J.H. Li

A simple method for calculating interaction of numerous microcracks and its applications

Abstract The effects of microcrack interaction on the failure behavior of materials present one problem of considerable interest in micromechanics, which has been extensively argued but has not been resolved as yet. In the present paper, a simple and effective method is presented based on the concept of the effective field to analyze the interaction of microcracks of a large number or of a high density. To determine the stress intensity factors of a microcrack embedded in a solid containing numerous or even countless microcracks, the solid is divided into two regions. The interaction of microcracks in a circular or elliptical region around the considered microcrack is calculated directly by using Kachanov’s micromechanics method, while the influence of all other microcracks is reflected by modifying the stress applied in the far field. Both the cases of tensile and compressive loading are considered. This simplified scheme may yield an estimate for stress intensity factors of satisfactory accuracy, and therefore provide a potential tool for elucidating some phenomena of material failure associated with microcracking. As two of its various promising applications, the above scheme is employed to investigate the size effects of material strength due to stochastic distribution of interacting microcracks and to calculate the effective elastic moduli of elastic solids containing distributed microcracks. Some conventional micromechanics methods for estimating the effective moduli of microcracked materials are evaluated by comparing with the numerical results. Only two-dimensional problems have been considered here, though the three-dimensional extension of the present method is of greater interest.

Long-range empirical potential model: extension to hexagonal close-packed metals

An n-body potential is developed and satisfactorily applied to hcp metals, Co, Hf, Mg, Re, Ti, and Zr, in the form of long-range empirical potential. The potential can well reproduce the lattice constants, c/a ratios, cohesive energies, and the bulk modulus for their stable structures (hcp) and metastable structures (bcc or fcc). Meanwhile, the potential can correctly predict the order of structural stability and distinguish the energy differences between their stable hcp structure and other structures. The energies and forces derived by the potential can smoothly go to zero at cutoff radius, thus completely avoiding the unphysical behaviors in the simulations. The developed potential is applied to study the vacancy, surface fault, stacking fault and self-interstitial atom in the hcp metals. The calculated formation energies of vacancy and divacancy and activation energies of self-diffusion by vacancies are in good agreement with the values in experiments and in other works. The calculated surface energies and stacking fault energies are also consistent with the experimental data and those obtained in other theoretical works. The calculated formation energies generally agree with the results in other works, although the stable configurations of self-interstitial atoms predicted in this work somewhat contrast with those predicted by other methods. The proposed potential is shown to be relevant for describing the interaction of bcc, fcc and hcp metal systems, bringing great convenience for researchers in constructing potentials for metal systems constituted by any combination of bcc, fcc and hcp metals.

Abnormal alloying behaviour observed in an immiscible Zr–Nb system

For the immiscible Zr?Nb system characterized by a positive heat of formation (+6?kJ?mol?1), thermodynamic calculation showed that the Gibbs free energy of the properly designed Zr?Nb multilayered films could be elevated to a higher level than that of the corresponding amorphous phase as well as the supersaturated solid solutions. Accordingly, nano-sized Zr?Nb multilayered films were prepared and then irradiated by 200?keV xenon ions. It was found that amorphous phases could be obtained within a composition range 12?92?at% of Nb. Also, two metastable crystalline phases of fcc structures with different lattice parameters were also obtained. Molecular dynamic simulation was carried out, based on a proven realistic Zr?Nb potential, to reveal the atomistic mechanism of the solid-state crystal-to-amorphous transition. A brief discussion on the formation of the two metastable crystalline phases is presented.

Interatomic potential to calculate the driving force, optimized composition, and atomic structure of the Cu-Hf-Al metallic glasses

An interatomic potential is proposed for the Cu-Hf-Al system and applied in molecular dynamics/statics simulations. Simulations predict a hexagonal composition region for the Cu-Hf-Al metallic glass formation. Kinetically, a local maximum driving force, defined by energy difference between the solid solution and disordered state, is predicted to be at Cu48Hf41Al11, close to the experimentally measured optimized composition. Moreover, Voronoi tessellation analysis shows that though the icosahedron and icosidihedron are dominant configurations, the fractions of both icosihexahedron and icosioctahedron decrease with increasing Al content, correlating closely with the atomic radii and the heat of mixing of the component metals.

Magnetic properties of some metastable Co–Ru alloys studied by ion beam mixing and ab initio calculation

Magnetic fcc-Co75Ru25 (L12) phase and nonmagnetic fcc-Co25Ru75 (L12) phase are obtained by ion beam mixing in the respective Co–Ru multilayered films. Interestingly, in the Co50Ru50 multilayered films, a magnetic dual-fcc phase, identified to be a mixture consisting of a magnetic fcc-Co75Ru25 and a nonmagnetic fcc-Co25Ru75 phases, is observed at an irradiation dose of 3×1015 Xe+/cm2 and, upon further irradiation to a dose of 7×1015 Xe+/cm2, transformed into a nonmagnetic single-fcc phase with an alloy composition back to be Co50Ru50.

Ab initio calculations to determine the formation enthalpy of Cu3Au phases

Ab initio calculations ascertain that the difference of formation enthalpy between order and disordered Cu3Au phases is at least 0.044 eV per atom. The calculations also suggest that the tetrahedral clustering configuration and its spatial distribution have drastic effect on the formation enthalpy of the disordered structure and that segregation of the Au atoms significantly lowers the formation enthalpy of the disordered phases.