Do it with electrons !

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Do it with electrons ! II
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TEM - transmission electron microscopy
Typical accel. volt. 100-400 kV (some
instruments - 1-3 MV) Spread broad probe across
specimen - form image from transmitted
electrons Diffraction data can be obtained from
image area Many image types possible (BF, DF,
HR, ...) - use aperture to select signal
sources Main limitation on resolution -
aberrations in main imaging lens Basis for
magnification - strength of post- specimen lenses
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TEM - transmission electron microscopy
Instrument components Electron gun (described
previously) Condenser system (lenses apertures
for controlling illumination on
specimen) Specimen chamber assembly Objective
lens system (image-forming lens - limits
resolution aperture - controls imaging
conditions) Projector lens system (magnifies
image or diffraction pattern onto final screen)
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TEM - transmission electron microscopy
Instrument components Electron gun (described
previously) Condenser system (lenses apertures
for controlling illumination on
specimen) Specimen chamber assembly Objective
lens system (image-forming lens - limits
resolution aperture - controls imaging
conditions) Projector lens system (magnifies
image or diffraction pattern onto final screen)
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TEM - transmission electron microscopy
Examples
Matrix - ?'-Ni2AlTi Precipitates - twinned L12
type ?'-Ni3Al
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TEM - transmission electron microscopy
Examples
Precipitation in an Al-Cu alloy
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TEM - transmission electron microscopy
Examples
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TEM - transmission electron microscopy
Examples
lamellar Cr2N precipitates in stainless steel
electron diffraction pattern
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TEM - transmission electron microscopy
Specimen preparation
Types replicas films slices powders,
fragments foils
as is, if thin enough ultramicrotomy crush
and/or disperse on carbon film
Foils 3 mm diam. disk very thin (lt0.1 - 1
micron - depends on material, voltage)

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TEM - transmission electron microscopy
Specimen preparation
Foils 3 mm diam. disk very thin (lt0.1 - 1
micron - depends on material, voltage)
mechanical thinning (grind) chemical
thinning (etch) ion milling (sputter)

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TEM - transmission electron microscopy
Diffraction
Use Bragg's law - ? 2d sin ? But ??much
smaller (0.0251Å at 200kV) if d 2.5Å, ?
0.288
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TEM - transmission electron microscopy
Diffraction
2q sin 2q R/L
l 2d sin q d (2q) R/L l/d Rd lL
specimen
image plane
L is "camera length" lL is "camera constant"
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TEM - transmission electron microscopy
Diffraction
Get pattern of spots around transmitted beam from
one grain (crystal)
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TEM - transmission electron microscopy
Diffraction
Symmetry of diffraction pattern
reflects symmetry of crystal around beam
direction
Why does 3-fold diffraction pattern look
hexagonal?
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TEM - transmission electron microscopy
Diffraction
Note all diffraction patterns are
centrosymmetric, even if crystal structure is
not centrosymmetric (Friedel's law)
Some 0-level patterns thus exhibit higher
rotational symmetry than structure has
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TEM - transmission electron microscopy
Diffraction
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TEM - transmission electron microscopy
Diffraction - Ewald construction
Remember crystallite size? when size is small,
x-ray reflection is broad To show this using
Ewald construction, reciprocal lattice
points must have a size
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TEM - transmission electron microscopy
Diffraction - Ewald construction
Many TEM specimens are thin in one direction -
thus, reciprocal lattice points elongated in one
direction to rods - "relrods"
Also, ? very small, 1/? very large
Only zero level in position to reflect
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TEM - transmission electron microscopy
Indexing electron diffraction patterns
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TEM - transmission electron microscopy
Indexing electron diffraction patterns
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TEM - transmission electron microscopy
Indexing electron diffraction patterns
Index other reflections by vector sums,
differences
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TEM - transmission electron microscopy
Indexing electron diffraction patterns
Find crystal system, lattice parameters, index
pattern, find zone axis
ACTF!!!
Note symmetry - if cubic, what direction has this
symmetry (mm2)?
Reciprocal lattice unit cell for cubic lattice
is a cube
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TEM - transmission electron microscopy
Why index?
Detect epitaxy Orientation relationships at
grain boundaries Orientation relationships
between matrix precipitates Determine
directions of rapid growth Other reasons
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TEM - transmission electron microscopy
Polycrystalline regions
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TEM - transmission electron microscopy
Indexing electron diffraction patterns -
polycrystalline regions
Same as X-rays smallest ring - lowest ? -
largest d
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TEM - transmission electron microscopy
Indexing electron diffraction patterns - comments
Helps to have some idea what phases
present d-values not as precise as those from
X-ray data
Systematic absences for lattice centering and
other translational symmetry same as for
X-rays Intensity information difficult to
interpret
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TEM - transmission electron microscopy
Sources of contrast
Diffraction contrast - some grains diffract more
strongly than others defects may affect
diffraction
Mass-thickness contrast - absorption/
scattering. Thicker areas or mat'ls w/
higher Z are dark
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TEM - transmission electron microscopy
Bright field imaging
Only main beam is used. Aperture in back focal
plane blocks diffracted beams Image contrast
mainly due to subtraction of intensity from the
main beam by diffraction
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TEM - transmission electron microscopy
Bright field imaging
Only main beam is used. Aperture in back focal
plane blocks diffracted beams Image contrast
mainly due to subtraction of intensity from the
main beam by diffraction
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TEM - transmission electron microscopy
Bright field imaging
Only main beam is used. Aperture in back focal
plane blocks diffracted beams Image contrast
mainly due to subtraction of intensity from the
main beam by diffraction
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TEM - transmission electron microscopy
Bright field imaging
Only main beam is used. Aperture in back focal
plane blocks diffracted beams Image contrast
mainly due to subtraction of intensity from the
main beam by diffraction
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TEM - transmission electron microscopy
What else is in the image?
Many artifacts surface
films local contamination
differential thinning others

Also get changes in image because of annealing
due to heating by beam
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TEM - transmission electron microscopy
Dark field imaging
Instead of main beam, use a diffracted beam Move
aperture to diffracted beam or tilt incident beam
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TEM - transmission electron microscopy
Dark field imaging
Instead of main beam, use a diffracted beam Move
aperture to diffracted beam or tilt incident beam
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TEM - transmission electron microscopy
Dark field imaging
Instead of main beam, use a diffracted beam Move
aperture to diffracted beam or tilt incident beam
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TEM - transmission electron microscopy
Lattice imaging
Use many diffracted beams Slightly
off-focus Need very thin specimen region Need
precise specimen alignment
See channels through foil Channels may be light
or dark in image Usually do image simulation
to determine features of structure
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TEM - transmission electron microscopy
Examples
M23X6 (figure at top left). L21 type b'-Ni2AlTi
(figure at top center). L12 type twinned g'-Ni3Al
(figure at bottom center). L10 type twinned NiAl
martensite (figure at bottom right).