Study On Strengthening Mechanisms And Three-dimensional Effect Of Annealing Twin In Face Centered Cubic Metals

High density twin boundaries in FCC metallic materials can significantly improve the strength and ductility of the materials,while maintaining good electrical conductivity and thermal stability.The improvement of the strength of nano-twinned materials is mainly due to the obstruction of the twin boundary to the gliding dislocations.The interaction between dislocations and ? 3 {111} coherent twin boundaries(CTBs)has been systematically studied in the past several decades,including dislocation theoretical analysis,in-situ TEM observation and molecular dynamics simulation.The crucial role of ? 3 {112} incoherent twin interface(ITB)in shock deformation and other special deformation modes has also been reported.However,most of the previous work focused on revealing the strengthening mechanism of ? 3{111} CTB.The annealing twin,however,is a three-dimensional domain.The role of twin lateral side boundaries in material strengthening cannot be ignored,and the influence of ? 3 {111} and ? 3 {112} and the overall effect of three-dimensional twin composition also needs to be revealed.In this paper,large-scale molecular dynamics simulation,dislocation theoretical analysis and in-situ TEM are used to study the strengthening mechanism of ITB in annealing twin,the strengthening mechanism of three-dimensional annealed twin and the reaction mechanism of slip band and twin boundary in alloys.Which will give us more comprehensive and in-depth understanding on nnealing twin induced strengthening mechanisms.The main research contents and conclusions are as follows:(1)Selecting Ni as the research object,an atomic bi-crystal model consisting of ITB and CTBs was constructed,and then molecular dynamics simulation was used to study the interaction mechanism between dislocations and ITB when the glide plane of dislocations perpendicular to the ITB.The interaction between dislocations and ITB are mainly determined by the basic characteristics of the dislocations and the location where the dislocation enters the ITB.When a screw dislocation enters the ITB,its motion path depends on the loading conditions.When the shear stress on the ITB equals to zero,the screw dislocation can enter the ITB and push the twinning dislocation out of the ITB.When the shear stress on the ITB is not zero,the screw dislocation will cross slip onto the ITB,and the twinning dislocation on the corresponding position on the ITB will be pushed out of the ITB.When a single mixed dislocation enters the ITB,it can only pass through the ITB at a special layer,and a new full dislocation is generated at twin domain.Its Burgers vector is the same as that of the entered dislocation.Dislocations pile-up at ITB can promote mixed dislocations to across the ITB.(2)Taking Cu,Ni and Al as the research objects,a three-dimensional annealing twin model was constructed.The interaction between dislocations and three-dimensional annealing twins was studied using molecular dynamics simulation.The results indicated that the interaction between dislocation and 3D annealing twin depends on the characteristics of dislocation,loading stress and material itself.In all simulations,the dislocation bypasses the 3D twin and forms a dislocation ring on the surface of the twin.In Cu,a screw dislocation can cross CTB,then annihilate in the twin and leave dislocation loops on the ITBs.It also can cross slip onto CTB and induce detwinning on CTB.In Ni,a screw dislocation will always cross slip onto CTB,resulting in de-twinning,while leaving dislocation segments on ITBs.In the case of Al,a screw dislocation can cross slip onto CTB inducing de-twinning or cross slip onto the{100} plane in the twin.When a mixed dislocation interacts with the 3D twin,the dislocation segments on the CTB in Cu will dissociate to form a Lomer dislocation on the {100} plane,and then the Lomer dislocation will dissociate to form a Lomer-Cottrell lock in the twin.In Ni and Al,the mixed dislocation can dissociate on CTB,forming a Shockley partial dislocation and a sessile Frank dislocation.(3)During the process of interaction between 3D twin and dislocation,ITBs migration are observed significantly,which will induce de-twinning.Especially in Cu and Ni,but not in Al.The migration ability of ITBs is related to the stress conditions of loading and the nature of the material itself.(4)Taking Cu as the research object,an atomic model including three-dimensional annealing twin and deformation twin was constructed under various crystallographic orientations.Molecular dynamics simulations were used to study the interaction mechanism between deformation twin and three-dimensional annealing twin in FCC metal.The interaction between deformation twin and 3D annealed twin depends on the orientation relationship between two twins and the basic characteristics of the twinning dislocation of the deformed twin.When the twinning plane of deformation twin and annealing twin are not the same twin plane,if the twinning dislocations of deformation twin are edge or screw dislocations,the deformation twin cannot transmit through the CTB of annealing twin.At the same time,crossing twin structure will be formed in the twin-twin crossing region,and the orientation difference between the crossing twin structure and annealing twin is 18°.At the edge of the crossing twin structure there are two dislocation walls composed of geometric necessary dislocations.If twinning dislocations of deformation twin are mixed dislocations,the deformation twin can transmitted through CTBs.When deformation twin and 3D annealing twin are in the same twinning plane,the intersection of two twins will induce de-twinning on the annealing twin and generate a large number of dislocation loops.(5)Co Cr Ni alloy was selected as the research object,and the interaction mechanism between the slip band and the CTB was studied by in-situ loading TEM.We found that when the dislocation band meets the CTB,the dislocation may dissociate,transmist and reflect at the CTB.For the metal with low stacking fault energy,a large number of dislocation bands generated by in-plane slip can propagate and slip to more{111} planes through reflection on the CTB,which can improve the strength of the material as well as its deformation compatibility.At the same time,a large number of deformation twins were produced in the plastic deformation area of the crack tip in the later stage of deformation,which will hinder the crack growth.