| In the process of practical application,crystal defects of metal materials lead that the actual strength and stiffness values are significantly lower than the theoretical expectations.For polycrystalline metals,due to the presence of grain boundary and the diversification of grain boundary orientation,the plastic deformation and fracture behavior of the material are of dominated by grain boundary mechanical behavior,such as grain boundary sliding,dislocations emission at the grain boundary,grain boundaries hindering dislocation slipand fracture along the grain boundary,etc.In the meanwhile,void is one of the important root reasons of material damage,and material mechanical properties are sharp decline due to the void growth and coalescence.Moreover,it has been paid less attention so far on ?-TiAl polycrystalline tensile deformation micro-mechanisms with atomic scale.So in this paper,?-TiAl polycrystalline cell was build based on the Voronoi algorithm,and the ?-TiAl polycrystalline tensile deformation was simulated using molecular dynamics(MD)method,in order to explore the influence of grain boundary and grain boundary voids on the tensile deformation of ?-TiAl polycrystalline micro-mechanisms,then analyzing ?-TiAl polycrystalline tensile deformation grain boundary mechanical behavior with atomic scale.This paper uses MD method to simulate uniaxial tensile deformationof ?-TiAl polycrystalline with different average grain sizes and strain rates.The simulation results show that the stress peak increase with the increase of the tensile strain rate,and the stress peak decrease with the decreasing of the grain size,showing a clear anti Hall-Petch law.Through the observation of atomic configuration,grain boundaries severely distort during the tensile deformation,indicating that grain boundary sliding is the main deformation mechanism.Furthermore,the dislocations only emit at the the triple grain boundary and slip inside the grains,which is a coordination of the grain boundary sliding.Due to the nonuniform distribution of the shear stress in the grain boundaries and at the grain boundaries,several grains have significant rotation,and the grain rotation angles at the cell boundaries are larger than that of the intermediate grains.Dislocation emission is later than the grain boundary sliding and rotation of grains,so grain boundary sliding and grain rotation at the early stage of tensile deformation provides orientation and shear stress conditions for dislocation emission.The spherical void is preset at the location of specific grain boundary(2D/3D grain boundary)in complete ?-TiAl polycrystalline cell,and the spherical radius are 0.5nm 0.8nm 1 nm,respectively.Then the tensile deformation of the polycrystalline cell with voids is simulated by MD method.The simulation result shows that the peak stress of cell has reduced with void at the grain boundaries,and the peak stress has close relationship with the position and size of the voids.Tensile plastic deformation mechanisms of ?-TiAl polycrystalline cells is also grain boundary sliding coordinated by dislocation slipping,and the grain rotation is also accompanied.Meanwhile,the location and size of voids have effect on the initial position of the dislocation nucleation.The crack nucleation occurs at the grain boundary with severe stress concentration,which is similar to that of perfect ?-TiAl polycrystalline cell.However,when the void is nearby the grain boundary,the cell's crack nucleation occurs earlier and its crack also propagates faster.The voids located at the grain boundary which is a certain angle with the tensile direction gradually distort and shrinks subjected to the ununiform shear stress caused by grain boundary sliding and relative rotation of the neighboring grains.Furthermore,the voids eventually healing when the void size is small,and coalesce the cracks propagated along the grain boundary when the void size is larger. |
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