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Published online by Cambridge University Press: 05 June 2012
Grain size is one of the important microstructural parameters that control rheological properties. Conversely, grain size often reflects deformation conditions. This chapter provides a detailed account of the physical processes by which grain size is controlled. We start from the basic physics of grain-boundary migration, and discuss the kinetics of grain growth, mechanisms of dynamic recrystallization and finally the physics by which grain size is controlled by nucleation growth. Some examples of the application of basic physics to understand the grain-size distribution and rheology of Earth's interior are presented.
Key words grain boundary, grain-boundary migration, grain growth, Zener pinning, Ostwald ripening, dynamic recrystallization, nucleation growth in phase transformations, paleopiezometers.
Introduction
Grain size is an important parameter that controls the rheology of rocks (see Chapter 8). The grain size of rocks can vary from ~10 μm (ultra-mylonite, a fine-grained rock found in shear zones; e.g., bell and etheridge, 1973; whiteet al., 1980; handy, 1989) to ~102–103 m (estimated grain size of the inner core; bergman, 1998, see also Chapter 17 of this book) depending on its thermal and mechanical history. This large variation in grain size causes significant changes in rheological properties. For example, for a small grain size and low stress, a polycrystalline material tends to deform by grain-size sensitive creep mechanisms, such as diffusional creep (see Chapter 8). In these cases, the viscosity of a rock, η, changes with grain size, L, as η ∝ Lm where m is a constant between 2 and 3.
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