Abstract
A sound knowledge of fundamental mechanical properties of water ice is of crucial importance to address a wide range of applications in earth science, engineering, as well as ice sculpture and winter sports, such as ice skating, ice fishing, ice climbing, bobsleighs, and so on. Here, we report large-scale molecular dynamics (MD) simulations of mechanical properties of bi- and poly-crystalline hexagonal ice (Ih) under mechanical loads. Results show that bicrystals, upon tension, exhibit either brittle or ductile fracture, depending on the microstructure of grain boundaries (GBs), whereas they show ductile fracture by amorphization and crystallographic slips emitted from GBs under compression. Under shearing, the strength of bicrystals exhibits a characteristic plateau or sawtooth behavior drawn out the initial elastic strains. Nanograined polycrystals are destabilized by strain-induced amorphization and collective GB sliding. Their mechanical responses depend on the grain size. Both tensile and compressive strengths decrease as grain size decreases, showing inverse Hall-Petch weakening behavior. Large fraction of amorphous water structure in polycrystals with small grain size is mainly responsible for the inverse Hall-Petch softening. Dislocation nucleation and propagation are also identified in nanograined ice, which is in good agreement with experimental measurements. Beyond the elastic strain, a combination of GB sliding, grain rotation, amorphization and recrystallization, phase transformation, and dislocation nucleation dominate the plastic deformation in both bicrystals and polycrystals.
Highlights
Water is one of the most essential substances to life on Earth
A water molecule is composed of one oxygen atom and two hydrogen atoms, and the oxygen atom is covalently bonded to the two hydrogen atoms
Repeated process of reformation and destruction of strongly cohesive grain boundaries (GBs) results in the perfect sawtooth-fluctuation of shear strength. These results indicate strong anisotropy in shear responses of GBs that is composed of pentagonal, hexagonal and heptagonal water structures. 1/3 1 2 1 0 dislocation intermittently occurs during the plastic deformation (Figure 5d)
Summary
This is attributed to the grain-size effect on the balance of operative deformation mechanisms.[47] when the strain rate exceeds the value of 4×10-6/s, it increases as the grain size increases This is explained by internal micro-fracturing and extensive deformation in GB regions.[47] Upon compression, polycrystalline ice exhibits significant differences in strength due to external and internal factors. Ice rheology at low stress was demonstrated to exhibit a strong dependence on the interactions between ice crystals.[70] ice properties have been studied in situ for the analysis of flow of glaciers and ice sheets.[71,72] Creep experiments on fine-grained ice[73] demonstrated that superplastic flow of ice at stresses less than 0.1 MPa governs the rate-limiting creep mechanism under a wide range of temperatures and grain sizes. This work provides the molecular insight to understand the differences in mechanical mechanisms of ice, clathrate hydrate and ice-like water-dominant materials that often coexist with ice in the permafrost regions
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