The Effects Of High Strain Rate And Severe Plastic Deformation On Ultrafining The Grain Of Materials

The strength and ductibility of metal materials are commonly increased with the decreasing of the grain size. Especially when the metal materials is fined to be nano-structured,its mechanical performance will improve greatly. The Severe Plastic Deformation method (SPD) is one of the effective means used to produce nano-scale grain metal materials.As a pure mechanical process, the SPD method is to increase the dislocation density in a metal by severe deformation. After the deformed material has been annealed, the energy stored in the dislocations will be released. The recrystallization process of the material will be driven by this energy. Due to the high density of the dislocations, the size of new grains will be fined to a small (in nano-scale) size. The loading adopted in SPD methods is used to being static. The deformation is considered as an only factor which will affect the size of the ultra-fined grains. But, some papers showed that the strain rate would also affect the density of dislocations in a metal as same as deformation. Hence, it is necessary to consider the effects of the strain rate as well as the deformation on the grain size.In this thesis, a pure oxygen-free copper in which original grain size is about 50~60um, is compressed under high strain rate (impacted). After specimens underwent severe plastic deformation by impacted, the microstructure of these specimens is examined by the transmission electron microscope (TEM).Compared to static loading, the dislocation density in the specimens compressed by dynamic loading is increased remarkably, and the distribution of the dislocation in the impacted specimens is more uniform. Furthermore, their grain size is also much smaller for the specimens impacted. TEM examinations show that the impacted specimens have the structure of nano-scale grain.It is found in this study that the strain rate is an important factor which will affect the grain size as well as deformation. It is easier to obtain the nano-structured material with impact SPD method than static SPD method.