The influence of grain size and water content on the high-temperature plasticity of olivine aggregates was studied, using a gas-medium high-pressure deformation apparatus. The specimens used were hot-pressed, dense olivine aggregates with controlled grain size ranging from a few to 70 μm, with or without added water. Mechanical tests were made at 1573 K and 300 MPa confining pressure and at strain rates of 10−3 to 10−6 s−1. The results reveal two distinct mechanisms of deformation, depending on stress level and grain size. At relatively high stress and large grain size, the strain rate is proportional to about the cube power of the stress and is nearly independent of grain size. In this regime, microstructural observations gave evidence of intragranular deformation involving dislocation motion. At low stress and small grain size, the strain rate depends almost linearly on stress and decreases markedly with increase in grain size. In the latter regime, little evidence was found for intragranular deformation. These observations suggest that the deformation mechanism in the grain size insensitive regime is dislocation creep, while that in the grain size sensitive regime is diffusion creep. In both regimes, water was found to enhance the creep rate. The absence of grain size sensitivity in the dislocation creep regime and comparison with single-crystal data indicate that the water weakening effect is mainly an intragranular process. However, the existence also of a water weakening effect in the diffusion creep regime indicates that water also enhances diffusion. The extrapolation of the present results to coarser grain sizes indicates that the transition from dislocation to diffusion creep occurs at 0.1–1 MPa for 10-mm grain size. Therefore it is suggested that this transition may occur in the upper mantle and that, in both regimes, the presence of trace amounts of water will result in significantly lower creep strength than under strictly “dry” conditions.

Rheology of synthetic olivine aggregates: Influence of grain size and water

Superplasticity—Recent advances and future directions

Creep experiments on fine-grained ice reveal the existence of three creep regimes: (1) a dislocation creep regime; (2) a superplastic flow regime in which grain boundary sliding is an important deformation process; and (3) a basal slip creep regime in which the strain rate is limited by basal slip. Dislocation creep in ice is likely climb-limited, is characterized by a stress exponent of 4.0, and is independent of grain size. Superplastic flow is characterized by a stress exponent of 1.8 and depends inversely on grain size to the 1.4 power. Basal slip limited creep is characterized by a stress exponent of 2.4 and is independent of grain size. A fourth creep mechanism, diffusional flow, which usually occurs at very low stresses, is inaccessible at practical laboratory strain rates even for our finest grain sizes of approximately 3 micrometers. A constitutive equation based on these experimental results that includes flow laws for these four creep mechanisms is described. This equation is in excellent agreement with published laboratory creep data for coarse-grained samples at high temperatures. Superplastic flow of ice is the rate-limiting creep mechanism over a wide range of temperatures and grain sizes at stresses less than or equal to 0.1 MPa, conditions which overlap those occurring in glaciers, ice sheets, and icy planetary interiors.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2000JB900336

Superplastic deformation of ice: Experimental observations

Creep of ceramics: Part 1 Mechanical characteristics

It is now recognized that grain boundary sliding (GBS) is often an important mode of deformation in polycrystalline materials. This paper reviews the developments in GBS over the last four decades including the procedures available for estimating the strain contributed by sliding to the total strain, ξ, and the division into Rachinger GBS in conventional creep and Lifshitz GBS in diffusion creep. It is shown that Rachinger GBS occurs under two distinct conditions in conventional creep depending upon whether the grain size, d, is larger or smaller than the equilibrium subgrain size, λ. A unified model is presented leading to separate rate equations for Rachinger GBS in power-law creep and superplasticity. It is demonstrated that these two equations are in excellent agreement with experimental observations. There are additional recent predictions, not fully resolved at the present time, concerning the role of GBS in nanostructured materials.

Grain boundary sliding revisited: Developments in sliding over four decades

A model for grain boundary sliding is developed in which sliding occurs by the movement of dislocations along, or adjacent to, the boundary by a combination of climb and glide. Under these conditions the strain rate due to sliding is proportional to [sgrave]2/d, where [sgrave] is the applied stress and d is the average grain diameter. It is shown that reports in theliterature of enhanced creep rates at low stresses and/or small grain sizes may be explained by assuming that the various deformation mechanisms, including sliding, operate independently.

Grain boundary sliding as a deformation mechanism during creep

Grain boundary displacements due to diffusional creep

The variation in secondary creep rate at large grain sizes

By assuming that triple point cracks are nucleated at zero time and grow only by grain boundary sliding, the time to fracture is estimated using two different fracture criteria. It is suggested that the method often used to calculate the effective surface energy of fracture may overestimate the true value by at least an order of magnitude.

Terence G. Langdon

Papers

Grain boundary sliding as a deformation mechanism during creep

Grain boundary displacements due to diffusional creep

The variation in secondary creep rate at large grain sizes

An estimate of the time to fracture due to triple point cracking

On the nature of superplastic deformation in the MgAl eutectic