The heterogeneous nature of slip in ice single crystals deformed under torsion

Abstract

International audienceIce single crystals were deformed under torsion and dislocation arrangements were analyzed by synchrotron topography at ESRF (European Synchrotron Radiation Facility). Profile analysis of the topographs revealed the scale-invariant character of the dislocation arrangement with long-range correlations. Dislocation density gradients are shown to be slightly anti-correlated as the intensity profile is similar to an anti-persistent random walk-like signal. This analysis reveals the influence of internal stresses on dislocation arrangement up to the sample scale. Similar observations in reversed torsion experiments, together with strong hardening behaviour, allow a mechanism of cross-slip of basal dislocations on prismatic planes to be suggested for interpretation of local dislocation interaction behaviour

Characterizing short-range vs. long-range spatial correlations in dislocation distributions

“…In X-ray-transparent solids such as ice, bulk characterization of dislocation dynamics processes is therefore feasible via X-ray diffraction techniques. Earlier work in that field [8] reported scale-invariance of the axial dislocation distribution in ice single crystals deformed in torsion creep at moderate strain, with longrange spatial correlations also attributed to the lattice distortion and elastic internal stress fields. In contrast, unstrained samples showed no evidence of spatial correlation.…”

“…[8], but since the technique is limited to low dislocation densities, hence to low strains <3%, hard X-ray diffraction was also employed at a larger strain (namely 4:35%). In the present work, radial slices 1 mm thick, of height Dx < h, were extracted from the samples after deformation for hard Xray diffraction analysis.…”

Elastoviscoplastic micromechanical modeling of the transient creep of ice

“…We cannot completely exclude that at least part of this apparent underestimation of the nonbasal reference stresses is attributed to the affine linearization scheme, which is known to overestimate the effective flow stress [Bornert et al, 2001;Masson et al, 2000;Castelnau et al, 2008a], and in particular at high local anisotropy as considered here. But, in any case, the cross slip of basal dislocations on prismatic planes is associated with the long-range internal stress field produced by basal dislocations [Montagnat et al, 2006]. Therefore, this deformation process is made very difficult when single crystals are deformed in compression along or perpendicular to the c axis since basal slip is avoided.…”

“…The cross slip of basal screw dislocations was recently invoked to explain dislocation multiplication and the scaleinvariant character of the dislocation arrangement observed in single crystals deformed under torsion [Montagnat et al, 2006]. The dissociation of screw dislocations on the basal plane [Hondoh et al, 1990] would not prevent this cross slip since the constriction of partial dislocations close to jogs should preexist [Friedel, 1964].…”

“…Concerning the hardening of prismatic slip systems introduced to reproduce the transient creep, it could be related to the interaction at short distance between nonbasal edge dislocations produced by cross slip or the climb of basal dislocations. Indeed, X-ray diffraction experiments [Montagnat et al, 2006] together with finite element modeling [Taupin et al, 2007] have demonstrated that internal stresses can initiate nonbasal slip in single crystal well oriented for basal slip. In such configurations, the nonbasal slip allows a softening of the basal system, and acts as an efficient mechanism of basal dislocation multiplication, while the nonbasal system is hardened by jog interactions with dislocations.…”