Mapping mesoscale heterogeneity in the plastic deformation of a copper single crystal

Abstract

Part of a ‘multiscale characterization’ study of heterogeneous deformation patterns in metals is reported. A copper single crystal was oriented for single slip in the (111)[] slip system and tested to ∼10% strain in roughly uniaxial compression. The macroscopic strain field was monitored during the test by optical ‘image correlation’. The strain field was measured on orthogonal surfaces, one of which (the x-face) was oriented perpendicular to [] and contained the [] direction of the preferred slip system. The macroscopic strain developed in an inhomogeneous pattern of broad, crossed shear bands in the x-face. One, the primary band, lay parallel to (111). The second, the ‘conjugate’ band, was oriented perpendicular to (111) with an overall () habit that contains no common slip plane of the fcc crystal. The mesoscopic deformation pattern was explored with selected area diffraction, using a focused synchrotron radiation polychromatic beam with a resolution of 1–3 µm. Areas within the primary, conjugate and mixed (primary + conjugate) strain regions of the x-face were identified and mapped for their orientation, excess defect density and shear stress. The mesoscopic defect structure was concentrated in broad, somewhat irregular primary bands that lay nominally parallel to (111) in an almost periodic distribution with a period of about 30 µm. These primary bands were dominant even in the region of conjugate strain. There were also broad conjugate defect bands, almost precisely perpendicular to the primary bands, which tended to bridge primary bands and terminate at them. The residual shear stresses were large (ranging to well above 500 MPa) and strongly correlated with the primary shear bands; interband stresses were small. The maximum resolved shear stresses within the primary bands were oriented out of the plane of the bands, and, hence, could not recover the dislocation structure in the bands. The maximum resolved shear stresses in the interband regions lay predominantly in {111} planes. The results are compared to the mesoscopic defect patterns found in Cu in etch pit studies done some decades ago, which also revealed a mesoscopic dislocation structure made up of orthogonal bands.