When electrons are accelerated up to high energy levels (few hundreds keV) and focused
on a material, they can scatter or backscatter elastically or inelastically, or produce many
interactions, source of different signals such as X-rays, Auger electrons or light (Fig. 3.1).
Some of them are used in transmission electron microscopy (TEM).
The purpose of this chapter is to introduce TEM and the different related techniques used
for the microstructural study of the AMCs. The TEM sample preparation of AMCs is described
in chapter 3.2. The chemical analyses by energy dispersive spectrometry (EDS) is presented in
chapter 3.3. Chapter 3.4 is concerned with the theoretical basis of TEM (diffusion and
diffraction). Chapter 3.5 deals with the contrast image formation in a conventional TEM
(bright/dark field modes, and diffraction patterns). A brief presentation of high resolution
transmission electron microscopy (HREM) with an introduction to electron crystallography is
given in chapters 3.6 and 3.7.
electron incident beam
backscattered electrons
X-rays
Auger electrons
light
absorbed electrons
direct beam
coherent elastic
scattered electrons
incoherent inelastic
scattered electrons
incoherent elastic
scattered electrons
Fig. 3.1 Interactions between electrons and material
3. Transmission Electron Microscopy
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22
3.1 Historical Introduction
The resolution ? of a microscope is defined as the distance between two details just
separable from one another. It can be calculated using the Abb theory of images formation
for optic systems. For incoherent light or electron beam:
where ? is the wavelength of the light, and ? the maximum angle between incident and
deflected beam in the limit of the lens aberrations.
For optical microscopy, the resolution is therefore limited by the wavelength of light
(410-660 nm). The X or ? rays have lower wavelength, but unfortunately, high-performance
lenses necessary to focus the beam to form an image do not exist yet (however, X-rays can
reveal structural information of materials by diffraction techniques). In 1923, De Broglie
showed that all particles have an associated wavelength linked to their momentum:
where m and v are the relativist mass and velocity respectively, and h the
Plank’s constant. In 1927, Hans Bush showed that a magnetic coil can focus an electron beam
in the same way that a glass lens for light. Five years later, a first image with a TEM was
obtained by Ernst Ruska and Max Knoll [42]. In a TEM, the electrons are accelerated at high
voltage (100-1000 kV) to a velocity approaching the speed of light (0.6-0.9 c); they must
therefore be considered as relativistic particles. The associated wavelength is five orders of
magnitude smaller than the light wavelength (0.04-0.008 ?). Nevertheless, the magnetic lens
aberrations limit the convergence angle of the electron beam to 0.5° (instead of 70° for the
glass lens used in optics), and reduce the TEM resolution to the ? order. This resolution
enables material imaging (section 3.5) and structure determination at the atomic level (section
3.6 and 3.7). In the 1950s, Raymond Castaing developed an electron probe and X-ray detector
for the chemical analyses. A modified version of his technique, the energy dispersive
spectrometry EDS (section 3.3) is nowadays usually added to the TEM. Many different
techniques based on TEM are used in materials science. Some of them will be detailed in the
following sections.
3.2 Preparation of the TEM Samples
For TEM observations, thin samples are required due to the important absorption of the
electrons in the material. High acceleration voltage reduces the absorption effects but can
cause radiation damage (estimated at 170 kV for Al). At these acceleration tensions, a
maximum thickness of 60 nm is required for TEM and HREM observations and
quantifications. For Al alloys, this thickness can be obtained by electropolishing with a
solution of 20% nitric acid and 80% methanol, but this method is not convenient for the
(Rayleigh criterion) (3.1) ? 0.61?
? sin
------------- =
? h mv ? =
3.3. Chemical Analyses by EDS
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23
preparation of AMCs due to the low reactivity of the reinforcements in comparison to the
unreinforced Al alloys. For AMCs, the following mechanical method was used: TEM foil
specimens were prepared by mechanical dimpling down to 20 µm, followed by argon ion
milling (Fig. 3.2) on a Gatan Duo-Mill machine, operating at an accelerating voltage of 5 kV
and 10° incidence angle, with a liquid nitrogen cooling stage to avoid sample heating and
microstructural changes associated with the annealing effect. Such effects have been
experienced on first samples prepared on an ion mill without cooling stage (PIPS), resulting in
an unexpected and substantial coarsening of the precipitation state.
Another preparation method called focus ion beam FIB has been tried. A thin slice of the
sample was cut by an ion beam on a scanning ion microscope. Unfortunately, the large
thickness of the sample (> 150 nm) impeded good HREM studies. The small observable area
(100 nm x 100 nm) permitted to study only one or two grains, which is generally not enough if
a special grain orientation is required.
3.3 Chemical Analyses by EDS
The first step in phase identification before the analysis of the diffraction patterns is a
chemical analysis that can been done in a TEM microscope by X-rays energy dispersive
spectrometry EDS, or electron energy loss spectrometry EELS. In addition to many other
advantages such as the possibility of obtaining information on the chemical bonding and its
Ar
3 mm
300 µm
100 µm
20 µm
mechanical grinding
+
polishing
dimple grinding
ion milling
grinding wheel
sample cutting
Fig. 3.2 TEM sample preparation of AMCs.
3.