Using atomic force microscopy to evaluate the development of mesoscopic shear planes in materials processed by severe plastic deformation

Samples of pure Al and a Zn-22% Al alloy were processed by equal-channel angular pressing (ECAP) to give grain sizes of ∼1.3 μm in both materials. Following ECAP, tensile specimens were cut from the as-pressed billets and tested at room temperature using an initial strain rate of 1×10−4 s−1. The surface morphology of these two materials was examined using atomic force microscopy (AFM) and it was found that long mesoscopic shear planes are formed in pure Al over a large fraction of the surface area but no mesoscopic shear planes are formed in the Zn-22% Al alloy. The results are examined with reference to earlier reports of the occurrence of mesoscopic shear planes in pure Cu and pure Ni after processing by ECAP. It is concluded that mesoscopic shear planes do not form in two-phase alloys because of the difficulties of achieving local re-arrangements of the grain boundaries and in other materials the extent of the mesoscopic shear planes depends upon the homologous testing temperature and the grain size of the material.