When a metallic material is exposed to a high temperature in an ambient atmosphere, oxidation takes place on the metallic surface. The formed oxides (both surface and grain boundary oxides) are mechanically brittle so that if the stress is high enough the oxides will be cracked. The grain boundary oxide formation in TAZ-8A nickel-base superalloy was studied. The effect of oxide crack nucleus on low cycle fatigue life will be analyzed. The TAZ-8A was subjected to high temperature oxidation tests in air under the stress-free condition. The oxidation temperatures were 600, 800, and 1000 C. The oxidation time varies from 10 to 1000 hours

Liu, H. W.

Oshida, Y.

English

NASA Technical Reports Server

N87-11215
Oxidation and Low Cycle Fatigue Life Prediction
Yoshiki Oshida and Hao-Wen Liu
Department of Mechanical Engineering
Syracuse University
When a metallic material is exposed to a high temperature in an ambient atmos-
phere, oxidation takes place on the metallic surface• The formed oxides (both surface
and grain boundary oxides) are mechanically brittle so that if the stress is high
enough the oxides will be cracked. This oxide crack will become the nucleus of a fa-
tigue crack if the allpied aJ is larger than AJth. Therefore oxidation may reduce
high temperature low cycle fatigue life.
Grain Boundary Oxidation
The grain boundary oxide formation in TAZ-8A nickel-base superalloy was studied.
The effect of oxide crack nucleus on low cycle fatigue life will be analyzed. TAZ-8A
was subjected to high temperature oxidation tests in air under the stress-free condi-
tion. The oxidation temperatures were 600 ° , 800 ° , and 1000°C. The oxidation time va-
ried from i0 to i000 hours. The temperatures were controlled within + 8°C.
Figure 1 shows the measured weight gain per unit area of exposure, AW [mg/cm 2] as
a function of exposure time for three different test temperatures• If oxidation of
TAZ-8A obeys the parabolic rate law, the square of AW should relate linearly to oxi-
dation time as shown in figure 2. Many other superalloys and refractory materials have
exhibited the parabolic rate law for oxidation• The parabolic oxidation rate indicates
that oxidation is controlled by diffusion kinetics.
The slope of these straight lines is called parabolic rate constant, a. Figure 3
shows the relationship between parabolic rate constants and inverse absolute tempera-
tures for a number of superalloys. TAZ-8A shows a superior oxidation resistance than
Rene 41 and Mar-M200. The data seem to follow two line segments similar to Mar-M200.
This change of the slope might be related to the change in diffusion mechanism. At the
higher temperatures (i.e., between 800 ° and 1000°C), oxidation is mainly controlled by
outward diffusion of Cr.
Grain boundary oxidation is faster and penetrates deeper. Therefore the grain
boundary oxide cracks are larger and they are more critical in reducing the low cycle
fatigue life. The oxidation rate was measured in terms of weight gain, and the oxida-
tion penetration was measured by direct optical observation• One of the main objecti-
ves of the current research program is to separate the grain boundary oxidation from
the total oxidation. The weight gain (AWT) was measured, and it is simply the sum of
weight gain due to surface oxidation (AW_) and the weight gain due to grain boundary
oxidation (AW) aw and AW G can be easiIy separated by calculating AW = V x 0 ,• S S S
where V_ is t_e total volume of surface oxide layer of a test coupon, and Pc is _he
specifig gravity of surface oxides. The calculated results are shown in figure 4. This
approach for separation of AW S and AW G is based on the assumption of homogeneous sur-
face oxide layer thickness.
A second way to separate AW and AW_ is by calculating weight gain due to grainS
boundary oxidation by morphological measurements of width (GPW) and depth (GPD) of
grain boundary oxide penetration. Figure 5 is one example of the results of this mor-
phological study on oxidized coupon (1000°C x 960 hr). The shape of grain boundary oxi-
des can be classified into two groups (round-edge and sharp-edge). This figure shows
the size as well as the shape of the grain boundary oxides• The frequencies of the ob-
served oxide size and shape are indicated by the size and shape of the data points in
the figure• The data have considerable scatter in the GPW vs. GPD relationships. Based
on the morphological analysis that the average value of the aspect ratio (6PW/GPD) of
grain boundary oxides is close to 1.0; so only the data point on the 45 degree dashed
line are used for calculation of grain boundary oxide weight. Each of the numbers on
the 45 degree dashed line is the percentage of the number of the data points for the
321
https://ntrs.nasa.gov/search.jsp?R=19870001782 2020-03-23T15:06:22+00:00Z
morphology over the total number of all of the data points along this dashed line. Let
us call these numbers as distribution percentage, Pi" The weight gain due to grain
boundary oxidation (AW G) can be estimated as
n
AW G = N x 0GO xi_iVGo i x Pi
where VGO i is the volume of grain boundary oxide per grain at the ith morphology
n
x is the average volume of grain boundary oxide per grainin figure 5,i PiIVGoi
N is the total number of grains on the surface of the test coupon which was obtained
as the total surface area of a coupon divided by average grain size area, and 0GO is
the specific gravity of grain boundary oxides which is assumed to be equal ofto 0_$. _____i
can be calculated from the GPW, GPD, and the shape. The calculated results Aw G
AW S (= AW T - AWG) based on this morphological approach are also plotted in figure 4.
Except in the early stage of oxidation, both approaches for separation of grain boun-
dary oxidation from total oxidation are satisfactory.
Figure 6 shows both the calculated results of AW according to the surface homo-
geneous oxide layer and AW G based on the morphologica_ approach for grain boundary
oxides at three different oxidation temperatures. These two measurements agree rea-
sonably well with each other.
In figure 7, the data on total oxidation are exactly the same as those shown in
figure 3. Of interest is t_at (i) grain boundary oxidation rate, in terms of its para-
bolic rate constant, is i0 time larger than the total oxidation and the surface oxi-
dation, and (ii) in both grain boundary and surface oxidation, the similar breaking
points at 800°C were found as that for the total oxidation.
The application of the statistical theory of extreme values to the analysis of
maximum pit depth due to pitting corrosion was successfully introduced by Aziz (ref.l)
and Hawn (ref.2). For some high temperature alloys, low cycle fatigue (LCF) cracks at
elevated temperatures were often initiated as grain boundary oxides, and the maximum
grain boundary oxide depth will control the LCF crack initiation period as well as the
early stage of crack propagation period. Therefore the most important measure of
"damage" to a specimen is grain boundary oxide penetration depth. This is analogous to
the maximum pit depth of Aziz (ref.l). Aziz found that the maximum pit depth obeys the
Poisson's distribution. The oxidized coupons were sectioned, and the cross sectional
surface was examined under an optical microscope and the maximum grain boundary oxide
penetration depth, ao, of this surface was recorded. After approximately 50 _m of the
surface layer was removed another maximum ao was recorded. A coupon was machined and
examined repeatedly layer after layer. All together 12 data points were collected for
each of the three coupons oxidized at different temperatures. Assuming a Poisson's
distribution, figure 8 shows the results for 800°C oxidation temperature at four di-
fferent oxidation times. The straight lines are drawn by the least square fit. Suppose
the surface area ratio of a real component to the test coupon is i00, the extrapolated
ao value can be read on the line at the return period = i00. For example in the case
of 800°C x 960 hr oxidation, on the small test coupon area, the deepest ao is 120 _m,
whereas on a surface area i00 times larger, the likely ao is 158 _m. According to
Aziz(ref.l), "rather than considering the return period, it is equally worthwhile to
consider the frequencies" on the left side of the figure. The probability of obtain-
ing an ao of 158 _m is 0.990. Therefore there is only one chance in a hundred to have
ao deeper than 158 _m on the total surface area of the 12 layers of the tested coupon
oxidized at 800°C for 960 hrs. This should be equivalent to the test of 12 sectioned
surfaces of 12 separate coupons.
322
Because of the brittle property of oxides, a crack might be initiated at a grain
boundary oxide. Once a crack is initiated and if the applied cyclic stress is high
enough, the crack will propagate by the fatigue loading. Therefore the grain boundary
oxides can be considered as a crack nucleus. It is reasonable to expect that the rates
of both surface oxidation and grain boundary oxidation will be accelerated by mecha-
nical stress, and that the grain boundary oxide penetration depth will be increased by
a high enough applied stress.
Grain Boundary Oxide Crack and Low Cycle Fatigue Life - A Preliminary Analysis
Fatigue life consists of crack nucleation and crack propagation periods. Fatigue
crack nucleation period is relatively shorter at higher stresses. Low cycle fatigue
crack nucleation period might even be shortened by severe environmental attack, such
as oxidation. In this case, fatigue life consists mainly of crack propagation period.
For general-yielding cyclic-loading, Dowling and Begley (ref.3) have shown that
fatigue crack growth rate correlates well with AJ; and near the threshold, da/dN can
be written in the form (ref.4 and 5).
da AJth AJ
d-N = A- ( aJth i)m (i)OYC
Zheng and Liu (ref.5) have shown that for a small crack in a large solid,
J = [nlDW + n2Oygy]a (2)
DW is deformation work density. For cyclic loading, it is related to the applied stress
and strain ranges.
.a2
DW = [_--+ _ycAgp] (3a)
Ae
Ao
= _[_-(Ag +2) ] (3b)P
where a_, Ag , and ag are the applied stress, elastic strain, and plastic strain ran-
e
ges. E and Oy_ are YoUng s modulus and cyclic yield strength.
Fatigue _ife can be obtained by integrating equation (i) (ref.5).
AJth)m- 1(DW)mNf = OYC m (
A(m - l)q I l
= C a(l. - m) (4)
I
where a.l is the crack nucleus size. Fatigue life, Nf is related to the size of the
crack nucleus a o.
Figures 9 _nd i0 show the correlations of low cycle fatigue lives of an aluminum
alloy with these two parameters of deformation work density (equations 3a and 3b). In
figures 9 and i0, fatigue lives of both smooth and pre-cracked specimens are plotted.
The size of the pre-cracks, ao is larger than a.. According to eq.(4), the fatigue
i
lives of the smooth and the pre-cracked specimens (Nf and Nfo) are related.
Nfo a l-m
) = constant (5)
-- ( a.
Nf i
Therefore, the log-log plots of the fatigue lives of the smooth and pre-cracked spe-
cimens are parallel as shown in figures 9 and i0.
323
The grain boundary oxide cracks can be considered as pre-cracks. Therefore the
fatigue lives of oxidized specimens will have a similar relation.
As shownby the data in figures 5 and 8, the oxide crack size may have a substan-
tial statistical scatter; and it is expected that this statistical scatter will be re-
flected in the fatigue lives of the oxidized specimens.
It should be emphasizedthat the analysis on the fatigue lives of oxidized speci-
mens is preliminary. However, it can serve as the guide for the future study.
References
(i) P.M.Aziz; Corrosion, 12(10), 1956, pp.495A-506A.(2) D.E.Hawn;Materials Performance, 1977, pp.29-32.
(3) N.E.Dowling and J.A.Begley; ASTMSTPVoi.590, 1976, pp.80-103.
(4) H.W.Liu and Dai Liu; Scripta Met., 18, 1984, pp.7-12.(5) M.Zhengand H.W.Liu; Technical Report NASAGrant No. NAG3-348"Crack Tip Field and
Fatigue Crack Growth in General Yielding and Low Cycle Fatigue", 1984.
TAZ- 8A
1.6_ oxidation in air =Zf _
I000_
I-4 ---_
1
/
/
- /
0.2
I
i
0 0-2 0-4 0-6
Fig. I
I I
0"8 l'O I-2 I-4
sec.
Oxidofion time
!
1-6
800_C
h
---_!,...
500| J I_
;, ° .,'s
1.8 2.0 3.6 x 106
Total weight gain vs. oxidation time
324
8-0
2.0
2-0
A
I._
t.O
0._
%
tOO _0
l I i i
I W l
_/,ooo_ ,_
I
-_, *TS-i I
•2 -4 FO I-2 1"4 1"6 I'0 _ 2"2 24 2'6 2-0 3'0 3°2 34 3-6 zlO 6
SOC.
Oxidation time
Fig. 2 Square of total weight gain vs oxidation time
/
B
('C)
lO00
Fig 3
0-7 0"0 0"0 I'0 I't I-2 zlO"3
I/T ('K "t )
temperature
Parabolic rate constant vs. inverse absolute temperature
325
!._00°<_ •
o_+lo 9+1
_^_+__^vI
", I_ =A=
"
. ,_, o o _1 t
• . <__,,_ <_
j; , <] o _,_ , ,
\
_._ _ ,,_
'° _1 _,,0N \
I--
__ _ - ® ©
8
o
3
n-
(wd) _de
LL
rl
(_w)
• m
LL
326
ORIGINALPAGE IS
DE POORQUAUTY
c_
,p-
o
q)
,i
CD_
8 °o
011
_4
oll
r'l
I
(Bw) eM V 'uo_.opTxo._ Ucu5 o_ anp u!c_;40.e_
o_
o
I=
0
LI-
327
oxidation
ORIGINAL PAGE IS
OF POOR QUALITY
100. (300
80,000
60,000
50.000
40.000
30.000
ZO.000
10,000
8,000
6.900
5.000
4.000
3.000
Z.O00
600
5130
4OO
3OO
2OO
ZO 40 60 80 1DO 120 140 160 180 ZOO 220 Z40 Zf_
maximum grain boundary oxide depth, a o (.,urn)
Fig 8 Maximum groin bOLrgk:ry =ide dg_h vs fmquslci=
' 1,01
I 001
1.0001
328
:E
Q.
<_
b
+
LD-IO AI Alloy
,_ Pre-Cracked Specimen
0 Smooth Specimen
-I
I0
102
Ftg. 9 Correlation of
I0 _ 104
2Nf (Cycle)
(_a 2
+ ayCACp) versus (2Nf) for LD-10 aluminum alloy.
IOI
._ IO
I
IO ,
Ftg. 10
,,,,,,I , ,,,,,,,I, , ,i,,,,l
I0 z I0 s I0 4
2Nf(Cycle )
AOw Ac e
Correlation of [--_ (hOp +T)] versus (2Nf) for LI)-10 aluminum alloy.
329


Oxidation and low cycle fatigue life prediction

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