Up-strain Deformation

Motivation

The viscosity of ice evolves with increasing strain, typically showing an initial strain hardening, followed by peak stress and subsequent strain weakening towards a steady flow stress. Most experimental data represent either the peak or the steady-state flow stress, while the microstructural evolution accompanying the strain weakening remains poorly constrained.

Experimental Procedure

Synthetic ice samples with an initially foam-like grain structure and random crystallographic preferred orientation (CPO) were deformed to progressive strains of ~3%, 5%, 8%, 12%, and 20% at temperatures of −10, −20, and −30°C. All experiments were conducted under a constant displacement rate corresponding to a strain rate of ~1 × 10⁻⁵ s⁻¹. The microstructures of the deformed samples were subsequently characterised using cryogenic electron backscatter diffraction (cryo-EBSD).

CPO and grain-size change

After deformation, many grains developed increasingly irregular grain boundaries, and the proportion of small grains rose with strain. At −30°C, a distinct core-and-mantle structure (“necklace” of finer grains encircling larger grains) emerged at higher strains. At warmer temperatures, the c-axes formed small, well-defined circles centred on the compression axis, whereas at colder temperatures they tended to cluster. Overall, grain size decreased systematically with increasing strain.

Neighbouring-grain misorientation

We selected a representative core-and-mantle structure from the −30 °C sample for detailed analysis. The c-axes of small grains are dispersed around those of larger reference grains, with the small grains exhibiting a much broader range of orientations. The boundary-misorientation axes between neighbouring small grains are distributed relatively uniformly. In rock deformation studies, small recrystallised grains often display crystallographic preferred orientations (CPOs) that are randomly dispersed equivalents of the stronger parent-grain CPOs. Such observations are commonly interpreted as evidence for an increasing contribution of grain-boundary sliding (GBS) during deformation.

CPO-pattern evolution

We summarise the opening angle of the c-axis CPO from both previous studies and this work, together with the observed decrease in CPO intensity with decreasing temperature. Earlier studies have suggested that the selective growth of grains favourably oriented for easy-slip systems becomes less active at lower temperatures, reflecting a reduction in grain-boundary migration (GBM) activity.

Our model

The c-axis cone narrows and CPO intensity decreases with falling temperature, reflecting reduced grain-boundary migration and enhanced grain rotation. CPO development is governed by concurrent deformation and recrystallisation processes, including basal slip, grain rotation, and strain-induced GBM. With increasing strain, the c-axis forms a tighter cone, driven by intracrystalline glide and subgrain rotation, while lower temperatures suppress selective growth of easy-slip orientations. Neighbour-pair misorientation data show evidence of recovery and subgrain rotation, suggesting both mechanisms act in parallel. These trends indicate that strain-induced GBM and lattice rotation jointly control CPO evolution, with temperature modulating their relative contributions.