| As a coherent interface, twin boundary (TB) is a lower-energy interface than the grain boundary, whose lattice structures are mirror images of each other in the plane of the boundary. Due to its special interactions with dislocation, it is an effective way to increase the strength and toughness of crystalline metals by introducing TBs into crystals. As one kind of internal boundaries to impede the motion of dislocation, TB imposes a strong strengthening effect on nano-crystalline metal. Meanwhile, TB can accommodate dislocations, which favors to sustain plasticity.In the thesis, TB migration is investigated through molecular dynamics simulations. Its detailed mechanism and the effects of temperature on it are also analyzed. What’s more, it is also shown the study of tensile deformation of nano-twinned (NT) materials with nano-voids, including the microscopic deformation mechanisms between nano-voids and coherent TBs, which concerns size effects of void dimension and TB spacing.Detailed atomic analysis reveals that TB migration in single face-centered-cubic (f.c.c.) metals is a process consisting of nucleation, expansion and annihilation of dislocation loops on successive{111} planes. With the aid of thermal fluctuations, the critical shear stress required for TB migration decreases with elevating temperature. A simple model is proposed to describe the strong temperature dependence of critical shear stress of TB migration.The results make it clear that the yielding stress of a crystalline metal embedded with nanoscale voids decreases with an increase of void diameter, while depends much less on the surrounding coherent TBs. However, the strength of NT copper is significantly enhanced by these coherent TBs, which can efficiently impede the motion of dislocations emitted from nanoscale voids. The influence of the spacing between two twin planes, however, seems to be less significant for the yielding stress and strength. |
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