Atomistic study of metallurgical bonding upon the high velocity impact of fcc core-shell particles

Large scale molecular dynamics simulations were carried out to simulate a particle with ductile, metallic core surrounded by a brittle, chemically inert layer impacting a metallic substrate. Both the particle and the substrate consisted of fcc single crystals. Particle impact velocities ranging from 500 m/s to 1000 m/s were considered. Despite the visible cracks, the brittle shell resisted to impact at velocities of up to 700 m/s. The breakage of the brittle shell seen for higher impact velocities exposed parts of the ductile core, allowing metal-to-metal contact with the substrate. It was found that particle adhesion requires the formation of a minimum amount of metallic bonds. Not fulfilling this condition resulted in the particle bouncing off. In this case, the formation and posterior rupture of metallic bonds that were not strong enough to keep the particle attached to the substrate eventually contributed to reduce the overall particle rebounding velocity. Particle adhesion occurred undoubtedly only for the highest velocity considered in this study, 1000 m/s. A significant degree of particle deformation, associated with usual fcc metal plasticity (i.e., creation/multiplication of dislocations), was observed for all impact velocities. Additionally, the strength of the impact caused partial destruction of the fcc crystalline structure near the particle-substrate contact zone. For the impact velocity of 1000 m/s, the flow of large portions of this amorphous material under shear resulted in jetting and, by partially removing the debris of the shattered brittle shell, large areas of metallurgical bonding, which maintained the particle adhered to the substrate. In spite of its role in the formation of metallurgical bonding, the amorphous material started to crystallize back to an fcc phase, which suggests the amorphous material was a short living, transient phase.