Chemistry Reference
In-Depth Information
Stress measurements are mostly performed on polycrystalline metal parts, and
typically on high angle peaks, so diffraction intensities are not very high. Photon
counting detectors, such as MWPC and microgap detectors, have high sensitivity with
essentially no noise, which is very suitable for a stress measurement system using a
laboratory source, such as sealed X-ray tubes or rotating anode generators. For stress
measurements of ferrous metals, Cr or Co radiation is normally used to avoid
fluorescence, and in this case, multiwire area detectors are, so far, the best choice.
If a Mo X-ray source or synchrotron beam is used, a CCD detector is the better choice
for its ability to handle high-energy X-rays and high count rate. The spatial resolution
requirement depends on the FWHM of the diffraction profile. In practice, a FWHM
of three to six times the spatial resolution should be sufficient to precisely determine the
peak position, provided that there are enough counts on the profile. Further reduction in
the spatial resolution does not necessarily improve the peak position accuracy. The
diffraction peak width for stress measurement is typically broad, so spatial resolution
of most area detectors is sufficient for stress analysis.
The selection of the goniometer and sample stage is based on sample size, weight,
and the stress or stress tensor components to be determined. For instance, one normal
stress component on a large sample is better handled by a large XYZ stage on a
goniometer with only twomain axes. The v scan can be achieved by the twomain axes
in either the u - 2u or u - u configuration. In a vertical u - u configuration, the v scan
is achieved bymoving the primary beamand detector, but keeping the sample still. This
is especially beneficial for large samples. The vertical u - u configuration requires both
the X-ray source and the detector to move for the v scan. The horizontal u - 2u
configuration has the advantage of not having to move the X-ray source, which is
convenient for a rotating anode generator. It is also easier to move a heavy 2D detector
in the horizontal rotation. In 2D stress measurement, the data collection scan can
imitate iso-inclination and side-inclination in the conventional method. The two main
axes can provide only the iso-inclination scan (v scan). The c axis is necessary to
achieve the side-inclination scan (c scan). Just as in the conventional method, the c
scan has less variation in the incident angle, so the depth of penetration is relatively
consistent compared to the v scan. For complete measurement of stress tensor
components, the f axis is necessary in addition to the v and c axes. An XYZ stage
is necessary to locate themeasurement point on the sample and to collect data for stress
mapping. The sample alignment device, such as a laser video system, is desired for
accurate alignment of the measurement spot to the instrument center.
Figure 9.14 (top) shows a two-dimensional X-ray diffraction system in a vertical
u - u configuration for stress measurement (Bruker D8-DISCOVER GADDS). The
sealed tube, monochromator, and collimator are mounted on the primary track (u
1
).
The MWPC (Hi-Star) 2D detector is mounted on the secondary track (u
2
). An
Eulerian cradle with an XYZ stage is used to rotate the sample in c and f, and to align
and move the sample in the XYZ directions. A laser video sample alignment device is
mounted in a tilted direction to yield space for scanning the primary beam and
detector. Figure 9.14 (bottom) shows a two-dimensional X-ray diffraction in the
horizontal u - 2u configuration for stress measurement (Bruker D8-DISCOVER
GADDS). The rotating anode generator, the X-ray optics (not shown), and the
