High purity Al (99.99%) is subjected to severe plastic deformation (SPD) at room temperature using a process of twist extrusion (TE). The microstructure evolution and the related change in microhardness are examined with respect to imposed strain. It is shown that subgrains develop after the first TE pass with a size of ∼1.6 μm and this size remains essentially the same for further application of TE passes. However, dislocations become less visible within grains and grain boundaries become straight and well-defined with misorientation angle higher as the imposed strain is increased. The hardness increases with imposed strain when the magnitude of the strain is small. However, as the imposed strain is large, the hardness decreases due to a reduction of dislocations within grains. It is confirmed that these results obtained with TE are consistent with those reported using equal-channel angular pressing (ECAP) and high pressure torsion (HPT), indicating that the microstructural change and the variation of related mechanical properties with straining observed in pure Al are not affected significantly by the methods of SPD processing.
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