AGU Fall Meeting 2001

Accounting for dynamic recrystallization in fabric evolution models
Throstur Thorsteinsson
Dept. Earth and Space Sciences
University of Washington
Seattle, WA 98195-1310, USA

When modeling the evolution of crystal orientation fabric in any material, the effects of dynamic recrystallization (polygonization and migration recrystallization) be taken into account. Dynamic recrystallization is observed in almost any material after a certain amount of deformation.

During polygonization, grains are effectively divided due to rearrangement of dislocations into sub-boundaries (dislocation walls). The effect on the fabric development from this process is to slow down the rotation towards the principal compression axis; the orientation of new crystal(s) usually deviates by less than 5 from the parent crystal. In ice, polygonization is believed to be responsible for the constant average grain size observed, for example, below 700 m in the GRIP ice core. The strain at that depth is only 25%; therefore polygonization must be active at very small bulk strains. 

Migration recrystallization is an important mechanism in fabric development, especially for temperatures (T) close to the melting point (T >-12C for ice). The high temperature allows the nucleation of new, strain-free grains and the rapid migration of grain boundaries. In studies of high temperature (-5C to 0C) creep of ice, Kamb (1972) found that after only about 0.04 shear strain there was already strong evidence of recrystallization.

To account for dynamic recrystallization, the evolution of dislocation density and grain size in each crystal is modeled. A criterion for polygonization is derived by comparing the magnitude of the resolved shear stress on the basal plane of a crystal to the bulk applied stress. For migration recrystallization, the energy associated with a dislocation density is compared with the grain boundary energy associated with a new grain. The model reproduces well the fabric and strain rate measured in uniaxial compression test.

It is difficult to assess the energy associated with a given dislocation density. This, and the lack of fabric data, leads to uncertainty about how to constrain other associated variables that control the rate of fabric evolution. More data on fabric evolution and recrystallization processes would help to constrain those variables.