The Dynamic Mechanical Properties. Ultrafine Grained Pure Copper

Grain size plays an important role on dynamic behavior, as in contrast to those of coarse grain size materilas, ultra-fine-grained (UFG) materials are proved to be higher stability subject to high-strain-rate deformation. Equal channel angular pressing (ECAP) is one of the most popular methods as which is highly effective in fabricating various of UFG materials while remaining the bulk size at the same time. Based on the evolution of microstructure during the high strain and high-strain-rate, studied the mechanical properties and adiabatic instability, provide a theoretical basis for the application of UFG copper under extreme environment.So in the present work a systematic investigation on grain refinement by ECAP processing was conducted, where a strain level of 1-8 passes were processed by ECAP on a 99.97wt% copper. Three strain rates of 1000 s-1,1500 s-1 and 1700 s-1 were chosen for dynamic loading by Split Hopkinson Pressure Bar (SHPB) test. And quasi-static compression test was also performed by a strain rate of 10-4 s-1 as a comparison. Optical microscope (OM),second electronic microscope (SEM) and transmission electron microscope (TEM) were used to characterize the microstructure after deformations.The results indicated that the grain size decreased with the increasing passes after ECAP, with the high extraordinary change from 65μm to 0.4μm after one pass, more equiaxed homogeneous microstructure formed after eight passes. Quasi-static mechanical deformation showed that the yield stress reach maximum (449MPa) after four passes. Acorrording to the results of high-strain-rate, obvious strain-rate hardening occurred, undergoing the 1700s-1 deformation, the sample of eight passes reached for 1245MPa. It showed the strain-rate-sensitivity (m) increased from 0.0058 (coarse copper) to 0.042 (UFG copper). Microstructure indicated that high density of dislocations and twins existed inside the grains, which were responsible for the high flow stress. And we also observed the occurrence of dynamic recrystallization in UFG copper during high-strain-rate deformation. This effect is interpreted as due to an acceletated thermal softening and inherent instability exhibited in the UFG structure. A modified Johnson-Cook constitutive equation was found to closely capture the dynamic response in the UFG materials.