48
METHODS OF MEASURING PROPERTIES
the mean free path. Much thinner films exhibit too little scattering to provide usehl
images, and in thick films multiple scattering events dominate, making the image
blurred and difficult to interpret. Thick specimens can be studied by detecting backscattered electrons.
A transmission electron microscope can form images by the use of the selectedarea electron diffraction (SAED) aperture located between the objective and
projector lenses shown in Fig. 3.10. The main part of the electron beam transmitted
by the sample consists of electrons that have not undergone any scattering. The beam
also contains electrons that have lost energy through inelastic. scattering with no
deviation of their paths, and electrons that have been reflected by various hkl
crystallographic planes. To produce what is called a bright-jeld image, the aperture
is inserted so that it allows only the main undeviated transmitted electron beam to
pass, as shown in Fig. 3.1 1. The bright-field image is observed at the detector or
viewing screen. If the aperture is positioned to select only one of the beams reflected
from a particular hkl plane, the result is the generation of a dark-field image at the
viewing screen. The details of the dark-field image that is formed can depend on the
particular diffracted beam ( particular hkl plane) that is selected for the imaging.
Figure 3.1 I shows the locations of the bright-field (BF) and dark-field (DF) aperture
positions. To illustrate this imaging technique we present in Fig. 3.12 images of an
iron base superalloy with a FCC austenite structure containing 2-3-nm y' preciPrimary
Backscattered
electron detector
BF Bright-field (BF)
and dark-field (DF)
transmitted electron
detectors
""1"" e :
:
i
F E .
s
detector
@
Fluorescent
screen
photographic
film
Figure 3.11. Positioning of signal detectors in electron microscope column. (From D. B.
Williams, Practical Analytical Electron Microscopy in Materials Science, Phillips Electronic
Instruments, Mahwah NJ, 1984.)
METHODS OF MEASURING PROPERTIES
the mean free path. Much thinner films exhibit too little scattering to provide usehl
images, and in thick films multiple scattering events dominate, making the image
blurred and difficult to interpret. Thick specimens can be studied by detecting backscattered electrons.
A transmission electron microscope can form images by the use of the selectedarea electron diffraction (SAED) aperture located between the objective and
projector lenses shown in Fig. 3.10. The main part of the electron beam transmitted
by the sample consists of electrons that have not undergone any scattering. The beam
also contains electrons that have lost energy through inelastic. scattering with no
deviation of their paths, and electrons that have been reflected by various hkl
crystallographic planes. To produce what is called a bright-jeld image, the aperture
is inserted so that it allows only the main undeviated transmitted electron beam to
pass, as shown in Fig. 3.1 1. The bright-field image is observed at the detector or
viewing screen. If the aperture is positioned to select only one of the beams reflected
from a particular hkl plane, the result is the generation of a dark-field image at the
viewing screen. The details of the dark-field image that is formed can depend on the
particular diffracted beam ( particular hkl plane) that is selected for the imaging.
Figure 3.1 I shows the locations of the bright-field (BF) and dark-field (DF) aperture
positions. To illustrate this imaging technique we present in Fig. 3.12 images of an
iron base superalloy with a FCC austenite structure containing 2-3-nm y' preciPrimary
Backscattered
electron detector
BF Bright-field (BF)
and dark-field (DF)
transmitted electron
detectors
""1"" e :
:
i
F E .
s
detector
@
Fluorescent
screen
photographic
film
Figure 3.11. Positioning of signal detectors in electron microscope column. (From D. B.
Williams, Practical Analytical Electron Microscopy in Materials Science, Phillips Electronic
Instruments, Mahwah NJ, 1984.)