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Effect of Cr on the microstructure and oxidation properties of Co-Al-W superalloys studied by in situ environmental TEM
Corrosion Science 161 (2019) 108179
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Effect of Cr on the microstructure and oxidation properties of Co-Al-W superalloys studied by in situ environmental TEM
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Yanhui Chena, Fei Xueb, Chunhui Wanga, Xueqiao Lia, Qingsong Denga, Xiaomeng Yanga,b, ⁎ Haibo Longa, Wei Lia, Luyan Yanga, Ang Lia, a b
Beijing Key Lab of Microstructure and Property of Advanced Materials, Beijing University of Technology, Beijing, 100124, China State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing, 100083, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Cr addition Co-Al-W superalloy Microstructure In situ oxidation ETEM
Modern in situ oxidation processes can be observed down to the nanoscale using environmental transmission electron microscope (TEM), and this technology has been applied to conventional corrosion science to provide more detailed intrinsic information on the oxidation properties of materials. The influence of chromium (0–10 at.%) addition on the microstructure and oxidation properties of a γ/γ′ two-phase Co82Al9W9-based superalloy was systematically investigated. The in situ oxidation behaviour of Cr-containing alloys was studied using environmental TEM, and the addition of moderate Cr amounts is suggested for the alloy design.
1. Introduction Chromium (Cr) and aluminium (Al) are the two most important elements used to improve the corrosion resistance of alloys, especially superalloys [1], by forming protective Cr2O3 and Al2O3 layers [2]. Al, however, acts as a basic structural stability element and forms an L12Ni3Al phase in Ni-based superalloys or an L12-Co3(Al/W) phase in Cobased superalloys; thus, Al is limited to only γ′ phase stability regions [3,4]. The concentration of Al is thus kept at 5–6 wt.% for first-generation to fourth-generation Ni-based superalloys without large concentration variations [5]. Tuning the concentration of Cr has thus become the main method used to tune the alloy’s corrosion resistance, and the Cr concentration can be changed from approximately 10 wt.% in the first generation to approximately 3 wt.% in the fourth generation by developing thermal barrier coating techniques [6]. However, the addition of Cr increases the quantity of precipitates in topologically closepacked phases and worsens the thermal mechanical properties [7,8]. A moderate concentration of Cr in an alloy, especially in a newly developed alloy, is thus important for the material design and future applications. The γ/γ´ phase Co-Al-W-based superalloys have the same microstructure as Ni-based superalloys and possess good high-temperature mechanical properties [9–11]. Improving the oxidation resistance of γ/ γ´-phase Co-Al-W-based superalloys is necessary for their applications and use in industry, in which materials are subjected to harsh conditions and high temperatures of up to thousands of degrees [12,13]. A
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Co-Al-W alloy was shown to have a poor corrosion resistance by oxidation experiments reported in the literature [14]. The addition of Cr to the alloy to improve its oxidation resistance is the most conventional and effective method used to design alloys [15,16]. The addition of an appropriate proportion of Cr to the γ/γ′ structure of a Co-Al-W superalloy requires practical consideration. Few studies have reported the addition of single Cr [9] or binary Ta-Cr [10–12] and B-Cr [17] elements to the Co-Al-W superalloy, and all the reported alloys were treated as reference samples; there are no systematic studies on the addition of Cr. The content of Cr added in these studies was comparatively low–up to 5 wt.%. As can be deduced from the γ/γ′ two-phase Nibased alloys, the morphology, γ′ size, γ′ volume fraction and hardness are important factors that influence the alloys’ mechanical properties [13]. Cr addition to Co-Ni-Al-W-based alloys had been reported to change the alloys’ morphologies and structures [18]. Limited information is available regarding Cr-containing Co-Al-W-based superalloys. Systematic studies on the microstructure stability, morphology, volume fraction of the γ' phase and oxidation behaviour of Cr-containing Co-AlW-based superalloys with Cr concentrations ranging from 0 to 10 at.% are necessary and important for developing γ/γ' two-phase Co-Al-Wbased superalloys. In situ techniques provide micrometre to even nanometre scale observation of material variation under stress or corrosive gases and afford a deep understanding of the properties of materials. The techniques are powerful tools for material characterization [19]. In situ environmental scanning electron microscopy has been used to study Co-
Corresponding author. E-mail address: [email protected] (A. Li).
https://doi.org/10.1016/j.corsci.2019.108179 Received 13 April 2019; Received in revised form 21 July 2019; Accepted 21 August 2019 Available online 23 September 2019 0010-938X/ © 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. SEM images of the alloys with different Cr contents after ageing at 900 °C for 50 h: (a) 0Cr, (b) 2Cr, (c) 4Cr, (d) 6Cr, (e) 8Cr and (f) 10Cr. All the images have the same scale bars as that shown at the bottom of Fig. 1f.
heating holder. The prepared sample was cut perpendicular to the {001} direction to observe the plane perpendicular to the γ/γ' phase boundary. The in situ oxidation experiments utilized a dedicated fieldemission ETEM (FEI-Titan-ETEM). The ETEM was equipped with a spherical aberration corrected objective lens, and the gas pressure around the sample was controlled using a differential pumping system. The gas pressure of the pure O2 (99.999%) atmosphere around the sample was set to 0.2 mbar. All the oxidation experiments performed by in situ TEM were imaged at 300 kV. The specimen temperature was maintained at 20–350 °C using a DENS Nano-Chip-based heating holder. The Nano-Chip allows fast heating and quenching with superior three-dimensional stability by using a four-point-probe method, which enables local measurements of the temperature with fast feedback to achieve immediate stabilization and accurate temperatures. Ex situ TEM was also performed with probe aberration-corrected scanning transmission electron microscopes (FEI-Titan-Themis) operated at 300 kV and equipped with a Super-X energy dispersive X-ray spectroscopy (EDS) four-quadrant detector. The alloy we used was a highly crystalline cubic γ' phase single crystal, and the side length of the single crystal was approximately 200 nm. We examined the centre of the 200 nm γ' phase to obtained the EDS mapping results. The region we measured was thin enough that only one γ or γ' phase was detected without any overlap. The quantitative data were obtained by averaging the data collected from the centre of the specified part. For the quantitative EDS analysis, the power law method was applied to remove the background. The K-series peaks were chosen for O, Al, Cr and Co, and the L-series peaks were chosen for W in the analysis.
Al-W-based alloys and provides micrometre-scale observations during the oxidation process [20]. Environmental transition electron microscopy (ETEM), which was recently developed, can provide nanoscale or atomic-scale investigation of material variation during reaction process and has been used to study some nanomaterials, providing information that traditional techniques cannot attain [21]. Research on the in situ oxidation corrosion process of Co-Al-W-based alloy is thus useful to gain a deep understanding of the properties of the alloy. In this study, a series of Co82Al9W9-based (at.%) alloys containing various concentrations of Cr ranging from 0 at.% to 10 at.% was investigated using scanning electron microscopy (SEM). The evolutions of the morphology and microstructure were analysed as a function of the Cr content. The oxidation resistance of the Co82Al9W9-based alloys was investigated by in situ oxidation using ETEM to reveal the effect of adding Cr. A moderate Cr concentration was suggested in the alloy design to optimally balance the structure, hardness, precipitation and corrosion resistance. 2. Experimental procedure The experimental alloys were vacuum arc melted into 20 g ingots. Six alloys composed of Co82Al9W9, with a Cr content ranging from 0 to 10 at.% at intervals of 2 at.% Cr were melted to systematically investigate the influence of the Cr content. The nominal compositions of the alloys are given in Table 1. The alloys are abbreviated based on their Cr content. The ingots were solution heat-treated at 1300 °C in a sealed tube for 168 h and then aged at 900 °C for 50 h or 300 h. The microstructures of the alloys were characterized using a ZEISS SUPRA scanning electron microscope. Energy dispersive X-ray (EDX) spectroscopy coupled with SEM and transmission electron microscopy (TEM) was used to investigate the compositions of the precipitates. The samples were first mechanically polished for the SEM observations, and the backscattered electrons (BSEs) were detected to study the elemental distribution. The γ′ volume fraction was quantitatively determined by secondary electron microscopy (SEM), and image processing software was used to calculate the area fraction. The analysed area was approximately 3.8 μm × 2.6 μm. The Vickers hardness of the alloys was measured using an indentation load of 200 g; the mean value of three measurements is presented in the paper. The in situ thermal oxidation TEM samples were prepared by a focused ion beam (FIB) cutting and transformation method (FEI-FIBHelios), and the samples were deposited on a DENS Nano-Chip-based
3. Results and discussion The SEM images show that all the alloys exhibit typical γ/γ′ structures after ageing at 900 °C for 50 h, as shown in Fig. 1. The morphology of the γ′ phase varies from a cuboid shape (0Cr, 2Cr and 4Cr alloys) to an irregular rectangular shape (6Cr alloy) and then to a nearly spherical shape (8Cr and 10Cr alloys) with an increase in the Cr content. The microstructures of the 0Cr, 2Cr and 4Cr alloys are similar to the microstructures reported by Sato et al [22]. and Suzuki et al [23]. In addition, the size (edge length or diameter) of the γ′ phase increased from 50 nm (0Cr) to 100 nm (10Cr) by increasing the Cr content. The bright contrast of the γ′ phase also indicates the aggregation of the refractory element W in the γ′ phase. Our study also indicates that the microstructures of the alloys treated with prolonged ageing at 900 °C 2
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Fig. 2. Alloys after ageing at 900 °C for 300 h: (a) 2Cr and (b) 10Cr alloys. Typical TEM images and diffraction patterns of all the precipitates: (c–d) DO19-Co3W phase, (e–f) μ-Co7W6 phase and (g–h) B2-CoAl phase.
Fig. 3. Evolution of the volume fraction of the γ′ phase as a function of ageing time at 900 °C for different Cr contents: (a) 2Cr and 6Cr alloys after ageing for different lengths of time and (b) comparison of the six alloys after ageing for 300 h.
Fig. 4. (a) Vickers hardness values of the 2Cr and 6Cr alloys under different heat treatment conditions. (b) Vickers hardness values of the six alloys after ageing at 900 °C for 50 h.
reveal a composition of approximately Co3W. The EDS analysis indicated that the bright, blocky, high-contrast phases in the 10Cr alloy, as shown in Fig. 2b, have a composition of approximately Co7W6, and the black needle-like high-contrast phases have a composition of CoAl. The results indicate that W and Al will supersaturate after long ageing times to form secondary precipitation phases. Cr addition prompted the precipitation of the Al- and W-containing secondary phases, such as CoW, CoAl and Co7W6. The precipitate phases of the aged alloys were
for 300 h have similar phase shape variations and enlarged phase sizes with an increase in the Cr content. The SEM analysis indicates that the 0–6Cr alloys all have a similar microstructure and precipitates, while the 8–10Cr alloys also all have a similar microstructure and precipitates. Fig. 2 shows typical SEM images of the precipitates in the 2Cr and 10Cr alloys aged at 900 °C for 300 h. The SEM analysis and corresponding EDX analysis of the secondary rod-like phases, as shown at the grain boundaries in Fig. 2a, 3
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Fig. 5. (a–g) Series of TEM images obtained during the oxidation process of the 0Cr alloy as it was heated from 20 °C to 350 °C. (h) HRTEM image from the border of the oxidized alloy and (i) the EDP corresponding to image (h).
hardness of the 2Cr alloy under all treatment conditions is higher than that of the 6Cr alloy. The decrease in the hardness with increasing Cr content is directly proportional to the volume fraction of the γ′ phase, as shown in Figs. 1 and 3. The γ′ phase is harder than the γ phase due to its high concentration of refractory tungsten, as determined from the EDX data shown in Table 1. This conclusion was further verified by measuring the hardness of the alloys with different Cr contents after ageing at 900 °C for 50 h, as shown in Fig. 4b. The results indicate that the hardness is inversely proportional to the Cr content but directly proportional to the volume fraction of the γ′ phase. From the abovementioned γ/γ′ structural analysis, the quantity of precipitates and the Vickers hardness variation, we can conclude that the addition of Cr should not be higher than 6 at.% to maintain a high enough γ′ volume fraction, a limited quantity of precipitates and a sufficient hardness. The oxidation behaviour of the 6Cr alloy was then studied to determine the effect of Cr addition, and the 0Cr alloy was also tested for comparison. An in situ thermal oxidation experiment under an oxygen pressure of 0.5 mbar was then carried out in an ETEM chamber to compare the effect of Cr addition on the oxidation properties of the alloys. Fig. 5 shows typical TEM images of the 0Cr alloy during thermal oxidation from 20 °C to 350 °C, in which thermal oxidation was evident. A video recorded the thermal oxidation process under different temperatures in the supplementary file V1, and typical images are presented in Fig. 5. No obvious structural variation was observed at 200 °C during oxidation, as shown in Fig. 5a–d. The heating treatment eliminates surface stresses, as indicated by the dark stress band in the TEM images (denoted by the arrows in Fig. 5a). A thin amorphous layer of
confirmed by TEM and the corresponding electron diffraction patterns (EDPs). A typical TEM image and the diffraction patterns of the precipitates in the 2Cr alloy are shown in Fig. 2c and d, respectively. The precipitates are mainly distributed at the grain boundary and have DO19 structures and a composition of Co3W. The precipitates in the 8Cr and 10Cr alloys occur as two kinds of phases. Spherical-shaped precipitates are shown in Fig. 2e and f and have μ-Co7W6 phase structures. The needle-like precipitates in the 8Cr and 10Cr alloys belong to the B2CoAl phase and have a BCC structure of a = b=c = 0.29 nm. The precipitate phases have special structural relationships with the base in the Co-Al-W-based alloys, as demonstrated in our early research: [111] B2//[110]γ´, (011)B2//(111)γ´, [001]μ//[110]B2//[001]DO19, (11-2) B2//(-100)μ//(100)DO19, (1–10)B2//(0–27)μ//(0–10)DO19 [24]. The volume fractions of the γ′ phase in the 2Cr and 6Cr alloys aged at 900 °C for different lengths of time were measured using image analysis software, and the data are shown in Fig. 3a. The volume fractions of the γ′ phase in both the alloys decreased remarkably with increasing ageing time. The data also indicate that the volume fractions of the γ′ phase in all six experimental alloys tended to decrease during the ageing process. Fig. 3b shows the evolution of the γ′ volume fraction as a function of the Cr content in alloys aged at 900 °C for 300 h. The result indicates that the γ′ volume fraction decreases from 59% in the 2Cr alloy to 33% in the 10Cr alloy with increasing Cr content. Fig. 4a shows the evolution of the hardness with ageing time in the 2Cr and 6Cr alloys aged at 900 °C. The maximum hardness was observed for both of the alloys aged at 900 °C for 50 h and decreased gradually with prolonged ageing time. This result indicates that the γ/γ′ structure enhances the hardness of the alloy. It is also noted that the 4
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Fig. 6. High-angle annular dark-field image and elemental distribution mappings of the 0Cr alloy: (a–d) the original alloy and (e–l) the oxidized alloy. Elemental linescan on the surface layer in (m) the γ phase and (n) the γ' phase.
oxygen is more concentrated in the γ phase (Fig. 6i). The colour-mixed elemental mappings of the metal and oxygen, as shown in Fig. 6j–l, also show the aggregation of oxygen in the γ phase. It is also noted that Co aggregates in the outermost layer, but the concentration of Co severely decreases at the middle border. Two concentration line-scans were plotted from the outermost to the innermost part of the alloy, as shown in Fig. 6e by the two arrows in the γ and γ' phases. The line-scans in the γ (Fig. 6m) and γ' (Fig. 6n) phases show that the elements in the γ phase do not vary from the outermost to the innermost part. The composition of the γ' phase is different: the concentration of Co is the highest at the outermost border, then decreases severely from approximately 50 at.% to 20 at.% and remains stable in the inner part of the alloy. The variation in the concentration of oxygen in the γ' phase, however, is different than that of Co and has the highest value in the middle section of the alloy, in which the concentration of Al is the highest value. The rate of forming CoO is faster than that of other kinds of metal oxides, such as Co3O4 and Al2O3, and thus, CoO forms first at the outermost border and has a low oxygen concentration. The formation of Al2O3 is slower and has a denser structure compared with CoO, and thus, the second oxide layer on the metal surface is Al2O3 and has the highest oxygen
approximately 2 nm existed before thermal oxidation, and this kind of amorphous layer can form at room temperature on the edge of thin samples. The surface stress decreases with an increase in the oxidation temperature and almost disappears at 250 °C. Nanocrystalline metal oxide particles formed above the amorphous layer after oxidation at 250 °C for 15 min (Fig. 2f). Increasing the temperature up to 350 °C to promote oxidation increased the quantity of the metal oxide nanoparticles, and the particle size increased from a few nanometres to tens of nanometres. The contrast between the γ/γ' phases became more distinct at 350 °C than at other temperatures. Fig. 5h shows a typical high-resolution TEM (HRTEM) image from the outermost surface of the oxidized border, indicating the multi-crystalline structure of the alloy. The corresponding fast Fourier transform of the HRTEM in Fig. 5i can be attributed to CoO, Al2O3 and CoAl2O4. The elemental mappings were obtained for all the elements in the original alloy (Fig. 6a–d) and the oxidized alloy (Fig. 6e–f). The elemental distributions in the original alloy indicate that Co prefers the γ phase, while W prefers the γ' phase, and Al has almost no preference for either of these two phases in the original alloy. After thermal oxidation, the γ phase is more oxidized than the γ' phase, and the distribution of 5
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Fig. 7. (a–g) Series of TEM images obtained during the oxidation process of the 6Cr alloy as it was heated from 20 °C to 350 °C. (h) HRTEM image from the border of the oxidized alloy and (i) the EDP corresponding to image (h).
oxygen into the inner alloy, and thus, the 6Cr alloy contains a lower oxygen concentration than the 0Cf alloy under the same oxidation conditions. Studies indicate that the γ′ phase is prone to adopting a cuboidal shape when the misfit is either positive or negative, while the γ′ phase is prone to adopting a spherical shape when the misfit is approximately zero. [15] For the Co-Al-W alloys, the γ′ phase contains higher concentrations of W, which is a heavy element, than the γ phase [25]. The γ′ phase in the 2Cr and 4Cr alloys retains its cuboidal structure, while it exhibits an irregular square structure in the 5Cr and 6Cr alloys and a spherical structure in the 8Cr and 10Cr alloys. The morphology indicates that the addition of Cr to the Co82Al9W9 alloy decreases the lattice misfit. For a Ni-based single-crystal superalloy, the addition of Cr changes the lattice misfit from a negative value to zero, which changes the γ′ morphology from a spherical shape to a cuboidal shape [26]. Therefore, the morphological evolution of the alloys (Fig. 2) is reasonably attributed to the addition of Cr, which gradually reduces the lattice misfit as the Cr content increases. In conventional γ/γ′ Ni-based single-crystal alloys, strengthening of the γ′ phase by precipitation is the most commonly used method. The hardness results show that the alloys aged for 50 h are the hardest among all the aged samples, as shown in Fig. 4a, which indicates that structure strengthening was the best strengthening method used for the experimental alloys. Furthermore, in the γ/γ′ two-phase Ni-based single-crystal alloys, alloys with higher γ′-volume fractions exhibit better mechanical properties than alloys with lower γ′-volume fractions. Cr was the γ-phase stabilizing element for the Co-Al-W-Cr alloys [22], and its addition to γ/γ′-strengthened alloys might decrease the γ′
concentration. Fig. 7 shows typical TEM images of the 6Cr alloy during thermal oxidation from 20 °C to 350 °C, in which thermal oxidation is evident. A video recorded the thermal oxidation process under different temperatures in the supplementary file V2, and typical images are presented in Fig. 7. The thermal oxidation-induced microstructural variation of the 6Cr alloy has a similar pattern to that of the 0Cr alloy: nanocrystalline metal oxides undergo nucleation at 250 °C, become distinct at 350 °C and exhibit a porous structure. The metal oxides can also be attributed to CoO, Al2O3 and CoAl2O4. The elemental mappings were obtained for all the elements in the original alloy (Fig. 8a–e) and the oxidized alloy (Fig. 8f–o). The elemental distributions in original alloy indicate that Co and Cr have preferences for the γ phase, while W prefers the γ' phase, and Al has almost no preference for either of these two phases in the original alloy. The oxidized 6Cr alloy has similar elemental distributions to the original alloy, and Cr still prefers the γ phase. The γ phase is more oxidized than the γ' phase in the thermally oxidized alloy, as shown in Fig. 8k–o. The concentration of oxygen in the 6Cr alloy is approximately 50 at.% and is lower in both the γ and γ' phases than in the 0Cr alloy (60–70 at. %). The line-scans of the elemental distributions plotted from the outermost to the innermost part of the 6 Cr alloy are different than those of the 0Cr alloy. A cavity structure with a porous CoO layer in the outermost part and then a dense Al2O3 structure does not appear in the 6Cr alloy, and the addition of Cr hinders the formation of this cavity structure. There is a high concentration of Cr at the outermost border of the 6Cr alloy, which means that a Cr2O3 layer formed on the surface. The formation of the Cr2O3 layers thus decreases the diffusion speed of 6
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Fig. 8. High-angle annular dark-field image and elemental distribution mappings of 6Cr alloy: (a–e) the original alloy and (f–o) the oxidized alloy. Elemental linescan on the surface layer in (p) the γ phase and (q) the γ' phase.
to spherical shapes. 2 Only two phases, γ and γ′, are present in the alloys with Cr contents lower than 6 at.%, whereas many CoAl and Co7W6 phases precipitate in alloys with Cr contents greater than 8 at.%. 3 For alloys aged at 900 °C, the γ′volume fraction decreases with increasing ageing time and increasing Cr content; the size of the γ′ phase, both in spherical and cuboidal shapes, increases with increasing ageing time. 4 The hardness of the alloys aged at 900 °C decreases with increasing Cr content. 5 The 6Cr alloy contains a lower oxygen concentration than the 0Cr alloy due to the formation of a protective Cr2O3 layer. 6 The corrosion resistance test indicates that 6Cr addition does not greatly improve the oxidation resistance due to the preferential oxidation of the γ phase and the fact that the 6Cr alloy contains a lower γ′-phase volume than the 0Cr alloy. 7 Improving the alloy’s oxidation resistance requires the addition of Cr to form a protective layer, and other elements can also be added to maintain a sufficient γ′ volume fraction.
volume fraction [27]. The decrease in the hardness of the experimental alloys, as shown in Fig. 4b, can be attributed to a decrease in the γ′ volume fraction with increasing Cr content. Research on the oxidation behaviour of a bulk Co82Al9W9 alloy [14] heated to 900 °C indicates that the formation of the γ phase is preferred over that of the γ′ phase, and this behaviour is similar to that observed from our in situ oxidation experiments. Studies on Co-Al-W-B-Cr alloys [17] indicate that 4Cr and 8Cr addition actually increases the thickness of the Cr2O3 layers that protect against further oxidation, but 8Cr addition prompts the γ′ phase to change to irregular round shapes and the precipitation of needle-shaped secondary phases, which decreases the oxidation resistance of the alloy. The γ phase in both the 0Cr and 6Cr alloys in our experiments contains more oxygen than the γ′ phase and occupies a larger oxidation area from the border into the inner region. It can be concluded that the alloys with a greater γ′-phase volume fraction have a better oxidation resistance. The addition of Cr decreases the γ′phase volume fraction of the alloy but decreases the oxygen content, especially in the inner part of the alloy, thus improving the oxidation resistance of the alloy by forming a protective Cr2O3 surface layer. It is thus reasonable to add a moderate amount of Cr to retain the oxidation resistance of the alloy, and other elements such as Ti and Ta can also be added to improve the γ′-phase volume fraction [28], which imparts a better oxidation resistance to the alloy compared with the γ phase.
Declaration of Competing Interest The authors declare no competing financial interests.
4. Conclusion Acknowledgements 1 Increasing the amount of added Cr causes the morphology of the γ′ phase to change from a cuboid shape to irregular squares and then
This work was supported by the Beijing Natural Science Foundation (No. 1182005), the Natural Science Foundation of China (No. 7
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11404014, No. 91860202 and No. 51872008), and the Beijing Natural Science Foundation (Z180014). Yanhui thanks Prof. Qiang Feng from the University of Technology, Beijing for his inspiring discussions. The authors thank Dr. Dongchang Wu from Thermo Fisher Scientific, Shanghai Nanoport for the useful discussions and assistance with the Titan-ETEM and Titan-Themis.
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Effect of Cr on the microstructure and oxidation properties of Co-Al-W superalloys studied by in situ environmental TEM
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Yanhui Chena, Fei Xueb, Chunhui Wanga, Xueqiao Lia, Qingsong Denga, Xiaomeng Yanga,b, ⁎ Haibo Longa, Wei Lia, Luyan Yanga, Ang Lia, a b
Beijing Key Lab of Microstructure and Property of Advanced Materials, Beijing University of Technology, Beijing, 100124, China State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing, 100083, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Cr addition Co-Al-W superalloy Microstructure In situ oxidation ETEM
Modern in situ oxidation processes can be observed down to the nanoscale using environmental transmission electron microscope (TEM), and this technology has been applied to conventional corrosion science to provide more detailed intrinsic information on the oxidation properties of materials. The influence of chromium (0–10 at.%) addition on the microstructure and oxidation properties of a γ/γ′ two-phase Co82Al9W9-based superalloy was systematically investigated. The in situ oxidation behaviour of Cr-containing alloys was studied using environmental TEM, and the addition of moderate Cr amounts is suggested for the alloy design.
1. Introduction Chromium (Cr) and aluminium (Al) are the two most important elements used to improve the corrosion resistance of alloys, especially superalloys [1], by forming protective Cr2O3 and Al2O3 layers [2]. Al, however, acts as a basic structural stability element and forms an L12Ni3Al phase in Ni-based superalloys or an L12-Co3(Al/W) phase in Cobased superalloys; thus, Al is limited to only γ′ phase stability regions [3,4]. The concentration of Al is thus kept at 5–6 wt.% for first-generation to fourth-generation Ni-based superalloys without large concentration variations [5]. Tuning the concentration of Cr has thus become the main method used to tune the alloy’s corrosion resistance, and the Cr concentration can be changed from approximately 10 wt.% in the first generation to approximately 3 wt.% in the fourth generation by developing thermal barrier coating techniques [6]. However, the addition of Cr increases the quantity of precipitates in topologically closepacked phases and worsens the thermal mechanical properties [7,8]. A moderate concentration of Cr in an alloy, especially in a newly developed alloy, is thus important for the material design and future applications. The γ/γ´ phase Co-Al-W-based superalloys have the same microstructure as Ni-based superalloys and possess good high-temperature mechanical properties [9–11]. Improving the oxidation resistance of γ/ γ´-phase Co-Al-W-based superalloys is necessary for their applications and use in industry, in which materials are subjected to harsh conditions and high temperatures of up to thousands of degrees [12,13]. A
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Co-Al-W alloy was shown to have a poor corrosion resistance by oxidation experiments reported in the literature [14]. The addition of Cr to the alloy to improve its oxidation resistance is the most conventional and effective method used to design alloys [15,16]. The addition of an appropriate proportion of Cr to the γ/γ′ structure of a Co-Al-W superalloy requires practical consideration. Few studies have reported the addition of single Cr [9] or binary Ta-Cr [10–12] and B-Cr [17] elements to the Co-Al-W superalloy, and all the reported alloys were treated as reference samples; there are no systematic studies on the addition of Cr. The content of Cr added in these studies was comparatively low–up to 5 wt.%. As can be deduced from the γ/γ′ two-phase Nibased alloys, the morphology, γ′ size, γ′ volume fraction and hardness are important factors that influence the alloys’ mechanical properties [13]. Cr addition to Co-Ni-Al-W-based alloys had been reported to change the alloys’ morphologies and structures [18]. Limited information is available regarding Cr-containing Co-Al-W-based superalloys. Systematic studies on the microstructure stability, morphology, volume fraction of the γ' phase and oxidation behaviour of Cr-containing Co-AlW-based superalloys with Cr concentrations ranging from 0 to 10 at.% are necessary and important for developing γ/γ' two-phase Co-Al-Wbased superalloys. In situ techniques provide micrometre to even nanometre scale observation of material variation under stress or corrosive gases and afford a deep understanding of the properties of materials. The techniques are powerful tools for material characterization [19]. In situ environmental scanning electron microscopy has been used to study Co-
Corresponding author. E-mail address: [email protected] (A. Li).
https://doi.org/10.1016/j.corsci.2019.108179 Received 13 April 2019; Received in revised form 21 July 2019; Accepted 21 August 2019 Available online 23 September 2019 0010-938X/ © 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. SEM images of the alloys with different Cr contents after ageing at 900 °C for 50 h: (a) 0Cr, (b) 2Cr, (c) 4Cr, (d) 6Cr, (e) 8Cr and (f) 10Cr. All the images have the same scale bars as that shown at the bottom of Fig. 1f.
heating holder. The prepared sample was cut perpendicular to the {001} direction to observe the plane perpendicular to the γ/γ' phase boundary. The in situ oxidation experiments utilized a dedicated fieldemission ETEM (FEI-Titan-ETEM). The ETEM was equipped with a spherical aberration corrected objective lens, and the gas pressure around the sample was controlled using a differential pumping system. The gas pressure of the pure O2 (99.999%) atmosphere around the sample was set to 0.2 mbar. All the oxidation experiments performed by in situ TEM were imaged at 300 kV. The specimen temperature was maintained at 20–350 °C using a DENS Nano-Chip-based heating holder. The Nano-Chip allows fast heating and quenching with superior three-dimensional stability by using a four-point-probe method, which enables local measurements of the temperature with fast feedback to achieve immediate stabilization and accurate temperatures. Ex situ TEM was also performed with probe aberration-corrected scanning transmission electron microscopes (FEI-Titan-Themis) operated at 300 kV and equipped with a Super-X energy dispersive X-ray spectroscopy (EDS) four-quadrant detector. The alloy we used was a highly crystalline cubic γ' phase single crystal, and the side length of the single crystal was approximately 200 nm. We examined the centre of the 200 nm γ' phase to obtained the EDS mapping results. The region we measured was thin enough that only one γ or γ' phase was detected without any overlap. The quantitative data were obtained by averaging the data collected from the centre of the specified part. For the quantitative EDS analysis, the power law method was applied to remove the background. The K-series peaks were chosen for O, Al, Cr and Co, and the L-series peaks were chosen for W in the analysis.
Al-W-based alloys and provides micrometre-scale observations during the oxidation process [20]. Environmental transition electron microscopy (ETEM), which was recently developed, can provide nanoscale or atomic-scale investigation of material variation during reaction process and has been used to study some nanomaterials, providing information that traditional techniques cannot attain [21]. Research on the in situ oxidation corrosion process of Co-Al-W-based alloy is thus useful to gain a deep understanding of the properties of the alloy. In this study, a series of Co82Al9W9-based (at.%) alloys containing various concentrations of Cr ranging from 0 at.% to 10 at.% was investigated using scanning electron microscopy (SEM). The evolutions of the morphology and microstructure were analysed as a function of the Cr content. The oxidation resistance of the Co82Al9W9-based alloys was investigated by in situ oxidation using ETEM to reveal the effect of adding Cr. A moderate Cr concentration was suggested in the alloy design to optimally balance the structure, hardness, precipitation and corrosion resistance. 2. Experimental procedure The experimental alloys were vacuum arc melted into 20 g ingots. Six alloys composed of Co82Al9W9, with a Cr content ranging from 0 to 10 at.% at intervals of 2 at.% Cr were melted to systematically investigate the influence of the Cr content. The nominal compositions of the alloys are given in Table 1. The alloys are abbreviated based on their Cr content. The ingots were solution heat-treated at 1300 °C in a sealed tube for 168 h and then aged at 900 °C for 50 h or 300 h. The microstructures of the alloys were characterized using a ZEISS SUPRA scanning electron microscope. Energy dispersive X-ray (EDX) spectroscopy coupled with SEM and transmission electron microscopy (TEM) was used to investigate the compositions of the precipitates. The samples were first mechanically polished for the SEM observations, and the backscattered electrons (BSEs) were detected to study the elemental distribution. The γ′ volume fraction was quantitatively determined by secondary electron microscopy (SEM), and image processing software was used to calculate the area fraction. The analysed area was approximately 3.8 μm × 2.6 μm. The Vickers hardness of the alloys was measured using an indentation load of 200 g; the mean value of three measurements is presented in the paper. The in situ thermal oxidation TEM samples were prepared by a focused ion beam (FIB) cutting and transformation method (FEI-FIBHelios), and the samples were deposited on a DENS Nano-Chip-based
3. Results and discussion The SEM images show that all the alloys exhibit typical γ/γ′ structures after ageing at 900 °C for 50 h, as shown in Fig. 1. The morphology of the γ′ phase varies from a cuboid shape (0Cr, 2Cr and 4Cr alloys) to an irregular rectangular shape (6Cr alloy) and then to a nearly spherical shape (8Cr and 10Cr alloys) with an increase in the Cr content. The microstructures of the 0Cr, 2Cr and 4Cr alloys are similar to the microstructures reported by Sato et al [22]. and Suzuki et al [23]. In addition, the size (edge length or diameter) of the γ′ phase increased from 50 nm (0Cr) to 100 nm (10Cr) by increasing the Cr content. The bright contrast of the γ′ phase also indicates the aggregation of the refractory element W in the γ′ phase. Our study also indicates that the microstructures of the alloys treated with prolonged ageing at 900 °C 2
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Fig. 2. Alloys after ageing at 900 °C for 300 h: (a) 2Cr and (b) 10Cr alloys. Typical TEM images and diffraction patterns of all the precipitates: (c–d) DO19-Co3W phase, (e–f) μ-Co7W6 phase and (g–h) B2-CoAl phase.
Fig. 3. Evolution of the volume fraction of the γ′ phase as a function of ageing time at 900 °C for different Cr contents: (a) 2Cr and 6Cr alloys after ageing for different lengths of time and (b) comparison of the six alloys after ageing for 300 h.
Fig. 4. (a) Vickers hardness values of the 2Cr and 6Cr alloys under different heat treatment conditions. (b) Vickers hardness values of the six alloys after ageing at 900 °C for 50 h.
reveal a composition of approximately Co3W. The EDS analysis indicated that the bright, blocky, high-contrast phases in the 10Cr alloy, as shown in Fig. 2b, have a composition of approximately Co7W6, and the black needle-like high-contrast phases have a composition of CoAl. The results indicate that W and Al will supersaturate after long ageing times to form secondary precipitation phases. Cr addition prompted the precipitation of the Al- and W-containing secondary phases, such as CoW, CoAl and Co7W6. The precipitate phases of the aged alloys were
for 300 h have similar phase shape variations and enlarged phase sizes with an increase in the Cr content. The SEM analysis indicates that the 0–6Cr alloys all have a similar microstructure and precipitates, while the 8–10Cr alloys also all have a similar microstructure and precipitates. Fig. 2 shows typical SEM images of the precipitates in the 2Cr and 10Cr alloys aged at 900 °C for 300 h. The SEM analysis and corresponding EDX analysis of the secondary rod-like phases, as shown at the grain boundaries in Fig. 2a, 3
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Fig. 5. (a–g) Series of TEM images obtained during the oxidation process of the 0Cr alloy as it was heated from 20 °C to 350 °C. (h) HRTEM image from the border of the oxidized alloy and (i) the EDP corresponding to image (h).
hardness of the 2Cr alloy under all treatment conditions is higher than that of the 6Cr alloy. The decrease in the hardness with increasing Cr content is directly proportional to the volume fraction of the γ′ phase, as shown in Figs. 1 and 3. The γ′ phase is harder than the γ phase due to its high concentration of refractory tungsten, as determined from the EDX data shown in Table 1. This conclusion was further verified by measuring the hardness of the alloys with different Cr contents after ageing at 900 °C for 50 h, as shown in Fig. 4b. The results indicate that the hardness is inversely proportional to the Cr content but directly proportional to the volume fraction of the γ′ phase. From the abovementioned γ/γ′ structural analysis, the quantity of precipitates and the Vickers hardness variation, we can conclude that the addition of Cr should not be higher than 6 at.% to maintain a high enough γ′ volume fraction, a limited quantity of precipitates and a sufficient hardness. The oxidation behaviour of the 6Cr alloy was then studied to determine the effect of Cr addition, and the 0Cr alloy was also tested for comparison. An in situ thermal oxidation experiment under an oxygen pressure of 0.5 mbar was then carried out in an ETEM chamber to compare the effect of Cr addition on the oxidation properties of the alloys. Fig. 5 shows typical TEM images of the 0Cr alloy during thermal oxidation from 20 °C to 350 °C, in which thermal oxidation was evident. A video recorded the thermal oxidation process under different temperatures in the supplementary file V1, and typical images are presented in Fig. 5. No obvious structural variation was observed at 200 °C during oxidation, as shown in Fig. 5a–d. The heating treatment eliminates surface stresses, as indicated by the dark stress band in the TEM images (denoted by the arrows in Fig. 5a). A thin amorphous layer of
confirmed by TEM and the corresponding electron diffraction patterns (EDPs). A typical TEM image and the diffraction patterns of the precipitates in the 2Cr alloy are shown in Fig. 2c and d, respectively. The precipitates are mainly distributed at the grain boundary and have DO19 structures and a composition of Co3W. The precipitates in the 8Cr and 10Cr alloys occur as two kinds of phases. Spherical-shaped precipitates are shown in Fig. 2e and f and have μ-Co7W6 phase structures. The needle-like precipitates in the 8Cr and 10Cr alloys belong to the B2CoAl phase and have a BCC structure of a = b=c = 0.29 nm. The precipitate phases have special structural relationships with the base in the Co-Al-W-based alloys, as demonstrated in our early research: [111] B2//[110]γ´, (011)B2//(111)γ´, [001]μ//[110]B2//[001]DO19, (11-2) B2//(-100)μ//(100)DO19, (1–10)B2//(0–27)μ//(0–10)DO19 [24]. The volume fractions of the γ′ phase in the 2Cr and 6Cr alloys aged at 900 °C for different lengths of time were measured using image analysis software, and the data are shown in Fig. 3a. The volume fractions of the γ′ phase in both the alloys decreased remarkably with increasing ageing time. The data also indicate that the volume fractions of the γ′ phase in all six experimental alloys tended to decrease during the ageing process. Fig. 3b shows the evolution of the γ′ volume fraction as a function of the Cr content in alloys aged at 900 °C for 300 h. The result indicates that the γ′ volume fraction decreases from 59% in the 2Cr alloy to 33% in the 10Cr alloy with increasing Cr content. Fig. 4a shows the evolution of the hardness with ageing time in the 2Cr and 6Cr alloys aged at 900 °C. The maximum hardness was observed for both of the alloys aged at 900 °C for 50 h and decreased gradually with prolonged ageing time. This result indicates that the γ/γ′ structure enhances the hardness of the alloy. It is also noted that the 4
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Fig. 6. High-angle annular dark-field image and elemental distribution mappings of the 0Cr alloy: (a–d) the original alloy and (e–l) the oxidized alloy. Elemental linescan on the surface layer in (m) the γ phase and (n) the γ' phase.
oxygen is more concentrated in the γ phase (Fig. 6i). The colour-mixed elemental mappings of the metal and oxygen, as shown in Fig. 6j–l, also show the aggregation of oxygen in the γ phase. It is also noted that Co aggregates in the outermost layer, but the concentration of Co severely decreases at the middle border. Two concentration line-scans were plotted from the outermost to the innermost part of the alloy, as shown in Fig. 6e by the two arrows in the γ and γ' phases. The line-scans in the γ (Fig. 6m) and γ' (Fig. 6n) phases show that the elements in the γ phase do not vary from the outermost to the innermost part. The composition of the γ' phase is different: the concentration of Co is the highest at the outermost border, then decreases severely from approximately 50 at.% to 20 at.% and remains stable in the inner part of the alloy. The variation in the concentration of oxygen in the γ' phase, however, is different than that of Co and has the highest value in the middle section of the alloy, in which the concentration of Al is the highest value. The rate of forming CoO is faster than that of other kinds of metal oxides, such as Co3O4 and Al2O3, and thus, CoO forms first at the outermost border and has a low oxygen concentration. The formation of Al2O3 is slower and has a denser structure compared with CoO, and thus, the second oxide layer on the metal surface is Al2O3 and has the highest oxygen
approximately 2 nm existed before thermal oxidation, and this kind of amorphous layer can form at room temperature on the edge of thin samples. The surface stress decreases with an increase in the oxidation temperature and almost disappears at 250 °C. Nanocrystalline metal oxide particles formed above the amorphous layer after oxidation at 250 °C for 15 min (Fig. 2f). Increasing the temperature up to 350 °C to promote oxidation increased the quantity of the metal oxide nanoparticles, and the particle size increased from a few nanometres to tens of nanometres. The contrast between the γ/γ' phases became more distinct at 350 °C than at other temperatures. Fig. 5h shows a typical high-resolution TEM (HRTEM) image from the outermost surface of the oxidized border, indicating the multi-crystalline structure of the alloy. The corresponding fast Fourier transform of the HRTEM in Fig. 5i can be attributed to CoO, Al2O3 and CoAl2O4. The elemental mappings were obtained for all the elements in the original alloy (Fig. 6a–d) and the oxidized alloy (Fig. 6e–f). The elemental distributions in the original alloy indicate that Co prefers the γ phase, while W prefers the γ' phase, and Al has almost no preference for either of these two phases in the original alloy. After thermal oxidation, the γ phase is more oxidized than the γ' phase, and the distribution of 5
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Fig. 7. (a–g) Series of TEM images obtained during the oxidation process of the 6Cr alloy as it was heated from 20 °C to 350 °C. (h) HRTEM image from the border of the oxidized alloy and (i) the EDP corresponding to image (h).
oxygen into the inner alloy, and thus, the 6Cr alloy contains a lower oxygen concentration than the 0Cf alloy under the same oxidation conditions. Studies indicate that the γ′ phase is prone to adopting a cuboidal shape when the misfit is either positive or negative, while the γ′ phase is prone to adopting a spherical shape when the misfit is approximately zero. [15] For the Co-Al-W alloys, the γ′ phase contains higher concentrations of W, which is a heavy element, than the γ phase [25]. The γ′ phase in the 2Cr and 4Cr alloys retains its cuboidal structure, while it exhibits an irregular square structure in the 5Cr and 6Cr alloys and a spherical structure in the 8Cr and 10Cr alloys. The morphology indicates that the addition of Cr to the Co82Al9W9 alloy decreases the lattice misfit. For a Ni-based single-crystal superalloy, the addition of Cr changes the lattice misfit from a negative value to zero, which changes the γ′ morphology from a spherical shape to a cuboidal shape [26]. Therefore, the morphological evolution of the alloys (Fig. 2) is reasonably attributed to the addition of Cr, which gradually reduces the lattice misfit as the Cr content increases. In conventional γ/γ′ Ni-based single-crystal alloys, strengthening of the γ′ phase by precipitation is the most commonly used method. The hardness results show that the alloys aged for 50 h are the hardest among all the aged samples, as shown in Fig. 4a, which indicates that structure strengthening was the best strengthening method used for the experimental alloys. Furthermore, in the γ/γ′ two-phase Ni-based single-crystal alloys, alloys with higher γ′-volume fractions exhibit better mechanical properties than alloys with lower γ′-volume fractions. Cr was the γ-phase stabilizing element for the Co-Al-W-Cr alloys [22], and its addition to γ/γ′-strengthened alloys might decrease the γ′
concentration. Fig. 7 shows typical TEM images of the 6Cr alloy during thermal oxidation from 20 °C to 350 °C, in which thermal oxidation is evident. A video recorded the thermal oxidation process under different temperatures in the supplementary file V2, and typical images are presented in Fig. 7. The thermal oxidation-induced microstructural variation of the 6Cr alloy has a similar pattern to that of the 0Cr alloy: nanocrystalline metal oxides undergo nucleation at 250 °C, become distinct at 350 °C and exhibit a porous structure. The metal oxides can also be attributed to CoO, Al2O3 and CoAl2O4. The elemental mappings were obtained for all the elements in the original alloy (Fig. 8a–e) and the oxidized alloy (Fig. 8f–o). The elemental distributions in original alloy indicate that Co and Cr have preferences for the γ phase, while W prefers the γ' phase, and Al has almost no preference for either of these two phases in the original alloy. The oxidized 6Cr alloy has similar elemental distributions to the original alloy, and Cr still prefers the γ phase. The γ phase is more oxidized than the γ' phase in the thermally oxidized alloy, as shown in Fig. 8k–o. The concentration of oxygen in the 6Cr alloy is approximately 50 at.% and is lower in both the γ and γ' phases than in the 0Cr alloy (60–70 at. %). The line-scans of the elemental distributions plotted from the outermost to the innermost part of the 6 Cr alloy are different than those of the 0Cr alloy. A cavity structure with a porous CoO layer in the outermost part and then a dense Al2O3 structure does not appear in the 6Cr alloy, and the addition of Cr hinders the formation of this cavity structure. There is a high concentration of Cr at the outermost border of the 6Cr alloy, which means that a Cr2O3 layer formed on the surface. The formation of the Cr2O3 layers thus decreases the diffusion speed of 6
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Fig. 8. High-angle annular dark-field image and elemental distribution mappings of 6Cr alloy: (a–e) the original alloy and (f–o) the oxidized alloy. Elemental linescan on the surface layer in (p) the γ phase and (q) the γ' phase.
to spherical shapes. 2 Only two phases, γ and γ′, are present in the alloys with Cr contents lower than 6 at.%, whereas many CoAl and Co7W6 phases precipitate in alloys with Cr contents greater than 8 at.%. 3 For alloys aged at 900 °C, the γ′volume fraction decreases with increasing ageing time and increasing Cr content; the size of the γ′ phase, both in spherical and cuboidal shapes, increases with increasing ageing time. 4 The hardness of the alloys aged at 900 °C decreases with increasing Cr content. 5 The 6Cr alloy contains a lower oxygen concentration than the 0Cr alloy due to the formation of a protective Cr2O3 layer. 6 The corrosion resistance test indicates that 6Cr addition does not greatly improve the oxidation resistance due to the preferential oxidation of the γ phase and the fact that the 6Cr alloy contains a lower γ′-phase volume than the 0Cr alloy. 7 Improving the alloy’s oxidation resistance requires the addition of Cr to form a protective layer, and other elements can also be added to maintain a sufficient γ′ volume fraction.
volume fraction [27]. The decrease in the hardness of the experimental alloys, as shown in Fig. 4b, can be attributed to a decrease in the γ′ volume fraction with increasing Cr content. Research on the oxidation behaviour of a bulk Co82Al9W9 alloy [14] heated to 900 °C indicates that the formation of the γ phase is preferred over that of the γ′ phase, and this behaviour is similar to that observed from our in situ oxidation experiments. Studies on Co-Al-W-B-Cr alloys [17] indicate that 4Cr and 8Cr addition actually increases the thickness of the Cr2O3 layers that protect against further oxidation, but 8Cr addition prompts the γ′ phase to change to irregular round shapes and the precipitation of needle-shaped secondary phases, which decreases the oxidation resistance of the alloy. The γ phase in both the 0Cr and 6Cr alloys in our experiments contains more oxygen than the γ′ phase and occupies a larger oxidation area from the border into the inner region. It can be concluded that the alloys with a greater γ′-phase volume fraction have a better oxidation resistance. The addition of Cr decreases the γ′phase volume fraction of the alloy but decreases the oxygen content, especially in the inner part of the alloy, thus improving the oxidation resistance of the alloy by forming a protective Cr2O3 surface layer. It is thus reasonable to add a moderate amount of Cr to retain the oxidation resistance of the alloy, and other elements such as Ti and Ta can also be added to improve the γ′-phase volume fraction [28], which imparts a better oxidation resistance to the alloy compared with the γ phase.
Declaration of Competing Interest The authors declare no competing financial interests.
4. Conclusion Acknowledgements 1 Increasing the amount of added Cr causes the morphology of the γ′ phase to change from a cuboid shape to irregular squares and then
This work was supported by the Beijing Natural Science Foundation (No. 1182005), the Natural Science Foundation of China (No. 7
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11404014, No. 91860202 and No. 51872008), and the Beijing Natural Science Foundation (Z180014). Yanhui thanks Prof. Qiang Feng from the University of Technology, Beijing for his inspiring discussions. The authors thank Dr. Dongchang Wu from Thermo Fisher Scientific, Shanghai Nanoport for the useful discussions and assistance with the Titan-ETEM and Titan-Themis.
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