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Nanoglasses (NGs), metallic glasses (MGs) with a nanoscale grain structure, have the potential to considerably increase the ductility of traditional MGs while retaining their outstanding mechanical properties. We investigated the effects of composition on the structural and mechanical properties of CuZr NG films with grain sizes between 3 to 15 nm using molecular dynamics simulations. Results indicate a transition from localized shear banding to homogeneous superplastic flow with decreasing grain size, although the critical average grain size depends on composition: 5 nm for Cu36Zr64 and 3 nm for Cu64Zr36. The flow stress of the superplastic NG at different compositions follows the trend of the yield stress of the parent MG, i.e., Cu36Zr64 yield/flow stress: 2.54 GPa/1.29 GPa and Cu64Zr36 yield/flow stress: 3.57 GPa /1.58 GPa. Structural analysis indicates that the differences in mechanical behavior as a function of composition are rooted at the distinct statistics of prominent atomic Voronoi polyhedra. The mechanical behavior of NGs is also affected by the grain boundary thickness and the fraction of atoms at interfaces for a given average grain size. The results suggest that the composition dependence of the mechanical behavior of NGs follows that of their parent MGs, e.g., a stronger MG will generate a stronger NG, while the intrinsic tendency for homogeneous deformation occurring at small grain size is not affected by composition. [ABSTRACT FROM AUTHOR]

Deformation mechanisms in α-alumina under hypervelocity impact are investigated using molecular dynamics simulations containing 540×106 atoms. A cylindrical projectile impacting normal to the (0001) surface at 18 km/s generates large temperature and pressure gradients around the impact face, and consequently local amorphization of the substrate in a surrounding hemispherical region is produced. Away from the impact face, a wide range of deformations emerge and disappear as a function of time under the influence of local stress fields, e.g., basal and pyramidal slips and basal and rhombohedral twins, all of which show good agreement with the experimental and theoretical results. New deformation modes are observed, such as twins along {0111}, which propagate at a roughly constant speed of 8 km/s and nucleate a large amount of defects where subsequent fractures initiate. The relation between deformation patterns and local stress levels is investigated. During unloading, we observe that microcracks nucleate extensively at the intersections of previous deformations within an hourglass-shaped volume that connects top and bottom free surfaces. From the simulation, the fracture toughness of alumina is estimated to be 2.0±0.5 MPa[Square_Root]m. The substrate eventually fails along the surface of the hourglass region during spallation when clusters of substrate material are ejected from both free surfaces. [ABSTRACT FROM AUTHOR]

Reports on molecular dynamics (MD) simulations performed to investigate structural, mechanical and vibrational properties of Ga[sub 1-x]In[sub x]As alloys in the crystalline and amorphous phases. Two-mode behavior for high-frequency optical phonons exhibited by the calculated phonon density of states; MD results for the amorphous spaces.

Molecular dynamics simulations are used for examining the deformation mechanisms in [alpha]-alumina under hypervelocity impact. The substrate has failed along the surface of the hourglass region during spallation when clusters of substrate material are ejected from both free surfaces.

The classical and ab initio molecular dynamics (MD) simulations of Ga(sub 1-x)In(sub x)As alloys are reported and the structural, mechanical and dynamical properties of these alloys for both crystalline and amorphous phases are investigated. The results show a local distortion in the crystalline lattice, which is characterized by two well-defined bond lengths.

Large-scale molecular dynamics simulations are carried out to investigate 3C-SiC single crystals under plane shock loading. From the analysis of the shock profile and the evolution of the atomic structures, we demonstrate the interplay between the shock induced elastic compression, plastic deformation, and structural phase transformation (SPT). Particle velocity ranges from 0.4-6.0 km/s. The predicted shock Hugoniot is divided into four regions. Below 2 km/s, only elastic shock induced compaction exists. Plastic deformation appears between 2-3 km/s, while the SPT emerges above 3 km/s, when the induced pressure reaches over 90 GPa. Plastic deformation in this cubic crystalline material is marked by extensive deformation twinning while at high pressure the original low pressure zinc blend structure is transformed into the more compact rocksalt structure. For particle velocity above 4.5 km/s, a single overdriven transformation wave propagates in the system. These simulation results provide an atomistic view of the dynamic effects of shock impact on single crystal high-strength ceramics. [ABSTRACT FROM AUTHOR]

Structural, mechanical, and vibrational properties of Ga[sub 1-x]In[sub x]As (0 ≤ x ≤ 1) random solid solutions are investigated with classical and ab initio molecular-dynamics simulations. We find that the Ga-As and In-As bond lengths change only slightly as a function of x, despite the large lattice mismatch (∼7%) between GaAs and InAs crystals. The nearest cation-cation distance has a broad distribution, whereas the nearest neighbor anion-anion distance distribution has two distinct peaks. The elastic constants exhibit a significant nonlinear dependence on x. The phonon density-of-states exhibits two high-frequency optical modes. These results are in excellent agreement with experiments. [ABSTRACT FROM AUTHOR]

The deformation of graphene due to the chemisorption of hydrogen atoms on its surface and the long-range elastic interaction between hydrogen atoms induced by these deformations are investigated using a multiscale approach based on first principles, empirical interactions, and continuum modeling. Focus is given to the intrinsic low-temperature structure and interactions. Therefore, all calculations are performed at T=0, neglecting possible temperature or thermal fluctuation effects. Results from different methods agree well and consistently describe the local deformation of graphene on multiple length scales reaching 500 Å. The results indicate that the elastic interaction mediated by this deformation is significant and depends on the deformation of the graphene sheet both in and out of plane. Surprisingly, despite the isotropic elasticity of graphene, within the linear elastic regime, atoms elastically attract or repel each other depending on (i) the specific site they are chemisorbed; (ii) the relative position of the sites; (iii) and if they are on the same or on opposite surface sides. The interaction energy sign and power-law decay calculated from molecular statics agree well with theoretical predictions from linear elasticity theory, considering in-plane or out-of-plane deformations as a superposition or in a coupled nonlinear approach. Deviations on the exact power law between molecular statics and the linear elastic analysis are evidence of the importance of nonlinear effects on the elasticity of monolayer graphene. These results have implications for the understanding of the generation of clusters and regular formations of hydrogen and other chemisorbed atoms on graphene. [ABSTRACT FROM AUTHOR]