Dynamic behaviour of ECAP-processed aluminium at room and sub-zero temperature

Dynamic behavior of ultrafine-grained (UFG) commercially pure aluminum (AA1070) samples prepared via two different equal-channel angular pressing (ECAP) routes was studied at high strain rates at room and sub-zero temperatures using the Hopkinson Split Bar technique, and at quasi-static strain rates at room temperature using a servohydraulic materials testing machine. For comparison, similar tests were also conducted on coarse-grained AA1070 aluminum samples. 3D- and 2D-ECAP dies were used to deform the initial coarse-grained aluminum to total equivalent plastic strains of ca. 10 using routes Bc and C+Bc, respectively. The ECAP processing increased the strength of the material substantially but at the same time reduced its strain hardening capability. After initial yielding, strain softening took place in all studied UFG materials at quasi- static strain rates, but at high strain rates (above 103 s-1) notable strain hardening was observed. Strain hardening was further enhanced by sub-zero temperatures, at least at high strain rates. The microstructures of the ultrafine-grained and coarse-grained materials were investigated prior to and after the compression tests using a transmission electron microscope. Increased dislocation activity at high strain rates and at sub-zero temperatures suggests dislocation mediated plasticity still to play a substantial role in the deformation of the studied ultrafine-grained materials.