Nanoindentation Of Al_0.3CoCrFeNi High Entropy Alloy Based On Molecular Dynamics Simulation

High-entropy alloys are new metallic materials that have emerged in the last two decades and have many excellent mechanical properties than traditional alloys.It is considered by researchers to have great application potential in many fields.The high-entropy alloy devices are inevitably subjected to kinds of mechanical loadings during practical services,causing plastic deformation of the material.When subjected to cyclic loading,the plastic deformation and the related damage evolution of high-entropy alloy materials have important impacts on the assessment of the service reliability of high-entropy alloy devices.Therefore,it is of great engineering and economic significance to carry out research on the plastic deformation behavior of high-entropy alloys and their deformation mechanisms.In recent years,many researches on deformation behavior of high entropy alloys have been carried out by scholars.However,the research on the plastic deformation mechanism of high-entropy alloys is still mostly on the experimental side,and the plastic deformation mechanism at the atomic scale still needs to be investigated.For the plastic deformation mechanism of Al_0.3CoCrFeNi high-entropy alloy under single loading and unloading and deterioration mechanism under cyclic loading and unloading,the following studies were carried out:（1）The mechanism of plastic deformation of nanoindentation at the atomic scale under single loading and unloading of Al_0.3CoCrFeNi high-entropy alloy was investigated by molecular dynamics simulation,and the effects of ambient temperature,indentation speed and crystal structure on the mechanical properties of Al_0.3CoCrFeNi high-entropy alloy nanoindentation were investigated.The simulation results show that the interaction of material softening caused by plastic yield and strain hardening caused by Stair-rod partial dislocations and Hirth partial dislocations leads to sawtooth-shaped fluctuations in the force-penetration curve.With increasing indentation depth,the region of plastic deformation expands outward and the surface stacked atoms keep increasing.Shockley partial dislocations firstly nucleate in the Al_0.3CoCrFeNi high-entropy alloy and induce the formation of stacking faults.Due to the influence of lattice distortion,Shockley partial dislocations and stacking faults grow and slip in an incomplete fourfold symmetry along the{111}plane.The generation of stacking faults,various dislocations such as perfect dislocations,Stair-rod partial dislocations,Hirth partial dislocations,Frank partial dislocations,etc.,and dislocation loops due to the dislocation-dislocation interactions are observed in the simulation cell.The nucleation and growth of Shockley partial dislocations and stacking faults are the main mechanisms for the plastic deformation of Al_0.3CoCrFeNi high-entropy alloy.At the low temperatures,twinning boundaries,HCP phases and Lomer-Cottrell locks impede dislocation motion and enhance material plastic deformation.As the temperature increases,the disordered structure increases and limits the growth and slip of dislocations,which further leads to a reduction in material strength.At larger indentation speeds,the high density of dislocation cells induced by massive dislocation entanglement hinders dislocation slip and leads to significant strengthening effects in Al_0.3CoCrFeNi high-entropy alloy.Grain boundaries and twinning boundaries in nanoindentation impede dislocation and stacking faults growth and slip,which has a significant effect on the plastic deformation of nanocrystalline and nanotwin polycrystalline Al_0.3CoCrFeNi high-entropy alloys.（2）The cyclic degradation（i.e.,the cyclic accumulation phenomenon of residual depth after unloading）of the Al_0.3CoCrFeNi high-entropy alloy nanoindentation during cyclic loading and unloading and its deformation mechanism at the atomic scale are investigated.The effects of peak load,ambient temperature,indentation speed and crystal structure on the cyclic degradation of the cyclic nanoindentation of Al_0.3CoCrFeNi high-entropy alloy are discussed.The simulation results show that the cyclic nanoindentation of Al_0.3CoCrFeNi high-entropy alloy exhibits significant cyclic degradation.The residual depth of cyclic indentation after unloading increases with the number of cycles.The accumulated residual depth of the single-crystal Al_0.3CoCrFeNi high-entropy alloy in cyclic nanoindentation loading and unloading is mainly caused by dislocation slip and the accumulation of residual HCP structures and disordered structures.The magnitude of the residual depth depends on the peak loading,ambient temperature and indentation speed.The residual depth after cyclic nanoindentation unloading of single-crystal Al_0.3CoCrFeNi high-entropy alloy increases with peak loading and temperature and decreases with indentation speed.For nanocrystalline structures,the slip and diffusion of grain boundaries are also responsible for the accumulation of residual depth in cyclic nanoindentation loading and unloading of Al_0.3CoCrFeNi high-entropy alloy.