Fabric, Recrystallization and Deformation

Hexagonal ice, I_h, which is the building material of all ice sheets on Earth has an extreme plastic anisotropy.  Plastic deformation by dislocation glide in the basal plane is 60-100 times easier than glide on the prismatic or pyramidal slip systems.  For this reason one can (very crudely) view the deformation of ice crystals as a deck of cards.  Intracrystalline slip leads to non-random orientation of the c-axes in ice, fabric development.

Deformation

Velocity gradient of single crystals deforming by glide on the basal plane.

Fabric Development

I have developed a model for the fabric evolution in materials.  The new features are explicit accounting of Nearest Neighbor Interaction, and inclusion of dynamic recrystallization, formulated in terms of dislocation density and grain size.

Dynamic Recrystallization

For most polycrystalline materials, there are at least 3 recrystallization regimes: 

1. Normal grain growth
2. Polygonization
3. Migration recrystallization.

The fabric (crystal orientation pattern) is not affected by normal grain growth.  During normal grain growth the mean crystal size increases with time according to a parabolic growth law.

As the grains strain, sub-boundaries (dislocation walls) may form due to heterogeneous deformation within grains that relieves stress concentrations. The formation of sub-boundaries can lead to the division of the parent grain into two new grains, as the misorientation of sub-grains increases. This is called polygonization, and it leads to the formation of two grains with a small misorientation angle.

Migration recrystallization is the formation of new strain free crystals, by rapid grain boundary migration.

Abstract for AGU Fall Meeting 2001
Throstur Thorsteinsson:  Accounting for dynamic recrystallization in fabric evolution models.

Stefnuhneigð, umkristöllun og aflögun
(sjá nánar á ensku)

Ís sem myndast náttúrulega á jörðinni er af gerðinn ís I_h.  Færsla ferilveilna á grunnplani (basal plane) er 60 - 100 sinnum auðveldari en á öðrum plönum (prismatic og pyramidal slip systems).  Þess vegna má, í grófri nálgun, líta á staka ískristalla sem spilastokk þegar kemur að aflögun.  Aflögun kristalla á þennan hátt, með færslu ferilveilna, myndar stefnuhneigð þannig að uppröðun c-ásanna verður ekki lengur einsleit.

Aflögun

Þinur sem lýsir stigli á hraða aflögunar fyrir staka kristalla sem aflagast eingöngu vegna færslu ferilveilna á grunnplani.

Þróun stefnuhneigðar

Þröstur hefur þróað líkan sem lýsir þróun stefnuhneigðar, sem tekur tillit til nágranna kristalla og umkristöllunar, með því að reikna þéttleika ferilveilna og taka tillit til stærðar kristalla.

Umkristöllun
(sjá ensku hér til hliðar)