Effects Of Dislocation Structure And Deformation Twin On Spallation Damage Of Single Crystal Copper

Spallation damage of ductile metals has been an important problem of scientific and applied interests in materials science,mechanics and physics.Spallation damage of metals usually occurs in bullet armor piercing,traffic accidents,bird crashes and aerospace accidents.In order to develop materials that can effectively resist dynamic damage in industry,researchers need to simulate similar impact events in laboratory,and to analyze deformation and damage mechanisms of such materials under shock loading,in an attempt to modify the structure of materials so as to obtain metallic materials with high strength and high toughness under dynamic loading.Under shock loading,before a metal is damaged by spallation,it is subjected to dynamic deformation caused by compressive and tensile pulses.These deformed substructures,such as dislocation structures and deformation twins,have a decisive influence on spallation damage.On one hand,these deformed substructures may be in favor of nucleation of damage,and on the other hand,they may hinder the growth of damage due to strain hardening.The synergistic effects of different deformed substructures on spallation damage need to be further examined.The present thesis mainly studies the dislocation structures and the formation of deformation twins in single-crystal copper under shock loading,and the effects of these deformed substructures on single-crystal copper spallation damage.Single-crystal copper is widely used in various fields due to its good ductility,thermal conductivity and electrical conductivity.In the present thesis,split Hopkinson bar(SHPB)and dynamic equal channel angular pressing(D-ECAP)technique are used to modify the microstructure of single-crystal copper,and the recovered samples are characterized with transmission electron microscopy(TEM)and electron backscatter diffraction(EBSD).It is expected that singlecrystal copper samples with different dislocation structures and twin structures can be obtained.Then spallation experiments are carried out on the single-crystal copper samples with these different deformation substructures using a single-stage light gas gun.Free surface velocity histories are measured by Doppler pins system,to deduce Hugoniot elastic limit(dynamic yield stress)and spall strength(dynamic fracture stress).The recovered samples from the spallation experiments are characterized with TEM,EBSD,and X-ray computed tomography(XCT)techniques.The conclusions of the present thesis are as follows.The single-crystal copper samples are loaded with a split Hopkinson bar.It is found that as the compression prestrain increases,the dislocation density in the single-crystal copper increases,and the dislocation organizations are in order of scattered dislocations,dislocation tangles,complex dislocation network structure,dense dislocation walls and dislocation cells.In the subsequent spallation experiments,as the prestrain increases,Hugoniot elastic limit increases considerably,while spall strength remains almost unchanged.These results indicate that dense dislocation cell structure can lead to the increase in yield stress of single-crystal copper,but has a weak overall effect on damage evolution under shock loading.XCT characterization of the recovered samples from the spallation experiment reveals that the spallation damage includs ellipsoidal voids and needle-shaped voids,and the ratio of ellipsoidal voids to needle-shaped voids decreases with the increasing of compression prestrain.TEM and EBSD analysis results show that the long axes of these ellipsoidal and needle-shaped voids are parallel to the crystallographic <110> direction.The ellipsoidal voids nucleate and grow in the low dislocation density area,which are insignificantly influenced by dislocations and the associated strain hardening.The needle-shaped voids nucleate on dislocation walls or dislocation cell walls.The dense dislocations result in severe strain hardening,which impedes the void growth in the crystal.The voids can only grow along the dislocation wall,and thus voids are severely elongated along the crystallographic<110> direction.Different twin structures in single-crystal copper are obtained with D-ECAP technique through different impact velocities.The quantity of twins and dislocation density increases with increasing impact velocity.Single-crystal copper samples with different twin structures and dislocation structures are subjected to mechanical tests on the materials testing system(MTS)and spallation tests on the gas gun.Mechanical test results show that the yield strength and fracture strength of single-crystal copper increases first,and then decreases slightly with the increase of twin density and dislocation density.In the spallation experiments,the dislocation structure or associated strain hardening generated by D-ECAP has a weak effect on the voids,and the spall damage tends to nucleate and grow along twin boundaries.As a result,spall strength of the single-crystal copper containing twins are much lower than that of the pristine single-crystal copper without pre-deformation.