Environmental Engineering Reference
In-Depth Information
1.0
AISI 304 SS
9Cr-1Mo steel - high stress
9Cr-1Mo steel - low stress
0.9
0.8
0.7
0.6
0.5
12
345678
Damage tolerance factor
9 0 1 2 3 4
3.24
Validity of Equation [3.60] studied in a 9Cr-1Mo steel and AISI 304
stainless steel. The solid line corresponds to the predictions of Equation
[3.60].
98
3.7.1 Effect of impurities
In Section 3.1 we identifi ed stress, temperature and grain size/microstructure
as the three important factors which determine the extent of creep defor-
mation that a material experiences. However there are examples where two
materials with similar compositions, grain sizes and second phase distributions
might creep at vastly different rates under a given stress and temperature.
Such anomalous behavior has been observed in titanium alloys by Mishra
et al
.
99
who found that alloys with nominally similar compositions crept at sig-
nifi cantly different rates. It was found that the presence of trace elements such
as Fe and Ni degrade the creep properties of the titanium alloy. Even though
these elements are present only in the order of ppm, they infl uence the dif-
fusion rates to an extent as to bring about signifi cant changes in creep rates.
The activation energy for diffusion in the higher Fe/Ni containing alloys was
found to be smaller and
vice versa
. Mishra
et al.
suggest that the Fe/Ni appear
to dissolve interstitially and form foreign atom-vacancy pairs which play a sig-
nifi cant role in accelerating the diffusion kinetics of the titanium.
3.7. 2 Diffusion creep in ionic solids or ceramics
In the section on diffusion creep mechanisms we discussed the importance
of grain size, temperature and stress in determining the rate controlling
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