Microstructure and mechanical properties of ultrafine-grained aluminum consolidated by high-pressure torsion

Coarse-grained aluminum powder with 99.5 wt% purity was consolidated by high-pressure torsion (HPT) technique at room temperature using a low carbon steel powder holder. In this process, the powder experiences a semi-constrained condition because the internal wall of the powder holder can expand under the load applied during HPT. After 4 turns of HPT a relative density of 99.83% was achieved. The microstructure was characterized by electron backscatter diffraction and X-ray line profile analysis. It was found that the grain size decreased while the dislocation density increased with both increasing the distance from the disk center and the number of HPT turns. The smallest grain size and the maximum dislocation density with the values of 0.41 mu m and 6.8x10(14) m(-2), respectively, were achieved at the periphery of the disks processed for 4 turns. Tensile tests showed that the consolidation of this Al powder by 4 turns of HPT yielded a high ultimate tensile strength and a good ductility (similar to 373 MPa and similar to 22%, respectively). It turned out that the yield strength versus grain size relationship obeys the Hall-Petch equation. This study demonstrates the capability of HPT technique for processing consolidated Al powder with high strength and good ductility.