Structural Defects

1
Crystal Growth 4
Structural Defects in Crystals
Nesse, Ch. 5, p. 84-90
Recovery and Recrystallisation
Nesse, Ch. 5, p. 86-91
2
What are defects?
What forms do they take?
What is their significance?
3
What are defects?
mistakes in the crystal lattice naturally
occurring, present in all crystals density,
forms depend on T, P, s, type of crystal, etc...
What forms do they take?
What is their significance?
4
What are defects?
mistakes in the crystal lattice naturally
occurring, present in all crystals density,
forms depend on T, P, s, type of crystal, etc...
What forms do they take?
classified according to geometry point ( 0D)
line ( 1D) planar ( 2D) different types of
each
What is their significance?
5
What are defects?
mistakes in the crystal lattice naturally
occurring, present in all crystals density,
forms depend on T, P, s, type of crystal, etc...
What forms do they take?
classified according to geometry point ( 0D)
line ( 1D) planar ( 2D) different types of
each
What is their significance?
control diffusion within the crystal
lattice control deformation within the crystal
lattice important in reactions, strain, other
solid-state processes
6
Defects and Deformation what is the problem?

how do rocks flow without breaking or melting?
7
Defects and Deformation what is the problem?

how do we make rocks do this?
8
Defects and Deformation what is the problem?

granite
how can undeformed protoliths be transformed into
fine-grained, strongly foliated, layered rocks?
mylonite
9
Defects and Deformation what is the solution?

deformation takes place within the crystal
lattice one atom at a time by migration of defects
10
What are defects?
mistakes in the crystal lattice naturally
occurring, present in all crystals density,
forms depend on T, P, s, type of crystal, etc...
What forms do they take?
classified according to geometry point ( 0D)
line ( 1D) planar ( 2D) different types of
each
What is their significance?
control diffusion within the crystal
lattice control deformation within the crystal
lattice important in reactions, strain, other
solid-state processes
11
POINT DEFECTS
mistakes in arrangement of atoms (points, 0D) in
lattice
impurity defects
interstitial (extra)
Schottky (vacancy) ve -ve
Frenkel (misplaced)
substitution (wrong kind)
Nesse, 2000 Fig. 5.11
12
POINT DEFECTS
interstitial
vacancy
Twiss Moores, 2000 Fig. 19.4
vacancies are particularly important in
controlling atomic-scale motions within the
crystal lattice
13
POINT DEFECTS
motion of a vacancy
Twiss Moores, 2000 Fig. 19.5
by exchanging places with atoms (randomly, or
driven by s) vacancies contribute to changing the
shapes of crystals
14
POINT DEFECTS
example vacancy migration and creep creep
slow, time-dependent strain diffusion creep
transfer of material from areas of
high compressive stress to areas of low
compressive stress
? the diffusion of point defects through a
crystal lattice ? the diffusion of atoms or ions
along grain boundaries ? the diffusion of
dissolved components in fluid along
grain boundaries
volume diffusion, grain boundary diffusion
15
POINT DEFECTS
volume diffusion diffusion of material through
the crystal lattice also termed Nabarro-Herring
creep controlled largely by vacancy
migration driven by gradients in stress or
composition favoured by high T
Twiss Moores, 2000 Fig. 19.6
16
LINE DEFECTS
linear (1D) misfits in crystal lattice dislocation
line separating adjacent, slightly
mismatched parts of the same crystal
2 types
edge dislocations
screw dislocations
Nesse (2000) Fig. 5.13
17
LINE DEFECTS
edge dislocation extra half plane (EHP) of atoms
in lattice dislocation migrates by exchanging
bonds at end of EHP dislocation migrates in same
direction as lattice offset net
result displacement of lattice by one full unit
cell analogy movement of caterpillar or moving
rug from under piano
van der Pluijm Marshak (1997) Fig. 9.12a
18
LINE DEFECTS
Twiss Moores, 2000 Fig. 19.9
19
LINE DEFECTS
screw dislocation lateral displacement of atoms
in lattice wedge-shaped zone of
offset dislocation migrates at right angles to
lattice offset net result displacement of
lattice by one full unit cell analogy tearing
a piece of paper
van der Pluijm Marshak (1997) Fig. 9.12b
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LINE DEFECTS
in combination, edge screw dislocations form
2-D loops allow displacement (slip or glide) of
lattice segments in any direction on slip (or
glide) plane
Burgers vector (b) direction and amount of
misfit (traverse error)
Twiss Moores, 2000 Fig. 19.11
21
LINE DEFECTS
in combination, edge screw dislocations form
2-D loops allow displacement (slip or glide) of
lattice segments in any direction on slip (or
glide) plane analogous to slipped part of fault
plane
dislocation loop
Nesse (2000) Fig. 5.14
22
LINE DEFECTS
dislocations in undeformed olivine (PPL, 2.5 mm)
decorated by oxidation
23
LINE DEFECTS
edge screw dislocations in undeformed olivine
(PPL, 0.25 mm) decorated by oxidation
24
What is the total length of dislocations in 1
cm3if they were strung end to end?
25
What is the total length of dislocations in 1
cm3if they were strung end to end?
100,000 km Two and half times around the world!
26
LINE DEFECTS
dislocation slip (glide) migration of
dislocations along slip (glide)
plane accomplished by switching bonds at end of
dislocation one atom at a time
slip system slip (glide) plane direction
within plane
Twiss Moores, 2000 Fig. 19.14
27
LINE DEFECTS
dislocation slip (glide) migration of
dislocations along slip (glide)
plane accomplished by switching bonds at end of
dislocation one atom at a time
slip system slip (glide) plane direction
within plane
Nesse (2000) Fig. 5.12
28
LINE DEFECTS
dislocation slip changes shape of crystal
lattice by incremental displacements problem
migrating dislocations can interfere with
each other, creating tangles
29
LINE DEFECTS
dislocation slip changes shape of crystal
lattice by incremental displacements problem
migrating dislocations can interfere with
each other, creating tangles strain
hardening increased resistance to
deformation in materials with high dislocation
density effect increase stress to continue
deformation change deformation
mechanism ? brittle failure dislocation
must migrate around the tangle
30
LINE DEFECTS
dislocation climb migration of dislocations out
of slip (glide) plane requires diffusion of
atoms (or vacancies) from one lattice plane to
another facilitated by high T, high vacancy
concentration resulting offsets termed jogs
Twiss Moores, 2000 Fig. 19.15
31
LINE DEFECTS
dislocation slip changes shape of crystal
lattice by incremental displacements problem
migrating dislocations can interfere with
each other, creating tangles strain
hardening increased resistance to
deformation in materials with high dislocation
density effect increase stress to continue
deformation change deformation
mechanism ? brittle failure dislocation
must migrate around the tangle dislocation
creep dislocation slip climb allows
deformation to continue most effective at high T
32
PLANAR DEFECTS
2-D (planar) arrays of dislocations
form during crystal growth by phase
transformations by dislocation
migration include grain boundaries twin
planes stacking faults antiphase
boundaries dislocation walls (low-angle tilt
boundaries)
33
PLANAR DEFECTS
2-D (planar) arrays of dislocations
antiphase boundary in clinopyroxene formed by
augite ? pigeonite transformation on cooling
Nesse (2000) Fig. 5.15
34
PLANAR DEFECTS
2-D (planar) arrays of dislocations
dislocation wall (low-angle tilt boundary) formed
by accumulation of edge dislocations
Twiss Moores, 2000 Fig. 19.17
35
PLANAR DEFECTS
dislocation walls in undeformed olivine (PPL,
0.25 mm) decorated by oxidation
36
PLANAR DEFECTS

• distortion of crystal lattice by dislocation
  migration
• ? misorientations of crystallographic axes
• (and therefore optical directions)
• between different parts of the same crystal

undulose extinction in quartz
37
PLANAR DEFECTS

• distortion of crystal lattice by dislocation
  migration
• ? misorientations of crystallographic axes
• (and therefore optical directions)
• between different parts of the same crystal

eventually accumulated dislocations
inhibit further deformation by dislocation
slip lattice distortion ? high internal strain
energy how can crystals get rid of dislocations?
38
RECOVERY RECRYSTALLISATION
recovery set of processes leading to reduction
in dislocation density and overall strain
energy recrystallisation creation of new
grain(s) from pre-existing grain(s) of same
mineral
39
RECOVERY RECRYSTALLISATION
dislocation wall (low-angle tilt boundary) formed
by accumulation of edge dislocations
separate misoriented lattice segments continued
migration of dislocations into low angle tilt
boundaries ? progressively more
misorientation, eventually forming subgrains
Twiss Moores, 2000 Fig. 19.17
40
RECOVERY RECRYSTALLISATION
continued migration of dislocations into low
angle tilt boundaries ? progressively more
misorientation, eventually forming
subgrains (lattice misorientation lt10o)
subgrains initially strain-free since
dislocations have migrated into their boundaries
Passchier Trouw, 1996 Fig. 3.14
41
RECOVERY RECRYSTALLISATION
continued migration of dislocations into low
angle tilt boundaries ? progressively more
misorientation, eventually forming
subgrains (lattice misorientation lt10o)
subgrains initially strain-free since
dislocations have migrated into their boundaries
formation of subgrains is therefore a recovery
process
with increasing misorientation (gt10o) subgrains ?
new grains
Passchier Trouw, 1996 Fig. 3.14
42
RECOVERY RECRYSTALLISATION
creation of subgrains ? new grains ? grain size
reduction (polygonisation) facilitates
deformation because new grains can now rotate,
move past each other, and/or change shape
independently facilitates diffusion (and thus
both reaction and deformation) by increasing
grain boundary path length and surface area
43
RECOVERY RECRYSTALLISATION
undulose extinction and subgrains in quartz
creation of subgrains ? new grains ? grain size
reduction (polygonisation)
44
RECOVERY RECRYSTALLISATION
undulose extinction ? subgrains ? new
grains grain size reduction by polygonisation a
type of recrystallisation
45
RECOVERY RECRYSTALLISATION
undulose extinction ? subgrains ? new
grains grain size reduction by polygonisation a
type of recrystallisation
46
RECOVERY RECRYSTALLISATION
where recovery keeps pace with grain size
reduction and strain, equilibrium grain size (for
given T, s) develops during deformation dynamic
recrystallisation polygonal or somewhat elongated
grains that may define strong fabrics
47
RECOVERY RECRYSTALLISATION
Passchier Trouw, 1996 Fig. 3.25
grain boundary migration another type of
recrystallisation driven by need to reduce grain
boundary surface energy
48
RECOVERY RECRYSTALLISATION
quartz mica
quartz only
under strain-free conditions, deformed crystals
in mono-mineralic aggregates will tend to
increase grain size (reduce SA/V) by grain
boundary area reduction (a form of
recrystallisation)
49
RECOVERY RECRYSTALLISATION
grain boundary migration another type of
recrystallisation driven by need to reduce grain
boundary surface energy
50
RECOVERY RECRYSTALLISATION
annealing static recrystallisation driven by
heating in strain-free environment
Nesse (2000) Fig. 5.22
51
RECOVERY RECRYSTALLISATION
granoblastic polygonal (foam) texture stable
microstructure produced by either dynamic
recrystallisation (strain recovery) or static
recrystallisation (no strain)
52
RECOVERY RECRYSTALLISATION
MR-13 polygonal texture in amphibole (2.5 mm)
granoblastic polygonal (foam) texture stable
microstructure produced by either dynamic
recrystallisation (strain recovery) or static
recrystallisation (no strain)
53
What are defects?
naturally occurring imperfections in the crystal
lattice
What forms do they take?
point ( 0D) e.g. vacancies, impurities line
( 1D) edge and screw dislocations planar
( 2D) e.g., low angle tilt boundaries, grain
boundaries
What is their significance?
control diffusion and deformation within the
crystal lattice dislocation slip and/or climb,
recovery, recrystallisation important in
reactions, strain, other solid-state processes