Dislocation behavior in copper single crystals with initial dislocation densities as much as $5.08\times10^6/cm^2$ was studied. Highly perfect copper crystals with dislocation as low as possible and very few sub-boundaries are desired in the field of crystal plasity. Dislocation densities were reduced by thermal cyclic annealing in whitch the temperature was changed between 1030°C and 850°C with a period of 40 min. The thermal cyclic annealing for 72 hr reduced the dislocation density to $8.94\times10^4/cm^2$ order without producing any sub-boundaries.
The fracture surface of $\Sigma$3 Cu-Bi bicrystal containing about 0.021Wt% Bi was studied with a Scanning Electron Microscope. The embrittling effect of bismuth in copper is well known. The reason for using bicrystals in the present work was to provide, through the grain boundary, a unique path for crack growth. But the fatigue free crack does not follow the boundary. Grain boundary faceting was discovered in the fracture surface of polycrystal. But Cu-Bi bicrystal was not. And $\Sigma$3 Cu-0.021wt% Bi bicrystal is fractured by ductile manner.