Atomic Study On Spallation In Single-Crystalline Ni: Effect Of Vacancies

It is very important to study the fracture behavior of materials under high pressure condition for engineering mechanics and technology application.Due to the existence of various defects in engineering materials,people have to pay attention to the impact of these defects on the mechanical properties of materials.In recent years,the influence of defects on the fracture properties of materials has attracted attention.However,the effect of high vacancy concentration on spallation for single crystal Ni has not been studied systematically.Therefore,the non-equilibrium molecular dynamics method,combined with an EAM potential,was used to systematically studied the effects of high concentration vacancy on spallation for single-crystalline Ni under shock loading.Through the simulation of the perfect single-crystalline Ni under the shock loading,it is found that when the impact strength is greater than the Hugoniot Elastic Limit,the shock wave will appear in the two-wave structure,leading to the relationship of Vs-Vp is not a straight line before the plastic wave is overdriven.Plastic deformation mainly produces defects such as stacking faults,dislocations and vacancies,but most of defects will recover in the process of shock wave relaxation,leaving only a small amount of defects for the tensile stress to provide nucleation sites for the tensile stress-induced plastic.With the increase of tensile stress,the new plasticity is intertwined,while the voids tend to nucleate in spall plane.By constructing the defect model with the vacancy concentration of 0%,0.5%,1.0%,1.5%and 2.0%,respectively,the higher the vacancy concentration is found,and the slower the shock wave velocity.The initial vacancies caused the shock wave to appear the two-wave structure so that the free surface particle velocity can’ t support a smooth impact of the plains,the material spall damage is about the same,a certain concentration of random vacancies provides a growth position for the compression plasticity,reducing the material strength,so that there is a certain vacancy concentration of the sample in the shock strength is low of the case can still occur spallation phenomenon.The spallation in shock wave loading along[1 1 0]and[1 1 1]is investigated.The effect of vacancy concentration on the spall damage of Ni showed a strong anisotropy.For the[001]direction,the vacancy concentration seemed to have no effect on the degree of spall damage,mainly because the vacancy concentration has two effects on the spalling process:plasticity increased from vacancy collapse,which not only dissipated some of the applied stress but also provided sites for void nucleation and accelerated dislocation motion.For the[001]direction,the two competing effects were comparable,thereby resulting in a similar spall damage for different vacancy concentrations.For the[110]and[111]directions,when cv≥1.5%,energy dissipation dominated,and it was difficult for the system to attain the stress required for void nucleation.