Tem

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


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)


Examples

Matrix - β'-Ni2AlTi

Precipitates - twinned L12 type γ'-Ni3Al


Examples

Precipitation in an
Al-Cu alloy


Examples

dislocations SiO2 precipitate
in superalloy particle in Si


Examples

lamellar Cr2N
precipitates in
stainless steel

electron
diffraction
pattern


Specimen preparation

Types
replicas
films as is, if thin enough
slices ultramicrotomy
powders, fragments crush and/or disperse on carbon film
foils

Foils
3 mm diam. disk
very thin (<0.1 - 1 micron - depends on material, voltage)


Specimen preparation

Foils
3 mm diam. disk
very thin (<0.1 - 1 micron - depends on material, voltage)

mechanical thinning (grind)
chemical thinning (etch)
ion milling (sputter)

examine region
around perforation


Diffraction

Use Bragg's law - λ = 2d sin θ

But λ much smaller

(0.0251Å at 200kV)

if d = 2.5Å, θ = 0.288°


Diffraction

2θ ≈ sin 2θ = R/L
specimen
λ = 2d sin θ ≈ d (2θ)

R/L = λ/d

Rd = λL

image plane
L is "camera length"

λL is "camera constant"


Diffraction

Get pattern of spots around transmitted beam from one grain (crystal)


Diffraction

Symmetry of diffraction pattern reflects
symmetry of crystal around beam direction

Example:
6-fold in hexagonal, 3-fold in cubic

[111] in cubic [001] in hexagonal

Why does 3-fold diffraction pattern look hexagonal?

Diffraction
P cubic reciprocal lattice
layers along [111] direction
Note: all diffraction
patterns are
centrosymmetric,
even if crystal structure l = +1 level
is not centrosymmetric
(Friedel's law)

Some 0-level patterns 0-level
thus exhibit higher
rotational symmetry than
structure has

l = -1 level


Diffraction

Cr23C6 - F cubic Ni2AlTi - P cubic
a = 10.659 Å a = 2.92 Å


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


Diffraction - Ewald construction

Many TEM specimens are thin in one direction - thus, reciprocal
lattice points elongated in one direction to rods - "relrods"

Also, λ very small, 1/λ very large

Only zero level in
position to reflect
Ewald
sphere


Indexing electron diffraction patterns

Measure R-values for at least 3 reflections



Index other reflections by vector sums, differences

Next find zone axis from cross product of any two (hkl)s

(202) x (220) ——> [444] ——> [111]



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


Why index?

Detect epitaxy
Orientation relationships at grain boundaries
Orientation relationships between matrix & precipitates
Determine directions of rapid growth
Other reasons


Polycrystalline regions

polycrystalline BaTiO3
spotty Debye rings


Indexing electron diffraction patterns - polycrystalline regions
Same as X-rays – smallest ring - lowest θ - largest d

Hafnium ( 铪 )


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


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


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


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


Dark field imaging

Instead of main
beam, use a
diffracted beam

Move aperture to
diffracted beam
or tilt incident
beam


Dark field imaging

Instead of main beam, use a diffracted beam

Move aperture to diffracted beam or tilt incident beam

strain field contrast


Dark field imaging

Instead of main beam, use a diffracted beam

Move aperture to diffracted beam or tilt incident beam


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
铝钌铜合金


Examples

M23X6 (figure at top
left).

L21 type β'-Ni2AlTi
(figure at top center).

L12 type twinned γ'-
Ni3Al (figure at bottom
center).

L10 type twinned NiAl
martensite (figure at
bottom right).

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