Oxidation Behavior and Foil Gage
4.3 Microstructure and Oxide Scale
The evolution of microstructure and oxide scale in the vacuum heat treated and oxidation tested conditions were investigated for two samples at 50 microns with different oxidation length change behaviors. The oxidation test results for the two samples are shown in Figure 5.
The oxidation weight gains were virtually identical for these two samples. However, the oxi- dation length changes of the two samples were significantly different. The “normal” (triangle markers) sample showed very small length change through the test while the “abnormal” sam- ple showed continuous length increase through the oxidation test.
Figure 5: Oxidation weight gain and length change for samples with normal and abnormal length changes The microstructures of the two samples are shown in Figure 6 from coupons withdrawn after being oxidation tested up to each test cycle. Optical micrographs of samples in the vacuum annealed and oxidation tested (5, 25, 100 and 200 hours) conditions show significant differ- ence in the microstructure development. The vacuum annealed sample with the “normal”
length change behavior had the microstructure of approximately two grains through the thick- ness while the “abnormal” sample showed the classic bamboo structure. The “normal” sample gradually developed a duplex grain structure as the oxidation test proceeded while the grain structure of the “abnormal” sample was quite stable through the oxidation test.
The Auger analysis on the surface of vacuum heat-treated samples of “normal” and “abnor- mal” showed a layer of approximately 200-angstrom thick oxide scale. There is no detectable difference in the composition of scale, which consists of only Al and oxygen.
The oxide scales on the two alloys are shown in Figure 7 for oxidation tested to 100 hours.
The scale on the sample of “normal” length change behavior shows a mixed morphology of equaxis and columnar oxides. However, the scales on the “abnormal” sample show dual layer morphology. The top layer, which was developed earlier, can be described, as equaxis while the lower layer is clearly columnar.
0 0.0002 0.0004 0.0006 0.0008 0.001
0 200 400 600 800 1000 1200
Square Root of Time, (sec1/2) Weight Gain/Area (g/cm2)
0 1 2 3 4 5
Length Change (%)
Abnormal, DWt Normal, DWt Abnormal, DL Normal, DL
Figure 6: Microstructure for samples show abnormal length changes (left) and normal length change (right) for vacuum heat treated, oxidation tested 5, 25, 100 and 200 hours (from top to bottom)
Figure 7: SEM micrographs of oxide scale on 100 hours oxidation samples show normal (top) and abnormal (bottom) length change behaviors
5 Discussion
The oxidation weight gain of Fe-Cr-Al, with rare earth addition, at 1100 °C in air, clearly fol- lowed the classic, diffusion controlled, parabolic behavior. The oxidation weight gain rate is thus a useful value to calculate the time to deplete the Al in the alloy. The oxidation tests car- ried out here has shown that the weight gain breakaway did not start at the depletion of Al.
However, the oxidation length change breakaway is closely related to the Al depletion. At least in the cases of cold rolled samples length change breakaway is always associated with the Al depletion.
The obvious effect of vacuum heat treatment on the oxidation behavior is the significant re- duction of weight gain. The direct consequence is to extend the duration of stable, protective oxide growth. It is not clear if the rapid oxidation weight gain in the cold rolled sample is due to either the doping of Fe/Cr in the scale or thin layer of aluminum oxide formed during the vacuum heat treatment. This thin layer of aluminum oxide might act as a protective layer dur- ing the first heating up and enable the formation of pure and highly protective aluminum oxide scale. Another possible effect of prior heat treatment is to avoid the recrystallization of cold rolled structure in the first test cycle, which might disrupt the formation of a stable scale.
However, the dimension stability is somewhat more complicate than the weight gain during the oxidation. Although the depletion of Al has been shown to coincide with the length change breakaway of cold rolled samples, the vacuum annealed samples behaved quite unpredictably.
An attempt is made to generalize the oxidation length change behaviors for the Fe-Cr-Al foils to provide the base for a mechanism that is useful to analyze the metallurgical variables.
Figure 8 depicts the four types of commonly observed length change behaviors. The most common length change is the one of which the foil shows stable length through out the oxida- tion test (Curve B, diamond). Curve C (check mark) shows that the foil starts out with small length increase then follow a shrinking trend to reach a stable dimension. This type of behavior is most commonly observed in cold rolled samples prior to the depletion of Al. This type of behavior is also observed in foil thinner than 40 micron or with higher Al contents. The third type of length change (Curve A, triangle) is termed as “abnormal” since the sample length increases continuously through the oxidation test. Curve D (square) shows the onset of breakaway length change when the Al was depleted after a period of continuous length in- crease. Curve D has to be considered for two scenarios. One is attributed to that when foil gauge is so thin that weight gain breakaway occurs prior to the end of oxidation test. The other scenario belongs to the abnormal behavior since the length change breakaway takes place when the Al is depleted (see Figure 3). It is necessary to mention that the length change breakaway will take place eventually when the weight gain breakaway occurs. Within these length change behaviors, it is most critical to understand the “abnormal” behaviors and ration- alize against, if available, controllable material variables in order to avoid the undesirable con- sequence on the catalytic substrate.
The sample dimension, during the oxidation test, was under the influence of several stresses of thermal-mechanical nature. Schutze and Przybilla [9] discussed the various origins of stress during the oxidation of Fe-Cr-Al alloy foils. A multitude of stresses can coexist in the alloy- scale system while the oxidation proceeds. However, two types of stress have aroused the most attention. The thermal expansion mismatch between the scale and alloy substrate will introduce a thermal stress on the alloy during the cooling, which will be in the tensile state. A growth
stress is also presented due to the combined effects of oxide growth as well as structure change might take place in either the scale or alloy itself. The thermal stress values from calculation and experimental measurement have shown to match reasonably well [10]. However, the stress states thus obtained, in the foil, are tensile and this implies that the foil shall be continuously growing as the oxidation proceeds.
As being mentioned earlier, the majority of length changes of Fe-Cr-Al foils is near zero or even negative. It is necessary to find a mechanism to provided the compressive stress in the scale-alloy system that can counter the tensile stress from the thermal/growth mechanism.
Schutze and Pryzbilla have suggested that there can be a “relaxation” mechanism, which re- lieves the foil from being stretched by the scale during the cooling. It is worthwhile noting that the relaxation mechanism, which causes the foil to show no significant dimensional change, will not provide a negative length change.
Figure 8: Generalized oxidation length change behaviors for (a) abnormal, (b) stable, (c) shrinking and (d) breakaway after Al depletion
Nevertheless, it might be the simple fact that the foil density is changing as the Al is con- sumed during the oxidation process. The density of Fe-20Cr-6Al alloy is 7.15 g/cm3 while that of Fe-20Cr alloy is 7.5 g/cm3. Since the Al oxidation follows the parabolic rate relation, the volume of alloy decreases follows the density increase. The dimensional change from density increase can amount to approximately a 2% decrease in the linear dimension of foil. Combin- ing the volumetric introduced dimensional change (~-2%) to that of the thermal stress induced creep of approximately 1%. The apparent dimensional change at the complete depletion of Al is about 1%, which has been observed for the majority of the vacuum annealed samples.
The aforementioned length change mechanism of which the stretching form the ther- mal/growth stress is balanced by the shrinking of materials, provides reasonably good length change value that matches the majority of measured values. To support this mechanism, it is worthwhile to examine the length changes of cold rolled sample, which always showed nega- tive length change (i.e. shrinking). If only the scale-induced stress is considered, the rapid
Typical Oxidation Length Change Behaviors
-1 0 1 2 3 4
0 100 200 300 400
Time, hours
Length Change, %
A: abnormal, quasi-parabolic B: stable
C: shrinking D: abnormal, early breakaw ay
weight gain (thickening of scale) in the cold rolled sample should have stretched the sample quickly as well. It is reasonable the oxide scales developed on the cold rolled samples were not as adherent (i.e. protective) as that of the vacuum heat-treated samples since the oxidation weight gains are much faster in cold rolled samples. Then the scales on the cold rolled samples will not be able to exert the tensile stress on the foils to cause the stretching of samples. Thus, the length of cold rolled sample will likely be controlled by the alloy density change due to the Al depletion.
The abnormal length changes, in the case of quasi-parabolic trend, can be as high than 6%
before the complete consumption of Al in the alloy. This level of length change can not be accounted for by the aforementioned mechanism. Schutze and Pryzbilla have pointed out that structure changes in alloy or scale might introduce additional dimensional changes. The mate- rials showing abnormal length change can usually be identified with several compositional and microstructural abnormalities. These alloys tend to have higher rare earth and/or austenite sta- bilizer contents and well-developed second phases (e.g. Fe-Cr carbide). These variables might affect the microstructures and scale development during the oxidation process.
Another explanation on the quasi-parabolic type, abnormal length change might be that the scale adhesion is so effective that no relaxation is possible between scale and foil. The scale on the abnormal length change sample showed columnar structure, which is usually an indication that the oxide adhere well and is growing without disruption. The microstructure of abnormal length change samples showed no change from vacuum annealing to the end of oxidation test and probably caused no disruption on the oxide growth.
The abnormal growth of foil seen through the cyclic test is the accumulated creep of foil af- ter each test cycle. The thermal expansion mismatch from cooling (1100°C to ambient) be- tween the scale of aluminum oxide (CTE = 9 PPM/°C) and foil (CTE = 15 PPM/°C), which are in equilibrium at the test temperature, will be approximately 0.66%. Since each test cycle will introduce this amount of stretching from cooling because there is no relaxation, the 10-cycle (400 hours) oxidation test will yield a 6.6% total length change. The accumulation mechanism is supported by testing sample of 6% total length change isothermally rather than cyclically.
The one cycle length change, for example 50 hours at ~0.5% is far less than that obtained
~1.5% after three cycles, for the same total oxidation duration, from the multiple cycle tests [10].
In summary, the “apparent” linear dimension of Fe-Cr-Al foils after each oxidation cycle is controlled by the combined effect of four mechanisms. The first mechanism is the creep of foil due to the thermal/growth induced tensile stress. The second mechanism is the volumetric re- duction and thus the shrinking due to the density increase since the Al is being removed from the alloy. The third mechanism is the adhesion of scale to foil. The importance of adhesion is that this will determine the effectiveness of transfer of the thermal/growth stress to the foil. At the extreme case when there is no adhesion, the foil dimension shows shrinking rather than the expected growth. The last mechanism is the structural changes of either foil or scale, as this will provide materials with different thermal expansion characteristics. This mechanism can also affect the adhesion of scale to alloy and complicate the relative effectiveness of mecha- nism one and two. The structural changes might be the root cause of the early length change breakaway. The Al and Cr additions to Fe are known to stabilize the ferritic structure and the depletion of these elements will cause the austenitic phase, which has different thermal expan- sion and physical properties to be presented at elevated temperature.
Although there are still many unanswered questions on the oxidation induced length change, a general guideline to obtain dimensionally stable foil of Fe-Cr-Al alloy can be drawn. The alloy has to be heat treated to provide a low oxidation weight gain. The alloy shall have as low as possible alloying elements that might cause any phase transformation or second phase pre- cipitation. The rare earth addition has to be at the level that provides sufficient scale adhesion for oxidation resistance. But the optimum amount is limited since the scale adhesion might be improved to the point that no “relaxation” is possible and thus full tensile stress is exerted on the foil to cause abnormal length change. Effects of other elements can be rationalized by con- sidering the effects on the “relaxation” of scale adhesion. For example, free sulfur might be needed as oppose to the conventional, total elimination of sulfur approach for the best oxida- tion resistance.
5 Conclusion
Oxidation weight gains of Fe-Cr-Al alloys follow the classic parabolic behavior. The oxidation weight gain is significantly reduced by the vacuum heat treatment.
The oxidation induced length change behaviors are categorized into four different groups.
The most commonly observed behavior is that the sample maintains a stable dimension prior to weight gain breakaway. However, there are also the abnormal cases of which either the length increases continuously or the length change breakaway occurs as soon as the Al is depleted.
To rationalize the commonly observed length change, be either stable or negative through the oxidation, mechanisms to counter the tensile stress states between the scale and foil is pro- posed. The density increase due to the Al consumption is proposed to provide the necessary
“shrinking” or volumetric decrease of the foil.
The foils showed abnormal length change in the case of continuous length increase are ex- amined. Based on the scale morphology and microstructure evolution, it is proposed that a
“perfect” adhesion between scales and foils existed in this type of samples. The perfect adhe- sion allows the thermal expansion mismatch between scale and foil to exert an approximately 0.6% stretch in each oxidation test cycle. The stretching is accumulated after each test cycle since there is no relaxation between scale and alloy and thus the observed “abnormal” length change.
From a theoretic point of view, the ideal foil dimensional change, in the cyclic oxidation condition, can be estimated by the multiplication of thermal expansion mismatch to the number of test cycles.
The commercial Fe-Cr-Al alloy, in order to achieve the least length change, has to strike a balance between the perfectly adherent scale for the maximum protection and the somewhat reduced oxidation resistance needed for the relaxation of adhesion.
6 Acknowledgment
Authors appreciate the extensive oxidation test performed by Linda Linehan and Bill Gorman.
The permission from the management of EMS to publish the results is greatly appreciated.
7 Reference
[1] S. Pelters, F. W. Kaiser and W. Maus, SAE Paper 89044, Society of Automotive Engi- neers, 1989
[2] N Birks and G. H. Meier in Introduction to High Temperature Oxidation of Metals, Ed- ward Arnold, London, 1983, 141
[3] K Ishii, S Satoh, M Kobayashi and T Kawasaki in Metal-Supported Automotive Cata- lytic Converters (Ed.: H. Bode) Werkstoff-Informationsgesellschaft mbH, 1997, 55 [4] K Tanaka and T. Saito, Tetsu to Hagane, 1995, 81, 79–84
[5] C S Chang, A Pandey and B Jha, , SAE Paper 960566, Society of Automotive Engineers, 1996
[6] I M Sukonnik, C S Chang and B Jha, US Patent 5980658, 1999
[7] I. M. Sukonnik, C S Chang and B Jha in Metal-Supported Automotive Catalytic Con- verters (Ed.: H. Bode) Werkstoff-Informationsgesellschaft mbH, 1997, 93
[8] N Birks and G. H. Meier in Introduction to High Temperature Oxidation of Metals, Ed- ward Arnold, London, 1983, 63
[9] M. Schutze and W. Przybilla in Metal-Supported Automotive Catalytic Converters (Ed.:
H. Bode) Werkstoff-Informationsgesellschaft mbH, 1997, 163 [10] S Chang, Internal Study, EMSI, 2001