99
in the alloys that were characterized in this study.
The proportionality constant can either be measured,
in which case the standard from which it is measured must
satisfy a criterion called the "thin film criterion," a
criterion that defines the maximum thickness of the
standard, or the constant can be calculated. Both of
these methods are outlined by Goldstein (1979) and Zalusec
(1979) It is primarily the effect of thickness on the k
term in the Cliff-Lorimer equation that defines the
specimen related effects. This thickness effect is a very
serious problem in Ni-Al alloys. The aluminum x-rays are
preferentially absorbed by the Ni, to the extent that the
thin film criterion in Ni-Mo-Al of RSR composition is not
satisfied for thicknesses in excess of about 600
Angstroms.
The ternary alloy //17 will be used as an example of
how thickness affects quantitation. A large beam was used
in an attempt to measure a "bulk" composition of the
alloy. The results are summarized below:
Nominal Composition
Ni
Mo
A1
wt.%
78.5
15.0
6.5
at .X
77.0
9.0
14.0
X-Ray Results
No thickness correction
wt.%
79.2
15.6
5.22
No thickness correction
at.%
79.1
9.5
11.3
1000 Angstrom correction
wt.%
78.8
15.5
5.6