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

Abstract

The work reported here is part of a 'multiscale characterization' study of heterogeneous deformation patterns in metals. A copper single crystal was oriented for single slip in the (111)[101] 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 [121] and contained the [101] 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 (101) 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, that 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.

Macrosegmentation and strain hardening stages in copper single crystals under compression

“…Even if special precautions were made to provide free displacement of samples in machine's grips the strain pattern in the subsurface of contact areas differs from that of the sample's bulk. That such is true has been shown both by the results of our experiments reported by Lychagin, (2006), Lychagin et al (2006Lychagin et al ( , 2011 and by the results of other researchers who studied strain in single crystals using a digital image correlation technique (Raabe et al, 2001(Raabe et al, , 2003Zhao et al, 2008;Roters et al, 2010;Magid et al, 2009;Ha and Kim, 2011;Florando et al, 2007).…”

“…These two processes serve to subdivide the single crystal into isolated segments of different scale size. It is worthwhile to note that shear deformation segments or domains described elsewhere (Magid et al, 2009;Lychagin, 2006) are not the same as those discussed above and even have nothing to do with them. The schematics of deformation-induced subdivision in one of the samples is shown in Fig.…”

Mesoscale strain measurement in deformed crystals: A comparison of X-ray microdiffraction with electron backscatter diffraction

“…Results of the surface strain measurements by digital image correlation along with the X-ray microdiffraction observations are given in detail in references [25,26]. The strain maps obtained from the DIC measurements are shown in Figure 2, where the largest amount of deformation is observed along the primary slip direction as well as a direction 90 to the primary.…”

“…The compression samples were painted with a thin layer of white paint, followed by a random pattern of fine black dots which are 2-10 pixels in size (roughly 40-200 mm). Details of the deformation experiment are given elsewhere [25,26].…”

“…This deformation technique (described elsewhere [25]) allows the crystal to deform without the typical end constraints of a fixed-platen test that can impose bending moments on the specimen. A description of the experiment and characterisation of the deformed specimens using X-ray microdiffraction has been recently published [26]. The specimens were deformed to just over 10% nominal strain as measured macroscopically and local strains were Philosophical Magazine 1453 measured over the specimen surface using digital image correlation.…”