| Nanocrystalline copper has ultra-high strength,which comes from the hindering effect of high-density grain boundaries on dislocation activity at nanometer size.However,the extremely poor mechanical stability of nanocrystalline Cu with grain size of tens of nanometers limits its research and application.Studies have shown that the grain boundary(GB)migration behavior under mechanical load can be significantly suppressed by appropriate pre-annealing treatment,and the mechanical stability of nanocrystalline Cu can be improved,which is manifested by the crystal structure of low energy states generated by inducing GB relaxation.However,the atomic mechanism of pre-annealing-induced changes in GB structure is still unknown.Therefore,a series of Cu bicrystal models with a total number of nineteen are established in this paper,they are<100>,<110>and<111>symmetric tilt grain boundaries(STGBs).The effects of pre-annealing at 400 K,600 K,800 K,1000 K,and 1200 K on the subsequent GB migration behavior were investigated based on molecular dynamics.Exploring the atomic mechanism of the effect of preannealing on GB has important research value for further stabilizing nanocrystalline metal materials,and has important guiding significance for further proposing the preparation method of low energy state and high stability nanocrystalline metal.This paper fully demonstrates the GB atomic structure and shear deformation behavior of the above model,and the reasons for the changes in migration behavior are discussed from the perspectives of dislocation activity,GB excess energy and free volume.Research indicates:(1)For the seven<100>STGBs,for Σ13(510)(θ=22.62°)GB after annealing at 800 K and higher temperatures,the shear stability become worse for the switch of coupling mode of shear-coupled migration motion,the average GB migration speed changes from-7.57 (?)/ns to 16.65 (?)/ns.For Σ17(410)(θ=28.07°)GB and Σ53(720)(θ=31.89°)GB,their shear mechanical responses turned into GB fluctuation when they are annealed at 800 K and higher temperatures,the migration of GB is seriously hindered.For Σ5(210)(θ=53.13°)GB,the shear stability of the GB enhanced only in the model after annealing at 1200 K,the migration of this GB was hindered in the later stage of shearing due to the existence of GB defects.(2)For the seven<110>STGBs,for the Σ33((?)8)(θ=20.1°)GB andΣ33((?)1)(θ=160.0°)GB annealed at 800 K and higher annealing temperature,their shear stabilities are worsen for broken GB structures under shearing,namely,in the initial stage of shearing,the coupled migration of part of the GB is followed by the destruction of the structural units of the GB,and the plastic mechanism gradually changes to the nucleation and emission of dislocations from the GB.The shear mechanical responses of Σ9((?)4)(θ=38.9°)GB after annealing at 1000 K and Σ9((?)1)(θ=141.1°)GB after annealing at 1200 K are similar to a certain degree,their shear stabilities are enhanced,namely,the plastic deformation is mainly dominated by the deformation of the GB structure.(3)For the five<111>STGBs,the annealing treatment has no effect on the shear migration volecities of the GBs with tilt angles ranging from 13.2° to 27.8°.For the Σ39(5(?)2)(θ=32.2°)GB after annealing at 1000 K and 1200 K,their shear stabilities are enhanced for the hysteresis of the occurrence of GB migration and higher shear stress.For the Σ19(3(?)2)(θ=46.8°)GB after annealing at 1000 K and 1200 K,the critical migration stress decreased by about 0.5 GPa,exbiting worsened shear stability,and it underwent obvious thermal migration during the annealing process of all temperatures.In this paper,the effect of pre-annealing on the migration behavior of STGBs in Cu bicrystal was explored,and a new GB migration mode was found.The atomic structure,GB excess energy and free volume were analyzed and discussed.This study provides a new idea for further stabilizing materials with GB s which have shear-coupled migration motion properties. |
PDF Full Text Request