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The influence of CrTaO4 layer on the oxidation behavior of a directionally-solidified nickel-based superalloy at 850–900 °C
Journal of Alloys and Compounds 724 (2017) 565e574
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The influence of CrTaO4 layer on the oxidation behavior of a directionally-solidified nickel-based superalloy at 850e900 C Weili Ren a, *, Fenfen Ouyang a, Biao Ding a, Yunbo Zhong a, Jianbo Yu a, **, Zhongming Ren a, Lanzhang Zhou b a b
State Key Laboratory of Advanced Special Steel, College of Materials Science and Engineering, Shanghai University, Shanghai 200072, PR China Metal Research Institute, Chinese Academy of Sciences, Shenyang 110015, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 April 2017 Received in revised form 3 July 2017 Accepted 6 July 2017 Available online 8 July 2017
The influence of CrTaO4 on the oxidation behavior of a directionally solidified nickel-based alloy has been investigated at the temperatures between 850 C and 900 C for the different time intervals (the longest time is 300 h). The oxide scale consists of three layers. The outer layer is mainly composed of Cr2O3, TiO2, and NiCr2O4, the middle layer is CrTaO4, and the inner oxide layer mainly consists of Al2O3. The continuous CrTaO4 layer is formed after oxidation for certain times. The formation time of continuous CrTaO4 layer shortens with the temperature increase. Furthermore, the layer causes the transformation of the oxidation kinetics from the parabolic law to the cubic law. The control of oxidation process converts from the outward diffusion of Cr, Ti, and Ni to the inward diffusion of O when the continuous CrTaO4 layer is established. The mechanism clarifies the influence of Ta on the oxidation behavior of Ni-based superalloys from the oxidation kinetics. © 2017 Published by Elsevier B.V.
Keywords: Ni-based superalloys High-temperature oxidation CrTaO4 layer Oxidation mechanism
1. Introduction Nickel-based superalloys have been widely applied in the hotsection components for the aerospace and power-generation engines, such as turbine disks and blades, due to their superior strength and adequate resistant-oxidation properties [1e3]. The excellent oxidation resistance of the alloys at elevated temperatures is associated with the formation of continuous Al2O3 and Cr2O3 oxide layers. Compared with other oxides, the oxide layer shows a dense structure, which can significantly decrease the element diffusion rate and consequently decrease the oxidation rate of the superalloys [4e6]. In order to ensure the strength and oxidation resistance of the Ni-based superalloys at elevated temperatures, the various alloying-elements are usually added, such as Al, Ti, Ta, Cr, Co, W, and Mo [7,8]. On the one hand, the addition of Ta in the Ni-based superalloys increases the contents of the strengthening phases through dissolving into the g' (Ni3Al) lattice and consequently enhances the superalloy strength [9,10]. On the other hand, it has a
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (W. Ren), [email protected] (J. Yu). http://dx.doi.org/10.1016/j.jallcom.2017.07.066 0925-8388/© 2017 Published by Elsevier B.V.
beneficial role in the oxidation resistance of the alloys with the appropriate configuration of the content and type of other elements and temperatures. Let us review briefly the investigations about the influence of Ta on the oxidation behavior of the Ni-based superalloys [11e17]. Firstly, the addition of appropriate amount of Ta in the alloy improves the oxidation resistance. Han et al. [11] found that the oxidation rate of Ni-based superalloys with 5.8 wt.% Ta is lower than that with 3.8 wt.% Ta. It was because the continuous dense CrTaO4 formed in the high Ta-content superalloy. Kim et al. [12] studied the oxidation behavior of five types of Nibased superalloys at 850 C and 1000 C and found that the superalloys containing higher Ta content were more oxidationresistant than those with the lower addition levels of Ta. They also attributed the reason to the formation of CrTaO4 and TaO layers in the superalloy with the high Ta content. By contrary, Yang [13] reported that an addition of Ta as small as 1 at.% (~3.0 wt.%) improved the oxidation resistance significantly by promoting early establishment of a protective Al2O3 layer. When 3 at.% (~8.8 wt.%) of Ta was added, complex oxides such as NiMoO4, NiTa2O6, TaO2, MAl2O4, and M2CrO4 were formed, causing degradation of the oxidation resistance. Secondly, the temperature affects the formation rate of Ta-containing oxides. Park et al. [14] found that the CrTaO4 layer was more continuous at 1000 C than that at 850 C. The CrTaO4 layer hinders the outward diffusion of the Al element,
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resulting in a formation of a continuous dense Al2O3 inner layer and further a slower oxidation rate. Li et al. [15] reported that the X-ray diffraction peak of CrTaO4 at 900 C was stronger than that at 800 C in the oxide scale. However, the CrTaO4erich oxide layer was very thin and discontinuous at 1000e1150 C [16]. It could be attributed to the rapid establishment of a continuous Al2O3 inner layer at high temperatures, which retarded the outward diffusion of Ta element. Wang et al. [17] studied the form of Ta in the oxide film of the N5 Ni-based single-crystal superalloy oxidized at different temperatures. It was found that the continuous CrTaO4 was not formed due to the low Cr content in the superalloy, while two types of Ta-containing oxides TaO2 and Ta2O5 were formed. Thirdly, Ta segregation at the grain boundaries of Al2O3 scale increases the spallation resistance of oxide scales, which was confirmed through the structure observation of oxide scales under the transmission electron microscope by Wang [17]. Finally, the oxidation resistance and spallation resistance of Ta is related with the homogeneous extent of its distribution in the superalloy matrix. At the Ta-rich region, Ta was oxidized prior to Al, TaOx was less protective than Al2O3, and the adhesion of the alloy substrate to TaOx was inferior to Al2O3. The inhomogenous dispersion and the excessive TaOx at some places upon superalloy substrate result in the formation of large cavities and the final spallation [17]. Moreover, a detrimental interspersed Ta-rich oxide particles developed in the top spinel scale on PWA-1484 superalloy [18]. Pedraza et al. found that the incorporation of TaxOy in the oxidation scales clearly worsened their oxidation resistance in aluminide coatings on different superalloy substrates [19]. The above investigation shows that the effects of Ta on the oxidation of the alloy exhibits the complex behaviors, and its beneficial role depends on whether it can form a continuous CrTaO4 layer. The formation of the continuous dense CrTaO4 could hinder the outward diffusion of other elements and decrease the oxidation rate of Ni-based superalloys. When the continuous dense CrTaO4 could not form due to the small Ta and Cr content [11,12,17] or the low temperature [14,15] or the Al2O3 inner scale formation [13,17], the effect of Ta decreasing the oxidation rate is not exerted. However, the formation time of CrTaO4 layer and its influence on the rate exponent of oxidation kinetics and on the activation energy of oxidation rate have not been reported up to now. The present paper studied that the influence of CrTaO4 layer on the oxidation behavior of a directionally-solidified nickel-based superalloy with 5.0 wt.% Ta at temperatures between 850 C and 900 C. The formation time of the CrTaO4 was determined at the different temperatures. The influence of CrTaO4 layer on the rate exponent and the activation energy were studied. The oxidation mechanism was proposed after the formation of the CrTaO4 layer. The investigation reflects the essentials of oxidation behavior, and clarifies the influence of Ta on the oxidation mechanism of Ni-based superalloys. 2. Experimental procedure The nominal composition of the investigated alloy is shown in Table 1. The alloy with a diameter of 16 mm was directionally solidified at a withdrawal speed of 7 mm/min. The oxidation specimens with the dimensions of 10 mm 10 mm 3 mm were cut from the heat-treated ingots, ground, and polished. The heattreatment procedure is 1210 C 2 h (AC) þ 1080 C 3 h
(AC) þ 850 C 24 h (AC) (AC: air cooling). The isothermal oxidation was carried out in static air at 850 C, 875 C, and 900 C. The corundum crucibles used in the oxidation were all heat-treated in the furnace at 1200 C for enough time to ensure that their weight would not be changed. The oxidation kinetics curves of the alloys were determined by the static weight gain method. After appropriate time intervals, the specimens were taken out of the furnace, cooled in air, and then weighed with an electronic balance with a resolution of 0.1 mg. Three specimens were simultaneously tested at each temperature. The oxidation time is up to 300 h. The phases formed on the oxidized specimens were determined by X-ray diffraction (XRD). The surface morphology of the oxidized samples was evaluated by the scanning electron microscope (SEM) equipped with an energy dispersive spectrometry (EDS) without any preparation. However, for evaluation of cross-sections, all specimens were initially covered with a thin layer of Ni, which aimed to prevent damage to the oxide layer during the specimen preparation for SEM observation. The chemical plating was used to cover the Ni layer. The samples were preprocessed in the solution of SnCl2 (13 g/L) þ HCl (40 ml/L)þ deionized water for 2 min, and further were put in the solution of PbCl2 (0.2 g/L) þ HCl (7 ml/ L) þ deionized water for 1 min at room temperature, which ensures the surface activation of the samples. Then, the plating process was carried out in the solution of NiSO4$6H2O (25 g/L) þ NaH2PO2$H2O (30 g/L) þ CH3COONa (12 g/L) þ Na3C6H5O7$2H2O (14 g/ L) þ C3H6O3 (13 g/L) þ deionized water for 90 min at 90 C. 3. Results 3.1. Phases in the oxide film The phases forming during the oxidation were characterized by XRD. Fig. 1(a) shows the XRD patterns of the specimens oxidized at 850 C, 875 C, and 900 C for 200 h. At each temperature, the diffraction peaks of the Cr2O3, TiO2, NiCr2O4, Al2O3, CrTaO4, and the matrix are observed. But the peak intensity of the CrTaO4 phase is weak. With the increase of the oxidation temperature from 850 to 900 C, the peak intensity of the matrix phases of Ni and Ni3Al decreases. Fig. 1(b) shows the XRD patterns of the specimen oxidized at 900 C for the different times in the range from 24 to 200 h. With the time increase, the peak intensity of the oxides increases, while that of the matrix phases decreases. No oxides of Co, W, and Mo were identified. The reason will be illustrated in the discussion part. 3.2. Surface morphology and composition No oxide spallation was observed during the sample cooling. The surface morphology of the oxide scale formed on the alloy after exposure time of 300 h at the different temperatures is presented in Fig. 2. The oxides uniformly cover the scale surface. With increasing the temperature, the oxide grains gradually increase. The composition of the scale surface was analyzed by EDS, which are shown in Table 2. It can be seen that the compositions are rich in Cr, Ti, Ni, and O, which are in agreement with the results of XRD and the cross-section of oxidation scale. It should be stated that Al and Ta are hardly detected in the surface oxide with EDS due to the shallow penetration depth of the electron beam. 3.3. The cross-section morphology and structure of the oxide film
Table 1 Nominal chemical composition of the superalloy. Element
Co
Cr
Mo
W
Al
Ti
Ta
C
Zr
Ni
wt.%
10.00
14.00
1.50
4.00
4.00
3.00
5.00
0.08
0.03
Bal.
In order to further clarify the structure of the oxidation scale of the superalloy, the cross-sections were studied. Fig. 3(aec) shows the morphology and structure of the cross-section of the samples
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Table 2 The composition of the oxidation scale surface by EDS (at.%).
Fig. 1. XRD patterns of the superalloy after the oxidation (a) at 850 C, 875 C, and 900 C for 200 h and (b) at 900 C for 24 h, 48 h, 96 h, 144 h, and 200 h.
oxidized at 850 C for the different times. After the oxidation time of 300 h as shown in Fig. 3(c) and the element redistribution as shown in Fig. 3(f), the oxide film is composed of three layers, i.e. the outer layer, the inner layer, and the middle layer (labeled as P1, P2,
Temperature ( C)
Al
Cr
Ta
Ti
O
Ni
Co
850 875 900
0.77 0.32 0.20
23.05 25.22 29.65
0.09 0.09 0.02
9.08 8.07 8.87
57.57 56.17 58.29
8.34 7.24 2.62
1.10 2.89 0.35
and P3, respectively, similarly hereinafter). The area labeled as P4 is the oxidation-affected zone in the matrix (similarly hereinafter). The compositions of EDS analysis in the different layers are shown in Table 3. The outer oxide layer P1 is mainly composed of Cr, Ti, Ni, and O, among which the Cr content is much more than that of Ti and Ni. Combined with the XRD results (Fig. 1), it can be concluded that the outer oxide layer is composed of the more Cr2O3, and the less TiO2 and NiCr2O4. The inner layer P2 consists of the black oxidized area and the white un-oxidized area. The compositions of the P2 layer in Table 3 are from the black areas, which are rich in Al and O. It indicates that the inner layer is enriched in the Al2O3. The middle layer P3 mainly rich in Cr, Ta, and O is CrTaO4 layer, which can be further identified from the element distribution map of the specimen for 300 h, as seen in Fig. 3(d). From the element distribution maps in Fig. 3(e and f), the minor content of Ti was detected in the middle layer. Considering the minor content, the layer is thought to be mainly composed of CrTaO4 in the present investigation. The zone P4 is the g'-depleted area, whose formation is due to the loss of Al, Ti, and Ta in the matrix during the oxidation and the disappearance of the g0 precipitates. The thickness of the outer layer initially increases sharply when the oxidation time prolongs. It almost keeps constant after the CrTaO4-layer formation. The thicknesses of the inner layer and the g'-depleted area increase all the time with the enhancement of the oxidation time. The continuous middle layer of CrTaO4 oxide is formed at the time of about 96 h, before which the layer is composed of the small scattered particles (Fig. 3(a)). Fig. 4(aec) and 5 (a-f) show the morphology of the cross-section of the superalloy samples oxidized at 875 C and 900 C for the different times, respectively. The results of element redistribution after oxidation at 875 C and 900 C for different times are shown in Fig. 4(d and e) and 5(g-i), respectively. The EDS compositions in the different layers in the specimens oxidized at 900 C are shown in Table 4. It can be seen that the structure of the oxide scales are the same as that of the samples oxidized at 850 C. However, the time required for the formation of the middle layer P3, i.e. the continuous CrTaO4 layer, decreases with the increase of temperature. At 875 C and 900 C, the continuous CrTaO4 layer is formed at the time of about 48 h (Fig. 4(b)) and 24 h (Fig. 5(a)), respectively. At
Fig. 2. The SEM micrographs of the oxidation-scale surfaces of the superalloy after the exposure time of 300 h at the different temperatures (a) 850 C, (b)875 C, (c) 900 C.
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Fig. 3. The SEM micrographs of the cross-sections of the superalloy oxidized at 850 C for the different hours (a) 48 h, (b) 96 h, (c) 300 h, (d) the elements redistribution maps of 48 h, (e) the elements redistribution maps of 96 h, and (f) the elements redistribution maps of 300 h. P1, P2, and P3 are used to label the outer, inner, and middle layer in the scales, respectively. P4 is the g'-depleted area in the matrix.
each temperature, the evolution trend of the P1, P2, and P4 layers with the oxidation time is consistent with that at 850 C. At the same oxidation time, the content of Al2O3 in the P2 layer increases with the increase of temperature. The continuous Al2O3 inner layer could form at each temperature as long as the oxidation time is long enough. The higher temperatures, the shorter time to form the continuous Al2O3 inner layer. The specimens oxidized for 300 h
(Fig. 4(c)) and 120 h (Fig. 5(d)) at 875 C and 900 C show the continuous Al2O3 layer, respectively. 3.4. Oxidation kinetics The oxidation rate of alloys is reflected by the kinetics curves, and it is usually characterized by the weight gain per unit area with
Table 3 The composition in the different layers in the oxidation scales by EDS at 850 C. The meanings of P1, P2, and P3 are same as those in Fig. 3. Oxide layer
P1
P2
P3
Oxidation time (h)
48 96 300 48 96 300 48 96 300
Elemental composition (at.%) Al
Cr
Ta
Ti
O
Ni
Co
2.10 2.37 3.88 29.70 32.40 36.64 e 6.22 3.21
36.36 40.60 42.55 3.63 4.80 3.83 e 25.00 20.56
0.29 3.10 3.68 0.14 2.50 e e 24.60 29.97
7.87 7.33 6.09 0.13 0.53 0.19 e 7.70 4.89
47.62 34.90 39.45 46.40 34.64 46.26 e 32.00 34.39
5.47 10.80 3.94 16.9 24.8 11.22 e 3.60 6.02
0.29 0.90 0.41 3.10 0.33 1.86 e 0.88 0.96
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Fig. 4. SEM micrographs of the cross-sections of the samples oxidized at 875 C for the different times (a) 24 h, (b) 48 h, (c) 300 h, (d) the elements redistribution maps of 48 h, and (e) the elements redistribution maps of 300 h.
the oxidation time. Fig. 6 shows the oxidation kinetics curves of the superalloy at 850 C, 875 C, and 900 C. The oxidation kinetics at each temperature follows the similar law, i.e, the weight gain increases sharply during the initial period and then tends to vary gently. At the same oxidation time, the mass gain increases with the temperature raise. The isothermal oxidation behavior can be described by the following equation [20,21]: (DW)n ¼ Kt
(1)
where DW is the weight gain per unit area, t is the oxidation time, n is the rate exponent that determines the rate law, and K is the oxidation rate constant. In order to determine the n and K value, taking logarithms of both sides of Eq. (1) gives:
lnDW ¼ 1=n $ lnt þ 1=n $ lnK
(2)
The slope reciprocal of a lnDW- lnt plot gives the n value. Fig. 7 shows the double logarithmic graphs of the mass gain and time at the different temperatures. It could be seen that the fit lines exhibit a turning point at each temperature. At 850 C, 875 C, and 900 C, the time to the turning point is 96 h, 48 h, and 24 h, respectively. Combined with the microstructure of the cross-section of the oxidized specimens (Figs. 3e5), the turning time at each temperature is in agreement with the formation time of continuous CrTaO4 layer, which is 96 h, 48 h, and 24 h at 850 C, 875 C, and 900 C, respectively. This indicates that the formation of the continuous CrTaO4 layer causes the change of the oxidation kinetics. The n value before and after the turning point are also presented in Table 5. It can be seen that the oxidation kinetics obey the parabolic
relationship before the turning time. The formation of the continuous CrTaO4 layer causes the oxidation kinetics to obey the cubic relationship. The oxidation rate constant K also affects the oxidation kinetics of the alloys. It could be obtained according to the intercept of the fit line of ln△W-lnt plot in Fig. 7, which is presented in Table 6. The K value is controlled by the oxidation activation energy, and their relationship with temperature can be expressed by the Arrhenius equation [22]:
K ¼ A $ expE=RT
(3)
where E is the oxidation activation energy, R is the gas constant, T is the oxidation temperature, and A is a constant for a certain alloy. In order to obtain the value of E, the relationship between lnK 1/T is shown in Fig. 8. The oxidation activation energy is 335 kJ/mol and 423 kJ/mol before and after the formation of the continuous CrTaO4 layer, respectively.
4. Discussion The experimental results show that the continuous CrTaO4 layer is formed when the superalloy is oxidized for a certain time. The time to the formation of continuous CrTaO4 layer corresponds to the turning point of oxide kinetics. The values of n and E are different before and after the turning point. Therefore, the formation of the continuous CrTaO4 layer changes the oxidation behavior of the superalloy. The influences of CrTaO4 layer on the oxidation behavior are discussed from the thermodynamic, the kinetics, and the mechanism aspects below.
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Fig. 5. SEM micrographs of the cross-sections of the samples oxidized at 900 C for the different times (a) 24 h, (b) 48 h, (c) 96 h, (d)120 h, (e)200 h, (f) 300 h, (g) the elements redistribution maps of 24 h, (h) the elements redistribution maps of 120 h, and (i) the elements redistribution maps of 300 h.
4.1. The thermodynamic of the CrTaO4 and other oxides The results of XRD, SEM, and EDS analysis indicate that the outer layer of the oxide film consists of Cr2O3, TiO2, and NiCr2O4, and the inner layer is mainly composed of Al2O3. As the oxidation proceeds, the middle continuous CrTaO4 layer is formed between the outer and inner layer. The chemical composition of the investigated superalloy is Ni-14Cr-10Co-4W-1.5Mo-4Al-3Ti-5Ta (wt.%). The
oxidation reactions which will occur at elevated temperatures are as follows [23,24]:
4=3CrðsÞ þ O2 ðgÞ ¼ 2=3Cr2 O3 ðsÞ
(4)
DGCr2 O3 ¼ 746840 þ 170:29T
(5)
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Table 4 The composition in the different layers in the oxidation scales by EDS at 900 C. Oxide layer
P1
P2
P3
Oxidation time (h)
24 48 120 300 24 48 120 300 24 48 120 300
Elemental composition (at.%) Al
Cr
Ta
Ti
O
Ni
Co
1.50 1.75 0.85 3.01 38.40 33.58 39.73 41.12 8.30 4.90 0.74 2.09
47.30 32.45 54.83 46.09 3.66 1.97 2.66 0.43 14.04 25.46 19.47 18.98
0.10 0.06 1.24 0.98 0.46 0.14 0.42 1.07 17.90 21.49 20.90 25.01
5.08 3.46 3.99 4.87 0.46 0.29 0.21 0.03 3.24 3.48 10.86 3.23
36.80 58.80 35.54 42.73 33.65 58.70 40.30 51.89 37.02 34.85 44.64 47.35
8.25 2.97 3.40 2.24 19.45 4.39 13.90 4.78 13.90 8.28 2.69 1.44
0.97 0.51 0.15 0.08 3.92 0.93 2.78 0.68 5.60 1.54 0.70 1.90
Table 5 The rate exponent n before and after the formation of continuous CrTaO4 layer. Temperature ( C)
The n value before the formation of CrTaO4 layer
The n value after the formation of CrTaO4 layer
850 875 900
1.9 1.8 2.1
3.2 3.1 3.4
Table 6 The rate constants K before and after the formation of continuous CrTaO4 layer. Temperature The K value before the formation The K value after the formation ( C) of CrTaO4 layer (mg2/cm4$h) of CrTaO4 layer (mg3/cm6$h) 850 875 900
7.04 104 2.00 103 3.10 103
8.65 105 3.08 104 4.71 104
Fig. 6. Isothermal oxidation kinetics curves of the superalloy samples at the different temperatures.
Fig. 8. Logarithm of the oxidation rate constant (lnK) plotted against the temperature reciprocal (1/T) before and after the formation of the continuous CrTaO4 layer.
Fig. 7. The double logarithmic graphs of the mass gain (DW) and oxidation time (t) at the different temperatures.
4=3AlðsÞ þ O2 ðgÞ ¼ 2=3Al2 O3 ðsÞ
(8)
TiðsÞ þ O2 ðgÞ ¼ TiO2 ðsÞ
(6)
DGAl2 O3 ¼ 1120480 þ 214:22T
(9)
DGTiO2 ¼ 943490 þ 179:08T
(7)
4=5TaðsÞ þ O2 ðgÞ ¼ 2=5Ta2 O5 ðsÞ
(10)
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DGTa2 O5 ¼ 810040 þ 165T
(11)
2CoðsÞ þ O2 ðgÞ ¼ 2CoOðsÞ
(12)
DGCoO ¼ 477810 þ 171:96T
(13)
2NiðsÞ þ O2 ðgÞ ¼ 2NiOðsÞ
(14)
DGNiO ¼ 476980 þ 168:62T
(15)
MoðsÞ þ O2 ðgÞ ¼ MoO2 ðsÞ
(16)
DGMoO2 ¼ 578200 þ 166:5T
(17)
WðsÞ þ O2 ðgÞ ¼ WO2 ðsÞ
(18)
DGWO2 ¼ 581200 þ 171T
(19)
It can be calculated that the DG value for each reaction is negative at temperatures in the range from 850e900 C, which indicates that all of the above reactions might occur. The order of the DG value for the different oxides are as follows: DGAl2O3
2TaðsÞ þ O2 ðgÞ ¼ 2TaOðsÞ
(20)
DGTaO ¼ 376600 173:2T
(21)
TaðsÞ þ O2 ðgÞ ¼ TaO2 ðsÞ
(22)
DGTaO2 ¼ 209200 20:5T
(23)
It can be calculated that the value of DGTa2O5 is smaller than that of DGTaO and DGTaO2 at 850e900 C, thus Ta2O5 is formed during the stage of oxidation. The formation equation of CrTaO4 is as follows [31,32]:
1=2Cr2 O3 ðsÞ þ 1=2Ta2 O5 ðsÞ ¼ CrTaO4 ðsÞ
DGCrTaO4 ¼ 1=2 DGTa2 O5 þ DGCr2 O3 ¼ 778440 þ 168T
(24) (25)
The reason that the continuous CrTaO4 layer lies in the middle should be attributed to the slow diffusion rate of Ta atom, which can be confirmed by the data in Table 7. Table 7 gathers the diffusion coefficients of Ta, Cr, and Ti in the pure Nickel [33] or the Waspaloy alloy [34] at 850e900 C. Therefore, Ta diffuses
Table 7 Diffusion coefficients of different elements in pure nickel or. Waspaloy alloy at 850 C 900 C. Element
Ta Ti Cr
diffusion coefficient (m2/s)
Ref.
850 C
875 C
900 C
4.63 1017 2.19 1016 8.25 1017
8.31 1017 3.67 1016 1.44 1016
1.45 1016 6.00 1016 2.46 1016
[33] [34]
outwardly at a smaller rate than Ti and Cr. The atomic mass also supports the guess since those of Ta, Ti, and Cr are 181, 48, and 52, respectively. The slow diffusion of Ta causes the requirement of an incubation period for the formation of the continuous CrTaO4 layer. From Table 7, it can be seen that the diffusion coefficient increases with increasing the temperature. Consequently, the incubation period for the formation of CrTaO4 would shorten, and the continuous CrTaO4 layer would appear early with the temperature increase. It should be noted that the AlTaO4 were hardly observed because the content of Cr was much higher than that of Al in the investigated alloy. The AlTaO4 was detected since the high content of Al (12.24 wt.%) and low content of Cr (5.42 wt.%) in the superalloy TMS-82þ [35]. 4.2. The effect of CrTaO4 layer on oxidation kinetics The oxidation kinetics is reflected by the rate exponent n and the isothermal oxidation rate constant K. The formation of continuous CrTaO4 layer causes the change of the above two factors. Before the formation of continuous CrTaO4 layer, the n value is about 2, and the kinetic curve follows the parabolic rate law. In contrast, after the continuous CrTaO4 layer is formed, the n value is around 3, and the kinetic curve follows the cubic law. In addition, the formation of continuous CrTaO4 layer reduces the K value by about one order of magnitude. For instance, the K value decreases from 7.04 104 mg2/cm4·h to 8.65 105 mg3/cm6·h when the continuous CrTaO4 layer is formed at 850 C, the K value decreases from 3.10 103 mg2/cm4·h to 4.71 104 mg3/cm6·h at 900 C. Therefore, the formation of the continuous CrTaO4 layer significantly decreases the oxidation rate, which is consistent with the thickness variation of the oxide film, i.e., the thickness of the outer layer remains nearly constant after the formation of the layer. The present result is in agreement with previous reports, i.e. Irving et al. [36] suggested that Ta reduces oxygen activity at the surface of a superalloy. Liu et al. [37] suggested that Ta slows down the outward diffusion of Al, suppressing the growth of oxide scales on the single-crystal Ni-based alloys. Park et al. [14] showed that Ta exerted a beneficial effect on oxidation resistance when its concentration was over 5 wt.% at 850 C, slowing down the formation of oxides in Ni-based superalloys. 4.3. The influence of the CrTaO4 layer on the oxidation mechanism The formation of continuous CrTaO4 layer reduces the oxidation rate of the alloy and also modifies the structure of the oxidation scales, which indicates that it changes the oxidation mechanism of the alloy. According to the morphology of the cross-section of the oxide film, the outer layer is composed of Cr2O3, and a small amount of TiO2 and NiCr2O4. The layer thickness increases with the oxidation time before the formation of continuous CrTaO4 layer. It can be concluded that the oxidation process is controlled by the outward diffusion of the Cr, Ti, and Ni atoms before the formation of continuous CrTaO4 layer. The schematic diagram is shown in
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Fig. 9. Schematic diagram of oxidation process (a) before the formation of CrTaO4 layer and (b) after the formation of CrTaO4 layer.
Fig. 9(a). It can be confirmed from the oxidation activation energy before the formation of continuous CrTaO4 layer, and the value of the oxidation activation energy E is 335 kJ/mol. It is slightly larger than the activation energy of Cr diffusion in Cr2O3, which is 280 kJ/ mol [38], since the diffusion of other elements, such as Ni and Ti, also contributes the activation energy of oxidation process. The continuous CrTaO4 layer inhibits the outward diffusion of Cr, Ti, Ni, and other metal elements [39], which would remarkably decrease the growth rate of the outer layer. So the thickness of the outer layer almost remains constant after the formation of the continuous CrTaO4 layer. Since the size of O atom (0.056 nm) is much smaller than that of Al atom (0.143 nm), O atom can diffuse faster through the CrTaO4 layer than the metal atoms. The content of Al2O3 in the inner oxide layer gradually increases after the formation of the continuous CrTaO4 layer. The continuous Al2O3 layer forms after the enough time. For example, the time to form the continuous Al2O3 layer at 900 C is 120 h (Fig. 5(c)). Therefore, the oxidation process is controlled by the inward diffusion of O after the formation of continuous CrTaO4 layer, and the schematic diagram is shown in Fig. 9(b). The oxidation mechanism is confirmed by the oxidation activation energy of 423 kJ/mol in the period, which is close to the activation energy, 482 kJ/mol, of Al diffusion in Al2O3 [40]. 5. Conclusions The effect of CrTaO4 layer on the oxidation behavior of a directionally solidified Ni-based superalloy at the temperatures between 850 C and 900 C was studied. The main conclusions are as follows: (1) The oxide film of the superalloy is mainly composed of three layers. The outer layer is mainly composed of Cr2O3, TiO2, and NiCr2O4, the middle layer is CrTaO4, and the inner oxide layer mainly consists of Al2O3. (2) The continuous CrTaO4 layer is formed after oxidation for a certain time. At 850 C, 875 C, and 900 C, the continuous CrTaO4 layer is formed at the oxidation times of about 96 h, 48 h, and 24 h, respectively.
(3) The formation of continuous CrTaO4 layer causes the transformation of the oxidation kinetics of the superalloy from the parabolic law to the cubic law. (4) The oxidation process is controlled by the outward diffusion of the Cr, Ti, and Ni atoms before the formation of continuous CrTaO4. In contrast, after the continuous CrTaO4 layer is formed, the inward diffusion of O control the oxidation process. The mechanisms uncover the effect of Ta on the oxidation behavior of Ni-based superalloys from the kinetics. Acknowledgments This research was supported by National Natural Science Foundation of China (No. 51371110) and the project from Guangdong Power Grid Co. Ltd. We also thank Instrumental Analysis and Research Center of Shanghai University for their support on material characterization. References [1] X.D. Lu, S.G. Tian, X.F. Yu, C.X. Wang, Oxidation behavior of a single-crystal Nibase superalloy in air at 900 and 1050 C, Rare Met. 30 (2011) 439e442. [2] X. Ren, F.H. Wang, X. Wang, High-temperature oxidation and hot corrosion behaviors of the NiCr-CrAl coating on a nickel-based superalloy, Surf. Coat. Technol. 198 (2005) 425e431. [3] X.G. Yang, S.L. Li, H.Y. Qi, Effect of high-temperature hot corrosion on the low cycle fatigue behavior of a directionally solidified nickel-base superalloy, Int. J. Fatigue 70 (2015) 106e113. [4] M.A. Azim, S. Burk, B. Gorr, H.J. Christ, D. Schliephake, M.H.R. Bornemann, P.H. Bolivar, Effect of Ti (macro-) alloying on the high-temperature oxidation behavior of ternary MoeSieB alloys at 820e1,300 C, Oxid. Met. 80 (2013) 231e242. [5] J.H. Chen, P.M. Rogers, J.A. Little, Oxidation behavior of several chromiaforming commercial nickel-base superalloys, Oxid. Met. 47 (1997) 381e410. [6] H.Y. Qi, X.B. Liang, S.L. Li, X.G. Yang, High-temperature oxidation behavior of DZ125 Ni-based superalloy under tensile stress, Rare Met. (2016) 1e6. [7] P. Caron, T. Khan, Evolution of Ni-based superalloys for single crystal gas turbine blade applications, Aerosp. Sci. Technol. 3 (1999) 513e523. [8] A.C. Yeh, S. Tin, Effects of Ru and Re additions on the high temperature flow stresses of Ni-base single crystal superalloys, Scr. Mater. 52 (2005) 519e524. [9] G.M. Janowski, R.W. Heckel, B.J. Pletka, The effects of tantalum on the microstructure of two polycrystalline nickel-base superalloys: B-1900 þ Hf and MAR-M247, Metall. Mater. Trans. A 17 (1986) 1891e1905. [10] R.C. Reed, T. Tao, N. Warnken, Alloys-By-Design: application to nickel-based single crystal superalloys, Acta Mater. 57 (2009) 5898e5913.
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[11] F.F. Han, J.X. Chang, H. Li, L.H. Lou, J. Zhang, Influence of Ta content on hot corrosion behaviour of a directionally solidified nickel base superalloy, J. Alloys Compd. 619 (2015) 102e108. [12] H.S. Kim, S.J. Park, S.M. Seo, Y.S. Yoo, H.W. Jeong, H.J. Jang, High temperature oxidation resistance of Ni-(5~13)Co-(10~16)Cr-(5~9)W-5Al- (1~1.5)Ti - (3~6) Ta alloys, Met. Mater. Int. 22 (2016) 789e796. [13] S.W. Yang, Effect of Ti and Ta on the oxidation of a complex superalloy, Oxid. Met. 15 (1981) 375e397. [14] S.J. Park, S.M. Seo, Y.S. Yoo, H.W. Jejong, H.J. Jang, Effects of Al and Ta on the high temperature oxidation of Ni-based superalloys, Corros. Sci. 90 (2015) 305e312. [15] M.H. Li, X.F. Sun, J.G. Li, Z.Y. Zhang, T. Jin, H.R. Guan, Z.Q. Hu, Oxidation behavior of a single-crystal Ni-base superalloy in air. I: at 800 and 900 C, Oxid. Met. 59 (2003) 591e605. [16] M.H. Li, X.F. Sun, T. Jin, H.R. Guan, Z.Q. Hu, Oxidation behavior of a singlecrystal Ni-base superalloy in Air-II: at 1000, 1100, and 1150 C, Oxid. Met. 60 (2003) 195e210. [17] J.L. Wang, M.H. Chen, S.L. Zhu, F.H. Wang, Ta effect on oxidation of a nickelbased single-crystal superalloy and its sputtered nanocrystalline coating at 900e1100 C, Appl. Surf. Sci. 345 (2015) 194e203. [18] J.A. Nychka, D.R. Clarke, G.H. Meier, Spallation and transient oxide growth on PWA 1484 superalloy, Mater. Sci. Eng. A 490 (2008) 359e368. [19] F. Pedraza, M. Mollard, B. Rannou, B. Bouchaud, J. Balmain, G. Bonnet, Oxidation resistance of thermal barrier coatings based on hollow alumina particles, Oxid. Met. 85 (2016) 231e244. [20] X.M. Hou, K.C. Chou, Quantitative interpretation of the parabolic and nonparabolic oxidation behavior of nitride ceramic, J. Eur. Ceram. Soc. 29 (2009) 517e523. [21] F.J. Liu, M.C. Zhang, J.X. Dong, Y.W. Zhang, High-temperature oxidation of FGH96 P/M superalloy, Acta Metall. Sin. 20 (2007) 102e110. [22] S.X. Liang, L.X. Yin, J.X. Li, M.Z. Ma, R.P. Liu, Isothermal oxidation behaviour of the TZAV-20 alloy, Mater. Des. 86 (2015) 458e463. [23] I. Barin, O. Knacke, O. Kubaschewski, Thermochemical Properties of Inorganic Substance, Springer-Verlag, Berlin, 1977. [24] L. Zheng, M.C. Zhang, J.X. Dong, Oxidation behavior and mechanism of powder metallurgy Rene95 nickel based superalloy between 800 and 1000 C, Appl. Surf. Sci. 256 (2010) 7510e7515. [25] K. Godlewski, E. Godlewska, Effect of chromium on the protective properties
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
The influence of CrTaO4 layer on the oxidation behavior of a directionally-solidified nickel-based superalloy at 850e900 C Weili Ren a, *, Fenfen Ouyang a, Biao Ding a, Yunbo Zhong a, Jianbo Yu a, **, Zhongming Ren a, Lanzhang Zhou b a b
State Key Laboratory of Advanced Special Steel, College of Materials Science and Engineering, Shanghai University, Shanghai 200072, PR China Metal Research Institute, Chinese Academy of Sciences, Shenyang 110015, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 April 2017 Received in revised form 3 July 2017 Accepted 6 July 2017 Available online 8 July 2017
The influence of CrTaO4 on the oxidation behavior of a directionally solidified nickel-based alloy has been investigated at the temperatures between 850 C and 900 C for the different time intervals (the longest time is 300 h). The oxide scale consists of three layers. The outer layer is mainly composed of Cr2O3, TiO2, and NiCr2O4, the middle layer is CrTaO4, and the inner oxide layer mainly consists of Al2O3. The continuous CrTaO4 layer is formed after oxidation for certain times. The formation time of continuous CrTaO4 layer shortens with the temperature increase. Furthermore, the layer causes the transformation of the oxidation kinetics from the parabolic law to the cubic law. The control of oxidation process converts from the outward diffusion of Cr, Ti, and Ni to the inward diffusion of O when the continuous CrTaO4 layer is established. The mechanism clarifies the influence of Ta on the oxidation behavior of Ni-based superalloys from the oxidation kinetics. © 2017 Published by Elsevier B.V.
Keywords: Ni-based superalloys High-temperature oxidation CrTaO4 layer Oxidation mechanism
1. Introduction Nickel-based superalloys have been widely applied in the hotsection components for the aerospace and power-generation engines, such as turbine disks and blades, due to their superior strength and adequate resistant-oxidation properties [1e3]. The excellent oxidation resistance of the alloys at elevated temperatures is associated with the formation of continuous Al2O3 and Cr2O3 oxide layers. Compared with other oxides, the oxide layer shows a dense structure, which can significantly decrease the element diffusion rate and consequently decrease the oxidation rate of the superalloys [4e6]. In order to ensure the strength and oxidation resistance of the Ni-based superalloys at elevated temperatures, the various alloying-elements are usually added, such as Al, Ti, Ta, Cr, Co, W, and Mo [7,8]. On the one hand, the addition of Ta in the Ni-based superalloys increases the contents of the strengthening phases through dissolving into the g' (Ni3Al) lattice and consequently enhances the superalloy strength [9,10]. On the other hand, it has a
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (W. Ren), [email protected] (J. Yu). http://dx.doi.org/10.1016/j.jallcom.2017.07.066 0925-8388/© 2017 Published by Elsevier B.V.
beneficial role in the oxidation resistance of the alloys with the appropriate configuration of the content and type of other elements and temperatures. Let us review briefly the investigations about the influence of Ta on the oxidation behavior of the Ni-based superalloys [11e17]. Firstly, the addition of appropriate amount of Ta in the alloy improves the oxidation resistance. Han et al. [11] found that the oxidation rate of Ni-based superalloys with 5.8 wt.% Ta is lower than that with 3.8 wt.% Ta. It was because the continuous dense CrTaO4 formed in the high Ta-content superalloy. Kim et al. [12] studied the oxidation behavior of five types of Nibased superalloys at 850 C and 1000 C and found that the superalloys containing higher Ta content were more oxidationresistant than those with the lower addition levels of Ta. They also attributed the reason to the formation of CrTaO4 and TaO layers in the superalloy with the high Ta content. By contrary, Yang [13] reported that an addition of Ta as small as 1 at.% (~3.0 wt.%) improved the oxidation resistance significantly by promoting early establishment of a protective Al2O3 layer. When 3 at.% (~8.8 wt.%) of Ta was added, complex oxides such as NiMoO4, NiTa2O6, TaO2, MAl2O4, and M2CrO4 were formed, causing degradation of the oxidation resistance. Secondly, the temperature affects the formation rate of Ta-containing oxides. Park et al. [14] found that the CrTaO4 layer was more continuous at 1000 C than that at 850 C. The CrTaO4 layer hinders the outward diffusion of the Al element,
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resulting in a formation of a continuous dense Al2O3 inner layer and further a slower oxidation rate. Li et al. [15] reported that the X-ray diffraction peak of CrTaO4 at 900 C was stronger than that at 800 C in the oxide scale. However, the CrTaO4erich oxide layer was very thin and discontinuous at 1000e1150 C [16]. It could be attributed to the rapid establishment of a continuous Al2O3 inner layer at high temperatures, which retarded the outward diffusion of Ta element. Wang et al. [17] studied the form of Ta in the oxide film of the N5 Ni-based single-crystal superalloy oxidized at different temperatures. It was found that the continuous CrTaO4 was not formed due to the low Cr content in the superalloy, while two types of Ta-containing oxides TaO2 and Ta2O5 were formed. Thirdly, Ta segregation at the grain boundaries of Al2O3 scale increases the spallation resistance of oxide scales, which was confirmed through the structure observation of oxide scales under the transmission electron microscope by Wang [17]. Finally, the oxidation resistance and spallation resistance of Ta is related with the homogeneous extent of its distribution in the superalloy matrix. At the Ta-rich region, Ta was oxidized prior to Al, TaOx was less protective than Al2O3, and the adhesion of the alloy substrate to TaOx was inferior to Al2O3. The inhomogenous dispersion and the excessive TaOx at some places upon superalloy substrate result in the formation of large cavities and the final spallation [17]. Moreover, a detrimental interspersed Ta-rich oxide particles developed in the top spinel scale on PWA-1484 superalloy [18]. Pedraza et al. found that the incorporation of TaxOy in the oxidation scales clearly worsened their oxidation resistance in aluminide coatings on different superalloy substrates [19]. The above investigation shows that the effects of Ta on the oxidation of the alloy exhibits the complex behaviors, and its beneficial role depends on whether it can form a continuous CrTaO4 layer. The formation of the continuous dense CrTaO4 could hinder the outward diffusion of other elements and decrease the oxidation rate of Ni-based superalloys. When the continuous dense CrTaO4 could not form due to the small Ta and Cr content [11,12,17] or the low temperature [14,15] or the Al2O3 inner scale formation [13,17], the effect of Ta decreasing the oxidation rate is not exerted. However, the formation time of CrTaO4 layer and its influence on the rate exponent of oxidation kinetics and on the activation energy of oxidation rate have not been reported up to now. The present paper studied that the influence of CrTaO4 layer on the oxidation behavior of a directionally-solidified nickel-based superalloy with 5.0 wt.% Ta at temperatures between 850 C and 900 C. The formation time of the CrTaO4 was determined at the different temperatures. The influence of CrTaO4 layer on the rate exponent and the activation energy were studied. The oxidation mechanism was proposed after the formation of the CrTaO4 layer. The investigation reflects the essentials of oxidation behavior, and clarifies the influence of Ta on the oxidation mechanism of Ni-based superalloys. 2. Experimental procedure The nominal composition of the investigated alloy is shown in Table 1. The alloy with a diameter of 16 mm was directionally solidified at a withdrawal speed of 7 mm/min. The oxidation specimens with the dimensions of 10 mm 10 mm 3 mm were cut from the heat-treated ingots, ground, and polished. The heattreatment procedure is 1210 C 2 h (AC) þ 1080 C 3 h
(AC) þ 850 C 24 h (AC) (AC: air cooling). The isothermal oxidation was carried out in static air at 850 C, 875 C, and 900 C. The corundum crucibles used in the oxidation were all heat-treated in the furnace at 1200 C for enough time to ensure that their weight would not be changed. The oxidation kinetics curves of the alloys were determined by the static weight gain method. After appropriate time intervals, the specimens were taken out of the furnace, cooled in air, and then weighed with an electronic balance with a resolution of 0.1 mg. Three specimens were simultaneously tested at each temperature. The oxidation time is up to 300 h. The phases formed on the oxidized specimens were determined by X-ray diffraction (XRD). The surface morphology of the oxidized samples was evaluated by the scanning electron microscope (SEM) equipped with an energy dispersive spectrometry (EDS) without any preparation. However, for evaluation of cross-sections, all specimens were initially covered with a thin layer of Ni, which aimed to prevent damage to the oxide layer during the specimen preparation for SEM observation. The chemical plating was used to cover the Ni layer. The samples were preprocessed in the solution of SnCl2 (13 g/L) þ HCl (40 ml/L)þ deionized water for 2 min, and further were put in the solution of PbCl2 (0.2 g/L) þ HCl (7 ml/ L) þ deionized water for 1 min at room temperature, which ensures the surface activation of the samples. Then, the plating process was carried out in the solution of NiSO4$6H2O (25 g/L) þ NaH2PO2$H2O (30 g/L) þ CH3COONa (12 g/L) þ Na3C6H5O7$2H2O (14 g/ L) þ C3H6O3 (13 g/L) þ deionized water for 90 min at 90 C. 3. Results 3.1. Phases in the oxide film The phases forming during the oxidation were characterized by XRD. Fig. 1(a) shows the XRD patterns of the specimens oxidized at 850 C, 875 C, and 900 C for 200 h. At each temperature, the diffraction peaks of the Cr2O3, TiO2, NiCr2O4, Al2O3, CrTaO4, and the matrix are observed. But the peak intensity of the CrTaO4 phase is weak. With the increase of the oxidation temperature from 850 to 900 C, the peak intensity of the matrix phases of Ni and Ni3Al decreases. Fig. 1(b) shows the XRD patterns of the specimen oxidized at 900 C for the different times in the range from 24 to 200 h. With the time increase, the peak intensity of the oxides increases, while that of the matrix phases decreases. No oxides of Co, W, and Mo were identified. The reason will be illustrated in the discussion part. 3.2. Surface morphology and composition No oxide spallation was observed during the sample cooling. The surface morphology of the oxide scale formed on the alloy after exposure time of 300 h at the different temperatures is presented in Fig. 2. The oxides uniformly cover the scale surface. With increasing the temperature, the oxide grains gradually increase. The composition of the scale surface was analyzed by EDS, which are shown in Table 2. It can be seen that the compositions are rich in Cr, Ti, Ni, and O, which are in agreement with the results of XRD and the cross-section of oxidation scale. It should be stated that Al and Ta are hardly detected in the surface oxide with EDS due to the shallow penetration depth of the electron beam. 3.3. The cross-section morphology and structure of the oxide film
Table 1 Nominal chemical composition of the superalloy. Element
Co
Cr
Mo
W
Al
Ti
Ta
C
Zr
Ni
wt.%
10.00
14.00
1.50
4.00
4.00
3.00
5.00
0.08
0.03
Bal.
In order to further clarify the structure of the oxidation scale of the superalloy, the cross-sections were studied. Fig. 3(aec) shows the morphology and structure of the cross-section of the samples
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Table 2 The composition of the oxidation scale surface by EDS (at.%).
Fig. 1. XRD patterns of the superalloy after the oxidation (a) at 850 C, 875 C, and 900 C for 200 h and (b) at 900 C for 24 h, 48 h, 96 h, 144 h, and 200 h.
oxidized at 850 C for the different times. After the oxidation time of 300 h as shown in Fig. 3(c) and the element redistribution as shown in Fig. 3(f), the oxide film is composed of three layers, i.e. the outer layer, the inner layer, and the middle layer (labeled as P1, P2,
Temperature ( C)
Al
Cr
Ta
Ti
O
Ni
Co
850 875 900
0.77 0.32 0.20
23.05 25.22 29.65
0.09 0.09 0.02
9.08 8.07 8.87
57.57 56.17 58.29
8.34 7.24 2.62
1.10 2.89 0.35
and P3, respectively, similarly hereinafter). The area labeled as P4 is the oxidation-affected zone in the matrix (similarly hereinafter). The compositions of EDS analysis in the different layers are shown in Table 3. The outer oxide layer P1 is mainly composed of Cr, Ti, Ni, and O, among which the Cr content is much more than that of Ti and Ni. Combined with the XRD results (Fig. 1), it can be concluded that the outer oxide layer is composed of the more Cr2O3, and the less TiO2 and NiCr2O4. The inner layer P2 consists of the black oxidized area and the white un-oxidized area. The compositions of the P2 layer in Table 3 are from the black areas, which are rich in Al and O. It indicates that the inner layer is enriched in the Al2O3. The middle layer P3 mainly rich in Cr, Ta, and O is CrTaO4 layer, which can be further identified from the element distribution map of the specimen for 300 h, as seen in Fig. 3(d). From the element distribution maps in Fig. 3(e and f), the minor content of Ti was detected in the middle layer. Considering the minor content, the layer is thought to be mainly composed of CrTaO4 in the present investigation. The zone P4 is the g'-depleted area, whose formation is due to the loss of Al, Ti, and Ta in the matrix during the oxidation and the disappearance of the g0 precipitates. The thickness of the outer layer initially increases sharply when the oxidation time prolongs. It almost keeps constant after the CrTaO4-layer formation. The thicknesses of the inner layer and the g'-depleted area increase all the time with the enhancement of the oxidation time. The continuous middle layer of CrTaO4 oxide is formed at the time of about 96 h, before which the layer is composed of the small scattered particles (Fig. 3(a)). Fig. 4(aec) and 5 (a-f) show the morphology of the cross-section of the superalloy samples oxidized at 875 C and 900 C for the different times, respectively. The results of element redistribution after oxidation at 875 C and 900 C for different times are shown in Fig. 4(d and e) and 5(g-i), respectively. The EDS compositions in the different layers in the specimens oxidized at 900 C are shown in Table 4. It can be seen that the structure of the oxide scales are the same as that of the samples oxidized at 850 C. However, the time required for the formation of the middle layer P3, i.e. the continuous CrTaO4 layer, decreases with the increase of temperature. At 875 C and 900 C, the continuous CrTaO4 layer is formed at the time of about 48 h (Fig. 4(b)) and 24 h (Fig. 5(a)), respectively. At
Fig. 2. The SEM micrographs of the oxidation-scale surfaces of the superalloy after the exposure time of 300 h at the different temperatures (a) 850 C, (b)875 C, (c) 900 C.
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Fig. 3. The SEM micrographs of the cross-sections of the superalloy oxidized at 850 C for the different hours (a) 48 h, (b) 96 h, (c) 300 h, (d) the elements redistribution maps of 48 h, (e) the elements redistribution maps of 96 h, and (f) the elements redistribution maps of 300 h. P1, P2, and P3 are used to label the outer, inner, and middle layer in the scales, respectively. P4 is the g'-depleted area in the matrix.
each temperature, the evolution trend of the P1, P2, and P4 layers with the oxidation time is consistent with that at 850 C. At the same oxidation time, the content of Al2O3 in the P2 layer increases with the increase of temperature. The continuous Al2O3 inner layer could form at each temperature as long as the oxidation time is long enough. The higher temperatures, the shorter time to form the continuous Al2O3 inner layer. The specimens oxidized for 300 h
(Fig. 4(c)) and 120 h (Fig. 5(d)) at 875 C and 900 C show the continuous Al2O3 layer, respectively. 3.4. Oxidation kinetics The oxidation rate of alloys is reflected by the kinetics curves, and it is usually characterized by the weight gain per unit area with
Table 3 The composition in the different layers in the oxidation scales by EDS at 850 C. The meanings of P1, P2, and P3 are same as those in Fig. 3. Oxide layer
P1
P2
P3
Oxidation time (h)
48 96 300 48 96 300 48 96 300
Elemental composition (at.%) Al
Cr
Ta
Ti
O
Ni
Co
2.10 2.37 3.88 29.70 32.40 36.64 e 6.22 3.21
36.36 40.60 42.55 3.63 4.80 3.83 e 25.00 20.56
0.29 3.10 3.68 0.14 2.50 e e 24.60 29.97
7.87 7.33 6.09 0.13 0.53 0.19 e 7.70 4.89
47.62 34.90 39.45 46.40 34.64 46.26 e 32.00 34.39
5.47 10.80 3.94 16.9 24.8 11.22 e 3.60 6.02
0.29 0.90 0.41 3.10 0.33 1.86 e 0.88 0.96
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Fig. 4. SEM micrographs of the cross-sections of the samples oxidized at 875 C for the different times (a) 24 h, (b) 48 h, (c) 300 h, (d) the elements redistribution maps of 48 h, and (e) the elements redistribution maps of 300 h.
the oxidation time. Fig. 6 shows the oxidation kinetics curves of the superalloy at 850 C, 875 C, and 900 C. The oxidation kinetics at each temperature follows the similar law, i.e, the weight gain increases sharply during the initial period and then tends to vary gently. At the same oxidation time, the mass gain increases with the temperature raise. The isothermal oxidation behavior can be described by the following equation [20,21]: (DW)n ¼ Kt
(1)
where DW is the weight gain per unit area, t is the oxidation time, n is the rate exponent that determines the rate law, and K is the oxidation rate constant. In order to determine the n and K value, taking logarithms of both sides of Eq. (1) gives:
lnDW ¼ 1=n $ lnt þ 1=n $ lnK
(2)
The slope reciprocal of a lnDW- lnt plot gives the n value. Fig. 7 shows the double logarithmic graphs of the mass gain and time at the different temperatures. It could be seen that the fit lines exhibit a turning point at each temperature. At 850 C, 875 C, and 900 C, the time to the turning point is 96 h, 48 h, and 24 h, respectively. Combined with the microstructure of the cross-section of the oxidized specimens (Figs. 3e5), the turning time at each temperature is in agreement with the formation time of continuous CrTaO4 layer, which is 96 h, 48 h, and 24 h at 850 C, 875 C, and 900 C, respectively. This indicates that the formation of the continuous CrTaO4 layer causes the change of the oxidation kinetics. The n value before and after the turning point are also presented in Table 5. It can be seen that the oxidation kinetics obey the parabolic
relationship before the turning time. The formation of the continuous CrTaO4 layer causes the oxidation kinetics to obey the cubic relationship. The oxidation rate constant K also affects the oxidation kinetics of the alloys. It could be obtained according to the intercept of the fit line of ln△W-lnt plot in Fig. 7, which is presented in Table 6. The K value is controlled by the oxidation activation energy, and their relationship with temperature can be expressed by the Arrhenius equation [22]:
K ¼ A $ expE=RT
(3)
where E is the oxidation activation energy, R is the gas constant, T is the oxidation temperature, and A is a constant for a certain alloy. In order to obtain the value of E, the relationship between lnK 1/T is shown in Fig. 8. The oxidation activation energy is 335 kJ/mol and 423 kJ/mol before and after the formation of the continuous CrTaO4 layer, respectively.
4. Discussion The experimental results show that the continuous CrTaO4 layer is formed when the superalloy is oxidized for a certain time. The time to the formation of continuous CrTaO4 layer corresponds to the turning point of oxide kinetics. The values of n and E are different before and after the turning point. Therefore, the formation of the continuous CrTaO4 layer changes the oxidation behavior of the superalloy. The influences of CrTaO4 layer on the oxidation behavior are discussed from the thermodynamic, the kinetics, and the mechanism aspects below.
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Fig. 5. SEM micrographs of the cross-sections of the samples oxidized at 900 C for the different times (a) 24 h, (b) 48 h, (c) 96 h, (d)120 h, (e)200 h, (f) 300 h, (g) the elements redistribution maps of 24 h, (h) the elements redistribution maps of 120 h, and (i) the elements redistribution maps of 300 h.
4.1. The thermodynamic of the CrTaO4 and other oxides The results of XRD, SEM, and EDS analysis indicate that the outer layer of the oxide film consists of Cr2O3, TiO2, and NiCr2O4, and the inner layer is mainly composed of Al2O3. As the oxidation proceeds, the middle continuous CrTaO4 layer is formed between the outer and inner layer. The chemical composition of the investigated superalloy is Ni-14Cr-10Co-4W-1.5Mo-4Al-3Ti-5Ta (wt.%). The
oxidation reactions which will occur at elevated temperatures are as follows [23,24]:
4=3CrðsÞ þ O2 ðgÞ ¼ 2=3Cr2 O3 ðsÞ
(4)
DGCr2 O3 ¼ 746840 þ 170:29T
(5)
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Table 4 The composition in the different layers in the oxidation scales by EDS at 900 C. Oxide layer
P1
P2
P3
Oxidation time (h)
24 48 120 300 24 48 120 300 24 48 120 300
Elemental composition (at.%) Al
Cr
Ta
Ti
O
Ni
Co
1.50 1.75 0.85 3.01 38.40 33.58 39.73 41.12 8.30 4.90 0.74 2.09
47.30 32.45 54.83 46.09 3.66 1.97 2.66 0.43 14.04 25.46 19.47 18.98
0.10 0.06 1.24 0.98 0.46 0.14 0.42 1.07 17.90 21.49 20.90 25.01
5.08 3.46 3.99 4.87 0.46 0.29 0.21 0.03 3.24 3.48 10.86 3.23
36.80 58.80 35.54 42.73 33.65 58.70 40.30 51.89 37.02 34.85 44.64 47.35
8.25 2.97 3.40 2.24 19.45 4.39 13.90 4.78 13.90 8.28 2.69 1.44
0.97 0.51 0.15 0.08 3.92 0.93 2.78 0.68 5.60 1.54 0.70 1.90
Table 5 The rate exponent n before and after the formation of continuous CrTaO4 layer. Temperature ( C)
The n value before the formation of CrTaO4 layer
The n value after the formation of CrTaO4 layer
850 875 900
1.9 1.8 2.1
3.2 3.1 3.4
Table 6 The rate constants K before and after the formation of continuous CrTaO4 layer. Temperature The K value before the formation The K value after the formation ( C) of CrTaO4 layer (mg2/cm4$h) of CrTaO4 layer (mg3/cm6$h) 850 875 900
7.04 104 2.00 103 3.10 103
8.65 105 3.08 104 4.71 104
Fig. 6. Isothermal oxidation kinetics curves of the superalloy samples at the different temperatures.
Fig. 8. Logarithm of the oxidation rate constant (lnK) plotted against the temperature reciprocal (1/T) before and after the formation of the continuous CrTaO4 layer.
Fig. 7. The double logarithmic graphs of the mass gain (DW) and oxidation time (t) at the different temperatures.
4=3AlðsÞ þ O2 ðgÞ ¼ 2=3Al2 O3 ðsÞ
(8)
TiðsÞ þ O2 ðgÞ ¼ TiO2 ðsÞ
(6)
DGAl2 O3 ¼ 1120480 þ 214:22T
(9)
DGTiO2 ¼ 943490 þ 179:08T
(7)
4=5TaðsÞ þ O2 ðgÞ ¼ 2=5Ta2 O5 ðsÞ
(10)
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DGTa2 O5 ¼ 810040 þ 165T
(11)
2CoðsÞ þ O2 ðgÞ ¼ 2CoOðsÞ
(12)
DGCoO ¼ 477810 þ 171:96T
(13)
2NiðsÞ þ O2 ðgÞ ¼ 2NiOðsÞ
(14)
DGNiO ¼ 476980 þ 168:62T
(15)
MoðsÞ þ O2 ðgÞ ¼ MoO2 ðsÞ
(16)
DGMoO2 ¼ 578200 þ 166:5T
(17)
WðsÞ þ O2 ðgÞ ¼ WO2 ðsÞ
(18)
DGWO2 ¼ 581200 þ 171T
(19)
It can be calculated that the DG value for each reaction is negative at temperatures in the range from 850e900 C, which indicates that all of the above reactions might occur. The order of the DG value for the different oxides are as follows: DGAl2O3
2TaðsÞ þ O2 ðgÞ ¼ 2TaOðsÞ
(20)
DGTaO ¼ 376600 173:2T
(21)
TaðsÞ þ O2 ðgÞ ¼ TaO2 ðsÞ
(22)
DGTaO2 ¼ 209200 20:5T
(23)
It can be calculated that the value of DGTa2O5 is smaller than that of DGTaO and DGTaO2 at 850e900 C, thus Ta2O5 is formed during the stage of oxidation. The formation equation of CrTaO4 is as follows [31,32]:
1=2Cr2 O3 ðsÞ þ 1=2Ta2 O5 ðsÞ ¼ CrTaO4 ðsÞ
DGCrTaO4 ¼ 1=2 DGTa2 O5 þ DGCr2 O3 ¼ 778440 þ 168T
(24) (25)
The reason that the continuous CrTaO4 layer lies in the middle should be attributed to the slow diffusion rate of Ta atom, which can be confirmed by the data in Table 7. Table 7 gathers the diffusion coefficients of Ta, Cr, and Ti in the pure Nickel [33] or the Waspaloy alloy [34] at 850e900 C. Therefore, Ta diffuses
Table 7 Diffusion coefficients of different elements in pure nickel or. Waspaloy alloy at 850 C 900 C. Element
Ta Ti Cr
diffusion coefficient (m2/s)
Ref.
850 C
875 C
900 C
4.63 1017 2.19 1016 8.25 1017
8.31 1017 3.67 1016 1.44 1016
1.45 1016 6.00 1016 2.46 1016
[33] [34]
outwardly at a smaller rate than Ti and Cr. The atomic mass also supports the guess since those of Ta, Ti, and Cr are 181, 48, and 52, respectively. The slow diffusion of Ta causes the requirement of an incubation period for the formation of the continuous CrTaO4 layer. From Table 7, it can be seen that the diffusion coefficient increases with increasing the temperature. Consequently, the incubation period for the formation of CrTaO4 would shorten, and the continuous CrTaO4 layer would appear early with the temperature increase. It should be noted that the AlTaO4 were hardly observed because the content of Cr was much higher than that of Al in the investigated alloy. The AlTaO4 was detected since the high content of Al (12.24 wt.%) and low content of Cr (5.42 wt.%) in the superalloy TMS-82þ [35]. 4.2. The effect of CrTaO4 layer on oxidation kinetics The oxidation kinetics is reflected by the rate exponent n and the isothermal oxidation rate constant K. The formation of continuous CrTaO4 layer causes the change of the above two factors. Before the formation of continuous CrTaO4 layer, the n value is about 2, and the kinetic curve follows the parabolic rate law. In contrast, after the continuous CrTaO4 layer is formed, the n value is around 3, and the kinetic curve follows the cubic law. In addition, the formation of continuous CrTaO4 layer reduces the K value by about one order of magnitude. For instance, the K value decreases from 7.04 104 mg2/cm4·h to 8.65 105 mg3/cm6·h when the continuous CrTaO4 layer is formed at 850 C, the K value decreases from 3.10 103 mg2/cm4·h to 4.71 104 mg3/cm6·h at 900 C. Therefore, the formation of the continuous CrTaO4 layer significantly decreases the oxidation rate, which is consistent with the thickness variation of the oxide film, i.e., the thickness of the outer layer remains nearly constant after the formation of the layer. The present result is in agreement with previous reports, i.e. Irving et al. [36] suggested that Ta reduces oxygen activity at the surface of a superalloy. Liu et al. [37] suggested that Ta slows down the outward diffusion of Al, suppressing the growth of oxide scales on the single-crystal Ni-based alloys. Park et al. [14] showed that Ta exerted a beneficial effect on oxidation resistance when its concentration was over 5 wt.% at 850 C, slowing down the formation of oxides in Ni-based superalloys. 4.3. The influence of the CrTaO4 layer on the oxidation mechanism The formation of continuous CrTaO4 layer reduces the oxidation rate of the alloy and also modifies the structure of the oxidation scales, which indicates that it changes the oxidation mechanism of the alloy. According to the morphology of the cross-section of the oxide film, the outer layer is composed of Cr2O3, and a small amount of TiO2 and NiCr2O4. The layer thickness increases with the oxidation time before the formation of continuous CrTaO4 layer. It can be concluded that the oxidation process is controlled by the outward diffusion of the Cr, Ti, and Ni atoms before the formation of continuous CrTaO4 layer. The schematic diagram is shown in
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Fig. 9. Schematic diagram of oxidation process (a) before the formation of CrTaO4 layer and (b) after the formation of CrTaO4 layer.
Fig. 9(a). It can be confirmed from the oxidation activation energy before the formation of continuous CrTaO4 layer, and the value of the oxidation activation energy E is 335 kJ/mol. It is slightly larger than the activation energy of Cr diffusion in Cr2O3, which is 280 kJ/ mol [38], since the diffusion of other elements, such as Ni and Ti, also contributes the activation energy of oxidation process. The continuous CrTaO4 layer inhibits the outward diffusion of Cr, Ti, Ni, and other metal elements [39], which would remarkably decrease the growth rate of the outer layer. So the thickness of the outer layer almost remains constant after the formation of the continuous CrTaO4 layer. Since the size of O atom (0.056 nm) is much smaller than that of Al atom (0.143 nm), O atom can diffuse faster through the CrTaO4 layer than the metal atoms. The content of Al2O3 in the inner oxide layer gradually increases after the formation of the continuous CrTaO4 layer. The continuous Al2O3 layer forms after the enough time. For example, the time to form the continuous Al2O3 layer at 900 C is 120 h (Fig. 5(c)). Therefore, the oxidation process is controlled by the inward diffusion of O after the formation of continuous CrTaO4 layer, and the schematic diagram is shown in Fig. 9(b). The oxidation mechanism is confirmed by the oxidation activation energy of 423 kJ/mol in the period, which is close to the activation energy, 482 kJ/mol, of Al diffusion in Al2O3 [40]. 5. Conclusions The effect of CrTaO4 layer on the oxidation behavior of a directionally solidified Ni-based superalloy at the temperatures between 850 C and 900 C was studied. The main conclusions are as follows: (1) The oxide film of the superalloy is mainly composed of three layers. The outer layer is mainly composed of Cr2O3, TiO2, and NiCr2O4, the middle layer is CrTaO4, and the inner oxide layer mainly consists of Al2O3. (2) The continuous CrTaO4 layer is formed after oxidation for a certain time. At 850 C, 875 C, and 900 C, the continuous CrTaO4 layer is formed at the oxidation times of about 96 h, 48 h, and 24 h, respectively.
(3) The formation of continuous CrTaO4 layer causes the transformation of the oxidation kinetics of the superalloy from the parabolic law to the cubic law. (4) The oxidation process is controlled by the outward diffusion of the Cr, Ti, and Ni atoms before the formation of continuous CrTaO4. In contrast, after the continuous CrTaO4 layer is formed, the inward diffusion of O control the oxidation process. The mechanisms uncover the effect of Ta on the oxidation behavior of Ni-based superalloys from the kinetics. Acknowledgments This research was supported by National Natural Science Foundation of China (No. 51371110) and the project from Guangdong Power Grid Co. Ltd. We also thank Instrumental Analysis and Research Center of Shanghai University for their support on material characterization. References [1] X.D. Lu, S.G. Tian, X.F. Yu, C.X. Wang, Oxidation behavior of a single-crystal Nibase superalloy in air at 900 and 1050 C, Rare Met. 30 (2011) 439e442. [2] X. Ren, F.H. Wang, X. Wang, High-temperature oxidation and hot corrosion behaviors of the NiCr-CrAl coating on a nickel-based superalloy, Surf. Coat. Technol. 198 (2005) 425e431. [3] X.G. Yang, S.L. Li, H.Y. Qi, Effect of high-temperature hot corrosion on the low cycle fatigue behavior of a directionally solidified nickel-base superalloy, Int. J. Fatigue 70 (2015) 106e113. [4] M.A. Azim, S. Burk, B. Gorr, H.J. Christ, D. Schliephake, M.H.R. Bornemann, P.H. Bolivar, Effect of Ti (macro-) alloying on the high-temperature oxidation behavior of ternary MoeSieB alloys at 820e1,300 C, Oxid. Met. 80 (2013) 231e242. [5] J.H. Chen, P.M. Rogers, J.A. Little, Oxidation behavior of several chromiaforming commercial nickel-base superalloys, Oxid. Met. 47 (1997) 381e410. [6] H.Y. Qi, X.B. Liang, S.L. Li, X.G. Yang, High-temperature oxidation behavior of DZ125 Ni-based superalloy under tensile stress, Rare Met. (2016) 1e6. [7] P. Caron, T. Khan, Evolution of Ni-based superalloys for single crystal gas turbine blade applications, Aerosp. Sci. Technol. 3 (1999) 513e523. [8] A.C. Yeh, S. Tin, Effects of Ru and Re additions on the high temperature flow stresses of Ni-base single crystal superalloys, Scr. Mater. 52 (2005) 519e524. [9] G.M. Janowski, R.W. Heckel, B.J. Pletka, The effects of tantalum on the microstructure of two polycrystalline nickel-base superalloys: B-1900 þ Hf and MAR-M247, Metall. Mater. Trans. A 17 (1986) 1891e1905. [10] R.C. Reed, T. Tao, N. Warnken, Alloys-By-Design: application to nickel-based single crystal superalloys, Acta Mater. 57 (2009) 5898e5913.
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