Recently, the stress-strain relationships of high purity Al and Cu were investigated over a wide range of strain by combining data obtained in conventional tensile and compression testing of annealed samples with data obtained after processing by equal-channel angular pressing (ECAP) to high imposed strains. It was shown that the nature of the macroscopic stress-strain relationship characteristically changes in different regions of the testing temperature. In the low temperature region the macroscopic stress-strain behavior shows a monotonously increasing tendency over the entire range of strain inherent in these experiments, while at high testing temperatures the flow stress increases only up to a certain strain. It was also shown that in the positive strain-hardening region the stress-strain relationship can be described by an exponential-power law constitutive equation which includes the main features of the conventional Hollomon power-law and the Voce exponential relationships. On the basis of this new equation, low and high temperature deformation regions can be defined. In the present work, additional results are reported on the mechanism of steady-state flow of pure Al deformed at low temperatures. Results obtained by compression and indentation tests on ECAP Al samples, in addition to atomic force microscopy (AFM), show that in the low temperature region grain boundary sliding (GBS) is a significant mechanism of plastic deformation at high strains.