Molecular Dynamic Simulation Of Shock-compression Response Of Ni+Al Nanopowders

Rective materials consist of two or more solid non-explosive materials,such as metal+metal,metal+metal oxide,metal+polymer,etc.which may undergo violent chemical reactions upon ignition or subjected to sufficiently strong shock loading.Such energetic materials integrate high energy density,high reaction temperature and mechanical strength.These materials are widely used in civil and military applications.Shock-compression experiments have shown that micron and/or nanoscale reactive metal powder mixtures can undergo ultra-fast chemical reactions under certent condition.However,the reaction is affected by many factors,such as shock conditions,distribution of components,particle size and morphology.However,due to the limitation of current real-time diagnostic techniques,the ultrafast mixing and chemical reaction mechanism are not fully understood.In this work,a typical reactive metal particle mixture system(Ni+Al)was selected and the molecular dynamics was used to simulate the propagation of shock waves in the particle mixtures.The deformation,mixing and reaction mechanism of the mixtures under different conditions were explored.In this study,Ni and Al spherical nanoparticles were first arranged in a NaCl-like structure(about 60%theoretical maximum density,TMD).Then the deformation and reaction characteristics from particle level to atomic level under different piston velocities were studied.The simulation results show that the shock-induced chemical reaction(SICR)occurs during the non-equilibrium process,accompanied by a sharp rise in temperature and rapid mixing of atoms.The preferentially deformed Al particles form a high-speed mass flow relative to the Ni near the shock front,which impinges on the Ni particles,and mixing of Ni and Al atoms occurs immediately at the interface.The particle velocity dispersion(PVD)that appears at the shock front has important implications for the initiation of shock-induced chemical reactions.Dislocations are mainly generated at the beginning of particle deformation or at the shock front,and do not directly affect the occurrence of SICR.The intimate contact of the molten Al and the amorphous Ni is found to be critical to the subsequent reactions for the extensive mixing of Ni and Al atoms.The mechanisms of SICRs involve mechanochemical process that near the shock front and the subsequent interdiffusion process that at the shocked region.The shock response of Ni+Al nanoparticle mixtures with different arrangements and densities were also explored.The powder configurations with varying arrangements and densities were constructed by stacking equal-sized Ni and Al particles based on five typical crystal structures,i.e.,zinc-blende,NaCl,CsCl,AuCu and the close-packed.Different particle coordinations or stacking modes result in different densities(coordination number from 4 to 12,the corresponding density is 40%to 91%TMD).The results indicate the important role of particle coordination number and density of mixture in shock response of energetic powder materials.Significant dependence of shock wave velocity,plastic deformation,temperature response,chemistry and microstructure change on particle packing and density is observed under shock loading at the same piston velocity,but the dependence on stacking mode with the same density is relatively weak.A reaction kinetic model was proposed to describe the rapid mixing and reaction behaviors during dynamic loading.This model can assess the extent of the mechanochemical effect and the rate of subsequent interdiffusion.In reality,for mechanical properties,reactive materials with high TMD are required in many applications and the materials with fully dense are desired.The nearly fully dense Ni+Al grain mixtures were created following the construction of polycrystalline in this study.The loading velocity ranges from 0.6 to 3.0 km/s was studied.Under the impact of medium and low strengths,the particle velocity dispersion phenomenon also exists at the shock front,and increases with the increase of shock strength.The displacement change and plastic deformation mainly occur at grain boundaries or the junction of grains during the shock rise.The temperature is higher in the region with severe plastic deformation,and hot spot tends to form.The composite exhibits higher strength and lower reactivity than mixtures with certain porosity.For U_p≥2.5 km/s,due to the shock-induced preferential melting of Al and the formation of a fluid plastic flow,the particle velocity dispersion has a tendency to expand from the shock front to the shocked region and the heterogeneous velocity field is presented in the shocked region,which may lead to local shear and erosion and thus accelerate the intermixing of atoms.The shock-induced preferential melting of a component can also initiate the mechanochemical behavior and play an important role in the ultra-fast chemical reactions.