Page 54 - Microtectonics

42 3 · Deformation Mechanisms
Fig. 3.26.
Three mechanisms of dynamic
recrystallisation on the grain
scale. a Bulging (BLG) recrystal-
lisation. If two neighbouring
grains have different dislocation
density, the grain boundary may
start to bulge into the grain with
the highest density (inset; grey
straight lines in crystals indicate
crystal lattice planes). On the
scale of individual grains, the
grain with higher dislocation
density (shaded) is consumed by
bulging of the less deformed
grain; the bulge may eventually
develop into an independent
grain. b Subgrain rotation (SGR)
recrystallisation. Rotation of a
subgrain in response to migra-
tion of dislocations into subgrain
walls during progressive defor-
mation can cause development
of high angle grain boundaries
and thus of new grains. Bars in
the subgrains indicate lattice
orientation. c High-temperature
grain boundary migration (GBM)
recrystallisation. At high tem-
perature, grain boundaries be-
come highly mobile and may
sweep the material in any direc-
tion to remove dislocations and
subgrain boundaries. Subgrain
rotation also occurs, but where
subgrain boundaries (s) are
transformed into grain bounda-
ries, the latter become also highly
mobile

3.7.2 (Means 1981; Urai et al. 1986), or by migration of a grain
Bulging (BLG) Recrystallisation boundary (Tungatt and Humphreys 1984; Stipp et al.
2002). BLG recrystallisation occurs mostly along the
At low temperature, grain boundary mobility may be lo- boundaries of old grains and at triple junctions. It is also
cal, and the grain boundary may bulge into the crystal possible, though probably less common in rocks (Drury
with high dislocation density and form new, independ- and Urai 1990), that a small dislocation-free core nucle-
ent small crystals (Figs. 3.25, 3.26a); this process is known ates inside a strongly deformed grain with high density
as low-temperature grain boundary migration or bulg- of dislocation tangles and grows at the cost of the old crys-
ing (BLG) recrystallisation (Baily and Hirsch 1962; Drury tal. Old grains may be little deformed or show fractures
et al. 1985; Shigematsu 1999; Stipp et al. 2002). BLG recrys- (at low temperature) or deformation lamellae (Box 3.3)
tallisation corresponds to Regime 1 of Hirth and Tullis and undulose extinction. Remains of old grains are com-
(1992). The bulges may separate from the host grain to monly surrounded by moats of recrystallised grains, a
form small independent new grains by formation of sub- feature known as a core-and-mantle structure (Gifkins
grain boundaries, which evolve into grain boundaries 1976; White 1976; Shigematsu 1999; Fig. 5.20; Sect. 5.6.5).