The impact compression of metal materials occurs in various situations, such as vehicle collision, armor penetration, and high-speed impact welding. The design of new materials with high impact resistance has broad application prospects. In recent decades, the impact response of Cu has been widely reported.
When the shock wave propagates in the direction, the plastic wave of single crystal copper is always overdriven. The impact speed of single crystal copper and polycrystalline copper with different orientations is also significantly different. Due to the existence of grain boundaries (GBs), nano-Cu exhibits unique mechanical properties and microstructure evolution under impact load.
In this context, the team launched research aimed at analyzing the impact resistance and self-healing properties of graphene/nano-twin copper composites.
Graphene/metal-based composite materials have attracted widespread attention due to their excellent mechanical properties, but few studies have introduced graphene into nano-twinned metal-based materials.
In this study, a single-layer graphene sheet was introduced into a nano-twinned Cu (nt-Cu) matrix to construct a nano-layer composite. The impact response of composite materials was studied by molecular dynamics simulation method. Compared with nt-Cu and single crystal Cu, graphene/nt-Cu composites have higher impact resistance and better self-healing ability.
The study found that at an impact velocity of 4.5 km/s, the wrinkling of graphene can trigger the rapid nucleation of a large number of dislocations in the composite material, and under the same conditions, only elastic waves are observed in the metal matrix. The subsequent dislocation propagation effectively absorbs the impact force, resulting in a rapid drop in impact stress and particle velocity.
At a higher impact velocity, there is a synergistic effect between graphene and twin boundaries, which improves the plastic sensitivity of the composite material, resulting in a larger dislocation density and higher impact resistance.
In addition, a fast and smooth self-healing process was also observed during the propagation of the reflected wave. Since graphene has high in-plane rigidity and is not easily disturbed by dislocations, the integrity of the crystal lattice in the graphene/nt-Cu composite material is gradually degraded and has good self-healing properties.
GBs can be used as stress and shear concentration areas to provide nucleation sites and holes for crystal plasticity. In recent years, nano twin Cu (nt-Cu) has attracted widespread attention due to its unique combination of high strength and high toughness. An interesting softening phenomenon (reverse Hall-Petch effect) was observed in nt-Cu.
With the decrease of the twin boundary distance (TBS), the intensity of nt-Cu increases, and reaches the maximum when the TBS is 15 nm, and the intensity decreases when the TBS is 15 nm. The researchers studied the effect of TBS on the nt-Cu shock response and got similar results. The average flow stress after the shock front reaches its maximum when the critical TBS is 1.04 nm, which is caused by two competing dislocation activities.
In summary
MD simulation was used to study the impact response of graphene/nt-Cu nanolayer composites. The composite material has higher impact resistance than pure nt-Cu and single crystal Cu. The wrinkling of graphene can trigger the nucleation of many dislocations, and the rapid propagation of dislocations effectively consumes the energy of the shock wave, thereby reducing the impact stress and particle velocity.
In addition, the propagation of the reflected wave has a self-healing process, but at a higher initial impact velocity, the self-healing tendency of the reflected wave is weakened. The composite material has better self-healing ability than pure nt-Cu, because the movement of dislocations is limited to adjacent graphene layers, and the dislocations cannot penetrate the entire material.
Since graphene/nt-Cu nano-layer composites have high impact resistance and good self-healing ability, it is believed that it can be an effective shock absorber, which has broad applications in the fields of automobiles, furniture, construction and shipbuilding.