Subgrain Rotation Recrystallization in Minerals |
| Journal |
Materials Science Forum (Volume 550) |
| Volume |
Fundamentals of Deformation and Annealing |
| Edited by |
P. B. Prangnell and P. S. Bate |
| Pages |
95-104 |
| DOI |
10.4028/www.scientific.net/MSF.550.95 |
| Online since |
July, 2007 |
| Authors |
M.R. Drury,
G.M. Pennock
|
| Keywords |
Dynamic Recrystalization (DRX), Lattice Misorientation, Mineral, Subgrain Boundary |
| Abstract |
Subgrain rotation is a common mechanism of continuous dynamic recrystallization in
minerals and some metals. The mechanism involves new grain boundary formation by progressive
rotation of subgrains or subgrain boundary migration in regions with an orientation gradient. This
paper reviews the status of our current knowledge of rotation recrystallization in minerals. In
minerals a misorientation angle (θ) of 10˚ is often taken as the transition from subgrain boundary to
grain boundary but recent studies on olivine indicate a much higher transition angle between 15-25˚.
In contrast to a high transition angle, the onset of subgrain boundary mobility may occur at much
lower angles between 3-10˚. In consequence, rotation recrystallization in minerals often involves an
initial stage of subgrain rotation followed by subgrain growth once medium angle boundaries have
formed. Current models assume that all subgrain boundaries increase in misorientation with strain.
However, recent studies show that many different types of subgrain boundary develop in minerals.
The formation of new high angle grain boundaries is only likely along some types of geometrically
necessary boundary (GNB). The mineral halite (NaCl) is often quoted as the classic example of
rotation recrystallization yet recent electron backscattered diffraction (EBSD) studies show that only
limited grain sub-division occurs in NaCl polycrystals. This grain sub-division occurs on the scale
of large subgrains that divide the old grain into a few domains and not by the rotation of the smaller
equiaxed subgrains, as envisaged in current models. The small scale, equiaxed, mainly low angle
network of subgrain boundaries that develop in many minerals may be incidental boundaries, as
found in metals, or could be smaller length-scale GNBs. As minerals have high plastic anisotropy
and a limited number of slip systems GNBs may dominate over incidental subgrain boundaries
formed by trapping of statistically stored dislocations. New and extended models for rotation
recrystallization are needed that consider i) incidental subgrain boundaries as well as different types
of GNB, ii) the potential high mobility of medium angle (3-15˚) subgrain boundaries and iii) a link
between the development of subgrain misorientation and texture development. |
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