Dynamic tensile extrusion behavior of coarse grained and ultrafine grained OFHC Cu

Dynamic tensile extrusion (DTE) tests were conducted on a coarse grained (CG) Cu and two types of ultrafine grained (UFG) Cu, one with an elongated UFG structure and another with an equiaxed UFG structure processed by the different routes of equal channel angular pressing (ECAP), in order to compare their metal jet formability under dynamic loading (the strain rate of 105 s−1 order). DTE tests were carried out by launching the sphere samples to the conical extrusion die at a speed of ∼475 m/s. The fragmentation behavior and microstructures of the soft-recovered fragments were examined. The DTE ductility was high in the order of the CG Cu (∼490%), the elongated UFG Cu (∼410%), and the equiaxed UFG Cu (∼340%). The numerical simulation results showed that adiabatic heating under the present DTE conditions can make the sample temperature over 800 K locally. In spite of such high adiabatic heating, dynamic recrystallization did not occur in the CG Cu fragments. Instead, mechanical twins as well as local shear bands were frequently observed in the severely elongated grains along the DTE direction. Mechanical twinning, in particular, is expected to delay dynamic recrystallization in the CG Cu samples. The fragments of UFG Cu samples showed dynamic recrystallization due to (a) their initial high energy state and the large grain boundary area induced by ECAP, and (b) no mechanical twinning due to the high critical stress for mechanical twinning associated with the ultrafine grain size.