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Effect of Al content on high temperature oxidation resistance of AlxCoCrCuFeNi high entropy alloys (x=0, 0.5, 1, 1.5, 2)
Vacuum 169 (2019) 108837
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Effect of Al content on high temperature oxidation resistance of AlxCoCrCuFeNi high entropy alloys (x=0, 0.5, 1, 1.5, 2)
T
Y.Y. Liua, Z. Chena,2,∗, Y.Z. Chen2, J.C. Shia, Z.Y. Wanga, S. Wanga, F. Liu2 a 2
School of Material Science and Engineering, China University of Mining and Technology, Xuzhou, Jiangsu, 221008, PR China State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an, Shaanxi, 710072, China
ARTICLE INFO
ABSTRACT
Keywords: High entropy alloys Oxidation behavior Oxidation kinetics
The oxidation behavior of AlxCoCrCuFeNi (x = 0, 0.5, 1, 1.5, 2) high entropy alloys are discussed in this work. Five kinds of HEAs with nominal compositions of are prepared by vacuum arc melting and subjected to isothermal oxidation at 1000°C for 100 h in static air. The results indicate that three typical CuNi-rich, AlNi-rich and matrix phases emerge after oxidation. In all oxidized HEAs there is a tendency to generate the Cr depletion or Al depletion region directly beneath the oxidation layer. With the increase of Al content, the oxidation scale mainly consists of alumina instead of Cr2O3 and the parabolic rate constant of Al2CoCrCuFeNi HEA reaches the minimum value, which clarifies the superior high temperature resistance.
1. Introduction High entropy alloys (HEAs), containing multiple and equal-mole elements, have received increasing concerns since the concept was proposed in 1995 for the first time, on account of its potential value in both commercial applications and theoretical studies [1–4]. The higher mixing entropy enables the formation of stable solid solution phases with simple FCC and BCC phase [5], while Lu et al. have proposed a novel Calculation of Phase Diagrams strategy to design eutectic high entropy alloys (EHEAs) efficiently and accurately, which can overcome the common shortcomings of HEA, such as weak liquidity and castability, and compositional inhomogeneity [6–11]. Compared to most conventional alloys that are designed with a single principal or several minor elements to optimize microstructures and properties, HEAs possess many superior mechanical behaviors owing to entropy-driven phase stability such as remarkable wear resistance, obvious sluggish diffusion effect and excellent corrosion and oxidation resistance [12–23]. Especially, the large degree of chaos at high temperature and the high entropy effect enable alloys to be more stable and HEAs are still given outstanding oxidation resistance [24,25]. Based on these characteristics, plenty of applications and researches on high temperature oxidation resistance of HEAs have been widely reported in last decade. A gradient NiCoCrAlSiY coating consisting of Al-enriched outer zone and Cr-enriched internal zone was prepared by Xu et al. The gradient coating exhibited better performance in the isothermal oxidations due
∗
to its high-Al content and the in-situ diffusion barrier formed during the oxidation [26]. Dabrowa et al. approved that the Cu addition made oxide scale ( -Al2O3) exhibit poor adhesion to the surface in AlCoCrCuxFeNi HEA system. There was also a visible tendency towards formation of the Al-depleted FCC phase directly beneath the scale in all samples [25]. Butler et al. studied the oxidation behaviors of a series of arc-melted Alx(NiCoCrFe)100-x(x = 8, 10, 12, 15, 20, 30 at.%) HEAs. They verified that the dominant structure of the low Al concentration high entropy alloys was determined to be FCC, while the high Al concentration HEAs were BCC dominant. Each high entropy alloy showed initial transient oxidation followed by various degrees of parabolic oxide growth. Increased Al content improved the continuity and internal position of the Al2O3 scale, resulting in enhanced oxidation resistances [27]. Massive researches have been reported about the microstructure and phase composition of AlCoCrCuFeNi, which verified that the growth rate of chromia and -alumina was very slow and the parabolic and oxidation constants are 2×10−11mg2cm−4s−1 −12 2 −4 −1 7×10 mg cm s at 1000°C respectively. Al and Cr affected significantly oxidation behavior of HEAs and refractory HEAs [28]. Thus, it is critically important to comprehend the relationship between the Al content of HEAs and its high temperature oxidation resistance. In this paper, the AlxCoCrCuFeNi (x = 0, 0.5, 1, 1.5, 2) HEAs were fabricated by arc melting under a Ti-gettered argon atmosphere. The microstructure, oxidation scale and phase composition were investigated by XRD, SEM and EPMA. The influence of Al concentration
Corresponding author.School of Material Science and Engineering, China University of Mining and Technology, Xuzhou, Jiangsu, 221008, PR China. E-mail address: [email protected] (Z. Chen).
https://doi.org/10.1016/j.vacuum.2019.108837 Received 21 May 2019; Received in revised form 22 June 2019; Accepted 24 July 2019 Available online 25 July 2019 0042-207X/ © 2019 Elsevier Ltd. All rights reserved.
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Table 1 The element composition of AlxCoCrCuFeNi HEAs (x = 0, 0.5, 1, 1.5, 2). EDX quantative analysis (at. %) of AlxCoCrCuFeNi HEAs Alloy
Al
Co
Al0CoCrCuFeNi Al0.5CoCrCuFeNi Al1CoCrCuFeNi Al1.5CoCrCuFeNi Al2CoCrCuFeNi
7.85 ± 0.5 15.52 ± 0.5 22.86 ± 0.5 27.64 ± 0.5
19.34 17.56 18.74 14.12 13.08
Cr ± ± ± ± ±
0.5 0.5 0.5 0.5 0.5
Cu
21.07 20.11 18.33 16.65 14.57
± ± ± ± ±
0.5 0.5 0.5 0.5 0.5
18.56 16.32 16.67 13.44 13.06
Fe ± ± ± ± ±
0.5 0.5 0.5 0.5 0.5
20.94 19.43 17.08 17.28 15.22
Ni ± ± ± ± ±
0.5 0.5 0.5 0.5 0.5
20.09 18.71 13.66 15.65 16.43
± ± ± ± ±
0.5 0.5 0.5 0.5 0.5
on high temperature oxidation behavior of AlxCoCrCuFeNi HEAs at 1000°C was declared. 2. Experiments The mixture of highly pure Al (99.99 wt%), Co (99.98 wt%), Cr (99.99 wt%), Cu (99.99 wt%), Fe (99.95 wt%), Ni (99.95 wt%) with a nominal composition of AlxCoCrCuFeNi (x = 0, 0.5, 1, 1.5, 2) were put into copper crucibles and prepared by arc melting under a Ti-gettered argon atmosphere for 5 times to assure homogenous chemical composition. The furnace was firstly pumped to high vacuum (10−4 Pa) and then backfilled with purified argon. The ingots approximately cooled at the speed of 10–20 K s−1 with a diameter of 25 mm and thickness of 15 mm and then were divided into initial cubical samples of 5 g. The high temperature oxidation experiments of AlxCoCrCuFeNi HEAs were carried by thermostatic weight gain method with GSL-1600X pipe furnace at 1000°C for 100 h. Discontinuous weighing method was used to measure the quality of the sample. The structural features of HEAs before and after oxidation were verified by X-ray diffraction (XRD, Bruker D8 Advance X) with Cu-Kα radiation. The overall sample composition, oxidation morphology and elemental distribution of samples were investigated by scanning electron microscope (SEM, Quanta™ 250) and electron probe micro-analyzer (EPMA -8050G). From the analysis of EPMA, the sample composition was consistent with nominal composition, as shown in Table 1 and Fig. 1 – Fig. 3.
Fig. 1. The optical microstructures of as-cast samples: (a) Al0CoCrCuFeNi; (b) Al0.5CoCrCuFeNi; (c) Al1CoCrCuFeNi; (d) Al1.5CoCrCuFeNi; (e) Al2CoCrCuFeNi.
3. Results and discussion 3.1. Phase constitution The optical images of as-cast AlxCoCrCuFeNi HEAs (x = 0, 0.5, 1, 1.5, 2) are shown via Fig. 1. The microstructures of as-cast samples are greatly affected by Al content and the initial coarse dendrites are transformed into coexistence of equiaxed grains and dendrites, and then completely isometric crystal finally. As presented in Fig. 2, the results of EDS line scan prove that elements distribution is relatively homogeneous and corresponding composition of as-cast samples is listed in Table 1. Fig. 3 exhibits SEM images of cross-section of as-cast Al1CoCrCuFeNi with element mappings, which testifies slight segregation of Cu and a huge difference in element distribution from samples oxidized after high temperature. The nature of as-cast and oxidized AlxCoCrCuFeNi (x = 0, 0.5, 1, 1.5, 2) are confirmed by XRD patterns in Fig. 4. In this paper FCC1 matrix phase and CuNi-rich FCC2 phase are uniformly described as FCC, disordered and ordered body-centered cubic as BCC phase [29]. The valance electron concentration (VEC) has vitally pronounced effect on the stability of FCC and BCC solid solutions. VEC for a multi-component alloy can be defined as:
Fig. 2. The results of EDS linescan conducted in the substrate interface on the as-cast samples (a) Al0CoCrCuFeNi; (b) Al0.5CoCrCuFeNi; (c) Al1CoCrCuFeNi; (d) Al1.5CoCrCuFeNi.
structure at VEC < 6.87. Mixed FCC and BCC phases co-exist when VEC ranges from 6.87 to 8. The addition of aluminium composition decreases the VEC value of HEAs and tunes the crystal structure from FCC to FCC + BCC and fully BCC structure owing to that the value of (VEC)Al is 3 [30]. After 100 h high temperature oxidation, the diffraction peak widens and shifts due to the segregation of copper elements into dendrites. In the case of the Al0CoCrCuFeNi, the XRD profiles of Fig. 4 (i, j) indicate the as-cast HEA is in single-phase FCC structure without intermetallic formation and BCC phases because the atomic radius and electronegativity of the five elements are quite similar. However, there are peaks splitting for oxidized sample which match Curich phase corresponding to (111), (200) and (220) planes in order, revealing the aggravating segregation of copper elements during high temperature process. In the case of oxidized Al0.5CoCrCuFeNi sample, the diffraction peak 2θ = 44.6° at (110) plane is identified as BCC phase
n
VEC =
ci (VEC )i i=1
(1)
Where ci and (VEC )i are atomic percentage and VEC for the individual element. The phase tends to be FCC structure at VEC≥8, and BCC 2
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Fig. 3. The SEM images of cross-section of as-cast Al1CoCrCuFeNi with element mappings.
Fig. 4. The XRD patterns of as-cast AlxCoCrCuFeNi (x = 0, 0.5, 1, 1.5, 2) HEAs and samples oxidized at 1000°C for 100 h with the magnification view.
in Fig. 1. (h) with further increasing aluminum content. When the molar mass of Al ratio increases to 1, XRD pattern affirms that the principal phases transform from FCC structure to BCC structure, and Wang et al. obtained the similar result [31]. Compared to the as-cast sample, FCC structure of CuNi-rich phase starts to emerge at oxidized counterpart and the main peak shifts from 44.5° to 43.9°. Al1.5 and Al2 consist of a majority of BCC phase with few FCC phase. After high temperature oxidation, phase identification of AlNi-rich can be acquired in this work for both Al1.5 and Al2. This may attribute to the reason that vacancy-induced segregation evolution of HEAs is depended on the atomic size of the alloy elements, in which oversized atoms diffusing faster while undersized atoms diffusing slower [32] and aluminum atom diameter is larger than others counterpart. Addition of Al engenders influence not only on as-cast microstructures, but also element diffusion at high temperature. The segregation and diffusion at high temperature contribute to a small quantity of FCC matrix phase, which will be certified by later EPMA mapping.
Fig. 5 compares the SEM microstructure of AlxCoCrCuFeNi (x = 0, 0.5, 1, 1.5, 2) after 1000°C-100 h high temperature oxidation with compositions fraction of dendrite and inter-dendrite regions below the SEM images. All the five HEAs share one common point that the dendrite mostly made up of copper because the mixing enthalpies between Cu and Al, Co, Cr, Fe and Ni are −1, +6, +12, +13 and + 4 respectively. The positive mixing enthalpies encourage copper to segregate in the inter-dendritic region and are incompatible with Co, Cr and Fe [33]. In the annealing process three typical structures including CuNi-rich FCC structure, AlNi-rich plate and FeCr-rich plate are observed with the occurrence of spinodal decomposition [34]. When the HEA contains higher Al content, the phenomenon is more obvious as shown in Fig. 9. 3.2. Oxides and microstructures after oxidation Fig. 6 shows the XRD patterns of AlxCoCrCuFeNi (x = 0, 0.5, 1, 1.5,
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Fig. 5. The micrographics of the oxidized samples: (a) Al0CoCrCuFeNi; (b) Al0.5CoCrCuFeNi; (c) Al1CoCrCuFeNi; (d) Al1.5CoCrCuFeNi; (e) Al2CoCrCuFeNi. The results of EPMA point analysis for each alloy are also presented.
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Fig. 6. The XRD patterns of oxidation scale of AlxCoCrCuFeNi (x = 0, 0.5, 1, 1.5, 2) HEAs at 1000 °C for 100 h.
2) HEAs after high temperature oxidation at 1000°Cfor 100 h. There are approximately four types of oxides on the surface of alloys: Al2O3, Cr2O3, spinel oxides (molecular formula uniformly written as (Co,Ni,Cu)(Al,Cr,Fe)2O4) and simple oxides ((Co,Ni,Cu)O). In the case of Al0CoCrCuFeNi, the primary oxides are Cr2O3 with small amount of spinel oxides and (Co,Ni,Cu)O simple oxides. The intensity of substrate diffraction peak seems to be weak because the thickness of oxidation layer is up to 6-7µm , as is shown in Fig. 8. Actually the thickness of the oxide layer is much thicker than observed in the EPMA image, which is due to the poor adhesion and the excessive spallation of the Cr2O3based oxide layer. When the Al content increases to 0.5, Al2O3 peak begins to appear in the XRD patterns. But the lack of Al element beneath the scale cannot generate adequate Al2O3 to impede oxidation as a result of the formation of simple oxides [35]. The phenomenon of exfoliation occurs continuously in the process of oxidation. With the concentration of Al increasing (Al1, Al1.5), the diffraction peak maximum of Al2O3, Cr2O3 and spinel oxides adds up gradually. However, in the case of Al2CoCrCuFeNi, substrate diffraction peaks are more obvious than the peaks of oxides. During the thermo-gravimetric analysis, the samples cool down and then oxides scale peels off due to differences in thermal expansion coefficients (CTEs), which is explained by Chou [24]. The oxidation morphologies of AlxCoCrCuFeNi (x = 0, 0.5, 1, 1.5, 2) HEAs are presented in Fig. 7, which shows complete difference between Al0CoCrCuFeNi and AlxCoCrCuFeNi (x = 0.5, 1, 1.5, 2) HEAs. As is shown in Fig. 7(a) and (b), the oxide layer of Al0CoCrCuFeNi contributes to a loose and hollow structure which leads to a large area of spallation (light color region in Fig. 7(a)) and unsatisfying high temperature resistance performance. When the content of Al increases, the
spallation gets relieved owing to that the surface generates compact Al2O3 oxide layer. In the case of Al0.5CoCrCuFeNi and Al1CoCrCuFeNi, the oxide scales has fewer oxidation hollows and pits because of the oxide peeling and new oxidation growing simultaneously. Compared to Al0.5 and Al1, the scale shows plenty of dense white granular Al2O3, which indicates that the rapid diffusion of Al in the substrate to the outer surface of the oxide film. This results in the expansion of granular alumina into an alumina film. During high temperature oxidation, the surface of Al2 HEA produces a large number of needle-like metastable transition aluminum as shown in Fig. 7(j) on account that this transition from metastable to steady Al2O3 is relevant with oxidation time and temperature. 3.3. The cross section after oxidation The EPMA element mapping conducted on the Al0CoCrCuFeNi HEA cross section which is oxidized for 100 h at 1000 °C is presented in Fig. 8, with XRD pattern presented in Fig. 4(b). It is clear to observe that the interface of the substrate-scale is not smooth any more after oxidation. This is caused by that only Cr without Al composition for Al0CoCrCuFeNi is difficult to establish compact oxidation scale. Hence, the external layer is consisting of hollow and loose Cr2O3 which possesses poor adhesion to result in spallation according to aforementioned discussion. In the oxidation process the diffusion of Al and Cr plays the critical role and the sluggish diffusion can be partly responsible for the apparent stability of HEAs [36]. The high temperature allows Cr element to diffuse slowly and steadily from substrate to scale surface, and then react with oxygen to form oxides directly, which contributes to a notable Cr depletion region with average depth of 14 µm beneath the
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Fig. 8. The BSE images of cross-section of Al0CoCrCuFeNi after oxidation at 1000 °C for 100 h with EPMA element mapping.
no Al and Cr depletion regions beneath the surface of substrate for ascast Al1. Besides the homogeneity are proved by Fig. 2 as well. The Cr2O3 and -Al2O3 have extremely slow growth rate and each of parabolic oxidation constants are 2×10−11mg2cm−4s−1 and 7×10−11mg2cm−4s−1 respectively [28]. Thus, the Al content decreases from outside to inside of oxidation layer with the increasing Cr element, while the other elements composition are slightly higher than substrate, which indicates that Al2O3 comprises the external scale and the internal layer is mainly based on Cr2O3 and spinel oxides. Besides, Cr element can provide Al with sites for reaction with oxygen ions. It is interesting that the oxidation layers of Al0.5, Al1, Al1.5 and Al2 HEAs are roughly same with the thickness of 4 µ m. It may be as a result of that there are at least two phases beneath the scale due to addition of Cu, which generates a huge difference between the thermal coefficient of expansion of alumina and HEAs substrate [37,38]. Consequently, a high stress within substrate and alumina is formed and induces the oxidation layer to break when the samples cools down. In the case of Al0.5 and Al1 HEAs, dendrites are basically AlNi-rich BCC phase and Cu-rich FCC phase that are supported by EPMA and XRD patterns. Moreover, the Al depletion region of Al0.5 and Al1 is much deeper than Al1.5 and Al2. The dominant phases of Al1.5 and Al2 are Al-rich and AlNi-rich phase with uniform dispersion of copper and matrix FCC phase.
Fig. 7. The SEM images of oxidation scale of AlxCoCrCuFeNi (x = 0, 0.5, 1, 1.5, 2) HEAs at 1000 °C for 100 h: (a-b) Al0CoCrCuFeNi; (c-d) Al0.5CoCrCuFeNi; (ef) Al1CoCrCuFeNi; (g-h) Al1.5CoCrCuFeNi; (i-j) Al2CoCrCuFeNi.
surface as presented in chromium mapping. EPMA element mapping clearly presents that the external oxidation scale mostly consists of Cr2O3 and the internal counterpart is made up of simple oxides ((Co,Ni,Cu)O) and few spinel oxides. The main CuNi-rich phase and matrix phase are verified in EPMA mapping with corresponding XRD result of Fig. 4(a). Different from Al0CoCrCuFeNi HEA, the oxidation layer and element distribution of Al0.5, Al1, Al1.5 and Al2 HEAs have diverse performance with the addition of Al composition, as exhibited in Fig. 9. Through comparing the cross section of as-cast Al1CoCrCuFeNi in Fig. 3 with oxidized sample, there are well-distributed element mappings and
3.4. The oxidation kinetics and mechanisms The oxidation kinetics is approximated using a single-stage parabolic law in a form for all studied alloys:
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m S
2
= 2KP t + C
(2)
where Δm is the sample gained weight by oxidation, S is the surface area, Δm/S for the oxidation gained weight per unit area. Kp is the parabolic rate constant, t is the oxidation time, C is a constant. The oxidation kinetics measurements of AlxCoCrCuFeNi (x = 0, 0.5, 1, 1.5, 2) after 100 h, 1000 °C high temperature are presented in Fig. 10 and corresponding Kp value in Fig. 11. The value of KP decreases with the increase of Al content based on aforementioned analysis, which means superior high temperature resistance. Al and Cr elements are inclined to react with oxygen ions in elevated temperature environment. Therefore, the two elements are firstly oxidized with their oxides covering the surface. When the HEA contains no Al composition or only a small molar mass, Cr2O3 becomes the main oxidation with other simple oxides to prevent the subsequent from oxidation. While the Al content continues to increasing, Al2O3 begins to replace Cr2O3 in the oxidation layer because the growth rate of Cr2O3 is slower than that of alumina. There is a premier Al depletion region beneath the oxidation scale, and the outer scale layer forms aluminum, while the inner layer is dominated by chromium and other spinel oxides. This process is mainly dependent on the diffusion of Al and Cr element. 4. Conclusion The influence of Al content on the microstructures, oxidation behaviors and high temperature oxidation resistance of AlxCoCrCuFeNi high entropy alloys (x = 0, 0.5, 1, 1.5, 2) were discussed. The conclusions can be summarized: 1) The principal CuNi-rich and matrix FCC phase transformed into BCC structure with the increase of Al content for arc-melted AlxCoCrCuFeNi HEAs. After 100 h, 1000 °C oxidation Cu element formed plenty of dendrites or granule segregations due to its positive mixing enthalpies with other elements. AlNi-rich plate and FeCr-rich plate were observed with the occurrence of spinodal decomposition.
Fig. 9. The BSE images of cross-section of AlxCoCrCuFeNi (x = 0.5, 1, 1.5, 2) after oxidation at 1000 °C for 100 h with EPMA element mapping (the same scale bars with BSE images in the individual mapping).
Fig. 10. Mass gain curves measured of AlxCoCrCuFeNi (x = 0, 0.5, 1, 1.5, 2) oxidized at 1000°C for 100 h.
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Fig. 11. The parabolic plots of AlxCoCrCuFeNi (x = 0, 0.5, 1, 1.5, 2) oxidized at 1000°C for 100 h.
2) Al2O3, Cr2O3, spinel oxides and simple oxides were verified in the oxidation process. In case of Al0CoCrCuFeNi, the oxide layer consisting of Cr2O3 shaped a loose and hollow structure which led to a large area of spallation. When the Al content increased, the spallation got relieved owing to compact Al2O3 scale which comprised the external scale and the internal layer was mainly based on Cr2O3 and spinel oxides.
3) Al element played an important role in high temperature resistance. The oxidation kinetics of the Al2CoCrCuFeNi alloys possessed the smallest KP value and revealed the best oxidation resistance of five HEAs. The parabolic constants KP value decreased with the increase of the Al content.
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Acknowledgments [17]
The authors are grateful for funding from the Fundamental Research Funds for the Central Universities (2018GF05), Natural Science Foundation of China (51771226) and the State Key Laboratory of Solidification Processing at NWPU (SKLSP201818).
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Appendix A. Supplementary data
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Supplementary data to this article can be found online at https:// doi.org/10.1016/j.vacuum.2019.108837.
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Effect of Al content on high temperature oxidation resistance of AlxCoCrCuFeNi high entropy alloys (x=0, 0.5, 1, 1.5, 2)
T
Y.Y. Liua, Z. Chena,2,∗, Y.Z. Chen2, J.C. Shia, Z.Y. Wanga, S. Wanga, F. Liu2 a 2
School of Material Science and Engineering, China University of Mining and Technology, Xuzhou, Jiangsu, 221008, PR China State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an, Shaanxi, 710072, China
ARTICLE INFO
ABSTRACT
Keywords: High entropy alloys Oxidation behavior Oxidation kinetics
The oxidation behavior of AlxCoCrCuFeNi (x = 0, 0.5, 1, 1.5, 2) high entropy alloys are discussed in this work. Five kinds of HEAs with nominal compositions of are prepared by vacuum arc melting and subjected to isothermal oxidation at 1000°C for 100 h in static air. The results indicate that three typical CuNi-rich, AlNi-rich and matrix phases emerge after oxidation. In all oxidized HEAs there is a tendency to generate the Cr depletion or Al depletion region directly beneath the oxidation layer. With the increase of Al content, the oxidation scale mainly consists of alumina instead of Cr2O3 and the parabolic rate constant of Al2CoCrCuFeNi HEA reaches the minimum value, which clarifies the superior high temperature resistance.
1. Introduction High entropy alloys (HEAs), containing multiple and equal-mole elements, have received increasing concerns since the concept was proposed in 1995 for the first time, on account of its potential value in both commercial applications and theoretical studies [1–4]. The higher mixing entropy enables the formation of stable solid solution phases with simple FCC and BCC phase [5], while Lu et al. have proposed a novel Calculation of Phase Diagrams strategy to design eutectic high entropy alloys (EHEAs) efficiently and accurately, which can overcome the common shortcomings of HEA, such as weak liquidity and castability, and compositional inhomogeneity [6–11]. Compared to most conventional alloys that are designed with a single principal or several minor elements to optimize microstructures and properties, HEAs possess many superior mechanical behaviors owing to entropy-driven phase stability such as remarkable wear resistance, obvious sluggish diffusion effect and excellent corrosion and oxidation resistance [12–23]. Especially, the large degree of chaos at high temperature and the high entropy effect enable alloys to be more stable and HEAs are still given outstanding oxidation resistance [24,25]. Based on these characteristics, plenty of applications and researches on high temperature oxidation resistance of HEAs have been widely reported in last decade. A gradient NiCoCrAlSiY coating consisting of Al-enriched outer zone and Cr-enriched internal zone was prepared by Xu et al. The gradient coating exhibited better performance in the isothermal oxidations due
∗
to its high-Al content and the in-situ diffusion barrier formed during the oxidation [26]. Dabrowa et al. approved that the Cu addition made oxide scale ( -Al2O3) exhibit poor adhesion to the surface in AlCoCrCuxFeNi HEA system. There was also a visible tendency towards formation of the Al-depleted FCC phase directly beneath the scale in all samples [25]. Butler et al. studied the oxidation behaviors of a series of arc-melted Alx(NiCoCrFe)100-x(x = 8, 10, 12, 15, 20, 30 at.%) HEAs. They verified that the dominant structure of the low Al concentration high entropy alloys was determined to be FCC, while the high Al concentration HEAs were BCC dominant. Each high entropy alloy showed initial transient oxidation followed by various degrees of parabolic oxide growth. Increased Al content improved the continuity and internal position of the Al2O3 scale, resulting in enhanced oxidation resistances [27]. Massive researches have been reported about the microstructure and phase composition of AlCoCrCuFeNi, which verified that the growth rate of chromia and -alumina was very slow and the parabolic and oxidation constants are 2×10−11mg2cm−4s−1 −12 2 −4 −1 7×10 mg cm s at 1000°C respectively. Al and Cr affected significantly oxidation behavior of HEAs and refractory HEAs [28]. Thus, it is critically important to comprehend the relationship between the Al content of HEAs and its high temperature oxidation resistance. In this paper, the AlxCoCrCuFeNi (x = 0, 0.5, 1, 1.5, 2) HEAs were fabricated by arc melting under a Ti-gettered argon atmosphere. The microstructure, oxidation scale and phase composition were investigated by XRD, SEM and EPMA. The influence of Al concentration
Corresponding author.School of Material Science and Engineering, China University of Mining and Technology, Xuzhou, Jiangsu, 221008, PR China. E-mail address: [email protected] (Z. Chen).
https://doi.org/10.1016/j.vacuum.2019.108837 Received 21 May 2019; Received in revised form 22 June 2019; Accepted 24 July 2019 Available online 25 July 2019 0042-207X/ © 2019 Elsevier Ltd. All rights reserved.
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Table 1 The element composition of AlxCoCrCuFeNi HEAs (x = 0, 0.5, 1, 1.5, 2). EDX quantative analysis (at. %) of AlxCoCrCuFeNi HEAs Alloy
Al
Co
Al0CoCrCuFeNi Al0.5CoCrCuFeNi Al1CoCrCuFeNi Al1.5CoCrCuFeNi Al2CoCrCuFeNi
7.85 ± 0.5 15.52 ± 0.5 22.86 ± 0.5 27.64 ± 0.5
19.34 17.56 18.74 14.12 13.08
Cr ± ± ± ± ±
0.5 0.5 0.5 0.5 0.5
Cu
21.07 20.11 18.33 16.65 14.57
± ± ± ± ±
0.5 0.5 0.5 0.5 0.5
18.56 16.32 16.67 13.44 13.06
Fe ± ± ± ± ±
0.5 0.5 0.5 0.5 0.5
20.94 19.43 17.08 17.28 15.22
Ni ± ± ± ± ±
0.5 0.5 0.5 0.5 0.5
20.09 18.71 13.66 15.65 16.43
± ± ± ± ±
0.5 0.5 0.5 0.5 0.5
on high temperature oxidation behavior of AlxCoCrCuFeNi HEAs at 1000°C was declared. 2. Experiments The mixture of highly pure Al (99.99 wt%), Co (99.98 wt%), Cr (99.99 wt%), Cu (99.99 wt%), Fe (99.95 wt%), Ni (99.95 wt%) with a nominal composition of AlxCoCrCuFeNi (x = 0, 0.5, 1, 1.5, 2) were put into copper crucibles and prepared by arc melting under a Ti-gettered argon atmosphere for 5 times to assure homogenous chemical composition. The furnace was firstly pumped to high vacuum (10−4 Pa) and then backfilled with purified argon. The ingots approximately cooled at the speed of 10–20 K s−1 with a diameter of 25 mm and thickness of 15 mm and then were divided into initial cubical samples of 5 g. The high temperature oxidation experiments of AlxCoCrCuFeNi HEAs were carried by thermostatic weight gain method with GSL-1600X pipe furnace at 1000°C for 100 h. Discontinuous weighing method was used to measure the quality of the sample. The structural features of HEAs before and after oxidation were verified by X-ray diffraction (XRD, Bruker D8 Advance X) with Cu-Kα radiation. The overall sample composition, oxidation morphology and elemental distribution of samples were investigated by scanning electron microscope (SEM, Quanta™ 250) and electron probe micro-analyzer (EPMA -8050G). From the analysis of EPMA, the sample composition was consistent with nominal composition, as shown in Table 1 and Fig. 1 – Fig. 3.
Fig. 1. The optical microstructures of as-cast samples: (a) Al0CoCrCuFeNi; (b) Al0.5CoCrCuFeNi; (c) Al1CoCrCuFeNi; (d) Al1.5CoCrCuFeNi; (e) Al2CoCrCuFeNi.
3. Results and discussion 3.1. Phase constitution The optical images of as-cast AlxCoCrCuFeNi HEAs (x = 0, 0.5, 1, 1.5, 2) are shown via Fig. 1. The microstructures of as-cast samples are greatly affected by Al content and the initial coarse dendrites are transformed into coexistence of equiaxed grains and dendrites, and then completely isometric crystal finally. As presented in Fig. 2, the results of EDS line scan prove that elements distribution is relatively homogeneous and corresponding composition of as-cast samples is listed in Table 1. Fig. 3 exhibits SEM images of cross-section of as-cast Al1CoCrCuFeNi with element mappings, which testifies slight segregation of Cu and a huge difference in element distribution from samples oxidized after high temperature. The nature of as-cast and oxidized AlxCoCrCuFeNi (x = 0, 0.5, 1, 1.5, 2) are confirmed by XRD patterns in Fig. 4. In this paper FCC1 matrix phase and CuNi-rich FCC2 phase are uniformly described as FCC, disordered and ordered body-centered cubic as BCC phase [29]. The valance electron concentration (VEC) has vitally pronounced effect on the stability of FCC and BCC solid solutions. VEC for a multi-component alloy can be defined as:
Fig. 2. The results of EDS linescan conducted in the substrate interface on the as-cast samples (a) Al0CoCrCuFeNi; (b) Al0.5CoCrCuFeNi; (c) Al1CoCrCuFeNi; (d) Al1.5CoCrCuFeNi.
structure at VEC < 6.87. Mixed FCC and BCC phases co-exist when VEC ranges from 6.87 to 8. The addition of aluminium composition decreases the VEC value of HEAs and tunes the crystal structure from FCC to FCC + BCC and fully BCC structure owing to that the value of (VEC)Al is 3 [30]. After 100 h high temperature oxidation, the diffraction peak widens and shifts due to the segregation of copper elements into dendrites. In the case of the Al0CoCrCuFeNi, the XRD profiles of Fig. 4 (i, j) indicate the as-cast HEA is in single-phase FCC structure without intermetallic formation and BCC phases because the atomic radius and electronegativity of the five elements are quite similar. However, there are peaks splitting for oxidized sample which match Curich phase corresponding to (111), (200) and (220) planes in order, revealing the aggravating segregation of copper elements during high temperature process. In the case of oxidized Al0.5CoCrCuFeNi sample, the diffraction peak 2θ = 44.6° at (110) plane is identified as BCC phase
n
VEC =
ci (VEC )i i=1
(1)
Where ci and (VEC )i are atomic percentage and VEC for the individual element. The phase tends to be FCC structure at VEC≥8, and BCC 2
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Fig. 3. The SEM images of cross-section of as-cast Al1CoCrCuFeNi with element mappings.
Fig. 4. The XRD patterns of as-cast AlxCoCrCuFeNi (x = 0, 0.5, 1, 1.5, 2) HEAs and samples oxidized at 1000°C for 100 h with the magnification view.
in Fig. 1. (h) with further increasing aluminum content. When the molar mass of Al ratio increases to 1, XRD pattern affirms that the principal phases transform from FCC structure to BCC structure, and Wang et al. obtained the similar result [31]. Compared to the as-cast sample, FCC structure of CuNi-rich phase starts to emerge at oxidized counterpart and the main peak shifts from 44.5° to 43.9°. Al1.5 and Al2 consist of a majority of BCC phase with few FCC phase. After high temperature oxidation, phase identification of AlNi-rich can be acquired in this work for both Al1.5 and Al2. This may attribute to the reason that vacancy-induced segregation evolution of HEAs is depended on the atomic size of the alloy elements, in which oversized atoms diffusing faster while undersized atoms diffusing slower [32] and aluminum atom diameter is larger than others counterpart. Addition of Al engenders influence not only on as-cast microstructures, but also element diffusion at high temperature. The segregation and diffusion at high temperature contribute to a small quantity of FCC matrix phase, which will be certified by later EPMA mapping.
Fig. 5 compares the SEM microstructure of AlxCoCrCuFeNi (x = 0, 0.5, 1, 1.5, 2) after 1000°C-100 h high temperature oxidation with compositions fraction of dendrite and inter-dendrite regions below the SEM images. All the five HEAs share one common point that the dendrite mostly made up of copper because the mixing enthalpies between Cu and Al, Co, Cr, Fe and Ni are −1, +6, +12, +13 and + 4 respectively. The positive mixing enthalpies encourage copper to segregate in the inter-dendritic region and are incompatible with Co, Cr and Fe [33]. In the annealing process three typical structures including CuNi-rich FCC structure, AlNi-rich plate and FeCr-rich plate are observed with the occurrence of spinodal decomposition [34]. When the HEA contains higher Al content, the phenomenon is more obvious as shown in Fig. 9. 3.2. Oxides and microstructures after oxidation Fig. 6 shows the XRD patterns of AlxCoCrCuFeNi (x = 0, 0.5, 1, 1.5,
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Fig. 5. The micrographics of the oxidized samples: (a) Al0CoCrCuFeNi; (b) Al0.5CoCrCuFeNi; (c) Al1CoCrCuFeNi; (d) Al1.5CoCrCuFeNi; (e) Al2CoCrCuFeNi. The results of EPMA point analysis for each alloy are also presented.
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Fig. 6. The XRD patterns of oxidation scale of AlxCoCrCuFeNi (x = 0, 0.5, 1, 1.5, 2) HEAs at 1000 °C for 100 h.
2) HEAs after high temperature oxidation at 1000°Cfor 100 h. There are approximately four types of oxides on the surface of alloys: Al2O3, Cr2O3, spinel oxides (molecular formula uniformly written as (Co,Ni,Cu)(Al,Cr,Fe)2O4) and simple oxides ((Co,Ni,Cu)O). In the case of Al0CoCrCuFeNi, the primary oxides are Cr2O3 with small amount of spinel oxides and (Co,Ni,Cu)O simple oxides. The intensity of substrate diffraction peak seems to be weak because the thickness of oxidation layer is up to 6-7µm , as is shown in Fig. 8. Actually the thickness of the oxide layer is much thicker than observed in the EPMA image, which is due to the poor adhesion and the excessive spallation of the Cr2O3based oxide layer. When the Al content increases to 0.5, Al2O3 peak begins to appear in the XRD patterns. But the lack of Al element beneath the scale cannot generate adequate Al2O3 to impede oxidation as a result of the formation of simple oxides [35]. The phenomenon of exfoliation occurs continuously in the process of oxidation. With the concentration of Al increasing (Al1, Al1.5), the diffraction peak maximum of Al2O3, Cr2O3 and spinel oxides adds up gradually. However, in the case of Al2CoCrCuFeNi, substrate diffraction peaks are more obvious than the peaks of oxides. During the thermo-gravimetric analysis, the samples cool down and then oxides scale peels off due to differences in thermal expansion coefficients (CTEs), which is explained by Chou [24]. The oxidation morphologies of AlxCoCrCuFeNi (x = 0, 0.5, 1, 1.5, 2) HEAs are presented in Fig. 7, which shows complete difference between Al0CoCrCuFeNi and AlxCoCrCuFeNi (x = 0.5, 1, 1.5, 2) HEAs. As is shown in Fig. 7(a) and (b), the oxide layer of Al0CoCrCuFeNi contributes to a loose and hollow structure which leads to a large area of spallation (light color region in Fig. 7(a)) and unsatisfying high temperature resistance performance. When the content of Al increases, the
spallation gets relieved owing to that the surface generates compact Al2O3 oxide layer. In the case of Al0.5CoCrCuFeNi and Al1CoCrCuFeNi, the oxide scales has fewer oxidation hollows and pits because of the oxide peeling and new oxidation growing simultaneously. Compared to Al0.5 and Al1, the scale shows plenty of dense white granular Al2O3, which indicates that the rapid diffusion of Al in the substrate to the outer surface of the oxide film. This results in the expansion of granular alumina into an alumina film. During high temperature oxidation, the surface of Al2 HEA produces a large number of needle-like metastable transition aluminum as shown in Fig. 7(j) on account that this transition from metastable to steady Al2O3 is relevant with oxidation time and temperature. 3.3. The cross section after oxidation The EPMA element mapping conducted on the Al0CoCrCuFeNi HEA cross section which is oxidized for 100 h at 1000 °C is presented in Fig. 8, with XRD pattern presented in Fig. 4(b). It is clear to observe that the interface of the substrate-scale is not smooth any more after oxidation. This is caused by that only Cr without Al composition for Al0CoCrCuFeNi is difficult to establish compact oxidation scale. Hence, the external layer is consisting of hollow and loose Cr2O3 which possesses poor adhesion to result in spallation according to aforementioned discussion. In the oxidation process the diffusion of Al and Cr plays the critical role and the sluggish diffusion can be partly responsible for the apparent stability of HEAs [36]. The high temperature allows Cr element to diffuse slowly and steadily from substrate to scale surface, and then react with oxygen to form oxides directly, which contributes to a notable Cr depletion region with average depth of 14 µm beneath the
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Fig. 8. The BSE images of cross-section of Al0CoCrCuFeNi after oxidation at 1000 °C for 100 h with EPMA element mapping.
no Al and Cr depletion regions beneath the surface of substrate for ascast Al1. Besides the homogeneity are proved by Fig. 2 as well. The Cr2O3 and -Al2O3 have extremely slow growth rate and each of parabolic oxidation constants are 2×10−11mg2cm−4s−1 and 7×10−11mg2cm−4s−1 respectively [28]. Thus, the Al content decreases from outside to inside of oxidation layer with the increasing Cr element, while the other elements composition are slightly higher than substrate, which indicates that Al2O3 comprises the external scale and the internal layer is mainly based on Cr2O3 and spinel oxides. Besides, Cr element can provide Al with sites for reaction with oxygen ions. It is interesting that the oxidation layers of Al0.5, Al1, Al1.5 and Al2 HEAs are roughly same with the thickness of 4 µ m. It may be as a result of that there are at least two phases beneath the scale due to addition of Cu, which generates a huge difference between the thermal coefficient of expansion of alumina and HEAs substrate [37,38]. Consequently, a high stress within substrate and alumina is formed and induces the oxidation layer to break when the samples cools down. In the case of Al0.5 and Al1 HEAs, dendrites are basically AlNi-rich BCC phase and Cu-rich FCC phase that are supported by EPMA and XRD patterns. Moreover, the Al depletion region of Al0.5 and Al1 is much deeper than Al1.5 and Al2. The dominant phases of Al1.5 and Al2 are Al-rich and AlNi-rich phase with uniform dispersion of copper and matrix FCC phase.
Fig. 7. The SEM images of oxidation scale of AlxCoCrCuFeNi (x = 0, 0.5, 1, 1.5, 2) HEAs at 1000 °C for 100 h: (a-b) Al0CoCrCuFeNi; (c-d) Al0.5CoCrCuFeNi; (ef) Al1CoCrCuFeNi; (g-h) Al1.5CoCrCuFeNi; (i-j) Al2CoCrCuFeNi.
surface as presented in chromium mapping. EPMA element mapping clearly presents that the external oxidation scale mostly consists of Cr2O3 and the internal counterpart is made up of simple oxides ((Co,Ni,Cu)O) and few spinel oxides. The main CuNi-rich phase and matrix phase are verified in EPMA mapping with corresponding XRD result of Fig. 4(a). Different from Al0CoCrCuFeNi HEA, the oxidation layer and element distribution of Al0.5, Al1, Al1.5 and Al2 HEAs have diverse performance with the addition of Al composition, as exhibited in Fig. 9. Through comparing the cross section of as-cast Al1CoCrCuFeNi in Fig. 3 with oxidized sample, there are well-distributed element mappings and
3.4. The oxidation kinetics and mechanisms The oxidation kinetics is approximated using a single-stage parabolic law in a form for all studied alloys:
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m S
2
= 2KP t + C
(2)
where Δm is the sample gained weight by oxidation, S is the surface area, Δm/S for the oxidation gained weight per unit area. Kp is the parabolic rate constant, t is the oxidation time, C is a constant. The oxidation kinetics measurements of AlxCoCrCuFeNi (x = 0, 0.5, 1, 1.5, 2) after 100 h, 1000 °C high temperature are presented in Fig. 10 and corresponding Kp value in Fig. 11. The value of KP decreases with the increase of Al content based on aforementioned analysis, which means superior high temperature resistance. Al and Cr elements are inclined to react with oxygen ions in elevated temperature environment. Therefore, the two elements are firstly oxidized with their oxides covering the surface. When the HEA contains no Al composition or only a small molar mass, Cr2O3 becomes the main oxidation with other simple oxides to prevent the subsequent from oxidation. While the Al content continues to increasing, Al2O3 begins to replace Cr2O3 in the oxidation layer because the growth rate of Cr2O3 is slower than that of alumina. There is a premier Al depletion region beneath the oxidation scale, and the outer scale layer forms aluminum, while the inner layer is dominated by chromium and other spinel oxides. This process is mainly dependent on the diffusion of Al and Cr element. 4. Conclusion The influence of Al content on the microstructures, oxidation behaviors and high temperature oxidation resistance of AlxCoCrCuFeNi high entropy alloys (x = 0, 0.5, 1, 1.5, 2) were discussed. The conclusions can be summarized: 1) The principal CuNi-rich and matrix FCC phase transformed into BCC structure with the increase of Al content for arc-melted AlxCoCrCuFeNi HEAs. After 100 h, 1000 °C oxidation Cu element formed plenty of dendrites or granule segregations due to its positive mixing enthalpies with other elements. AlNi-rich plate and FeCr-rich plate were observed with the occurrence of spinodal decomposition.
Fig. 9. The BSE images of cross-section of AlxCoCrCuFeNi (x = 0.5, 1, 1.5, 2) after oxidation at 1000 °C for 100 h with EPMA element mapping (the same scale bars with BSE images in the individual mapping).
Fig. 10. Mass gain curves measured of AlxCoCrCuFeNi (x = 0, 0.5, 1, 1.5, 2) oxidized at 1000°C for 100 h.
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Fig. 11. The parabolic plots of AlxCoCrCuFeNi (x = 0, 0.5, 1, 1.5, 2) oxidized at 1000°C for 100 h.
2) Al2O3, Cr2O3, spinel oxides and simple oxides were verified in the oxidation process. In case of Al0CoCrCuFeNi, the oxide layer consisting of Cr2O3 shaped a loose and hollow structure which led to a large area of spallation. When the Al content increased, the spallation got relieved owing to compact Al2O3 scale which comprised the external scale and the internal layer was mainly based on Cr2O3 and spinel oxides.
3) Al element played an important role in high temperature resistance. The oxidation kinetics of the Al2CoCrCuFeNi alloys possessed the smallest KP value and revealed the best oxidation resistance of five HEAs. The parabolic constants KP value decreased with the increase of the Al content.
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Acknowledgments [17]
The authors are grateful for funding from the Fundamental Research Funds for the Central Universities (2018GF05), Natural Science Foundation of China (51771226) and the State Key Laboratory of Solidification Processing at NWPU (SKLSP201818).
[18] [19]
Appendix A. Supplementary data
[20]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.vacuum.2019.108837.
[21]
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