Chemistry Reference
In-Depth Information
9.4.2 Data Collection Strategy
X-ray diffraction measures stress by measuring the d-spacing change caused by the
stress. The diffraction vector is in the normal direction of the measured crystalline
planes. It is not always possible to have the diffraction vector in the desired measure-
ment direction. In reflection mode X-ray diffraction, it is easy to have the diffraction
vector normal to the sample surface, or tilted away from the normal, but impossible to
have the vector on the surface plane. The stress on the surface plane, or biaxial stress, is
calculated by elasticity theory from the measured strain in other directions. The final
stress measurement results can be considered an extrapolation from the measured
values. In the conventional sin
2
c method, several c-tilt angles are required, typically
from 45
to þ45
. The same is true with an XRD
2
system. The diffraction vectors
corresponding to the data scan can be projected in a 2D plot in the sameway as the pole
density distribution in a pole figure. The 2D plot is called a data collection strategy
scheme or simply scheme.
Figure 9.15 shows a comparison between the conventional method and the 2D
method in terms of the diffraction vector (H
hkl
) and sample coordinates S
1
S
2
S
3
during the data collection scan. For conventional X-ray diffraction (XRD), seven
normal strains are measured at the diffraction vector (H
hkl
) directions corresponding
to seven c-tilt angles at c ¼45
45
achieved by v-
rotation. The seven directions are mapped onto the scheme as seven solid dots.
The directions of the measured stress components are plotted into the scheme as
circles. Since the seven solid dots are aligned along the horizontal direction (S
1
), the
stress component s
11
can be determined by the extrapolation of the seven measured
data points (sin
2
c method). The two gray filled circles represent the direction of s
11
.
It would be impossible to calculate the stress component s
22
from the seven data
; 30
; 15
0
15
30
;
;
;
;
s
22
S
2
g
s
11
s
11
S
1
H
hkl
XRD
w
H
hkl
S
3
s
22
s
22
S
2
XRD
2
S
2
S
1
s
11
S
1
s
11
s
22
FIGURE 9.15 Diffraction vector distribution in stress measurement with conventional
method and 2D method.
