Shock Response Of Open-Cell Nanoporous Ta And Cu:The Molecular Dynamics Simulations

The nanoporous metals are the emerging materials,possessing many attractive properties,such as the low density,large surface area,and thermal and chemical resistance.They are generally applied in structure engineering and shock physics.Complimenting the full-density,nanoporous metals have exhibited different shock response,owing to their different properties and specific surface areas.In this paper,we implement large-scale non-equilibrium molecular dynamics simulation to study the wave propagation,plasticity,heat-release,and void collapse of nanoporous tantalum(np-Ta),with body-centered cubic(bcc)structure,and nanoporous copper(np-Cu),with face-centered cubic(fcc)structure,under shock impact.The main conclusions are listed below:(1)For np-Ta,we systematically investigate the wave propagation,plasticity and void collapse,as well as the effects of porosity,specific surface area and impact velocity,during shock compression.The shock wave propagation presents an impedance,sensitive to porosity,but not to specific surface area.Such surprising phenomena are due to the similar sensitivities in density and stress variations to porosity or specific surface area.Upon impact,shock front shapes change from ramped to steep ones,with increasing porosity,specific surface area or impact velocity,owing to the transition from the heterogeneous to homogeneous plasticity along transverse directions.This transition of plasticity arises by(i)the strong impedance on large deformation bands as porosity increases;and(ii)the transition from deformation twinning to dislocation slips,and to amorphization,as the specific surface area or impact velocity increases.Shock-induced plasticity,including their nucleation,growth and interactions,also facilitates the collapse of voids.(2)For np-Cu,we systematically discuss the heat-release,deformation modes and void collapse,as well as the effects of porosity and specific surface area during shock compression.The heat-release presents insensitivities to porosity under low impact velocities,and the dislocation slip is dominant in deformation.Upon high impact velocity,heat-release shows the sensitivities to porosity,and the melting promotes the deformation of material,which is dependent on the ligament size.With increasing porosity,it undergoes a transition from “heterogeneity” to “homogeneity”.The heat-release also depends on specific surface area,owing to their surface energy release degree and activation energy barrier.The deformation twinning occurs inside under low velocity impact,while the solid disordering,owing to the intersection of higher-density perfect dislocation and the lower-density Stair-rod dislocation,facilitates the structural deformation under high velocity impact.Such plasticity transition change the temperature distribution,i.e.,from “heterogeneity” to“homogeneity”.The void collapse is mainly controlled by two mechanisms,i.e.,the plasticity under low-velocity impact and the internal jetting under high-velocity impact.