Grain-size-sensitive creep of ice in the 'dislocation creep' regime

Abstract:

Deformation experiments on polycrystalline ice reveal multiple creep mechanisms, each of which can be described by a power law relationship of the form ė ∝ σⁿd^-p, where ė is strain rate, σ differential stress, n the stress exponent, d grain size, and p the grain size exponent. For ice-sheet modeling, the rheological behavior of ice can be described by a composite flow law containing two parallel creep mechanisms - grain boundary sliding creep, which is grain size-sensitive, with n ~ 2 and p ~ 1.4, and dislocation creep, which is grain size-insensitive, with n ~ 4 and p = 0.

Here we demonstrate that deformation in the putative dislocation creep regime, with n ~ 4, is also grain size-sensitive, even for samples with relatively coarse grain sizes of up to 2.5 mm. Nominally constant-strain-rate experiments were conducted on samples of uniform grain sizes of ~0.01, 0.23, 1.3 and 2.5 mm, at temperatures of 243 and 263 K, strain rates from 10^-5 to 10^-3 s^-1, and to axial strains of ~20%, yielding stresses of 4 to 20 MPa. In each experiment, a peak stress was observed at ~2 to 3% strain, when the microstructure (grain size and crystallographic preferred orientation, CPO) is homogenous and little-changed from the starting microstructure. The peak stress increases markedly with increasing grain size; analysis of the data yields ė ∝ σ^3.8d^-.5. The observed grain-size dependence may indicate that grain boundary sliding is an active deformation process in the n ~ 4 creep regime, consistent with theoretical models. With continuing deformation, all samples except those with an initial grain size of 0.010 mm exhibit dramatic strain weakening, caused by a reduction in grain size due to dynamic recrystallization, and the development of a CPO. After a strain of ~20%, a steady-state 'flow stress' is observed, and data for samples of different initial grain size collapse onto a single linear trend of log σ vs. log ė, because all samples for a given strain rate have nominally the same grain size and CPO. The flow-stress data yield a grain size-insensitive, constant-microstructure flow law of the form ė ∝ σ^3.8 that can be applied to modeling high-strain natural environments. Our experiments demonstrate the importance of grain size in the enhancement of ice creep, even for relatively coarse-grained ice deforming via what has previously been described as dislocation creep.