Decoupling between strain localisation and the microstructural record revealed by in-situ strain measurements in polycrystalline ice

Abstract

We explore the links between the evolutions of the strain field and microstructure in coarse-grained Ih ice. Using digital image correlation (DIC), we monitored the evolution of the strain field in two columnar ice samples deformed by creep (uniaxial compression at constant load) at -7 ºC and 0.5 MPa up to 9.5% bulk shortening. After a brief transient (<0.2% bulk strain), in which strain localises nearby grain boundaries, viscoplastic strain concentrates in a few narrow intracrystalline shear bands that eventually extend over multiple grains. Comparison of pre and post-deformation crystal orientation maps shows that strain localisation in shear bands is mainly accommodated by basal glide without creating significant dislocation-related substructures. Severe dynamic recrystallization develops locally at grain boundaries that act as barriers to dislocation motion, especially where transfer of basal shear is ineffective. Full-field simulations reproducing the initial microstructure and experimental setup suggest that dynamic recrystallization is preferentially triggered at locations characterized by high work rates. Overall, these observations indicate that during deformation of coarse-grained Ih ice at high homologous temperatures and fast (experimental) strain rates: (1) recrystallization does not drive strain localisation but accommodates strain incompatibility and (2) large strains can be accommodated by unimpeded basal slip without producing dislocation concentrations. Observation (2) implies that intragranular orientation gradients are unreliable gauges of viscoplastic strain intensity and that the proportion of dislocation types in subgrains does not gauge the relative contribution of different slip systems to deformation.