High temperature oxidation behavior of a novel cobalt-nickel-base superalloy

Accepted Manuscript High temperature oxidation behavior of a novel cobalt-nickel-base superalloy Bohua Yu, Yunping Li, Yan Nie, Hua Mei PII:

S0925-8388(18)32396-X

DOI:

10.1016/j.jallcom.2018.06.275

Reference:

JALCOM 46611

To appear in:

Journal of Alloys and Compounds

Received Date: 1 May 2018 Revised Date:

18 June 2018

Accepted Date: 23 June 2018

Please cite this article as: B. Yu, Y. Li, Y. Nie, H. Mei, High temperature oxidation behavior of a novel cobalt-nickel-base superalloy, Journal of Alloys and Compounds (2018), doi: 10.1016/ j.jallcom.2018.06.275. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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High temperature oxidation behavior of a novel cobalt-nickel-base superalloy

Bohua Yu1, Yunping Li , Yan Nie3, Hua Mei4 1

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2*

School of Materials Science and Engineering, Central South University, Changsha, China

2

China 3

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State Key Lab for Powder Metallurgy, Central South University, Changsha,

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YuanMeng Precision Technology (Shenzhen) Institute, Shenzhen, China 4

Zhuzhou Cemented Carbide Cutting Tools Co. LTD, Zhuzhou, China

Abstract

Oxidation behavior of a novel cobalt-nickel-base superalloy in air from 600 to

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900°Cis investigated in details. The results indica te a two-layer oxide film formed on the surface after oxidation: the external layer of oxide film is mainly composed of Cr2O3 and a few of other oxides, and the inner layer dominantly

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contains Al2O3. During oxidation at higher temperature and long duration, evaporation of Mo-containing oxide leads to void formation and local failure in

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external oxide layer, characterized by roughening of oxide film. A formation mechanism of oxide film on this alloy during oxidation is proposed for the first time.

Keywords:Cobalt-nickel-base superalloy; SEM; Interfaces; Oxidation

1. Introduction Ni-base and Co-base superalloys, strengthened by superfine γ′ phase, are widely used as turbine disk materials, and high temperature components 1

ACCEPTED MANUSCRIPT etc., owing to their superior strength and high oxidation resistance at elevated temperatures [1-7]. However, mechanical properties of superalloys are typically deteriorated by the coarsening of γ′ phase, which is the primary strengthening component at high temperatures[8,9]. With this purpose, a novel

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cobalt-nickel-based superalloy with a nanoscale coherent γ′ phase was developed recently, where the nanoscale coherent γ′ phase is isolated by stacking-fault ribbons rich in Suzuki segregation of alloying elements [10,11]. Additionally, this specific combination of microstructure can slow down the

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coarsening of γ′ phase at high temperatures, giving rise to considerably higher mechanical properties at elevated temperature than those of the commonly

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used nickel- and cobalt-base superalloys.

Apart from the mechanical properties, oxidation behavior of superalloy is also of great importance to both scientific research and practical use. The oxidation behavior of this novel alloy cannot be simply explained and predicted by the existing theories of either nickel base alloy [12] or cobalt base alloys

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[13] due to its complex composition from large number of elements [10]. For example, this alloy contains a large number of alloying elements such as Cr, Al, Ti, Nb, and Mo etc., which tend to interact with each other and play significant

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roles in influencing the oxide formation [14]. The oxidation behavior of this alloy becomes more complex than most of the traditional alloys [15-19].

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Therefore, in the present work,oxidation behavior of this novel Co-Ni-base superalloy in air with temperatures ranging from at 600°C to 900 °C was investigated

in

details

by

using

scanning

electronic

microscope,

energy-dispersive X-ray spectroscopy, laser microscope etc..

2. Experimental method The detailed composition of alloy used in this research is tabulated in Table 1. The ingots are fabricated by vacuum melting, followed by a solution 2

ACCEPTED MANUSCRIPT treatment at 1200 °C for 2 h. Sheet samples of5 × 1 0 × 2 mm3 for oxidation test and cylindrical sample of Φ 5×1mm2 for thermo gravimetric (TG) analysiswere cut by electrical discharge machining (EDM), respectively. All samples were ground with abrasive paper, followed by polishing with 0.5-µm alumina

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suspensions. Finally, after they were cleaned in ethanol using an ultrasonic cleaner and dried with a blower, the mirror-polished samples were ready for the oxidation test. Isothermal oxidation testing was performed in air from 600 to 900 °C for 2, 5, 12, 24, 36, 48, 72, 100 h, resp ectively, using muffle furnace.

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After exposure to the designated temperature for a given duration, the sample was removed and cooled in air to room temperature. The mass of samples

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after oxidation for various durations were measured by electronic balance (FA2004) with a precision of 0.0001 g. TG analysis was performed in air from 20 to 1005°C with a heating rate of 5 °C/min in the rmogravimetric analyzer (SDT-Q600, TA Instruments-Waters LLC, Shanghai).

The microstructures were observed by using a field-emission scanning

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electron microscope (FE-SEM; Quanta FEG 250, FEI, Tokyo). X-ray diffraction (XRD) patterns were obtained by using a Cu Kα (0.1547 nm) radiation source(D/max 2500, Rigaku, Tokyo).The applied current and voltage were 40

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kV and 250 mA, respectively. During the measurements, each sample was scanned from 20 to 80°.The morphology of the oxide scale on the surface was

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investigated by laser microscope (VK-X200, Keyence, Tokyo).The chemical composition was analyzed using an energy-dispersive spectroscope (EDX) equipped in the scanning electron microscope.

3. Results 3.1. Initial microstructure

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ACCEPTED MANUSCRIPT The surface morphology of the as-polished alloy is shown in Fig. 1(a). Equiaxed grains of γ phase with mean size of 300-500 µm are observed after solution treatment. The microstructure is characterized by bight contrasts of precipitates both along the grain boundaries and in the interior of matrix.

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However, the area fraction of precipitates is not so high. This can be verified from the XRD patterns of the alloy, where only γ phase matrix is obviously observed as show in Fig. 1(b). To gain more information on the precipitates, EDX is used to analyse the composition of both the precipitates (A) and matrix

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(B) as shown in Fig. 2. Detailed compositions of the two areas are tabulated in Table 2. The results reveal that the precipitates in both grain boundary and the

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interior of matrix are intrinsically identical in composition and contain much higher contents of Mo and Nb. However, due to the formation of these Mo, Nb-rich precipitates, Mo, and Nb contents in alloy matrix are a little lower than the nominal contents of alloy (Table 1). From the phase diagram of alloy calculated using the commercial thermodynamic calculation software

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Thermo-Calc 5.0 with Ni7 database, it is suggested that the alloy dominantly consists of γ phase and minor fraction of δ-Ni3Nb phase after solution treatment at present condition [10]. Combined with the EDX analysis, it is possible that some lattice sites of Ni and Nb are also replaced by high fraction

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of Co and Mo (Ti) atoms in δ phase, respectively.

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3.2 Phase characterization of oxide film XRD patterns of the alloy after oxidation at 600, 700, 800 and 900 °C are

shown in Figs. 3, 4, for 5 and 72h, respectively.Analysis on the patterns reveals a dominant Cr2O3and a small amount of Al2O3in film after oxidizing for a short time (5 h) at low temperature (Fig. 3). The peak intensity of the alloy matrix is relatively high in all conditions, and Cr2O3 peak intensities increase markedly with higher oxidationtemperature (900 °C) and longer time, implying the thickening of the oxide film. In addition, the relative intensities of Al2O3peaks increase slowly with oxidation time or temperature compared to 4

ACCEPTED MANUSCRIPT that of Cr2O3 peak, as indicated in Fig. 3and Fig. 4, suggesting that the gradual increase in thickness or fractionofAl2O3at higher temperature or prolonged oxidation. A weak CoCr2O4peak is also found in alloy after oxidation at 900 °C for 5h (Fig. 3(d)) as well as800 and 900 °C for 72 h (Fig. 4(c-d)). This suggests

formation of CoCr2O4. 3.3. Surface morphology after oxidation

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that prolonged oxidation or oxidation at higher temperature enhances the

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The surface of the as-polished sample is bright silver in colour. After oxidation at low temperatures (<800 °C) for less th an 5 h, all the sample

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surfaces appear dark green. After prolonged oxidation (longer than 48 h), the sample surfaces turn darker, which is similar to the results ofCo-29Cr-Mo alloy after comparable oxidation[20-22].Oxidation at higher temperature (>800 °C) yields similar surface profiles to that in prolonged oxidation at lower temperature, suggesting that the temperature and oxidation duration strongly

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influence the thickness or composition of the oxide film. In the condition of higher temperature and longer oxidation (900 °C for 100 h),the sample surface becomes extremely rougher. This is also accompanied by local failure of oxide

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film during cooling, which can be even observed by naked eyes. The surface morphologies of alloy oxidized at 600, 700, 800 and 900 °C

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for 2, 12 and 100 h are shown in Fig. 5 by BSE mode. Slight change of sample surface at 600 °C is observed with the progress of oxidation. After oxidation for short duration (2 and 12 h), oxidation preferentially takes place at the grain boundaries and are characterized by bright contrasts of Cr, Mo-rich oxide dots as verified by using EDX. When the oxidation time increases to 100 h, oxidation also takes place in the matrix. Nevertheless, the entire morphology of surface does not change markedly compared to the initial microstructure, suggesting the slight oxidation at this temperature. In addition, with the progress of oxidation, the bright oxide dots disappear gradually, which is in 5

ACCEPTED MANUSCRIPT turn replaced by dark ones. The number and size of dark dots are observed to increase gradually as indicated by arrows. With the oxidation temperature increasing to 700 °C , the change in sample surface is roughly comparable to that at 600 °C, but is characterized by

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larger number of dark contrasts both along grain boundaries and in the matrix, especially in the prolonged oxidation, suggesting the more severe oxidation. At oxidation temperature of 800 °C, apart from the gra in boundaries, change in the matrix surface takes places much earlier, and the sample surface becomes

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much darker after oxidation for 100 h. After oxidation at 900 °C, sample surface is oxidized homogeneouslyat 12 h. When the oxidation time reaches

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100 h, the oxide film becomes much rougher and is characterized by large size of bright and dark contrast, while this is not observed after oxidation for a shorter time (12 h) or lower temperature.

Fig. 6 shows the surface height maps, measured by the laser scanning

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microscope (LSM), for samples after oxidation for100h and at(a) 600 °C, (b) 700, (c) 800 and (d) 900°C, respectively.It can be seen that, with temperature increasing from 600 to 800 °C, the sample surface b ecomes smoother, implying the more compact and homogeneous film formed. This is considered

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due to that the occurrence of oxidation varied from local oxidation to uniform oxidation. After oxidation at 900 °C, the sample su rface turns to uneven. In

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order to gain more information on the evolution of the oxide film at temperatures 900 °C, LSM is used to observe the sam ple surfaces after oxidation for various durations, as shown in Fig. 7. After oxidation for a short duration (2 h) no significant change in surface height is observed, suggesting the slight or homogeneous oxidation on samples surface. After oxidation for 12 h, the sample surface becomes a little rougher. This is in contrast to that after oxidation longer than 48 h, where significant variation on sample surface height is observed.

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ACCEPTED MANUSCRIPT 3.3. EDX analysis EDX analysis is applied to study the composition variation in the entire surface of after oxidation at900 °C for different d urations, as shown in Fig. 8. The results indicate that sample surface after oxidation at 900 °C, 2 h is mainly

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composed of oxides of Cr, and a few of Al, Mo, Nb and Ti. After oxidation for 12 h, similar to that at 2 h, the sample surface is dominantly composed of Cr2O3, but its fraction increases obviously compared to that at 2 h. In addition, the fraction of Mo in sample surface also decreased dramatically. After oxidation

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for a longer duration(100 h), with the roughing of oxide film, the fraction of Al

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oxide in sample surface increases from 1.15% at 12 h to 22.2% at 100 h. A more detailed EDX analysis on two typical areas of the bright area (A) and dark area (B) under SEM image is shown in Fig. 9 for the sample surface at 900 °C for 100 h. The results indicate that brig ht area (A) is dominantly composed of Cr2O3, while dark area B mainly contains Al2O3. By combining the

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LSM results in Fig. 8, a two-layer microstructure of oxide film is considered in sample surface in present condition: the outmost layer dominantly consisting of Cr2O3 and the inner layer dominantly consisting of Al2O3. With the progress of oxidation, the outmost Cr2O3 layer tends to break locally during cooling,

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giving rise to the exposure of inner Al2O3.

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We summarize the elemental concentration variation on outmost surfaces of sample as a function of oxidation duration at various temperatures as shown in Fig. 10 by using EDX analysis on the entire surface. At 600 °C, with the progress of oxidation, except for the increased O concentration, the concentrations of other elements do not vary in large scale, revealing the slight oxidation and extremely thin film occurred on sample surface. At 700 °C, oxidation on sample surface is accelerat ed, characterized by significant increase in the concentration of both O and Cr compared to that at 600 °C. Accordingly, concentrations of Ni, Co and M o are decreased. At800 7

ACCEPTED MANUSCRIPT °C,the increase rate of Cr element is obviously acc elerated while the Al element increases first and then decreases, indicating the gradual thickening of outmost Cr2O3layer on Al2O3layer. At 900 °C, the Cr 2O3 oxide layer is formed rapidly but replaced by Al2O3 film suddenly after oxidation for 50 h

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accompanied by the roughening of oxide surface as shown in Fig. 7. The weight vs. temperature for the alloy exposed in flowing air from ambient temperature to 1005°C with a heating rate o f 5 °C/min is given in Fig. 11(a). The mass of sample increases gradually from room temperature to

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about 800°C, indicating the oxidation throughout th e heating process. After that, the mass gain is observed to be followed by a sudden drop. Isothermal

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oxidation behaviours at different temperatures for 100 h are given in Fig. 11(b). In contrast to the oxidation behaviour of Co-29Cr-6Mo alloys characterized by a parabolic behaviour with oxidation time [20], present alloy reveals the different feature. Almost no changes at 600and a slight increase at 700 °C in sample mass are observed. At 800 °C, the curve of m ass gain follows a

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parabolic law, while at 900 °C the mass gain of sam ple increases more rapidly before 12 h than that at 800 oC and reaches a maximum value at 48h. After that, the mass gain begins to decrease gradually with the further oxidation.

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4. Discussion

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It can be inferred from the aforementioned results that the oxide film of the present cobalt-nickel-based superalloy exhibits a two-layer structure, characterized by a Cr(M)2O3(M=Mo, Ti, etc.) dominant external layer and Al2O3 inner layer. This structure is very compact at lower temperature and short period, while higher temperature and longer time give rise to local failure of the external layer, leading to the roughening of surface. Note that the content of Mo in oxide film especially at 900 °C decreased much more dramatically with the progress of oxidation compared to the other elements such as Ni and Co (Fig. 10 (d)). It has been reported that the melting 8

ACCEPTED MANUSCRIPT point of MoO3is approximately 800 °C and it begins to volatilize even at 600 °C[23]. Since Mo after oxidation was observed not i n the form of MoO3 but as a substitution element in Cr2O3 from the XRD analysis in Fig. 3 and 4, we believe that the evaporation of MoO3 takes place in the following reaction at high

Cr(Mo)2O3(s) MoO3(g)↑+Cr2O3(s)

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temperature, leading to the significant mass loss as shown in Fig. 11. (1)

The vapour pressure of MoO3 as given by Jones [24] can be expressed as ିଵଵଶ଼଴ ்

-7.04log(T)+30.494

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LogP(atm)=

(2)

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We calculated the vapour pressure of MoO3 at temperature range of 600-1000 oC, and can see it keeps almost constant before 850 oC but increases substantially after melting (>850

o

C) (Fig. 12). The significant

evaporation of MoO3 at 900 oC is considered closely related to the roughing of oxide film. This is because the fast evaporation of MoO3above the melting

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point will inevitably lead to the formation of voids in the external oxide film. This may be confirmed by the surface morphologies at lower temperature where the grain boundaries rich in Mo are characterized by the darker dots (voids) apart

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from brighter contrasts (Mo containing oxide). Due to the presence of these voids, the inwards diffusion of oxygen through the oxide film will be

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accelerated; on the other hand, the external Cr2O3 film layer becomes loose, brittle and easy to break from the inner layer during cooling with the progress of evaporation.

It is reported that the oxidation behaviour of alloy strongly depends on its

composition. For example, oxide film of Co-Al-W alloy at 800 and 900 oC in air demonstrates a three-layer microstructure: the external layer of the oxide film is dominantly composed of Co3O4 and a small amount of CoO [25]. Although the Co has a weaker affinity with O than Al, due to the much lower fraction of Al than Co, formation of a compact Al2O3 film on sample surface is difficult. In turn, 9

ACCEPTED MANUSCRIPT the outmost surface of oxide film is dominantly composed of Co3O4, while the inner layer gradually comes from the aggregation of Al oxides at the external layer/matrix interface. This is similar to the present study that the oxide film of the cobalt-nickel base alloy consists of the Al2O3-dominant internal layer, and

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the Cr2O3-dominant external layer with a few of (Ni,Co)Cr2O4 and TiO2. The initial oxidation may involve the simultaneous formation of oxides of most elements, which gradually evolved into a Cr2O3-dominant scale [20]. Since the Al (2 wt. %) is not sufficient for the formation of compact oxide film, Al2O3 may

gradually expend into an inner layer [16,26].

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formed in some defects between the matrix and external oxide layer, and

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From the aforementioned results, at low temperature (600 oC), oxidation predominantly takes placealong the grain boundaries. With the progress of oxidation, the oxidation extends into the interior of grains of sample surface, forming an oxide layer covering the surface of the substrate. When oxidation is carried out at higher temperatures (700-800 oC), the initial oxidation proceeds

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simultaneously in grain boundaries and grains, which leads to a more rapid formation of oxide film. This indicates that the early oxidation is more intense at higher temperatures, resulting in the same results with long time oxidation at

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600 oC.

After oxidation at 900 oC for long time, local failure of Cr2O3-dominant

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external layeroccurs. By considering the aforementioned results and previous research, the failure of oxide film can be ascribed to the following two factors: as mentioned in Fig. 2 and Table 1, the microstructure of present alloy is composed of a dominant γ phase matrix and a few of δ phase mostly along the grain boundaries. This will give rise to the oxide film on sample surface, characterized by a non-homogeneous microstructure with Mo rich oxide along the grain boundaries and Cr2O3 rich in the interior of grains. At high temperature, the oxide film will be subjected to a selective volatilization of MoO3 [23,24], giving rise to large number of voids formed along the grain 10

ACCEPTED MANUSCRIPT boundaries (Fig. 5). At the temperature of 900 oC exceeding the melting point of MoO3 (Figs. 11, 12), void formation will inevitably lead to a local failure of the Cr2O3film around the grain boundary. Since the inner layer (Al2O3) formed beneath the external layer and possibly contains much lower fraction of Mo

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compared to the external layer, no breakage is observed in the inner layer. In addition, the failure of oxide film due to its thickening has been observed in various alloys [27-29]. It is considered that the difference in thermal expansion coefficients between Cr2O3 (6.4×10-6 K-1) and Al2O3 (7.7×

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10-6 K-1) [30] may also play a role in promoting the failure of external layer. The different thermal coefficients between external layer and internal layer may

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produce shear stress, which will be more obvious and give rise to film breakage during cooling with the thickening of oxide film under high temperature and with prolonged oxidation. In contrast, the internal Al2O3 layer formed after long distance diffusion of Al through grain boundary plays a significant role of eliminating the cavities located at the oxide film/matrix

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interface [20]. Moreover, the internal oxide layer keeps a high bonding surface with matrix. This can be manifested by the results of Yan et al. [31], who suggested that internal oxides develop at the oxide-scale–alloy interface and

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protrude into the grain boundaries during oxidation. On the basis of this, the two layer oxide film formation during the oxidation of present cobalt-nickel

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base superalloy is proposed asshown in Fig. 13.

5. Conclusion

A two-layer oxidation film formed on the surface of the novel cobalt-nickel base superalloy after oxidation at 600 to 900 oC with oxidation duration ranging from 2 to 100 h: the external oxide layer is mainly composed of Cr2O3, while the inner layer is dominantly composed of Al2O3.

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ACCEPTED MANUSCRIPT The selective evaporation of MoO3 in the external oxide film and the thermal stress generated between the internal and external layers give rise to a local failure of the external layer around grain boundaries.

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Acknowledgements The authors gratefully acknowledge the financial support from project of The Science Fund for Distinguished Young Scholars of Hunan Province, China

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(2016JJ1016), and the project of Innovation and Entrepreneur Team Introduced by Guangdong Province, China (201301G0105337290).

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ACCEPTED MANUSCRIPT Figure and table captions Fig. 1 (a) BSE image and (b) XRD pattern of present cobalt-nickel-base superalloy after solution treatment. Fig. 2Amplified BSE images of present cobalt-nickel-base superalloy after solution treatment.

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Fig. 3 XRD patterns of cobalt-nickel-base superalloy after oxidation for 5 h at different temperatures. Fig. 4 XRD patterns of cobalt-nickel-base superalloy after oxidation for 72 h at different temperatures.

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Fig. 5BSE image on the surface morphology of cobalt-nickel-base superalloy after oxidation at 600, 700, 800 and 900 °C for 2, 12, and 100 h, respectively.

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Fig. 6Laser scanning microscope profiles of cobalt-nickel-base superalloy after oxidation at temperatures of (a) 600, (b) 700, (c) 800 and (d) 900 °C for 100 h. Fig. 7Laser scanning microscope profiles of cobalt-nickel-base superalloy after oxidation at 900 °Cfor different time. Fig. 8Energy spectrum (EDX) analysis on the whole sample surface of cobalt-nickel-base superalloy after oxidation for (a) 2, (b) 12, and (c) 100 h at 900°C.

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Fig. 9EDX analysis on the the bright area (A) and dark area (B) of oxide film on cobalt-nickel-base superalloy after oxidation at 900°C for 100 h.

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Fig. 10The variation on the mean element concentration as a function of oxidation duration in sample outmost surface of cobalt-nickel-base superalloy after oxidation at (a) 600 oC, (b) 700 oC, (c) 800 oC, and (d) 900 oC.

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Fig. 11(a)Mass change of cobalt-nickel-base superalloy in flowing air as a function of oxidation temperature from room temperature to 1005°C. (b) Isothermal oxidation curves at 600, 700, 800 and 900 °C for oxidation duration of 300 min, respectively. Fig. 12 Calculated vapour pressure of MoO3 at various temperatures. Fig. 13Proposed mechanism for the formation and failure of oxide film in cobalt-nickel-base superalloy at various temperature and various duration. Table 1 Compositions of cobalt-nickel-base superalloy used in this study. Table 2 EDX analysis results (wt. %) at location A and B for cobalt-nickel-base superalloy after solution treatment as shown in Fig. 2.

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Grain boundary

200μm

Fig. 1 (a) BSE image and (b) XRD pattern of present cobalt-nickel-base superalloy after solution treatment.

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B

EP

A

5μm

AC C

Fig. 2 Amplified BSE images of present cobalt-nickel-base superalloy after solution treatment.

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Fig. 3 XRD patterns of cobalt-nickel-base superalloy after oxidation for 5 h at different temperatures.

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Fig. 4 XRD patterns of cobalt-nickel-base superalloy after oxidation for 72 h at different temperatures.

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Fig. 5 BSE image on the surface morphology of cobalt-nickel-base superalloy after oxidation at 600, 700, 800 and 900 °C for 2, 12, and 100 h, respectively.

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Fig. 6 Laser scanning microscope profiles of cobalt-nickel-base superalloy after oxidation at temperatures of (a) 600, (b) 700, (c) 800 and (d) 900 oC for 100 h.

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12h

High

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2h

AC C

EP

48h

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200μm

Fig. 7Laser scanning microscope profiles of cobalt-nickel-base superalloy after oxidation at 900 °C for different time.

Low

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Fig. 8 Energy spectrum (EDS) analysis on the whole sample surface of cobalt-nickelbase superalloy after oxidation for (a) 2, (b) 12, and (c) 100 h at 900 oC.

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Fig. 9 EDS analysis on the bright area (A) and dark area (B) of oxide film on cobalt-nickel-base superalloy after oxidation at 900 oC for 100 h.

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Fig. 10 The variation on the mean element concentration as a function of oxidation duration in sample outmost surface of cobalt-nickel-base superalloy after oxidation at (a) 600 oC, (b) 700 oC, (c) 800 oC, and (d) 900 oC.

0

Time, t/min

50

100

150

200

1.2 -2

20.817mg

20.78 20.768mg 20.76

o

795.3 800 oCC o

20.74

1005.1 C

20.72 400

600

Temperature, T /

800

1000

EP

200

0.8 0.6 0.4 0.2 0.0

0

20

40

60

80

100

Time/h

AC C

0

1.0

900 800 700 600

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Weight, m/mg

20.80

Mass Gain/mg cm

20.82

(b)

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(a)

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Fig. 11 (a) Mass change of cobalt-nickel-base superalloy in flowing air as a function of oxidation temperature from room temperature to 1005 oC. (b) Isothermal oxidation curves at 600, 700, 800 and 900 oC for oxidation duration of 100 h, respectively.

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175 150 125 100 75

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Vapor pressure, mm Hg

200

50

0

650

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600

EP

25

700

750 800 850 Temperature, °C

900

950 1000

Fig. 12 Calculated vapor pressure of MoO3 at various temperatures.

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Fig. 13 Proposed mechanism for the formation and failure of oxide film in cobalt-nickel-base superalloy at various temperature and various duration.

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Element

Ni

Co

35.02

31.85

EP AC C

(wt.%)

Cr

Mo

Nb

Al

Fe

Ti

17.5

7.98

3.05

1.97

1.60

0.81

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Alloy composition

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Table. 1

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B

Matrix

Cr

Mo

Nb

Al

Fe

17.77

21.15

15.01

25.60

16.44

1.37

1.61

1.05

35.09

32.11

18.24

7.57

2.84

1.93

1.76

0.76

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Co

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A

Ni

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Element

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Table. 2

Ti

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Oxidation behavior of a novel superalloy was investigated; Two-layer microstructure of oxide film was observed; Failure of external layer (Cr2O3) during prolonged high temperature oxidation;

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Selective evaporation of Mo and thermal stress account for the local failure.