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Microstructure and its high temperature oxidation behavior of W-Cr alloys prepared by spark plasma sintering
Materialia 6 (2019) 100332
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Microstructure and its high temperature oxidation behavior of W-Cr alloys prepared by spark plasma sintering Qing-Qing Hou a, Ke Huang a, Lai-Ma Luo a,b,∗, Xiao-Yue Tan a,b, Xiang Zan a,b, Qiu Xu c, Xiao-Yong Zhu a,b, Yu–Cheng Wu a,b,d,∗ a
School of Materials Science and Engineering, Hefei University of Technology, Hefei 230009, China Laboratory of Nonferrous Metal Material and Processing Engineering of Anhui Province, Hefei 230009, China c Institute for Integrated Radiation and Nuclear Science, Kyoto University, Osaka-fu 590-0494, Japan d National-Local Joint Engineering Research Centre of Nonferrous Metals and Processing Technology, Hefei 230009, China b
a r t i c l e
i n f o
Keywords: W-Cr alloy Semi-coherent second phase Oxidation behavior
a b s t r a c t W-10 wt.% Cr (W-10Cr) and W-20 wt.% Cr (W-20Cr) alloys were fabricated through mechanical alloying and spark plasma sintering technology. The microstructure of W-Cr alloys was characterized by XRD, SEM, TEM. The W-20Cr alloy significantly increased the microhardness from 333.5 HV of pure W to 961.3 HV. Oxidation experiments at 800 °C and 1000 °C confirmed that W-20Cr alloy shows better oxidation resistance. A suitable amount of Cr content is favorable for the formation of the oxide scale Cr2 O3 and the quality assurance at the initial stage of oxidation, which is able to slow down the oxidation rate. The quality of the oxide scale CrWO4 is better than that of Cr2 WO6 , indicating that the formation of the oxide scale CrWO4 is beneficial to weaken the further oxidation of the W-20Cr alloy.
1. Introduction As the metal material with the highest melting point, W has the advantages of high density, good high temperature strength, excellent thermal conductivity, low thermal expansion coefficient and high anti-nuclear radiation. It is widely used in military defense, aerospace, electronics industry, radiation shielding and other fields [1–5]. In addition, due to its good resistance to particle sputtering and low hydrogen (H/D/T) retention, W is considered to be the first candidate for direct plasma-oriented devices for future nuclear fusion devices [6–8]. However, taking into account accidents that may be caused by various factors ——Loss-of-Coolant Accidents (LOCA) [9,10]. The occurrence of LOCA is accompanied by air and water vapor entering the vacuum chamber, which will cause the W-based material of the first wall to oxidize to a volatile WO3 . After neutron irradiation, W contains a certain amount of radioactivity, and radioactivity can enter the atmosphere through the volatility of WO3 , thereby causing human harm [11–13]. In order to address the risk of nuclear leakage after the occurrence of LOCA, a self-passivating W alloy was proposed [14]. The self-passivating W alloy prevents the oxidation of W by forming a dense protective scale on the surface of the sample by adding alloying elements, thereby avoiding the danger of nuclear radioactive leakage caused by the volatilization
∗
of WO3 [15,16]. With the addition of chromium (Cr) [17], silicon (Si) [18], and titanium (Ti) [19,20] as passivation elements, compact passivated layer, which prevented further oxidation by acting as effective barrier against ionic migration, was formed preferentially on the surface of the material during high temperature oxidation. Satisfactory results have been obtained with alloys prepared by magnetron sputtering [16,21,22]. However, the layer produced by this method is very thin and therefore not suitable for use in a fusion reactor. In addition, the Cr element can also be dissolved in the W lattice by mechanical alloying. Mechanical alloying is a solid powder processing technique that allows tailoring the grain size, formation of dense dislocation structures and formation of full solid solution with elements that possess otherwise limited solubility in tungsten lattice [16,23,24]. However, subsequent powder consolidation by conventional methods often results in the decomposition of the solid solution into the second phase [24]. As processing by conventional sintering methods such as HIP leads to the development of a Cr-rich phase [11,24,25]. Therefore, current efforts have focused on rapid sintering techniques such as spark plasma sintering. Spark plasma sintering (SPS) technology can significantly inhibit grain coarsening due to its characteristics of rapid sintering and densification, and obtain a material with high density and uniform microstructure [26–28]. Vilémová et al. demonstrated that spark plasma sintering is capable to preserve W-Cr solid solution prepared by
Corresponding authors at: School of Materials Science and Engineering, Hefei University of Technology, Hefei 230009, China. E-mail addresses: [email protected] (L.-M. Luo), [email protected] (Y. Wu).
https://doi.org/10.1016/j.mtla.2019.100332 Received 26 November 2018; Accepted 18 April 2019 Available online 24 April 2019 2589-1529/© 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
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mechanical alloying, and no chromium was expelled to form Cr-rich phase [11]. On the one hand, according to W-Cr binary alloy phase diagram, W-Cr alloy is an isomorphous system with a miscibility gap at below 1677 °C [29]. On the other hand, the Cr oxidation activity is much active than that of W. W alloyed with the active Cr is preferentially oxidized to form a compact and continuous film Cr2 O3 . Thus, doping W-based alloy with Cr is an effective approach. Consequently, developing W-Cr based alloys with good overall performance has become a hot topic in research. In recent years, the antioxidant properties of W-Cr alloys have been widely studied. Telu [17] fabricated Wx -Cr1− x (x = 0.3, 0.5, 0.7) binary alloys by mechanical alloying (MA) and conventional sintering. Cyclic oxidation experiments revealed that the antioxidant property of W-Cr alloys depends on the diffusion rate of O2− and Cr3+ ions. The fast outward diffusion of Cr3+ ions combine with O2− ions to form Cr2 O3 and Cr2 WO6 , which act as a barrier to the inward O2− ions and significantly improves the antioxidant properties of the material. Similar results are presented in other reports [30–32]. Many studies on W-Cr alloys with low Cr concentration focus on their mechanical properties. Zhou et al. [33] indicated that the occurrence of spinodal decomposition in W-5 wt.% Cr alloys after annealing contributes to nanostructure formation, which significantly improves the mechanical properties. In the present work, W-Cr alloys with high Cr concentration were fabricated through MA and SPS. The effect of microstructure on the oxidation resistance of W-Cr alloy and the oxidation behavior of the alloy were investigated. 2. Experimental Pure W, W-10 wt.% Cr (W-10Cr), and W-20 wt.% Cr (W-20Cr) alloys were fabricated from raw W and Cr powders. Cr powder with 0.8– 4.2 μm grain diameter and W powder with 1.2–4.0 μm grain diameter were used. These powders were milled in a planetary mill for 20 h at a ball-to-powder weight ratio of 10:1 and rotation speed of 400 rpm. The material of jar and balls is of WC. The samples were consolidated at 1400 °C by SPS (SE-607, Germany). After sintering, phase composition was characterized by X-ray diffraction (XRD, X’Pert PROMPD, Netherlands). The fracture is artificially cut off with pliers at room temperature. And the surface morphology was metallographic polished for observing. The fracture and surface morphologies were examined with field emission scanning electron microscope (FE-SEM, ZEISS SIGMA, Germany) and energy dispersive spectroscopy (EDS). The density of the sintered samples was measured via the Archimedes method. The polished sintered samples were subsequently subjected to Vickers microhardness testing by MH-3 L; microhardness was measured from the center to the edges of the sample at a load of 300 gf and a dwelling time of 10 s. Each of the investigated samples was exposed to the synthetic air, which contained 20 vol.% of O2 and 80 vol.% of N2 at 800 °C and 1000 °C for 8 h (eight cycles) in a tube furnace (GSL-1700X, MTI- Group, Hefei, China). Commercial rolled pure W was as a contrast material. The cyclic oxidation behavior of the commercial rolled pure W and W-Cr alloys at different temperatures was studied by recording the variation of weight gain per unit surface area (Δm/A) with time (t). The exposed alloy samples that were the most resistant to oxidation according to the kinetic study were cross-sectioned, cold mounted using resins, and metallographically polished for FE-SEM observation. 3. Results and discussion 3.1. Sintered samples characterization XRD was used to determine the constituent phases of pure W, W10Cr and W-20Cr alloys (Fig. 1). Examination of these patterns confirms the presence of 𝛼(W, Cr) and 𝛼(Cr, W) phases in W-10Cr and W20Cr alloys. These patterns depict broadening of 𝛼(W, Cr) peaks with
Materialia 6 (2019) 100332
Fig. 1. XRD patterns of the pure W and W-Cr sintered alloys. Table 1 The real density, theoretical density, relative density and Vickers hardness of the sintered composites. Composites
Theoretical density (g/cm3 )
Experiment density (g/cm3 )
Relative Density
Vickers hardness (Hv)
Pure W W-10%Cr W-20%Cr
19.35 16.68 14.46
17.53 15.91 13.73
90.6% 95.4% 95.0%
333.5 871.8 961.3
reduction in their intensities, which may be attributed to the refinement of W-Cr crystallite size in the powders subjected to ball milling. Fig. 2 shows the SEM surface images of the sintered samples. For pure W, the sintering temperature at 1400 °C was insufficient for complete densification, and many pores were also present in its surface morphology (Fig. 2a). The surface morphologies of the W-Cr alloys showed no pores. Thus, chromium addition promoted the sintering densification process. The combined SEM images are presented in Fig. 2b and c. In addition to the W-rich phase and the Cr-rich phase corresponding to the XRD results, black particles are also distributed in the surface morphology. The black particles corresponded to the Cr oxide phases in both W-10Cr and W-20Cr alloys. This result indicated that the reduced Cr captured oxygen impurities in the W to form the oxide during the sintering process. Fig. 3 displays the SEM images of the fracture surface of the pure W, W-10Cr, and W-20Cr samples at room temperature. As shown in Fig. 3a and in conjunction with Fig. 2a, there are multiple smaller pores at the pure W grain boundary. In contrast, W-Cr alloys show fewer pores in Fig. 3b and c. The results are also consistent with Fig. 2b and c. In terms of crack propagation mode, the fracture modes of pure W and W-10Cr alloys appear as intergranular fracture modes, and the fracture appearance is straight, as shown in Fig. 3a–c shows the fracture surface of the W-20Cr composite, which is a typical transgranular fracture mode as seen from the surface topography. W-based materials are brittle materials at room temperature, and they possess weak grain boundaries. When the W-based material breaks, the microcracks preferentially expand at the grain boundary, which reflects intergranular fracture under macroscopic view. The fracture mode was changed from intergranular to transgranular by increasing the Cr content from 10 wt.% to 20 wt.%. Table 1 shows the densities and Vickers hardness of the different samples. The densities and Vickers hardness of the sintered sample were dependent on the Cr content. Chromium addition promoted the
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Materialia 6 (2019) 100332
Fig. 2. The surface morphology of the sintered samples, (a) pure W, (b) W-10Cr, (c) W-20Cr. Fig. 3. The fracture morphology of the sintered samples, (a) pure W, (b) W-10Cr, (c) W-20Cr.
Fig. 4. Plots of Δm/A against t obtained from cyclic oxidation tests carried out on pure W and W-Cr alloys for duration of 8 h, (a) at 800 °C; (b) at 1000 °C. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
sintering densification and significantly improved the hardness of the Wbased alloys. The highest average Vickers microhardness value among those of the W alloys was observed in the W-20Cr sintered sample (approximately 961.3 HV). The microhardness value of pure W, with a relative density of 98%, is 412 HV, as reported in other literature [34]. The addition of Cr forms a solid solution with W and achieves the effect of solid solution strengthening. As the Cr content of the solute atoms increases, the hardness of the W-based alloy remarkably improves.
3.2. Oxidation result The oxidation results after exposure to 800 °C and 1000 °C for 8 h are depicted in Fig. 4. The weight gain values obtained for the tests at different temperatures are presented in Table 2. Pure W was severely
oxidized, but the W-Cr alloy samples were not almost oxidized. From the weight gain curve, the slope of the pure W weight gain curve is much larger than the W-Cr alloy. For the surface appearances after oxidation, the pure W surface was a yellow oxide layer (yellow tungsten, WO3 ). After oxidation at 800 °C for 8 h in an atmosphere of 20 vol.% of O2 and 80 vol.% of N2 , the W-Cr alloy surfaces still achieved a metallic luster without a significant oxide layer. The self-passivation effect of W-20Cr alloy is much better than that of pure W, and the weight gain is only 0.3 × 10−3 mg.cm−2 . When the sample was exposed to an oxidation test at 1000 °C, the W-Cr alloy was rapidly oxidized and an oxide layer was formed on the surface. However, the oxidation weight gain was less than that of pure W. The weight gain result of W-20Cr alloy was only 1/10 of pure W. First, the oxidation curve combined with the weight gain data, the anti-oxidation behavior of W-20Cr alloy is higher than W-10Cr alloy. It
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Fig. 5. XRD patterns of the W-Cr oxidized alloys, (a) W-10Cr alloy; (b) W-20Cr alloy.
Fig. 6. The surface morphology of the oxidized sample, (a) W-10Cr alloy; (b) W-20Cr alloy.
Table 2 Oxidation weight gain (mg.cm−2 ) for pure W and W-Cr alloys at different temperatures. Alloy
Temperature (°C)
Weight gain, △m /A (mg.cm−2)
Pure W W-10Cr W-20Cr Pure W W-10Cr W-20Cr
800 800 800 1000 1000 1000
0.40 1.4 × 10−3 0.3 × 10−3 130.41 19.25 12.35
shows that with the increase of Cr content, the behavior of oxidation resistance of W-Cr alloy is effectively improved. Second, the W-20Cr alloy is almost unoxidized at temperatures of 800 °C, compared to pure tungsten. When the temperature was 1000 °C, a positive 10-fold decrease in the oxidation weight gain of W-20Cr alloy was achieved compared with that of pure tungsten. When the temperature is raised from 800 °C to 1000 °C, the oxidation resistance of the W-Cr alloy is significantly decreased. Given that the W-Cr alloy’s oxidation weight gain was largely small at 800 °C, and no significant difference in oxidation weight gain
was observed between W-10Cr and W-20Cr, the samples oxidized for 8 h at 1000 °C were selected for the oxidation mechanism study. The XRD patterns obtained from the oxidized samples of the W-Cr alloys exposed for 8 h at 1000 °C are shown in Fig. 5. The oxide layer phase of the W-10Cr alloy was mainly composed of Cr2 WO6 , WO3 , and CrWO4 . The W-20Cr alloy phase consisted of Cr2 WO6 , WO3 , CrWO4 , and Cr2 O3 . Cr2 O3 was only present in the W-20Cr alloy oxide layer, which played an important role in the oxidation resistance of materials. Fig. 6 shows the SEM images of sample surface exposed to a W-Cr alloy at 1000 °C for 8 h. W-10Cr alloy has many protrusions and protrusions cracked on the surface after oxidation (Fig. 6a). In addition, some slope-like oxides are present on the surface of the alloy. According to the EDS spectrum and XRD results, the protrusion-like oxide and the slope-like oxide were identified as WO3 and Cr2 WO6 , respectively. The W-rich phase present in the sintered W-Cr alloy forms a large amount of WO3 during oxidation, and the considerable growth stress of WO3 results in the formation of a protrusion-like structure. As the oxidation progresses, the WO3 protrusion continues to be oxidized and then grows up until the crack breaks. This promotes further oxidation of the material. The surface of the W-20Cr alloy after oxidation has a part of the protrusions and cracked protrusions of WO3 , as well as a relatively
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Materialia 6 (2019) 100332
Fig. 7. Cross-section viewing of the oxidized W-10Cr alloy after 8 h of oxidation in 80 vol.% Ar + 20 vol.% O2 at 1000 °C: (a) low magnification image; (b) EDS elemental mapping of (a); (c) and (d) high magnification image. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
complete planar oxide layer and a granular oxide which has been ruptured by the surface layer (Fig. 6b). The planar oxide is mainly Cr2 O3 , and the granular oxide is Cr2 WO6 . Compared with WO3 and Cr2 WO6 , the Cr2 O3 oxide layer is relatively dense. This may be one of the reasons why the oxidation resistance of W-20Cr alloy is better than that of W-10Cr alloy. 3.3. Oxidation behavior The SEM images of the oxide scale cross-sections (Figs. 7 and 8) display the alloy oxide interfaces in the W-Cr alloy samples exposed at 1000 °C for 8 h. The thickness of the W-10Cr alloy oxide layer is about 185 μm as shown in Fig. 8a. According to the results of SEM and EDS, the scale is composed of a W-Cr-O region and a W-O region. Combined with the results of XRD, the W-Cr-O region and the W-O region are mainly Cr2 WO6 and WO3 , respectively. In addition, a large number of transverse cracks (marked by red arrows) were found in the W-Cr-O mixed region; these cracks separated the oxide layer from the substrate and exerted a significant adverse effect on the oxidation resistance of the materials. In the W-O region of the subsurface, numerous voids and longitudinal cracks were observed (Fig. 7c). Fig. 8a shows that the W-20Cr alloy oxide layer has a thickness of about 150 μm, which is thinner than the scale of the W-10Cr alloy. Dif-
ferent from the oxide layer of W–10Cr alloy, the surface of the W–20Cr oxide layer was covered with a protective Cr2 O3 scale. Only a small amount of Cr2 O3 protective layer can be observed in the oxide crosssection, which may be related to the Cr content. The entire oxide layer was also mainly composed of a W-Cr-O mixed region. Confirmed by XRD and EDS surface scanning, W-Cr-O mixed area is mainly CrWO4 . There is a small number of cracks in the upper part of the CrWO4 oxide layer, and the pores penetrate almost the entire oxide layer. Figs. 7d and 8d are W-Cr substrates after oxidation of W-10Cr alloy and W-20Cr alloy, respectively. Among them, the black enrichment zone is mainly Cr2 O3 , and it is obvious that there are more enrichment zones of W-20Cr alloy, which should be the reason of Cr content. The Cr element acts as a passivating element to effectively improve the oxidation resistance of the W alloy. The quality of the scale can reflect the oxidation resistance of the material. Cr is more oxytropism than W. In the initial stage of oxidation, the W-Cr alloys first form protective scale Cr2 O3 . As the oxidation experiment continues, the Cr ion supply of the W-10Cr alloy is insufficient. The external O ions continue to diffuse and pass through the scale, causing internal oxidation to form WO3 . WO3 reacts with Cr2 O3 and O2 to form Cr2 WO6 . The oxide scale Cr2 WO6 is very loose in appearance and there are a large number of transverse cracks, indicating that the scale quality is very poor, and it is difficult to achieve the protection material from further oxidation. Therefore, the
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Materialia 6 (2019) 100332
Fig. 8. Cross-section viewing of the oxidized W-20Cr alloy after 8 h of oxidation in 80 vol.% Ar + 20 vol.% O2 at 1000 °C: (a) low magnification image; (b) EDS elemental mapping of (a); (c) and (d) high magnification image.
interior of the W-10Cr alloy is further oxidized to form WO3 . In contrast, W-20Cr alloys have higher Cr content than W-10Cr alloys. The Cr ion supply is sufficient during the oxidation process, so that a betterquality protective scale Cr2 O3 can be formed. A good scale can inhibit the external O ions from passing through the scale and weaken the further oxidation of the alloy. Continuous oxidation further consumes Cr ions, and W reacts with Cr2 O3 and O2 to form CrWO4 . From the aspect of appearance, the oxide scale CrWO4 has pores and a small number of cracks, and the overall quality is better than that of Cr2 WO6 , which can also play a certain protective role. If the W-20Cr alloy continues to be oxidized at a high temperature, the scale Cr2 WO6 will be formed. As the oxidation continues, the scale will crack or even fall off, and the O ions will easily pass through the scale to further oxidize the internal material.
3.4. Probable oxidation reactions On the basis of the oxidation results, we constructed a schematic of the oxidation process, which corresponded to the possible chemical reaction. This schematic also describes the oxidation process and the mechanism of the W-Cr alloy (Figs. 9 and 10). In the initial stage of W-10Cr alloy oxidation process, the chemical reactions that occurred at the top surface of the oxide layer (Fig. 9b)
were as follows: T = 1273 K 2Cr (s) + (3∕2)O2 (g) → Cr2 O3 (s);
(1)
W(s)+(3∕2)O2 (g) → WO3 (s).
(2)
In the late stage of the oxidation process, the following chemical reactions occurred in the oxide layer, as shown in Fig. 10c: W(s) + Cr2 O3 (s) + (3∕2)O2 (g) → Cr2 WO6 (s);
(3)
Cr2 O3 (s) + WO3 (g) → Cr2 WO6 (s).
(4)
In the initial stage of W-20Cr alloy oxidation process, the chemical reaction that occurred at the top surface of the oxide layer, as shown in Fig. 10b, was as follows: Reaction (5) and (1) are identical.
(5)
In the late stage of the oxidation process, the chemical reactions that occurred in the oxide layer (Fig. 10c) were as follows: Reaction (6) and (2) are identical.
(6)
W(s) + (1∕2)Cr2 O3 (s) + (5∕3)O2 (g) → CrWO4 (s);
(7)
Reaction (8) and (3) are identical.
(8)
Reaction (9) and (4) are identical.
(9)
Reaction (10) and (1) are identical.
(10)
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Materialia 6 (2019) 100332
Fig. 9. The oxidation process schematic diagram of W-10Cr alloy, (a) the sintered alloy before oxidation; (b) initial stage; (c) late stage.
Fig. 10. The oxidation process schematic diagram of W-20Cr alloy, (a) the sintered alloy before oxidation; (b) initial stage; (c) late stage.
4. Conclusion In this work, W-Cr alloys were prepared through mechanical alloying and spark plasma sintering technology. The addition of Cr element effectively improves the mechanical properties and oxidation resistance of W-Cr alloy. The W-20Cr alloy significantly increased the microhardness from 333.5 HV of pure W to 961.3 HV. Oxidation experiments at 800 °C and 1000 °C confirmed the effect of Cr on the oxidation resistance of W-Cr alloy. A suitable amount of Cr content is favorable for the formation of the oxide scale Cr2 O3 and the quality assurance at the initial stage of oxidation. The diffusion of O ions is hindered to some extent by the scale of Cr2 O3 , which aims to slow down the oxidation rate. According to the present results, the Cr mass ratio in the W-Cr self-passivation alloys should be higher than 10 wt.%. Otherwise, the low Cr content in W-Cr alloys will cause many adverse effects. On the one hand, the Cr concentration in the W-Cr alloy is very low, which results in a low chromium content in the tungsten lattice. Therefore, the W-rich phase is larger. W-rich phase exerts adverse effect on the antioxidant property of W-Cr alloys. On the other hand, the supply of the Cr element is insufficient to form a dense Cr2 O3 oxide layer at the initial stage of oxidation. This promotes the formation of WO3 and the occurrence of internal oxidation. The oxide scale of W-20Cr alloy is denser, with less cracks and less thickness compared to W-10Cr. The quality of the oxide scale CrWO4 is better than that of Cr2 WO6 , indicating that the formation of the oxide scale CrWO4 is beneficial to weaken the further oxidation of the W-20Cr alloy. Acknowledgments This work is supported by National Natural Science Foundation of China (Grant no. 51574101 and 51474083), The National Key Research and Development Program of China (No.
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Full Length Article
Microstructure and its high temperature oxidation behavior of W-Cr alloys prepared by spark plasma sintering Qing-Qing Hou a, Ke Huang a, Lai-Ma Luo a,b,∗, Xiao-Yue Tan a,b, Xiang Zan a,b, Qiu Xu c, Xiao-Yong Zhu a,b, Yu–Cheng Wu a,b,d,∗ a
School of Materials Science and Engineering, Hefei University of Technology, Hefei 230009, China Laboratory of Nonferrous Metal Material and Processing Engineering of Anhui Province, Hefei 230009, China c Institute for Integrated Radiation and Nuclear Science, Kyoto University, Osaka-fu 590-0494, Japan d National-Local Joint Engineering Research Centre of Nonferrous Metals and Processing Technology, Hefei 230009, China b
a r t i c l e
i n f o
Keywords: W-Cr alloy Semi-coherent second phase Oxidation behavior
a b s t r a c t W-10 wt.% Cr (W-10Cr) and W-20 wt.% Cr (W-20Cr) alloys were fabricated through mechanical alloying and spark plasma sintering technology. The microstructure of W-Cr alloys was characterized by XRD, SEM, TEM. The W-20Cr alloy significantly increased the microhardness from 333.5 HV of pure W to 961.3 HV. Oxidation experiments at 800 °C and 1000 °C confirmed that W-20Cr alloy shows better oxidation resistance. A suitable amount of Cr content is favorable for the formation of the oxide scale Cr2 O3 and the quality assurance at the initial stage of oxidation, which is able to slow down the oxidation rate. The quality of the oxide scale CrWO4 is better than that of Cr2 WO6 , indicating that the formation of the oxide scale CrWO4 is beneficial to weaken the further oxidation of the W-20Cr alloy.
1. Introduction As the metal material with the highest melting point, W has the advantages of high density, good high temperature strength, excellent thermal conductivity, low thermal expansion coefficient and high anti-nuclear radiation. It is widely used in military defense, aerospace, electronics industry, radiation shielding and other fields [1–5]. In addition, due to its good resistance to particle sputtering and low hydrogen (H/D/T) retention, W is considered to be the first candidate for direct plasma-oriented devices for future nuclear fusion devices [6–8]. However, taking into account accidents that may be caused by various factors ——Loss-of-Coolant Accidents (LOCA) [9,10]. The occurrence of LOCA is accompanied by air and water vapor entering the vacuum chamber, which will cause the W-based material of the first wall to oxidize to a volatile WO3 . After neutron irradiation, W contains a certain amount of radioactivity, and radioactivity can enter the atmosphere through the volatility of WO3 , thereby causing human harm [11–13]. In order to address the risk of nuclear leakage after the occurrence of LOCA, a self-passivating W alloy was proposed [14]. The self-passivating W alloy prevents the oxidation of W by forming a dense protective scale on the surface of the sample by adding alloying elements, thereby avoiding the danger of nuclear radioactive leakage caused by the volatilization
∗
of WO3 [15,16]. With the addition of chromium (Cr) [17], silicon (Si) [18], and titanium (Ti) [19,20] as passivation elements, compact passivated layer, which prevented further oxidation by acting as effective barrier against ionic migration, was formed preferentially on the surface of the material during high temperature oxidation. Satisfactory results have been obtained with alloys prepared by magnetron sputtering [16,21,22]. However, the layer produced by this method is very thin and therefore not suitable for use in a fusion reactor. In addition, the Cr element can also be dissolved in the W lattice by mechanical alloying. Mechanical alloying is a solid powder processing technique that allows tailoring the grain size, formation of dense dislocation structures and formation of full solid solution with elements that possess otherwise limited solubility in tungsten lattice [16,23,24]. However, subsequent powder consolidation by conventional methods often results in the decomposition of the solid solution into the second phase [24]. As processing by conventional sintering methods such as HIP leads to the development of a Cr-rich phase [11,24,25]. Therefore, current efforts have focused on rapid sintering techniques such as spark plasma sintering. Spark plasma sintering (SPS) technology can significantly inhibit grain coarsening due to its characteristics of rapid sintering and densification, and obtain a material with high density and uniform microstructure [26–28]. Vilémová et al. demonstrated that spark plasma sintering is capable to preserve W-Cr solid solution prepared by
Corresponding authors at: School of Materials Science and Engineering, Hefei University of Technology, Hefei 230009, China. E-mail addresses: [email protected] (L.-M. Luo), [email protected] (Y. Wu).
https://doi.org/10.1016/j.mtla.2019.100332 Received 26 November 2018; Accepted 18 April 2019 Available online 24 April 2019 2589-1529/© 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
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mechanical alloying, and no chromium was expelled to form Cr-rich phase [11]. On the one hand, according to W-Cr binary alloy phase diagram, W-Cr alloy is an isomorphous system with a miscibility gap at below 1677 °C [29]. On the other hand, the Cr oxidation activity is much active than that of W. W alloyed with the active Cr is preferentially oxidized to form a compact and continuous film Cr2 O3 . Thus, doping W-based alloy with Cr is an effective approach. Consequently, developing W-Cr based alloys with good overall performance has become a hot topic in research. In recent years, the antioxidant properties of W-Cr alloys have been widely studied. Telu [17] fabricated Wx -Cr1− x (x = 0.3, 0.5, 0.7) binary alloys by mechanical alloying (MA) and conventional sintering. Cyclic oxidation experiments revealed that the antioxidant property of W-Cr alloys depends on the diffusion rate of O2− and Cr3+ ions. The fast outward diffusion of Cr3+ ions combine with O2− ions to form Cr2 O3 and Cr2 WO6 , which act as a barrier to the inward O2− ions and significantly improves the antioxidant properties of the material. Similar results are presented in other reports [30–32]. Many studies on W-Cr alloys with low Cr concentration focus on their mechanical properties. Zhou et al. [33] indicated that the occurrence of spinodal decomposition in W-5 wt.% Cr alloys after annealing contributes to nanostructure formation, which significantly improves the mechanical properties. In the present work, W-Cr alloys with high Cr concentration were fabricated through MA and SPS. The effect of microstructure on the oxidation resistance of W-Cr alloy and the oxidation behavior of the alloy were investigated. 2. Experimental Pure W, W-10 wt.% Cr (W-10Cr), and W-20 wt.% Cr (W-20Cr) alloys were fabricated from raw W and Cr powders. Cr powder with 0.8– 4.2 μm grain diameter and W powder with 1.2–4.0 μm grain diameter were used. These powders were milled in a planetary mill for 20 h at a ball-to-powder weight ratio of 10:1 and rotation speed of 400 rpm. The material of jar and balls is of WC. The samples were consolidated at 1400 °C by SPS (SE-607, Germany). After sintering, phase composition was characterized by X-ray diffraction (XRD, X’Pert PROMPD, Netherlands). The fracture is artificially cut off with pliers at room temperature. And the surface morphology was metallographic polished for observing. The fracture and surface morphologies were examined with field emission scanning electron microscope (FE-SEM, ZEISS SIGMA, Germany) and energy dispersive spectroscopy (EDS). The density of the sintered samples was measured via the Archimedes method. The polished sintered samples were subsequently subjected to Vickers microhardness testing by MH-3 L; microhardness was measured from the center to the edges of the sample at a load of 300 gf and a dwelling time of 10 s. Each of the investigated samples was exposed to the synthetic air, which contained 20 vol.% of O2 and 80 vol.% of N2 at 800 °C and 1000 °C for 8 h (eight cycles) in a tube furnace (GSL-1700X, MTI- Group, Hefei, China). Commercial rolled pure W was as a contrast material. The cyclic oxidation behavior of the commercial rolled pure W and W-Cr alloys at different temperatures was studied by recording the variation of weight gain per unit surface area (Δm/A) with time (t). The exposed alloy samples that were the most resistant to oxidation according to the kinetic study were cross-sectioned, cold mounted using resins, and metallographically polished for FE-SEM observation. 3. Results and discussion 3.1. Sintered samples characterization XRD was used to determine the constituent phases of pure W, W10Cr and W-20Cr alloys (Fig. 1). Examination of these patterns confirms the presence of 𝛼(W, Cr) and 𝛼(Cr, W) phases in W-10Cr and W20Cr alloys. These patterns depict broadening of 𝛼(W, Cr) peaks with
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Fig. 1. XRD patterns of the pure W and W-Cr sintered alloys. Table 1 The real density, theoretical density, relative density and Vickers hardness of the sintered composites. Composites
Theoretical density (g/cm3 )
Experiment density (g/cm3 )
Relative Density
Vickers hardness (Hv)
Pure W W-10%Cr W-20%Cr
19.35 16.68 14.46
17.53 15.91 13.73
90.6% 95.4% 95.0%
333.5 871.8 961.3
reduction in their intensities, which may be attributed to the refinement of W-Cr crystallite size in the powders subjected to ball milling. Fig. 2 shows the SEM surface images of the sintered samples. For pure W, the sintering temperature at 1400 °C was insufficient for complete densification, and many pores were also present in its surface morphology (Fig. 2a). The surface morphologies of the W-Cr alloys showed no pores. Thus, chromium addition promoted the sintering densification process. The combined SEM images are presented in Fig. 2b and c. In addition to the W-rich phase and the Cr-rich phase corresponding to the XRD results, black particles are also distributed in the surface morphology. The black particles corresponded to the Cr oxide phases in both W-10Cr and W-20Cr alloys. This result indicated that the reduced Cr captured oxygen impurities in the W to form the oxide during the sintering process. Fig. 3 displays the SEM images of the fracture surface of the pure W, W-10Cr, and W-20Cr samples at room temperature. As shown in Fig. 3a and in conjunction with Fig. 2a, there are multiple smaller pores at the pure W grain boundary. In contrast, W-Cr alloys show fewer pores in Fig. 3b and c. The results are also consistent with Fig. 2b and c. In terms of crack propagation mode, the fracture modes of pure W and W-10Cr alloys appear as intergranular fracture modes, and the fracture appearance is straight, as shown in Fig. 3a–c shows the fracture surface of the W-20Cr composite, which is a typical transgranular fracture mode as seen from the surface topography. W-based materials are brittle materials at room temperature, and they possess weak grain boundaries. When the W-based material breaks, the microcracks preferentially expand at the grain boundary, which reflects intergranular fracture under macroscopic view. The fracture mode was changed from intergranular to transgranular by increasing the Cr content from 10 wt.% to 20 wt.%. Table 1 shows the densities and Vickers hardness of the different samples. The densities and Vickers hardness of the sintered sample were dependent on the Cr content. Chromium addition promoted the
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Fig. 2. The surface morphology of the sintered samples, (a) pure W, (b) W-10Cr, (c) W-20Cr. Fig. 3. The fracture morphology of the sintered samples, (a) pure W, (b) W-10Cr, (c) W-20Cr.
Fig. 4. Plots of Δm/A against t obtained from cyclic oxidation tests carried out on pure W and W-Cr alloys for duration of 8 h, (a) at 800 °C; (b) at 1000 °C. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
sintering densification and significantly improved the hardness of the Wbased alloys. The highest average Vickers microhardness value among those of the W alloys was observed in the W-20Cr sintered sample (approximately 961.3 HV). The microhardness value of pure W, with a relative density of 98%, is 412 HV, as reported in other literature [34]. The addition of Cr forms a solid solution with W and achieves the effect of solid solution strengthening. As the Cr content of the solute atoms increases, the hardness of the W-based alloy remarkably improves.
3.2. Oxidation result The oxidation results after exposure to 800 °C and 1000 °C for 8 h are depicted in Fig. 4. The weight gain values obtained for the tests at different temperatures are presented in Table 2. Pure W was severely
oxidized, but the W-Cr alloy samples were not almost oxidized. From the weight gain curve, the slope of the pure W weight gain curve is much larger than the W-Cr alloy. For the surface appearances after oxidation, the pure W surface was a yellow oxide layer (yellow tungsten, WO3 ). After oxidation at 800 °C for 8 h in an atmosphere of 20 vol.% of O2 and 80 vol.% of N2 , the W-Cr alloy surfaces still achieved a metallic luster without a significant oxide layer. The self-passivation effect of W-20Cr alloy is much better than that of pure W, and the weight gain is only 0.3 × 10−3 mg.cm−2 . When the sample was exposed to an oxidation test at 1000 °C, the W-Cr alloy was rapidly oxidized and an oxide layer was formed on the surface. However, the oxidation weight gain was less than that of pure W. The weight gain result of W-20Cr alloy was only 1/10 of pure W. First, the oxidation curve combined with the weight gain data, the anti-oxidation behavior of W-20Cr alloy is higher than W-10Cr alloy. It
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Fig. 5. XRD patterns of the W-Cr oxidized alloys, (a) W-10Cr alloy; (b) W-20Cr alloy.
Fig. 6. The surface morphology of the oxidized sample, (a) W-10Cr alloy; (b) W-20Cr alloy.
Table 2 Oxidation weight gain (mg.cm−2 ) for pure W and W-Cr alloys at different temperatures. Alloy
Temperature (°C)
Weight gain, △m /A (mg.cm−2)
Pure W W-10Cr W-20Cr Pure W W-10Cr W-20Cr
800 800 800 1000 1000 1000
0.40 1.4 × 10−3 0.3 × 10−3 130.41 19.25 12.35
shows that with the increase of Cr content, the behavior of oxidation resistance of W-Cr alloy is effectively improved. Second, the W-20Cr alloy is almost unoxidized at temperatures of 800 °C, compared to pure tungsten. When the temperature was 1000 °C, a positive 10-fold decrease in the oxidation weight gain of W-20Cr alloy was achieved compared with that of pure tungsten. When the temperature is raised from 800 °C to 1000 °C, the oxidation resistance of the W-Cr alloy is significantly decreased. Given that the W-Cr alloy’s oxidation weight gain was largely small at 800 °C, and no significant difference in oxidation weight gain
was observed between W-10Cr and W-20Cr, the samples oxidized for 8 h at 1000 °C were selected for the oxidation mechanism study. The XRD patterns obtained from the oxidized samples of the W-Cr alloys exposed for 8 h at 1000 °C are shown in Fig. 5. The oxide layer phase of the W-10Cr alloy was mainly composed of Cr2 WO6 , WO3 , and CrWO4 . The W-20Cr alloy phase consisted of Cr2 WO6 , WO3 , CrWO4 , and Cr2 O3 . Cr2 O3 was only present in the W-20Cr alloy oxide layer, which played an important role in the oxidation resistance of materials. Fig. 6 shows the SEM images of sample surface exposed to a W-Cr alloy at 1000 °C for 8 h. W-10Cr alloy has many protrusions and protrusions cracked on the surface after oxidation (Fig. 6a). In addition, some slope-like oxides are present on the surface of the alloy. According to the EDS spectrum and XRD results, the protrusion-like oxide and the slope-like oxide were identified as WO3 and Cr2 WO6 , respectively. The W-rich phase present in the sintered W-Cr alloy forms a large amount of WO3 during oxidation, and the considerable growth stress of WO3 results in the formation of a protrusion-like structure. As the oxidation progresses, the WO3 protrusion continues to be oxidized and then grows up until the crack breaks. This promotes further oxidation of the material. The surface of the W-20Cr alloy after oxidation has a part of the protrusions and cracked protrusions of WO3 , as well as a relatively
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Fig. 7. Cross-section viewing of the oxidized W-10Cr alloy after 8 h of oxidation in 80 vol.% Ar + 20 vol.% O2 at 1000 °C: (a) low magnification image; (b) EDS elemental mapping of (a); (c) and (d) high magnification image. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
complete planar oxide layer and a granular oxide which has been ruptured by the surface layer (Fig. 6b). The planar oxide is mainly Cr2 O3 , and the granular oxide is Cr2 WO6 . Compared with WO3 and Cr2 WO6 , the Cr2 O3 oxide layer is relatively dense. This may be one of the reasons why the oxidation resistance of W-20Cr alloy is better than that of W-10Cr alloy. 3.3. Oxidation behavior The SEM images of the oxide scale cross-sections (Figs. 7 and 8) display the alloy oxide interfaces in the W-Cr alloy samples exposed at 1000 °C for 8 h. The thickness of the W-10Cr alloy oxide layer is about 185 μm as shown in Fig. 8a. According to the results of SEM and EDS, the scale is composed of a W-Cr-O region and a W-O region. Combined with the results of XRD, the W-Cr-O region and the W-O region are mainly Cr2 WO6 and WO3 , respectively. In addition, a large number of transverse cracks (marked by red arrows) were found in the W-Cr-O mixed region; these cracks separated the oxide layer from the substrate and exerted a significant adverse effect on the oxidation resistance of the materials. In the W-O region of the subsurface, numerous voids and longitudinal cracks were observed (Fig. 7c). Fig. 8a shows that the W-20Cr alloy oxide layer has a thickness of about 150 μm, which is thinner than the scale of the W-10Cr alloy. Dif-
ferent from the oxide layer of W–10Cr alloy, the surface of the W–20Cr oxide layer was covered with a protective Cr2 O3 scale. Only a small amount of Cr2 O3 protective layer can be observed in the oxide crosssection, which may be related to the Cr content. The entire oxide layer was also mainly composed of a W-Cr-O mixed region. Confirmed by XRD and EDS surface scanning, W-Cr-O mixed area is mainly CrWO4 . There is a small number of cracks in the upper part of the CrWO4 oxide layer, and the pores penetrate almost the entire oxide layer. Figs. 7d and 8d are W-Cr substrates after oxidation of W-10Cr alloy and W-20Cr alloy, respectively. Among them, the black enrichment zone is mainly Cr2 O3 , and it is obvious that there are more enrichment zones of W-20Cr alloy, which should be the reason of Cr content. The Cr element acts as a passivating element to effectively improve the oxidation resistance of the W alloy. The quality of the scale can reflect the oxidation resistance of the material. Cr is more oxytropism than W. In the initial stage of oxidation, the W-Cr alloys first form protective scale Cr2 O3 . As the oxidation experiment continues, the Cr ion supply of the W-10Cr alloy is insufficient. The external O ions continue to diffuse and pass through the scale, causing internal oxidation to form WO3 . WO3 reacts with Cr2 O3 and O2 to form Cr2 WO6 . The oxide scale Cr2 WO6 is very loose in appearance and there are a large number of transverse cracks, indicating that the scale quality is very poor, and it is difficult to achieve the protection material from further oxidation. Therefore, the
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Fig. 8. Cross-section viewing of the oxidized W-20Cr alloy after 8 h of oxidation in 80 vol.% Ar + 20 vol.% O2 at 1000 °C: (a) low magnification image; (b) EDS elemental mapping of (a); (c) and (d) high magnification image.
interior of the W-10Cr alloy is further oxidized to form WO3 . In contrast, W-20Cr alloys have higher Cr content than W-10Cr alloys. The Cr ion supply is sufficient during the oxidation process, so that a betterquality protective scale Cr2 O3 can be formed. A good scale can inhibit the external O ions from passing through the scale and weaken the further oxidation of the alloy. Continuous oxidation further consumes Cr ions, and W reacts with Cr2 O3 and O2 to form CrWO4 . From the aspect of appearance, the oxide scale CrWO4 has pores and a small number of cracks, and the overall quality is better than that of Cr2 WO6 , which can also play a certain protective role. If the W-20Cr alloy continues to be oxidized at a high temperature, the scale Cr2 WO6 will be formed. As the oxidation continues, the scale will crack or even fall off, and the O ions will easily pass through the scale to further oxidize the internal material.
3.4. Probable oxidation reactions On the basis of the oxidation results, we constructed a schematic of the oxidation process, which corresponded to the possible chemical reaction. This schematic also describes the oxidation process and the mechanism of the W-Cr alloy (Figs. 9 and 10). In the initial stage of W-10Cr alloy oxidation process, the chemical reactions that occurred at the top surface of the oxide layer (Fig. 9b)
were as follows: T = 1273 K 2Cr (s) + (3∕2)O2 (g) → Cr2 O3 (s);
(1)
W(s)+(3∕2)O2 (g) → WO3 (s).
(2)
In the late stage of the oxidation process, the following chemical reactions occurred in the oxide layer, as shown in Fig. 10c: W(s) + Cr2 O3 (s) + (3∕2)O2 (g) → Cr2 WO6 (s);
(3)
Cr2 O3 (s) + WO3 (g) → Cr2 WO6 (s).
(4)
In the initial stage of W-20Cr alloy oxidation process, the chemical reaction that occurred at the top surface of the oxide layer, as shown in Fig. 10b, was as follows: Reaction (5) and (1) are identical.
(5)
In the late stage of the oxidation process, the chemical reactions that occurred in the oxide layer (Fig. 10c) were as follows: Reaction (6) and (2) are identical.
(6)
W(s) + (1∕2)Cr2 O3 (s) + (5∕3)O2 (g) → CrWO4 (s);
(7)
Reaction (8) and (3) are identical.
(8)
Reaction (9) and (4) are identical.
(9)
Reaction (10) and (1) are identical.
(10)
Q.-Q. Hou, K. Huang and L.-M. Luo et al.
Materialia 6 (2019) 100332
Fig. 9. The oxidation process schematic diagram of W-10Cr alloy, (a) the sintered alloy before oxidation; (b) initial stage; (c) late stage.
Fig. 10. The oxidation process schematic diagram of W-20Cr alloy, (a) the sintered alloy before oxidation; (b) initial stage; (c) late stage.
4. Conclusion In this work, W-Cr alloys were prepared through mechanical alloying and spark plasma sintering technology. The addition of Cr element effectively improves the mechanical properties and oxidation resistance of W-Cr alloy. The W-20Cr alloy significantly increased the microhardness from 333.5 HV of pure W to 961.3 HV. Oxidation experiments at 800 °C and 1000 °C confirmed the effect of Cr on the oxidation resistance of W-Cr alloy. A suitable amount of Cr content is favorable for the formation of the oxide scale Cr2 O3 and the quality assurance at the initial stage of oxidation. The diffusion of O ions is hindered to some extent by the scale of Cr2 O3 , which aims to slow down the oxidation rate. According to the present results, the Cr mass ratio in the W-Cr self-passivation alloys should be higher than 10 wt.%. Otherwise, the low Cr content in W-Cr alloys will cause many adverse effects. On the one hand, the Cr concentration in the W-Cr alloy is very low, which results in a low chromium content in the tungsten lattice. Therefore, the W-rich phase is larger. W-rich phase exerts adverse effect on the antioxidant property of W-Cr alloys. On the other hand, the supply of the Cr element is insufficient to form a dense Cr2 O3 oxide layer at the initial stage of oxidation. This promotes the formation of WO3 and the occurrence of internal oxidation. The oxide scale of W-20Cr alloy is denser, with less cracks and less thickness compared to W-10Cr. The quality of the oxide scale CrWO4 is better than that of Cr2 WO6 , indicating that the formation of the oxide scale CrWO4 is beneficial to weaken the further oxidation of the W-20Cr alloy. Acknowledgments This work is supported by National Natural Science Foundation of China (Grant no. 51574101 and 51474083), The National Key Research and Development Program of China (No.
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