp and fcc phase the faster the cooling rate is the more imperfect they are, which may be considered as the precursor of glass-forming.

The cooling rate dependence of crystallization for liquid copper: A molecular dynamics study

Damping measurements were performed to examine the damping behavior of foamed aluminum (FA). It was found that the damping capacity of FA is higher than that of its parent metal, and the major contribution of pores to the damping of the material is to raise the internal friction background, particularly at elevated temperatures. The damping mechanisms can be interpreted by the nonuniform stress and strain distributions originating from the inhomogeneous pore structure morphology, the number of boundaries formed by the pores, the high density of defects, and their movement and interaction.

Liu Changsong

Papers

Electron- or hole-assisted reactions of H defects in hydrogen-bonded KDP.

Liquid-liquid phase transition in Pb-Sn melts

Scaling law for diffusion coefficients in simple melts

The cooling rate dependence of crystallization for liquid copper: A molecular dynamics study

Damping behavior of foamed aluminum