Deformation and microstructural evolution of ultrafine- and fine-grained OFHC Cu during dynamic tensile extrusion

Abstract

The deformation behavior and microstructural evolution of ultrafine- (UFG) and fine-grained (FG) Cu during the dynamic tensile extrusion (DTE) process were investigated. The DTE tests were conducted with identical projectile velocities using an all-vacuum gas gun. The DTE ductility increases as the grain size increases, in contrast to the previous outcome with coarse-grained Cu exhibiting that the ductility increased with a decrease in the grain size. The fragments were softly recovered and were examined by a micro-Vickers hardness test and microstructural characterization assessments. The hardness profiles of the fragments exhibit a drastic decrease in the UFG-B remnant, while hardness variations are found in the middle fragments for FG-200. A strong dual <001> + <111> texture is developed during the DTE regardless of the UFG and FG sizes. The UFG-B fragments show that the <111> fibers are replaced by the <001> fibers as a result of meta-dynamic recrystallization (mDRX), while the <111> fibers in FG-200 saturate without any extensive reduction. Evidence of the mDRX was found in an analysis of the misorientation-angle distribution, grain morphology, and grain orientation spread. A numerical simulation reveals that the mDRX can occur due to adiabatic heating, with the faster kinetics in the UFG-B originated from the accumulation of higher levels of deformation energy during the DTE process. The present study also confirms that occurrence of mDRX in the UFG and FG Cu triggers a ductile failure and local necking with a decrease in the DTE ductility.