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An important theoretical ductility criterion for group V and VI metal-based refractory alloys in body-centered cubic (bcc) lattices is the mechanical failure mode of their perfect crystals under tension along the weakest direction [100]. Pure Mo and W fail by cleavage and are deemed intrinsically brittle. However, first-principles calculations show that alloying with group IV or V transition metals can transform these materials into ones that display intrinsically ductile behavior, failing in shear under [100] tension. Remarkably, this transition can be understood as an electron filling effect with the intrinsically ductile response the manifestation of a Jahn-Teller distortion. [ABSTRACT FROM AUTHOR]

A continuum theory is formulated to describe dislocations within graphene. The bending modes and defect formation energies predicted by the theory are in good agreement with atomic scale total energy calculations. Dislocations induce out-of-plane buckling that screens their strain fields and enables the identification of a dislocation formation energy. Further, the theory provides an efficient means to model nanoidentation of suspended graphene. [ABSTRACT FROM AUTHOR]

Monte Carlo simulations within the isoenthalpic-isotension-isobaric ensemble are employed to compute the elastic properties and relevant thermodynamic functions of freestanding monolayer graphenes in order to explore the influence of thermal ripples on the elastic properties of graphene. Thermal ripples soften the elastic constants, and lead to the negative thermal expansion coefficient observed experimentally at low temperatures. [ABSTRACT FROM AUTHOR]

We present direct observations of room temperature dislocation plasticity in single crystalline silicon. Previous studies have shown that phase transformation and fracture are the relevant mechanisms of deformation in silicon. In contrast, using in-situ nanoindentation in a transmission electron microscope we find dislocation nucleation and metal-like flow. The results of finite-element modelling suggest that the presence of free surfaces in our unique sample geometry leads to preferential surface nucleation of dislocations and the suppression of phase transformation. [ABSTRACT FROM AUTHOR]

A simple statistical analysis of the structure or superdislocations in L1[sub 2] compounds displaying the yield strength anomaly is developed. This analysis is based on the superkink model of dislocation dynamics, where it is assumed that a superdislocation propagates through the lateral motion of superkinks. The analysis is used to assess the probability that a superdislocation is immobile, and this probability is found to depend exponentially on the nominal length of the superdislocation. The statistical analysis requires knowledge of the global distribution of superkink lengths to obtain quantitative relative immobilization rates; this information is extracted from published experimental work and also from previous simulations. The implications of both superkink distributions are explored. The predictions of the analysis appear consistent with the results from simulations. [ABSTRACT FROM AUTHOR]

Ion-beam-synthesized 74Ge nanocrystals embedded in an amorphous silica matrix exhibit large compressive stresses in the as-grown state. The compressive stress is determined quantitatively by evaluating the Raman line shift referenced to the line position of free-standing nanocrystals. Postgrowth thermal treatments lead to stress reduction. The stress relief process is shown to be governed by the diffusive flux of matrix atoms away from the local nanocrystal growth region. A theoretical model that quantitatively describes this process is presented. [ABSTRACT FROM AUTHOR]

Provides a corrected reprint to an article previously published in the October 2000 issue of 'Philosophical Magazine A.'

We predict that C, N, and O dopants in TIBr can display large, localized magnetic moments. Density functional theory based electronic structure calculations show that the moments arise from partial filling of the crystal-field-split localized p states of the dopant atoms. A simple model is introduced to explain the magnitude of the moments. [ABSTRACT FROM AUTHOR]

A simple mathematical argument explains a recently identified route for the ion beam synthesis of nanoclusters with a narrowed size distribution. The key idea is that growth conditions for which the average nanocluster size is increasing rapidly can lead to narrowed size distributions. Modeling candidate processes using a self-consistent, mean-field theory shows that normalized nanocluster size distributions with full-width at half-maximum of 17% of the average can be attained. [ABSTRACT FROM AUTHOR]

Pulsed-laser melting (PLM) is commonly used to achieve a fast quench rate in both thin films and nanoparticles. A model for the size evolution during PLM of nanoparticles confined in a transparent matrix, such as those created by ion-beam synthesis, is presented. A self-consistent mean-field rate equations approach that has been used successfully to model ion beam synthesis of germanium nanoparticles in silica is extended to include the PLM process. The PLM model includes classical optical absorption, multiscale heat transport by both analytical and finite difference methods, and melting kinetics for confined nanoparticles. The treatment of nucleation and coarsening behavior developed for the ion beam synthesis model is modified to allow for a nonuniform temperature gradient and for interacting liquid and solid particles with different properties. The model allows prediction of the particle size distribution after PLM under various laser fluences, starting from any particle size distribution including as-implanted or annealed simulated samples. A route for narrowing the size distribution of embedded nanoparticles is suggested, with simulated distribution widths as low as 15% of the average size. [ABSTRACT FROM AUTHOR]

Isotopically pure 74Ge nanocrystals were formed in a sapphire matrix by the ion beam synthesis method. In contrast to those embedded in amorphous silica, sapphire-embedded nanocrystals are clearly faceted and are preferentially oriented with respect to the crystalline matrix. In situ transmission electron microscopy of heated samples reveals that the nanocrystals melt at 955±15 °C, very near to the bulk Ge melting point. The Raman spectra indicate that the sapphire-embedded Ge nanocrystals are under compressive stress in the range of 3–4 GPa. The magnitude of the stress is consistent with that expected for hydrostatic pressure arising from solidification. Stress relaxation was not observed for sapphire-embedded Ge nanocrystals; this is attributed to the slow self-diffusion rate of the alumina matrix atoms at temperatures below the nanocrystal melting point. [ABSTRACT FROM AUTHOR]