W.R. Jian

Shock-induced melting of honeycomb-shaped Cu nanofoams: Effects of porosity

We investigate shock-induced melting in honeycomb-shaped Cu nanofoams with extensive molecular dynamics simulations. A total of ten porosities ( ϕ) are explored, ranging from 0 to 0.9 at an increment of 0.1. Upon shock compression, void collapse leads to local melting followed by supercooling at low shock strengths. Superheating occurs at ϕ≤0.1. Both supercooling of melts and superheating of solid remnants are transient, and the equilibrated shock states eventually fall on the equilibrium melting curve for partial melting. However, phase equilibrium has not been achieved on the time scale of simulations in supercooled Cu liquid (from completely melted nanofoams). The temperatures for incipient and complete melting are related to porosity via a power law, (1−ϕ)k, and approach the melting temperature at zero pressure as ϕ→1.

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.

Shock response of open-cell nanoporous Cu foams: Effects of porosity and specific surface area

We investigate the effects of porosity or relative mass density and specific surface area on shock response of open-cell nanoporous Cu foams with molecular dynamics simulations, including compression, shock velocity–particle velocity, and shock temperature curves, as well as shock-induced melting. While porosity still plays the key role in shock response, specific surface area at nanoscales can have remarkable effects on shock temperature and pressure, but its effects on shock velocity and specific volume are negligible. Shock-induced melting of nanofoams still follows the equilibrium melting curve for full-density Cu, and the incipient and complete melting temperatures are established as a function of both relative mass density and specific surface area.

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 60 GPa 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 60 GPa 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 60 GPa) 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.