Microstructural evolution in pure aluminum processed by high-pressure torsion

Pure Al (99.99%) was processed using high-pressure torsion (HPT), and Vickers microhardness was measured along the diameter of the disk samples. When all hardness values were plotted as a function of equivalent strain, they fell on a single line having three distinctive regions. The hardness increases with the strain in the first region and, after taking a maximum at an equivalent strain of ∼2, the hardness decreases with further straining in the second region. The third region appears at an equivalent strain of ∼6 or higher as a steady state where the hardness remains unchanged. Electron backscatter diffraction analysis and transmission electron microscopy were conducted in the corresponding regions and microstructural evolution with straining was examined. Grain refining mechanism using HPT was discussed based on the change in the hardness and microstructures.