Study On The Microscopic Mechanism Of The Low-energy Transition Of Cu Σ15 Grain Boundary Structure And Its Influencing Factors

Nano grained(NG)metallic materials have attracted a lot of attention from researchers at home and abroad due to their excellent strength and wear resistance,etc.However,the lower stability of NG metals impedes the further expansion of their application scope,such as the softening and grain growth induced by grain boundary(GB)migration.In addition to the common alloying methods,heat treatment or plastic deformation-induced GB relaxation can also improve the stability of NG metallic materials,and the latter is especially important for pure NG metals.Mani studies have demonstrated that GB relaxation can proceed not only by GB dissociation mechanism,but also by GB structural transformation.Unlike the GB dissociation mechanism in which a high-energy GB dissociates into two or more low-energy GBs,copper ∑15[112](521)can transform into ∑11 GB with a lower-energy state under a mechanical loading,but the microscopic mechanisms and factors influencing the structural transformation are not yet clear so far.Therefore,in order to investigate the microscopic processes and the essential causes of the structural transformation mediate by dislocations and to improve the comprehensive understanding of the GB relaxation model,the molecular dynamics simulation method was employed in this thesis to investigate the uniaxial tensile deformation of Cu ∑15[112](521)symmetric tilt GB in the direction normal to the GB plane,and to study the dislocation evolution and atomic motion law during the GB structural transformation on the atomic scale.On this basis,the influence of different temperature,grain size,and tensile strain rates on the GB structure transformation was systematically explored.Our work not only contributes to a deep-going understanding of the microscopic mechanism of the transformation process of the Cu ∑15[112](521)GB structure and a comprehensive understanding of the potential GB relaxation models induced by mechanical strain,but also provides a new way and reference for the stabilization of NG metals.The main findings of this thesis are as follows:(1)The Cu ∑15[112](521)GB undergoes an energy-lowering structural transformation to the ∑11[110](113)GB under uniaxial tension.The structural transformation process is mainly completed under the joint action of two Shockley partial dislocations b1=1/6[211]and b2=1/6[121].Firstly,the b1 dislocation nucleates and emits from atoms 5 and 6 of the hexagonal structure unit of ∑15 GB.Then it slips on the {111} plane and causes a misalignment of face-centered cubic(FCC)atoms to form an intrinsic stacking fault.The b2 dislocation then nucleates and emits at the same position with b1,causing a rearrangement of atoms on {111} plane,where the displacement of atoms on the slip plane under the action of these two partial dislocations is equivalent to the motion results of a perfect dislocation 1/2[110],leading to the formation and disappearance of the stacking fault.After the above dislocation movements,the atoms 5 and 6,which originally formed the hexagonal structural unit of the Cu ∑15 GB,break away from the original GB structural unit and enter into grain interior along the {111} plane,i.e.,these two atoms turn into FCC bulk atoms,while the remaining four atoms consisting of the initial GB structure units rearrange and then form the quadrilateral structural unit of the ∑11 GB,in which way the energy-lowering transformation of the GB structure completes.(2)The uniaxial tension results of Cu ∑15 GB at different temperatures(100～300 K)shows that the increase in temperature increases the atomic motion and makes dislocations easier to nucleates and emits,resulting in the structural transformation of the Cu GB at smaller tensile strains;at the same time,due to the increase in atomic motion,the disorder of atomic arrangement increases with the increase in temperature during the structural transformation of the Cu GBs,and thus some stacking faults cannot disappear under the action of dislocations,resulting in an insignificant reduction in the energy of whole GB system.(3)The low strain rate(2×10-4 ps-1)is favorable for the atoms to fully relax to low-energy equilibrium positions,so the energy state after GB structural transition is lower;with smaller grain size(7.9 nm),the generation and disappearance of stacking faults during the structural transition is relatively easier,resulting in the completion of the GB structural relaxation at a smaller tensile strain.