Severe local strain and the plastic deformation of Guinier-Preston zones in the Al-Ag system revealed by three-dimensional electron tomography

A severe bulk strain, γ ∼ 1, intentionally introduced into an Al-Ag specimen by the equal-channel angular pressing process caused nearly spherical Guinier-Preston (GP) zones and {1 1 1} planar γ' phases to be sheared by dislocation motion. Although distortion of GP zones in the Al-Ag system was observed by Nicholson and Nutting in 1961, the three-dimensional morphology or the amount of strain of the deformed GP zones has not been studied to date. The presence of fine-scale distorted (ellipsoidal) GP zones parallel to the {1 1 1} slip planes is revealed using three-dimensional atomic number, Z-contrast electron tomography. The GP zones lie within localized shear bands, which result from the introduction of the severe strain. The local shear strain, measured at the nanoscale, within the shear band was determined to be 1.83 ± 0.272, a value considerably more than previously expected