Quantitative investigation on deformation mechanism and dynamic recrystallization during localized adiabatic shearing of single crystal copper

The adiabatic shearing of single crystals is realized by using large-sized columnar crystalline pure copper specimens prepared by directional solidification and designing hat-shaped sample to make the shearing region inside a grain under shock loading with split Hopkinson pressure bar (SHPB) in the present work. The pure copper hat-shaped specimens are loaded at strain rates of 6.5, 8.2, and 9.1 x 10(4)s(-1) respectively by SHPB, then the microstructure of sheared zone is investigated with optical microscopic (OM) as well as transmission electron microscopic (TEM) and its evolution process is quantitatively analyzed. The results showed that the flow stress increase with the increasing of strain rate, and the highest flow stress are 618.6, 460.6 and 410.3 MPa, respectively. There is no twinning at 6.5, 8.2 x 10(4)s(-1) due to the highest flow stress cannot reach the twinning critical stress of 517.3 MPa. Although the highest flow stress reached 618.6 MPa that exceed the twinning critical stress, the force acting on the twinning dislocations and slipping dislocations could not satisfy the twinning condition Ftwin > Fslip due to the strong < 001 > texture of the directional solidification, so there is no twinning at 9.1 x 10(4)s(-1). Slip is the dominant deformation mechanism at those three high strain rates. The large-sized dislocation cells at the edges of the ASB gradually transformed into elongated dislocation cells in the middle at strain rate of 6.5 x 10(4)s(-1). There are equiaxed grains with size of 100-200 nm in the middle region of the ASB. The instantaneous grains refinement from 428 mu m to 100-200 nm is the result of dynamic recrystallization of pure copper under its recrystallization temperature (542 K) caused by adiabatic shearing. The sub-grains were formed at 91 mu s, and then changed from sub-grains to equiaxed recrystallized grains with size of 100-200 nm grains at 108 mu s within the whole shearing time of 141 mu s