Isothermal Oxidation Behavior of Haynes 230 Alloy in Air at 1100 °C

Rare Metal Materials and Engineering Volume 37, Issue 9, September 2008 Online English edition of the Chinese language journal ARTICLE

Cite this article as: Rare Metal Materials and Engineering, 2008, 37(9): 15451548.

Isothermal Oxidation Behavior of Haynes 230 Alloy in Air at 1100 °C Liu Dongmei,

Hu Rui,

Li Jinshan,

Liu Yi,

Kou Hongchao,

Fu Hengzhi

Northwestern Polytechnical University, Xi’an 710072, China

Abstract: The isothermal oxidation behavior of Haynes 230 alloy (Ni-Cr-W-Mo alloy) in air at 1100 °C was investigated by means of SAXD and SEM (EDAX). The predominant phase in the oxide scales were Cr2O3 and (Ni, Mn, Cr)3O4. Multilayer was observed with increasing of oxidation time. The outer (Ni, Mn, Cr)3O4 scale may reduce the volatilization of inner Cr2O3 scale and its oxidation behavior almost followed parabolic law. Due to preferential diffusion of manganese and chromium, alloy composition beneath the oxide scale changed, which led to subsurface degradation such as intergranular oxidation, void formation, and no carbide zone formation. Key words: oxidation; haynes 230alloy; Cr2O3; (Ni, Mn, Cr)3O4; subsurface degradation

Haynes 230 is a solid solution and carbide strengthened wrought nickel-base alloy used in the combustion region of jet engines. When it is exposed to air at high temperatures, its chromium concentration is high enough (22 wt %) to form a continuous Cr2O3 scale[1í5]. Cr2O3 is thermodynamically stable and has high melting points with respect to the metal, while the transport processes through the oxide scale are relatively slow. So Cr2O3 can act as a protective barrier against oxidation[6]. Unfortunately, when temperatures exceed 950 °C, chromium vaporizes and liberates volatile CrO3 instead of forming more desirable barrier Cr2O3 scale [7]. Minor addition of manganese can lead to the formation of Mn-rich spinel[3í6] which may effectively retard the Cr2O3 scale vaporization at the temperatures near 1000 °C [5]. In general, the cyclic oxidation behavior at temperatures between 871í1093 °C [2], long-term isothermal oxidation kinetics between 650í850 °C [3,4] and isothermal oxidation behavior at 1150 °C [5] of Haynes 230 alloy were studies in detail previously. Nevertheless, the mechanism of long-term isothermal oxidation behavior at 1100 °C has not been fully understood. The purpose of this study is therefore to investigate the isothermal oxidation behavior of Haynes 230 alloy at 1100 °C with emphasis placed on the element diffusion mechanism

and its resulting phenomena.

1

Experiment

Haynes 230 alloy forged plate was provided by HAYNES INTL-CHINA-LTD. The chemical composition of the sheet used in isothermal oxidation tests is listed in Tab.1. The sample size was 20 mm×10 mm×3 mm. Before testing, the samples were wetly ground up to 800-grit, and washed in a ultrasonic cleaner with ethyl alcohol. Isothermal oxidation was carried out by exposing the samples to dry air in the furnace. Oxidation time at 1100 °C was 3, 10, 25, 50 and 100 h. Every sample was located in a silica crucible during oxidation. All the crucibles were previously roasted to constant weight. Before the oxidation tests, the precise total mass of the every sample and crucible was recorded. After isothermal oxidation for the above time, every crucible was covered with a crucible cover which was also previously roasted to constant mass and then air-cooled to room temperature. The crucible and sample was precisely weighed again. In each experiment, three samples were used. For every oxidation time, the value of mass gain is the average of three samples oxidized for one oxidation time. After oxidation tests, small angle X-ray diffraction (SAXD)

Received date: December 17, 2007 Foundation item: National Natural Science Foundation(50432020) Corresponding author: Liu Dongmei, Master, State Key Laboratory of Solidification Processing, Northwest Polytechnical University, Xi’an 710072, P. R. China, Tel: 0086-29-88493484, E-mail:[email protected] Copyright © 2008, Northwest Institute for Nonferrous Metal Research. Published by Elsevier BV. All rights reserved.

Liu Dongmei et al. / Rare Metal Materials and Engineering, 2008, 37(9): 15451548

Table 1 Chemical composition of Haynes 230 alloy(Ȧ/%) Elements

Ni

Content

Bal.

Cr

W

22.4994 13.8300

Mo

Si

Al

La

Mn

C

P

Fe

B

Co

S

1.2507

0.3400

0.3435

0.0260

0.5465

0.1020

<0.0050

0.7766

0.0020

0.0925

0.0020

technique, Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Analyses (EDXA) were employed to analyze the oxide scale.

2

Results and Discussion

2.1

Microstructure of Haynes 230 alloy before oxidation

Fig.1 is the typical microstructure of Haynes 230 alloy used in oxidation tests before oxidation. The carbides distributed in the grains and at grain boundaries were identified as tungsten and molybdenum-rich M6C.

2.2

Evolution of surface-oxide scale

Fig.2 shows the morphology of the initial oxide formed on the surface of the sample. When the samples were exposed in air at 1100 °C for 5 min, as projecting oxides were rich in tungsten compared with the other area, it could be inferred that the carbide was preferentially oxidized. It was reported that when the carbide was oxidized, the volume could be up to 2.3í2.5 times larger than its initial volume which led to the extrusion of the oxide[8]. As the oxidation time increased to 10 min, the oxides formed on the surface were very fine and the oxide scale above the grain boundaries was relatively thinner than that above the grains. When viewed at higher magnification, the oxide particles appeared as square particles, which were attributed to the lattice diffusion. The oxide scale consisted of nickel and chromium, without any manganese. Fig.3

illustrates the morphology of the surface-oxide scale developed by the alloy after oxidation at 1100 °Cfor relative long oxidation time. With increasing of oxidation time, the scale locally spalled off and EDAX results show that the average concentration of nickel where was no spalling decreased and that of the manganese and the chromium increased. When oxidation time was 10 h (Fig.3d), disastrous spallation happened which led to reveal the bulk where hole-likely voids were obvious. Vacancies could be created by the different diffusion rates of nickel back into the bulk and chromium and manganese outwards to form oxide scale and the vacancies condensed at the scale/alloy interface to form voids, which led to the disastrous spallation of the scale. When oxidation time was 25 h, the spalling-off area was more than that of 10 h, but the revealed area was covered by finer oxide particles identified as chromium-rich oxides as shown in Fig.3f. The surface-oxide scale morphologies of 50 and 100 h were similar to that of 25 h. Fig.4 was surface-oxide morphology after 100 h oxidation viewed at high magnification. There were two kinds of oxide crystals: larger pyramidal-type with well-developed flat faces which indicated both lattice- and grain-boundary diffusion[5] a

b

37 μm

11 μm

c

d

150 μm

16 μm

e

f

160 μm

16 μm

10 μm Fig.1 Microstructure of Haynes 230 alloy before oxidation

a

16 μm

b

37 μm

Fig.3

Secondary-electron SEM images, illustrating the morphology of surface-oxide scale: (a)3 h, (b) the spalled area of 3 h oxi-

Fig.2 Secondary-electron SEM images, illustrating the morphology

dation at high magnification, (c)10 h, (d) the spalled area of

of surface-oxide scale at the beginning of the oxidation for

10 h oxidation at high magnification, (e) 25 h, and (f) the

5 min(a) and 10 min(b)

spalled area of 25 h oxidation at high magnification

Liu Dongmei et al. / Rare Metal Materials and Engineering, 2008, 37(9): 15451548

8 μm Fig.4 Secondary-electron SEM image, illustrating the morphology of surface scale developed at 1100 °C for 100 h, viewed at high magnification

and much smaller round-type. The atomic ratio of Cr/Mn of the two kinds of the crystals was both close to 2, which indicated they were MnCr2O4. The predominant phase in the oxide scales at all time are Cr2O3 (rhombohedral: a=b=0.49 588 nm, c=1.359 42 nm) and M3O4 type spinel oxide identified by the small angle X-ray diffraction technique, where the M could be Ni, Cr or Mn. There are a few kinds of isomorphous oxide presenting the M3O4 type, such as NiMn2O4, Ni2MnO4, NiCr0.5Mn1.5O4, Cr1.5Mn1.5O4, MnCr2O4 etc. All of these M3O4 type spinel oxides are isomorphous with Mn3O4 oxide(cubic: a=0.842 00 nm). When the oxidation time was 1 h, surface-oxide scale contains a high content of nickel and chromium, while the manganese content is relatively lower. But there is no evidence for the presence of NiO or NiCr2O4 oxide. The results of the X-ray diffraction show that the predominant phases in oxide scale are Cr2O3 and NiMn2O4(cubic: a=0.838 20 nm). As the concentration of Mn is so low that the chromium might dissolve in NiMn2O4 to form Ni(Mn, Cr)2O4 and the Cr has little or no effect on the lattice parameter. When the oxidation is up to 25 h, the predominant phases in oxide scale are Cr2O3 and MnCr2O4 (cubic: a=0.845 50 nm) and did not change as oxidation time increase up to 50 and 100 h. Fig.5 shows the backscattered electron SEM image of cross-section microstructures of the oxide scales after oxidation at 110°C in air for 25, 50 and 100 h. The concentration of aluminum which is more easily oxidized in Haynes 230 alloy is 0.3435 wt%. When oxidation time was 25 h, as the outer

manganese-chromium oxide scale was porous and the oxygen could diffuse inward to react with the aluminum, preferential intergranular Al-rich oxide developed. As oxidation time increased, the oxide scale thickened and transferred from one layer to multilayer. When oxidation time was up to 50 h, multilayer was obvious. There were two layers: the outer layer identified as MnCr2O4 as mentioned above was porous and thicker than the inner layer and it inclined to spall off during the cooling; the inner layer was identified as Cr2O3 resolved with a low concentration of Al and Si as shown in Fig.5d. The outer MnCr2O4 scale could retard the chromia volatilization and thus improve the oxidation resistance of the alloy. After long time at 1100 °C deep, thick subscale loops of Cr2O3 incorporating alloy particles into the scale were observed. As for the formation of multilayer, it could be explained as follows: the concentration of manganese in Haynes 230 alloy was only 0.5 wt% and with increasing of oxidation time, the preferential outward diffusion of manganese could lead to the manganese impoverishment in the alloy beneath the oxide scale and the supply of the manganese could not maintain the forming of the manganese-chromium oxide; however, the chromium content was still enough to form the Cr2O3 scale. The oxygen could diffuse inward and react with the surface metal and then the Cr2O3 scale formed beneath the outer MnCr2O4 scale. Therefore, multilayer developed as oxidation time increased. The white bright blocks and strips in Fig.5 were identified as carbides. It is obvious that no carbide zone where its initial carbides were lost and deterioration was caused by internal oxidation developed just beneath the scale. The no carbide zone became deeper and deeper as oxidation time increased. There were three possibilities to explain its formation: (i) the scale spalled off during oxidation and the carbon diffused outward to react with oxygen to form CO or CO2 gases; (ii) the internal Al-rich oxides suggested oxygen inward diffusion, and the oxygen might react with the carbon to form CO or CO2 gases which might induce the nucleation of voids; (iii) as chromium and manganese diffused outward to form an oxide scale which could lead to chromium and manganese depletion beneath the scale, the carbides then could resolve back into the matrix.

b

c

d Intensity/a.u.

a

50 μm

50 μm

50 μm

Cr

Si Al 1 2

Cr 3

4

5

6

Energy/keV Fig.5 Backscattered electron SEM images of cross-section microstructure of scale after oxidation at 1100 °C in air for: (a) 25 h, (b) 50 h, (c)100 h, and (d) energy-dispersive X-ray spectrum from the inner oxide scale

Liu Dongmei et al. / Rare Metal Materials and Engineering, 2008, 37(9): 15451548

2.3

3

Mass change behavior

Fig.6 illustrates the dependence of the mass gain of the samples on the oxidation time at 1100 °C in air. The parabolic law is almost followed. It can be explained as follows: the outer MnCr2O4 scale could retard the volatilization of Cr2O3 to CrO3 by reducing the contact area between the inner Cr2O3 scale and the air. Consequently, the oxide scale can restrain the further oxidation of the alloy to a certain extent, so the oxidation rate decreases with increasing of the oxidation time. The experimentally obtained parabolic rate constant kw at 1100 °C which is 4.70853 g2·m-4·h-1.

Mass Gain/g·m-2

25

Conclusion

1) W-rich M6C-type carbides are preferentially is oxidized at the beginning of the oxidation. 2) During oxidation, Ni diffuses inward into the bulk, while Mn and Cr diffuse outward resulting in both the void formation and the transition from the initial (Ni, Mn, Cr)3O4 oxide to MnCr2O4. As oxidation time increases, Mn depletion beneath the scale will lead to the formation of Cr2O3 scale. 3) As the outer MnCr2O4 can retard the volatilization of Cr2O3, the parabolic law is almost followed when the alloy is oxidized at 1100 °C in air.

References

20

1 Twancy, H. M.; Klarstrom, D. L.; Rothman, M. F. J. Met., 1984, 36: 58

15

2 Fen-Ren, Chien.; Brown, R. J. Mater. Sci., 1992, 27: 1514 3 Diane, England. M.; Anil, Virkar. V. J. Electrchem. Soc., 1999,

10

146: 3196 4 Li, Jian.; Pu, Jian.; Hua, Bing. J Power Sources, 2006, 159: 641

5 0

5 Twancy, H. M. Oxid. Met., 1996, 45: 323 0

20

40

60

80

100

Time/h Fig.6 Mass gain as a function of oxidation time of Haynes 230 alloy oxidized at 1100 °C in air

6 Stott, F. H. The Role of Active Elements in the Oxidation Behavior of High Temperature Metals and Alloys[M]. London: Elsevier Applied Science, 1989: 3 7 Caplan, D.; Cohen, M. J Electrchem Soc, 1961, 108: 438 8 Litz, J.; Rahmel, A.; Schorr, M. Oxid. Met., 1988, 30: 9