| The plastic deformation and mechanical properties depend on their microstructure evolution in metallic materials,which mostly depends on the movement behavior of dislocations.Therefore,the accurate description of the behavior of dislocations is the key to understand the micro-deformation mechanism of materials and its impact on mechanical properties.In face centered cubic(FCC)metals,the dislocation behavior is affected by the dissociation configuration.The dissociation of dislocation is the basis of understanding the mechanism of the cross slip,annihilation and corresponding micro deformation.It is generally believed that stacking fault energy(SFE)is the key parameter affecting dislocation dissociation.However,in practice,the measurement results of the stacking fault energy are very divergent and lack of reasonable explanation.In addition,there are still a lack of clear understandings of the microscopic mechanism of the cross slip and annihilation processes after dislocation decomposition,and there are doubts about their relationship with the macroscopic properties of materials.On the other hand,even for the single screw dislocation,the mechanism of cross slip is still unclear.Besides,the intrinsic parameters in dislocation annihilation and its impact on the macroscopic properties of materials are also in great doubt.Therefore,in this thesis,we choose four common FCC metals,and use the atomistic simulation method of molecular statics and molecular dynamics to study the effects of dislocation configuration on the dissociation behavior of dislocation and the SFE,as well as the cross slip mechanism and annihilation behavior of screw dislocation.By analyzing the movement behavior of dislocations,we can understand the micro mechanism of typical experimental phenomena,and provide theoretical guidance for the optimal design of metallic structural materials.The main results are as follows:1.The effects of dislocation configuration on the dissociation behavior of dislocation and the SFE are discussed.The results show that in a certain dislocation array configuration,with the increase of dislocation density,the effective SFE of materials increases continuously,which can explain the dispersion of the actual measurement value of SFE.At the same time,the normalized expression of the effective SFE with the change of dislocation density are given in this thesis,through which the effective SFE of materials with different deformation degrees can be well predicted.In addition,the results show that the applied stress has the similar effect to the SFE of the material,and it can adjust the dissociation width of the dislocation on the twin boundary to a greater extent,thus affecting the deformation behavior and macro mechanical properties of the corresponding material.2.The effects of glide stress and temperature on the cross slip mechanism of a single screw dislocation were investigated.The results show that the single screw dislocation in Ag undergoes cross slip through Fleischer(FL)obtuse angle mechanism at low temperature and low stress.It changes to Friedel-Escaig(FE)mechanism and the FL acute angle mechanism with the increase of stress.When the temperature increases,the dislocations tend to be localized,and the FE mechanism is the main cross slip mechanism,but the FL acute angle mechanism still exists in the high stress range.With the increase of the SFE,the obtuse angle mechanism of FL disappears gradually,while the FE mechanism is dominant,but the acute angle mechanism of FL still exists under high stress.In this thesis,we systematically analyzed the causes of various mechanism changes,and found new cross slip mechanisms,which enriched the understanding of the single screw dislocation cross slip mechanism.In addition,the quantitative study of the two FL mechanisms shows that the distance between two dislocations on the original slip plane before cross slip determines the transformation of the two FL mechanisms:when the distance is large,it shows the FL acute angle mechanism;when the distance decreases,it changes to the FL obtuse angle mechanism,and the theoretical model well explains the results of atomic simulation.3.A new cross slip mode of single screw dislocation under non-glide stress is found.Through the atomistic simulations,it is found that the screw dissociation width firstly decreases to zero,whereas the finite dissociation width of edge component remains,when the dislocation cross slips under non-glide stress.Based on the Peierls Nabarro model,it is confirmed that the dissociation width of screw component approaches to zero is the more dominant cross slip path in energy,which explains the simulation results well.At the same time,the results show that the path is most affected by the unstable SFE of materials,followed by the intrinsic SFE,whereas the influence of the critical SFE is the least.4.The relationship between the intrinsic parameters of dislocation dipole arrays and the macroscopic deformation of materials is explored.The results show that the critical annihilation distance decreases with the increase of the critical pile-up distance,which explains the existence of the inverse relationship between the strength and plasticity in the tensile deformation of FCC metal and provides microscopic evidence for further understanding the strengthening and toughening mechanism of FCC metals.On the other hand,the results show that the critical passage stress of dipole decreases with the increase of the number of dislocations in the slip plane of the dipole array,so the local slip is easy to occur in the actual deformation of materials.In addition,it also shows that under the action of glide stress,the unlike screw dislocations in Ag with lower SFE annihilate through the FL acute angle and FE mechanisms,while for Cu,Ni and Al with higher SFE,they annihilate mainly through the FE mechanism;under non-glide stress,the cross slip of unlike screw dislocations has the same effect as that of single dislocation,and the critical condition of cross slip is screw type dissociation width equals zero. |
PDF Full Text Request