X.H. Yao

Improved ductility of Cu64Zr36 metallic glass/Cu nanocomposites via phase and grain boundaries

We investigate tensile deformation of metallic glass/crystalline interpenetrating phase nanocomposites as regards the effects of specific area of amorphous/crystalline phase interfaces, and grain boundaries. As an illustrative case, large-scale molecular dynamics simulations are performed on Cu64Zr36 metallic glass/Cu nanocomposites with different specific interface areas and grain boundary characteristics. Plastic deformation is achieved via shear bands, shear transformation zones, and crystal plasticity. Three-dimensional amorphous/crystalline interfaces serve as effective barriers to the propagation of shear transformation zones and shear bands if formed, diffuse strain localizations, and give rise to improved ductility. Ductility increases with increasing specific interface area. In addition, introducing grain boundaries into the second phase facilitates crystal plasticity, which helps reduce or eliminate mature shear bands in the glass matrix.

Short- and medium-range orders in Cu46Zr54 metallic glasses under shock compression

We investigate short- and medium-range orders in Cu46Zr54 metallic glasses, as represented by icosahedra and icosahedron networks, respectively, under shock compression with molecular dynamics simulations. Complementary isothermal compression and isobaric heating simulations reveal that compression below 60u2009GPa gives rise to increased coordination and thus high-coordination-number Voronoi polyhedra, such as icosahedra; however, pressure-induced collapse or thermal disintegration of icosahedra (and subsequently, icosahedron networks) occurs at pressures above 60u2009GPa or at melting, accompanied by free volume increase. The evolutions of the short- and medium-range orders upon shock loading are the effects of compression combined with shock-induced melting. The structural changes are partially reversible for weak shocks without melting (below 60u2009GPa) and irreversible for strong shocks. Crystallization does not occur under isothermal or shock compression at molecular dynamics scales.

Balancing strength, hardness and ductility of Cu64Zr36 nanoglasses via embedded nanocrystals

Superplasticity can be achieved in nanoglasses but at the expense of strength, and such a loss can be mitigated via embedding stronger nanocrystals, i.e., forming nanoglass/nanocrystal composites. As an illustrative case, we investigate plastic deformation of Cu64Zr36 nanoglass/nanocrystalline Cu composites during uniaxial tension and nanoindentation tests with molecular dynamics simulations. With an increasing fraction of nanocrystalline grains, the tensile strength of the composite is enhanced, while its ductility decreases. The dominant interface type changes from a glass-glass interface to glass-crystal interface to grain boundary, corresponding to a failure mode transition from superplastic flow to shear banding to brittle intercrystal fracture, respectively. Accordingly, the indentation hardness increases continuously and strain localization beneath the indenter is more and more severe. For an appropriate fraction of nanocrystalline grains, a good balance among strength, hardness and ductility can be realized, which is useful for the synthesis of novel nanograined glass/crystalline composites with high strength, high hardness and superior ductility.Superplasticity can be achieved in nanoglasses but at the expense of strength, and such a loss can be mitigated via embedding stronger nanocrystals, i.e., forming nanoglass/nanocrystal composites. As an illustrative case, we investigate plastic deformation of Cu64Zr36 nanoglass/nanocrystalline Cu composites during uniaxial tension and nanoindentation tests with molecular dynamics simulations. With increasing fraction of nanocrystalline grains, the tensile strength of the composite is enhanced, while its ductility decreases. The dominant interface type changes from glass-glass interface to glass-crystal interface to grain boundary, corresponding to a failure mode transition from superplastic flow to shear banding to brittle intercrystal fracture, respectively. Accordingly, the indentation hardness increases continuously and strain localization beneath the indenter is more and more severe. For an appropriate fraction of nanocrystalline grains, a good balance among strength, hardness and ductility can be realized, which is useful for the synthesis of novel nanograined glass/crystalline composites with high strength, high hardness and superior ductility.

Spin transition of ferropericlase under shock compression

Planar shock compression experiments are performed at 9–105 GPa on polycrystalline ferropericlase (Mg0.94Fe0.06)O to investigate its Fe2+ spin transition. Forward and reverse impact configurations are used to obtain Hugoniot and shock-state sound velocities. While wave profiles, shock velocity–particle velocity and pressure–density measurements show negligible/weak indications of a phase transition, the shock-state sound speed data clearly manifest a phase transition in the range of 36–62 GPa at the nanosecond time scales. These shock data reveal the phase transition as the spin transition identified in static compression experiments and first-principles calculations.

Torsional buckling analysis of multiwalled carbon nanotubes subjected to thermoelectromechanical loadings

Abstract The theoretical torsional buckling analysis of multiwalled carbon nanotubes (MWCNTs) under thermoelectromechanical loadings has been studied, which is of great significance to the application of carbon nanotubes in the electrical and thermal condition. Based on the Donnell shell theory, the theoretical governing equations of torsional buckling for MWCNTs subjected to thermoelectromechanical loadings have been established. The effects of torsional load, applied electrical and temperature loads, surrounding elastic medium and van der Waals forces between two adjacent nanotubes are taken into account at the same time. The critical buckling condition has been derived in terms of the buckling modes and the parameters that indicate the effects of thermoelectrical change, surrounding elastic medium and the van der Waals forces. Numerical results in general case are obtained for the thermal and electrical effects on torsional buckling of MWCNTs. It is shown that the critical buckling load under certain electrical and temperature field only depends on one factor, that is, the wavenumber of torsional buckling modes. Again, the critical buckling torques of MWCNTs change with the electrical and thermal field. Furthermore, the electrical field has much more powerful influence on the critical buckling torques than the thermal field.