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W alloys
International Journal of Refractory Metals & Hard Materials 86 (2020) 105082
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International Journal of Refractory Metals & Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Effect of Al2O3 content and swaging on microstructure and mechanical properties of Al2O3/W alloys ⁎
T
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Changji Wanga, Laiqi Zhanga, , Kunming Panb, Shizhong Weib, , Xiaochao Wuc, Qingkui Lic a
State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 10083, China National Joint Engineering Research Center for Abrasion Control and Molding of Metal Materials, Henan University of Science and Technology, Luoyang, Henan 471003, China c Henan Province Industrial Technology Research Institute of Resource and Materials, Zhengzhou University, Henan 450001, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Tungsten Nano-indentation Fracture toughness Al2O3-reinfored
Al2O3-reinfored tungsten alloys were fabricated by powder metallurgy method and hot swaging technology. The investigation was made on the microstructure, relative density, nano-hardness and fracture toughness (KIC) of the sintered and swaged Al2O3/W alloys. The swaging process and addition of Al2O3 are beneficial to comprehensive properties of the sintered and swaged alloys. After swaging, the Al2O3/W alloys can achieve the full density. According to the nano-indentation test and three-point bend test, the swaged W-0.25 wt% Al2O3 alloy possesses the highest hardness of 7.02 GPa, the greatest modulus of 435.09 GPa and the maximum fracture toughness of 21 MPa·m1/2. The observation of fracture morphology shows that the recrystallization behavior and grain growth occur above 1400 °C in the swaged pure W alloy, which leads to recrystallization brittleness. At the same time, the microstructure of the swaged W-0.25 wt% Al2O3 alloy does not change apparently.
1. Introduction Oxide dispersion strengthened (ODS) alloys were prepared by adding oxide particles (e.g. Y2O3, ZrO2, La2O3) using powder metallurgy technology, which has been widely applied in the fields of aerospace, energy, chemicals and metallurgy [1–5]. Due to low-temperature brittleness, low recrystallization temperatures, radiation brittleness and high ductile-brittle transition temperatures, these oxides have been widely used in tungsten alloys as the strengthening phases to improve the high-temperature strength and recrystallization temperature of the tungsten alloys [6–9]. It was reported that the addition of La2O3 particles improved the fracture strength and recrystallization beginning temperature of the La2O3/W alloys in comparison with pure tungsten [10]. The W-0.5% Y2O3 alloy was prepared using the wet chemical method and spark plasma sintering (SPS) technology by both Dong [11] and Tan et al. [12]. It was found that the composite powders fabricated by the wet chemical method possess the fine particles, improving the sintering properties of the materials. However, most of these works focused on the effects of the second phases on preparation, microstructure, micro-hardness and high temperature strength [13]. The nano-hardness and fracture toughness of tungsten alloys have been barely studied. Moreover, most oxide reinforced tungsten alloys were prepared by
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powder metallurgy method and conventional induction sintering. Even with the high sintering temperature, it is difficult to achieve the desired density of the tungsten alloy [14]. Therefore, the tungsten alloys needed to be hot working (swaging or rolling) to improve mechanical properties after sintering. For tungsten alloy, the influence of swaging on its properties is a crucial research. Because of high thermodynamic stability and excellent high temperature softening resistance, especially its low cost, Al2O3 have been widely used as the reinforced phases to prepare alloys [15–17]. In this paper, Al2O3-reinfored tungsten alloys were fabricated by powder metallurgy method and hot swaging technology. The effects of the Al2O3 content and swaging process on the microstructure, relative density, nano-hardness and fracture toughness of the Al2O3/W alloys were investigated. The changes in fracture morphology were also studied to analyze the high-temperature recrystallization behavior of the Al2O3/W alloys. 2. Materials and methods The sintered Al2O3/W rods were prepared via the hydrothermal reaction and powder metallurgy method. Subsequently, the consolidated Al2O3/W rods were obtained by hot working using a swaging machine. The specific process flow and process parameters were shown
Corresponding authors. E-mail addresses: [email protected] (L. Zhang), [email protected] (S. Wei).
https://doi.org/10.1016/j.ijrmhm.2019.105082 Received 5 May 2019; Received in revised form 27 July 2019; Accepted 2 September 2019 Available online 03 September 2019 0263-4368/ © 2019 Elsevier Ltd. All rights reserved.
International Journal of Refractory Metals & Hard Materials 86 (2020) 105082
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Fig. 1. Schematic of preparation technology.
3. Results and discussion
in Fig. 1. In order to do comparative tests, some swaged bars were annealed in a tubular furnace under argon protection for one hour. The annealing temperatures were 1300 °C, 1400 °C, 1500 °C and 1600 °C, respectively. After preparation, the Archimedes method was used to measure the relative densities of the sintered and swaged samples. The micro-hardnesses of the sintered and swaged samples were evaluated by Vickers micro-hardness testing at a loading force of 200 g. The room temperature fracture toughness was evaluated by threepoint bending tests with a span of 40 mm using an electro-hydraulic servo fatigue testing machine (MTS-810) at a constant crosshead speed of 10−3 mm/s. The samples for three-point bending tests were cuboid with a length of 50 mm, a width of 10 mm and a thickness of 5 mm. Because of the room temperature brittleness for pure W and Al2O3/W alloy, single-edge notched technique was adopted for testing, and the sketch of samples for three-point bending tests is shown in Fig. 2. Nano-indentation test was carried out on the swaged samples using a Keysight G200 nano indentation apparatus and conducted by continuous stiffness test method (CSM) with maximum indentation depth controlled. The Berkovich indenter was used in the entire test. The microstructure and fracture morphology observations on the sintered and swaged samples were carried out using a TESCAN VEGA3-SBH scanning electron microscope (SEM) equipped with an EDAX Octane Prime energy-dispersive X-ray spectrometer (EDXS).
3.1. Microstructure of the sintered and swaged Al2O3/W alloys The SEM micrographs of the sintered samples with different Al2O3 contents are shown in Fig. 3. As can be seen from the figures, the addition of Al2O3 leads to a refinement of the W grain size from about 18 μm for pure tungsten to 7.5 μm for W-0.75 wt% Al2O3. The result shows that the distribution of Al2O3 in the tungsten matrix can effectively prevent the migration of tungsten grain boundaries during high temperature sintering of tungsten alloys, thus inhibiting the growth of tungsten grains. From the EDXS point spectra, it can be seen that the black phase is Al2O3, as shown in Fig. 3(e) and (f). However, the size of Al2O3 gradually grows up and its morphology becomes long or irregular with the increase of Al2O3 content. The coarse Al2O3 particles are located at grain boundaries and some fine Al2O3 particles distribute into the W grains. These results can be confirmed by fracture morphology and EDXS point spectra in Fig. 4. Fig. 5 shows that the optical images of the swaged samples with different Al2O3 contents oriented in the radial direction. Due to the hot working temperature (1550 °C) exceeds the recrystallization temperature of tungsten, the dynamic recrystallization and grain growth of the pure W alloy are observed, as shown in Fig. 5(a). However, the microstructure of the swaged Al2O3-dispersed tungsten alloys does not obvious changes. Only a small amount of the dynamic recrystallization in Fig. 5(c, d) marked by red arrows occurs in the swaged W-0.5 wt% Al2O3 and W-0.75 wt% Al2O3 alloys. This may be because the aggregation and growth of Al2O3 weaken the local pinning effect.
3.2. Relative density and micro-hardness of the sintered and swaged Al2O3/ W alloys Fig. 6 shows that the relative density of the sintered and swaged alloys with different Al2O3 contents. From the change of relative density for the sintered alloys, it can be seen that the Al2O3 addition can improve the relative density of the sintered Al2O3/W alloys. The highest density of the sintered alloy is about 95% for the W-0.75 wt% Al2O3,
Fig. 2. Sketch of samples for three-point bending tests. 2
International Journal of Refractory Metals & Hard Materials 86 (2020) 105082
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Fig. 3. (a–d) SEM micrographs of the sintered samples with different Al2O3 contents: (a) Pure W; (b) 0.25 wt%; (c) 0.50 wt%; (d) 0.75 wt%. (e, f) EDXS point spectra from point 1 and 2, respectively, in (c, d).
lower relative density, the sintered Al2O3/W alloys exhibit the lower micro-hardness ranging from about 200 HV for the sintered W-0.75 wt % to about 375 HV for the sintered W-0.25 wt%. After swaging, the micro-hardness of the Al2O3/W alloys greatly increases in comparison with the sintered alloys. Moreover, the swaged W-0.25 wt% Al2O3 alloy
indicating that it is difficult to obtain the full density even at high sintering temperature (2350 °C). However, the sintered Al2O3/W alloys can achieve the full density after swaging process, as shown in Fig. 6. The measured values of micro-hardness for the sintered and swaged alloys with different Al2O3 contents are shown in Fig. 7. Owing to the
Fig. 4. Fracture morphology of the sintered W-0.5 wt% Al2O3 alloy and corresponding EDXS point spectra. 3
International Journal of Refractory Metals & Hard Materials 86 (2020) 105082
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Fig. 5. Optical images of the swaged samples with different Al2O3 contents oriented in the radial direction. (a) Pure W, (b) 0.25 wt%, (c) 0.50 wt%, (d) 0.75 wt%.
Fig. 7. Micro-hardness of the sintered and swaged alloys with different Al2O3 contents. Fig. 6. Relative density of the sintered and swaged alloys with different Al2O3 contents.
other micro-mechanical properties. The elastic modulus and hardness of the testing materials can be measured from the relationship between indentation load and depth in the process of loading and unloading [18,19]. Fig. 8 shows a typical load-depth curve of nano-indentation test. The key parameters of nano-indentation test can be obtained from the load-depth curve: Pmax is the maximum indentation load; hmax is the maximum indentation depth; hc is the contact depth; S is the contact stiffness, which can be expressed by the slope of the top portion of the unloading curve. Therefore, S can be calculated by Eq. (1),
possesses the highest hardness at 559 HV, and its micro-hardness is 30% higher that of the swaged pure tungsten. This is presumably due to the swaging process leading to a full density microstructure and grain refinement strengthening, giving a prominent micro-hardness.
3.3. Nano-indentation test of the swaged Al2O3/W alloys Nano-indentation characterization technology can measure the hardness of the micro-region within a single grain and continuously determine the change of nano-hardness with indentation depth in comparison with micro-hardness and macro-hardness. Therefore, nanohardness can eliminate the effect of grain boundary on hardness, and thus can directly reflect the effect of the microstructure on hardness and
dP S=⎛ ⎞ ⎝ dh ⎠h = hmax The contact depth (hc) can be obtained by Eq. (2),
4
(1)
International Journal of Refractory Metals & Hard Materials 86 (2020) 105082
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Fig. 11. Modulus-depth curves of nano-indentation test for the swaged alloys with different Al2O3 contents.
Fig. 8. Typical load-depth curve of nano-indentation test.
Fig. 9. Load-depth curves of nano-indentation test for the swaged alloys with different Al2O3 contents.
Fig. 12. Fracture toughness (KIC) of sintered and swaged alloys with different Al2O3 contents.
Fig. 10. Hardness-depth curves of nano-indentation test for the swaged alloys with different Al2O3 contents.
hc = hmax − ε∙
Pmax S
Fig. 13. Fracture toughness (KIC) of swaged alloys with different Al2O3 contents annealed at variant temperature.
Where θ is the face angle (θ = 65.35o for Berkovich indenter). The hardness of the tested materials can be measured from Eq. (4),
(2)
Where ε is a constant related to the shape of the indenter (ε = 0.75 for Berkovich indenter). The contact area (Ac) can be calculated by Eq. (3),
H=
Ac = 3 3 ∙hc 2∙tan2 θ
According to the traditional elastic-plastic theory, the elastic modulus of the tested material can be calculated from Eq. (5),
(3) 5
Pmax Ac
(4)
International Journal of Refractory Metals & Hard Materials 86 (2020) 105082
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Fig. 14. Fracture morphology of the sintered alloys with different Al2O3 contents. (a) Pure W, (b) 0.25 wt%, (c) 0.50 wt%, (d) 0.75 wt%.
Fig. 15. Fracture morphology of the swaged alloys with different Al2O3 contents. (a) Pure W, (b) 0.25 wt%, (c) 0.50 wt%, (d) 0.75 wt%.
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Fig. 16. Fracture morphology of the swaged pure tungsten annealed at different temperatures. (a) 1300 °C, (b) 1400 °C, (c) 1500 °C, (d) 1600 °C.
1 1 − v2 1 − vi 2 = + Er E Ei
interaction mechanism between the dispersed particles (Al2O3) and dislocation motion, which is mainly bypassing mechanism (Orowan mechanism). During the pressing process of the indenter, the dislocations caused by the force were blocked by the dispersed particles, leading to the hardness improvement of the Al2O3/W alloys.
(5)
Where Er is the reduced modulus; E and Ei are the elastic modulus of the experimental sample and indenter, respectively; v and vi are the Poisson ratio of the experimental sample and indenter, respectively. For Berkovich indenter, Ei and vi are equal to 1114 GPa and 0.07, respectively. Through the nano-indentation test, the elastic modulus of the tested materials can be calculated from Eq. (6),
Er =
π S ∙ 2 Ac
3.4. Fracture behavior of the sintered and swaged Al2O3/W alloys Fracture is the most dangerous failure mode of the engineering components, especially brittle fracture, which has no obvious sign before break. This often leads to the catastrophic damage accidents. Tungsten-based high temperature structural materials are commonly used as engineering component materials, so it is very important to evaluate the fracture mode [20]. Considering the brittleness of tungsten alloys at room temperature, the fracture toughness of tungsten alloys can be calculated by the evaluation method of fracture toughness of brittle materials, such as ceramics. The calculation formula of fracture toughness is as follow [21],
(6)
Fig. 9 shows load-depth curves of nano-indentation test for the swaged alloys with different Al2O3 contents, indicating that the corresponding loading force of W-0.25 wt% Al2O3 alloy is larger than that of other tungsten alloys. This shows that the W-0.25 wt% Al2O3 alloy has good properties in resisting external attack. Figs. 10 and 11 show that hardness-depth and modulus-depth curves of nano-indentation test for the swaged alloys with different Al2O3 contents, respectively. From the figures, the measured values of hardness and modulus gradually tend to be stable after indentation depth is greater than 500 nm. The average values of hardness and modulus were calculated using measuring values of the 500–1900 nm indentation depth. The W-0.25 wt% Al2O3 alloy possesses the highest hardness of 7.02 GPa and the greatest modulus of 435.09 GPa. Moreover, the hardness and modulus of Al2O3 doped tungsten alloys are higher than those of the pure tungsten alloy. The pure tungsten alloy has the lowest hardness of 6.44 GPa and modulus of 405.72 GPa. The hardness and modulus of W-0.25 wt% Al2O3 alloy have increased by 9% and 7% in comparison with that of the pure tungsten, respectively. The improvement of hardness may be due to the
KIC =
PC ∙S 3
f (a/ W )
(7)
B∙W 2
In Eq. (7), the calculation formula of the f(a/W) is as follow,
f (a/ W ) 1
3
5
7
9
a 2 a 2 a 2 a 2 a 2 = 2.9 ⎛ ⎞ − 4.6 ⎛ ⎞ + 21.8 ⎛ ⎞ − 37.6 ⎛ ⎞ + 38.7 ⎛ ⎞ W W W W W ⎝ ⎠ ⎝ ⎠ ⎝ ⎠ ⎝ ⎠ ⎝ ⎠ (8) 1/2
Here KIC is the fracture toughness (MPa·m ), PC is the ultimate load of fracture (N), S is the span length (mm), B is the specimen width 7
International Journal of Refractory Metals & Hard Materials 86 (2020) 105082
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Fig. 17. Fracture morphology of the swaged W-0.25 wt% Al2O3 alloy annealed at different temperatures. (a) 1300 °C, (b) 1400 °C, (c) 1500 °C, (d) 1600 °C.
cohesion strength of grain boundary. The SEM micrographs of fracture surfaces for the swaged alloys with different Al2O3 contents are shown in Fig. 15. The fracture mode is predominant cleavage fracture which is characterized by river pattern. The fracture of the swaged alloys exhibit entirely transgranular fracture. This is also one of the main reasons for the great improvement of the fracture toughness. From the above analysis, it has been found that the swaged W0.25 wt% Al2O3 alloy possesses the higher miro-hardness and fracture toughness in comparison with pure W. Therefore, only the fracture morphology of the swaged W-0.25 wt% Al2O3 and pure W alloys annealed at high temperature are shown to analyze the effect of annealing temperature on fracture toughness and recrystallization temperature. Figs. 16 and 17 show the fracture morphology of the swaged W-0.25 wt % Al2O3 and pure W alloys annealed at different temperatures, respectively. For the swaged pure W alloy annealed at 1300 °C, the dominating kind of fracture is transcrystalline and some regions of the fracture surface show intercrystalline fracture, as shown in Fig. 16(a). After annealing at 1400 °C, the main fracture mode of the swaged pure W alloy is intercrystalline fracture and transgranular fracture occurs in only a small part of the fracture surface region. When annealing temperature is above 1500 °C, the fracture morphology of the swaged pure W alloy presents all intergranular fracture. Moreover, the swaged W0.25 wt% Al2O3 alloys annealed at different temperatures exhibit a similar morphology. The whole transgranular fracture appeared on the fracture surface of the annealed alloys, as shown in Fig. 17. From the above analysis, the recrystallization behavior and grain growth occurred in the swaged pure W alloy at or above 1400 °C. These behaviors lead to recrystallization brittleness, which is the primary reason why the fracture toughness of the swaged pure W alloy decreases sharply after annealing above 1400 °C. Nevertheless, the microstructure of the
(mm), W is the specimen height (mm) and a is the specimen notch depth (mm). Fig. 12 shows the values of the fracture toughness (KIC) for the sintered and swaged alloys with different Al2O3 contents. Either the sintered or swaged alloys, the Al2O3 reinforced tungsten alloys display excellent fracture toughness compared with pure tungsten. In addition, the fracture toughness of the alloys was highly improved by the swaging process. The W-0.25 wt% Al2O3 alloy has the highest fracture toughness value of 21 MPa·m1/2, which is 52% higher than that of the swaged pure tungsten. Nevertheless, the fracture toughnesses are comparable with increasing Al2O3 content. This can be caused by the agglomeration of Al2O3, weakening the barrier effect of Al2O3 on crack propagation. The average values of fracture toughness for the swaged alloys with different Al2O3 contents annealed at variant temperatures are shown in Fig. 13. The values of fracture toughness for all alloys decreased slightly after annealing at 1300 °C. Presumably, the residual stress is released by the hot-swaging. After annealing above 1400 °C, the fracture toughness of the swaged Al2O3/W alloys tends to be stable. On the contrary, the fracture toughness of the swaged pure W decreases. Fig. 14 shows the fracture morphology of the sintered alloys with different Al2O3 contents. The addition of Al2O3 has a significant influence on the refinement of tungsten grains. Moreover, the change in tungsten grain size is remarkably consistent with that the counted values from the SEM images of the etched samples (see Section 3.1). The fracture surface presents brittle facture, and the fracture mode is typical intergranular fracture for the all tested samples. It is worth noting that a small amount of transgranular fracture can be observed from Fig. 14(c) and (d). This presumably attributes to the addition of Al2O3 by improving sintering activity of tungsten alloy and hence resulting in better density in comparison with pure tungsten, which enhancing the 8
International Journal of Refractory Metals & Hard Materials 86 (2020) 105082
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Technology Project of Henan Province (No.152102410037).
swaged W-0.25 wt% Al2O3 alloy did not change apparently, which also leads to a relatively stable fracture toughness. These results indicate that the swaged W-0.25 wt% Al2O3 alloy possess better thermal stability in comparison with the swaged pure W.
References [1] K. Unocic, B. Pint, D. Hoelzer, Advanced TEM characterization of oxide nanoparticles in ODS Fe-12Cr-5Al alloys, J. Mater. Sci. 51 (2016) 9190–9206. [2] A. Certain, S. Kuchibhatla, V. Shutthanandan, et al., Radiation stability of nanoclusters in nano-structured oxide dispersion strengthened (ODS) steels, J. Nucl. Mater. 434 (1) (2013) 311–321. [3] Y. Kim, K.H. Lee, E.-P. Kim, et al., Fabrication of high temperature oxides dispersion strengthened tungsten composites by spark plasma sintering process, Int. J. Refract. Met. Hard Mater. 27 (5) (2009) 842–846. [4] R. Liu, Z.M. Xie, Q.F. Fang, et al., Nanostructured yttria dispersion-strengthened tungsten synthesized by sol–gel method, J. Alloys Compd. 657 (2016) 73–80. [5] N. Liu, Z. Dong, Z. Ma, et al., Eliminating bimodal structures of W-Y2O3 composite nanopowders synthesized by wet chemical method via controlling reaction conditions, J. Alloys. Compd. 774 (2019) 122–128. [6] C. Wang, L. Zhang, S. Wei, et al., Effect of ZrO2 content on microstructure and mechanical properties of W alloys fabricated by spark plasma sintering, Int. J. Refract. Met. Hard Mater. 79 (2019) 79–89. [7] Y. Zhang, A.V. Ganeev, J.T. Wang, et al., Observations on the ductile-to-brittle transition in ultrafine-grained tungsten of commercial purity, Mater. Sci. Eng. A 503 (2009) 37–40. [8] Y. Kim, M.-H. Hong, S. Ho Lee, et al., The effect of yttrium oxide on the sintering behavior and hardness of tungsten, Met. Mater. Int. 12 (2006) 245–248. [9] C.J. Wang, L.Q. Zhang, S.Z. Wei, et al., Preparation, microstructure, and constitutive equation of W-0.25 wt% Al2O3 alloy, Mater. Sci. Eng. A 744 (2019) 79–85. [10] I. Wesemann, W. Spielmann, P. Heel, et al., Fracture strength and microstructure of ODS tungsten alloys, Int. J. Refract. Met. Hard Mater. 28 (6) (2010) 687–691. [11] Z. Dong, N. Liu, Z.Q. Ma, et al., Synthesis of nanosized composite powders via a wet chemical process for sintering high performance W-Y2O3 alloy, Int. J. Refract. Met. Hard Mater. 69 (2017) 266–272. [12] X.Y. Tan, L.M. Luo, H.Y. Chen, et al., Mechanical properties and microstructural change of W-Y2O3 alloy under helium irradiation, Sci. Pep. 5 (2015) 18. [13] M. Battabyal, R. Schaublin, P. Spatig, et al., W-2 wt.%Y2O3 composite: microstructure and mechanical properties, Mater. Sci. Eng. A 538 (2012) 53–57. [14] Z.M. Xie, R. Liu, S. Miao, et al., Effect of high temperature swaging and annealing on the mechanical properties and thermal conductivity of W-Y2O3, J. Nucl. Mater. 464 (2015) 193–199. [15] Y. Pan, S.Q. Xiao, X. Lu, et al., Fabrication, mechanical properties and electrical conductivity of Al2O3 reinforced Cu/CNTs composites, J. Alloys Compd. 782 (2019) 1015–1023. [16] K.H. Yeon, H.S. Park, M.S. Kim, et al., Characterization of hot deformation behavior of nanosized Al2O3 reinforced Al6061 composites, J. Nanosci. Nanotechnol. 19 (7) (2019) 3929–3934. [17] X.X. Pang, Y.J. Xian, W. Wang, et al., Tensile properties and strengthening effects of 6061Al/12 wt%B4C composites reinforced with nano-Al2O3 particles, J. Alloys Compd. 768 (2018) 476–484. [18] A.C. Fischer-Cripps, A simple phenomenological approach to nanoindentation creep, Mater. Sci. Eng. A 385 (1) (2004) 74–82. [19] W.C. Oliver, G.M. Pharr, An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments, J. Mater. Res. 7 (6) (1992) 1564–1583. [20] M. Faleschini, H. Kreuzer, D. Kiener, et al., Fracture toughness investigations of tungsten alloys and SPD tungsten alloys, J. Nucl. Mater. 367-370 ( (2007) 800–805. [21] L. Zhang, K. Pan, J. Lin, Fracture toughness and fracture mechanisms in Mo5SiB2 at ambient to elevated temperatures, Intermetallics 38 (2013) 49–54.
4. Conclusion In this work, the consolidated Al2O3-reinfored tungsten alloys were fabricated by powder metallurgy and hot swaging. The results can be summarized as following: (1) The addition of Al2O3 leads to a refinement of the W grains in the sintered alloys. The dispersed Al2O3 particles in the tungsten matrix can effectively prevent the migration of tungsten grain boundaries during high temperature sintering process, thus inhibit the growth of tungsten grains. (2) After swaging, the micro-hardness of the Al2O3/W alloys increases in comparison with the sintered alloys. The micro-hardnesss of the swaged W-0.25 wt% Al2O3 alloy is 559 HV, which is 30% higher that of the swaged pure tungsten. The swaging process can lead to a fully density and grain refinement strengthening, resulting in a prominent micro-hardness. (3) For the swaged W-0.25 wt% Al2O3 alloy, the highest nano-hardness is 7 GPa and the greatest modulus is 435 GPa. Moreover, both the hardness and modulus of the Al2O3 doped tungsten alloys are higher than those of pure tungsten. (4) The value of fracture toughness for the swaged W-0.25 wt% Al2O3 alloy was 21 MPa·m1/2, which is 52% higher than that of pure W. This is probably because of the dispersed Al2O3 particles refine the W grains and improve the resistance to crack propagation. After annealing above 1400 °C, the fracture toughness of the swaged Al2O3/W alloys tends to be stable. On the contrary, the fracture toughness of the swaged pure W decreases. (5) Above 1400 °C, the recrystallization and grain growth occurred in the swaged pure W alloy. Nevertheless, the microstructures of the swaged W-0.25 wt% Al2O3 alloys annealed at different temperatures were almost same. These results indicate that the swaged W0.25 wt% Al2O3 alloy has better thermal stability in comparison with the swaged pure W. Acknowledgement The research is supported by the National Natural Science Foundation of China (No. 51672070), Key Projects of the State Key Research and Development Plan (No. 2017YFB0306000) and Science &
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Effect of Al2O3 content and swaging on microstructure and mechanical properties of Al2O3/W alloys ⁎
T
⁎
Changji Wanga, Laiqi Zhanga, , Kunming Panb, Shizhong Weib, , Xiaochao Wuc, Qingkui Lic a
State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 10083, China National Joint Engineering Research Center for Abrasion Control and Molding of Metal Materials, Henan University of Science and Technology, Luoyang, Henan 471003, China c Henan Province Industrial Technology Research Institute of Resource and Materials, Zhengzhou University, Henan 450001, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Tungsten Nano-indentation Fracture toughness Al2O3-reinfored
Al2O3-reinfored tungsten alloys were fabricated by powder metallurgy method and hot swaging technology. The investigation was made on the microstructure, relative density, nano-hardness and fracture toughness (KIC) of the sintered and swaged Al2O3/W alloys. The swaging process and addition of Al2O3 are beneficial to comprehensive properties of the sintered and swaged alloys. After swaging, the Al2O3/W alloys can achieve the full density. According to the nano-indentation test and three-point bend test, the swaged W-0.25 wt% Al2O3 alloy possesses the highest hardness of 7.02 GPa, the greatest modulus of 435.09 GPa and the maximum fracture toughness of 21 MPa·m1/2. The observation of fracture morphology shows that the recrystallization behavior and grain growth occur above 1400 °C in the swaged pure W alloy, which leads to recrystallization brittleness. At the same time, the microstructure of the swaged W-0.25 wt% Al2O3 alloy does not change apparently.
1. Introduction Oxide dispersion strengthened (ODS) alloys were prepared by adding oxide particles (e.g. Y2O3, ZrO2, La2O3) using powder metallurgy technology, which has been widely applied in the fields of aerospace, energy, chemicals and metallurgy [1–5]. Due to low-temperature brittleness, low recrystallization temperatures, radiation brittleness and high ductile-brittle transition temperatures, these oxides have been widely used in tungsten alloys as the strengthening phases to improve the high-temperature strength and recrystallization temperature of the tungsten alloys [6–9]. It was reported that the addition of La2O3 particles improved the fracture strength and recrystallization beginning temperature of the La2O3/W alloys in comparison with pure tungsten [10]. The W-0.5% Y2O3 alloy was prepared using the wet chemical method and spark plasma sintering (SPS) technology by both Dong [11] and Tan et al. [12]. It was found that the composite powders fabricated by the wet chemical method possess the fine particles, improving the sintering properties of the materials. However, most of these works focused on the effects of the second phases on preparation, microstructure, micro-hardness and high temperature strength [13]. The nano-hardness and fracture toughness of tungsten alloys have been barely studied. Moreover, most oxide reinforced tungsten alloys were prepared by
⁎
powder metallurgy method and conventional induction sintering. Even with the high sintering temperature, it is difficult to achieve the desired density of the tungsten alloy [14]. Therefore, the tungsten alloys needed to be hot working (swaging or rolling) to improve mechanical properties after sintering. For tungsten alloy, the influence of swaging on its properties is a crucial research. Because of high thermodynamic stability and excellent high temperature softening resistance, especially its low cost, Al2O3 have been widely used as the reinforced phases to prepare alloys [15–17]. In this paper, Al2O3-reinfored tungsten alloys were fabricated by powder metallurgy method and hot swaging technology. The effects of the Al2O3 content and swaging process on the microstructure, relative density, nano-hardness and fracture toughness of the Al2O3/W alloys were investigated. The changes in fracture morphology were also studied to analyze the high-temperature recrystallization behavior of the Al2O3/W alloys. 2. Materials and methods The sintered Al2O3/W rods were prepared via the hydrothermal reaction and powder metallurgy method. Subsequently, the consolidated Al2O3/W rods were obtained by hot working using a swaging machine. The specific process flow and process parameters were shown
Corresponding authors. E-mail addresses: [email protected] (L. Zhang), [email protected] (S. Wei).
https://doi.org/10.1016/j.ijrmhm.2019.105082 Received 5 May 2019; Received in revised form 27 July 2019; Accepted 2 September 2019 Available online 03 September 2019 0263-4368/ © 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. Schematic of preparation technology.
3. Results and discussion
in Fig. 1. In order to do comparative tests, some swaged bars were annealed in a tubular furnace under argon protection for one hour. The annealing temperatures were 1300 °C, 1400 °C, 1500 °C and 1600 °C, respectively. After preparation, the Archimedes method was used to measure the relative densities of the sintered and swaged samples. The micro-hardnesses of the sintered and swaged samples were evaluated by Vickers micro-hardness testing at a loading force of 200 g. The room temperature fracture toughness was evaluated by threepoint bending tests with a span of 40 mm using an electro-hydraulic servo fatigue testing machine (MTS-810) at a constant crosshead speed of 10−3 mm/s. The samples for three-point bending tests were cuboid with a length of 50 mm, a width of 10 mm and a thickness of 5 mm. Because of the room temperature brittleness for pure W and Al2O3/W alloy, single-edge notched technique was adopted for testing, and the sketch of samples for three-point bending tests is shown in Fig. 2. Nano-indentation test was carried out on the swaged samples using a Keysight G200 nano indentation apparatus and conducted by continuous stiffness test method (CSM) with maximum indentation depth controlled. The Berkovich indenter was used in the entire test. The microstructure and fracture morphology observations on the sintered and swaged samples were carried out using a TESCAN VEGA3-SBH scanning electron microscope (SEM) equipped with an EDAX Octane Prime energy-dispersive X-ray spectrometer (EDXS).
3.1. Microstructure of the sintered and swaged Al2O3/W alloys The SEM micrographs of the sintered samples with different Al2O3 contents are shown in Fig. 3. As can be seen from the figures, the addition of Al2O3 leads to a refinement of the W grain size from about 18 μm for pure tungsten to 7.5 μm for W-0.75 wt% Al2O3. The result shows that the distribution of Al2O3 in the tungsten matrix can effectively prevent the migration of tungsten grain boundaries during high temperature sintering of tungsten alloys, thus inhibiting the growth of tungsten grains. From the EDXS point spectra, it can be seen that the black phase is Al2O3, as shown in Fig. 3(e) and (f). However, the size of Al2O3 gradually grows up and its morphology becomes long or irregular with the increase of Al2O3 content. The coarse Al2O3 particles are located at grain boundaries and some fine Al2O3 particles distribute into the W grains. These results can be confirmed by fracture morphology and EDXS point spectra in Fig. 4. Fig. 5 shows that the optical images of the swaged samples with different Al2O3 contents oriented in the radial direction. Due to the hot working temperature (1550 °C) exceeds the recrystallization temperature of tungsten, the dynamic recrystallization and grain growth of the pure W alloy are observed, as shown in Fig. 5(a). However, the microstructure of the swaged Al2O3-dispersed tungsten alloys does not obvious changes. Only a small amount of the dynamic recrystallization in Fig. 5(c, d) marked by red arrows occurs in the swaged W-0.5 wt% Al2O3 and W-0.75 wt% Al2O3 alloys. This may be because the aggregation and growth of Al2O3 weaken the local pinning effect.
3.2. Relative density and micro-hardness of the sintered and swaged Al2O3/ W alloys Fig. 6 shows that the relative density of the sintered and swaged alloys with different Al2O3 contents. From the change of relative density for the sintered alloys, it can be seen that the Al2O3 addition can improve the relative density of the sintered Al2O3/W alloys. The highest density of the sintered alloy is about 95% for the W-0.75 wt% Al2O3,
Fig. 2. Sketch of samples for three-point bending tests. 2
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Fig. 3. (a–d) SEM micrographs of the sintered samples with different Al2O3 contents: (a) Pure W; (b) 0.25 wt%; (c) 0.50 wt%; (d) 0.75 wt%. (e, f) EDXS point spectra from point 1 and 2, respectively, in (c, d).
lower relative density, the sintered Al2O3/W alloys exhibit the lower micro-hardness ranging from about 200 HV for the sintered W-0.75 wt % to about 375 HV for the sintered W-0.25 wt%. After swaging, the micro-hardness of the Al2O3/W alloys greatly increases in comparison with the sintered alloys. Moreover, the swaged W-0.25 wt% Al2O3 alloy
indicating that it is difficult to obtain the full density even at high sintering temperature (2350 °C). However, the sintered Al2O3/W alloys can achieve the full density after swaging process, as shown in Fig. 6. The measured values of micro-hardness for the sintered and swaged alloys with different Al2O3 contents are shown in Fig. 7. Owing to the
Fig. 4. Fracture morphology of the sintered W-0.5 wt% Al2O3 alloy and corresponding EDXS point spectra. 3
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Fig. 5. Optical images of the swaged samples with different Al2O3 contents oriented in the radial direction. (a) Pure W, (b) 0.25 wt%, (c) 0.50 wt%, (d) 0.75 wt%.
Fig. 7. Micro-hardness of the sintered and swaged alloys with different Al2O3 contents. Fig. 6. Relative density of the sintered and swaged alloys with different Al2O3 contents.
other micro-mechanical properties. The elastic modulus and hardness of the testing materials can be measured from the relationship between indentation load and depth in the process of loading and unloading [18,19]. Fig. 8 shows a typical load-depth curve of nano-indentation test. The key parameters of nano-indentation test can be obtained from the load-depth curve: Pmax is the maximum indentation load; hmax is the maximum indentation depth; hc is the contact depth; S is the contact stiffness, which can be expressed by the slope of the top portion of the unloading curve. Therefore, S can be calculated by Eq. (1),
possesses the highest hardness at 559 HV, and its micro-hardness is 30% higher that of the swaged pure tungsten. This is presumably due to the swaging process leading to a full density microstructure and grain refinement strengthening, giving a prominent micro-hardness.
3.3. Nano-indentation test of the swaged Al2O3/W alloys Nano-indentation characterization technology can measure the hardness of the micro-region within a single grain and continuously determine the change of nano-hardness with indentation depth in comparison with micro-hardness and macro-hardness. Therefore, nanohardness can eliminate the effect of grain boundary on hardness, and thus can directly reflect the effect of the microstructure on hardness and
dP S=⎛ ⎞ ⎝ dh ⎠h = hmax The contact depth (hc) can be obtained by Eq. (2),
4
(1)
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Fig. 11. Modulus-depth curves of nano-indentation test for the swaged alloys with different Al2O3 contents.
Fig. 8. Typical load-depth curve of nano-indentation test.
Fig. 9. Load-depth curves of nano-indentation test for the swaged alloys with different Al2O3 contents.
Fig. 12. Fracture toughness (KIC) of sintered and swaged alloys with different Al2O3 contents.
Fig. 10. Hardness-depth curves of nano-indentation test for the swaged alloys with different Al2O3 contents.
hc = hmax − ε∙
Pmax S
Fig. 13. Fracture toughness (KIC) of swaged alloys with different Al2O3 contents annealed at variant temperature.
Where θ is the face angle (θ = 65.35o for Berkovich indenter). The hardness of the tested materials can be measured from Eq. (4),
(2)
Where ε is a constant related to the shape of the indenter (ε = 0.75 for Berkovich indenter). The contact area (Ac) can be calculated by Eq. (3),
H=
Ac = 3 3 ∙hc 2∙tan2 θ
According to the traditional elastic-plastic theory, the elastic modulus of the tested material can be calculated from Eq. (5),
(3) 5
Pmax Ac
(4)
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Fig. 14. Fracture morphology of the sintered alloys with different Al2O3 contents. (a) Pure W, (b) 0.25 wt%, (c) 0.50 wt%, (d) 0.75 wt%.
Fig. 15. Fracture morphology of the swaged alloys with different Al2O3 contents. (a) Pure W, (b) 0.25 wt%, (c) 0.50 wt%, (d) 0.75 wt%.
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Fig. 16. Fracture morphology of the swaged pure tungsten annealed at different temperatures. (a) 1300 °C, (b) 1400 °C, (c) 1500 °C, (d) 1600 °C.
1 1 − v2 1 − vi 2 = + Er E Ei
interaction mechanism between the dispersed particles (Al2O3) and dislocation motion, which is mainly bypassing mechanism (Orowan mechanism). During the pressing process of the indenter, the dislocations caused by the force were blocked by the dispersed particles, leading to the hardness improvement of the Al2O3/W alloys.
(5)
Where Er is the reduced modulus; E and Ei are the elastic modulus of the experimental sample and indenter, respectively; v and vi are the Poisson ratio of the experimental sample and indenter, respectively. For Berkovich indenter, Ei and vi are equal to 1114 GPa and 0.07, respectively. Through the nano-indentation test, the elastic modulus of the tested materials can be calculated from Eq. (6),
Er =
π S ∙ 2 Ac
3.4. Fracture behavior of the sintered and swaged Al2O3/W alloys Fracture is the most dangerous failure mode of the engineering components, especially brittle fracture, which has no obvious sign before break. This often leads to the catastrophic damage accidents. Tungsten-based high temperature structural materials are commonly used as engineering component materials, so it is very important to evaluate the fracture mode [20]. Considering the brittleness of tungsten alloys at room temperature, the fracture toughness of tungsten alloys can be calculated by the evaluation method of fracture toughness of brittle materials, such as ceramics. The calculation formula of fracture toughness is as follow [21],
(6)
Fig. 9 shows load-depth curves of nano-indentation test for the swaged alloys with different Al2O3 contents, indicating that the corresponding loading force of W-0.25 wt% Al2O3 alloy is larger than that of other tungsten alloys. This shows that the W-0.25 wt% Al2O3 alloy has good properties in resisting external attack. Figs. 10 and 11 show that hardness-depth and modulus-depth curves of nano-indentation test for the swaged alloys with different Al2O3 contents, respectively. From the figures, the measured values of hardness and modulus gradually tend to be stable after indentation depth is greater than 500 nm. The average values of hardness and modulus were calculated using measuring values of the 500–1900 nm indentation depth. The W-0.25 wt% Al2O3 alloy possesses the highest hardness of 7.02 GPa and the greatest modulus of 435.09 GPa. Moreover, the hardness and modulus of Al2O3 doped tungsten alloys are higher than those of the pure tungsten alloy. The pure tungsten alloy has the lowest hardness of 6.44 GPa and modulus of 405.72 GPa. The hardness and modulus of W-0.25 wt% Al2O3 alloy have increased by 9% and 7% in comparison with that of the pure tungsten, respectively. The improvement of hardness may be due to the
KIC =
PC ∙S 3
f (a/ W )
(7)
B∙W 2
In Eq. (7), the calculation formula of the f(a/W) is as follow,
f (a/ W ) 1
3
5
7
9
a 2 a 2 a 2 a 2 a 2 = 2.9 ⎛ ⎞ − 4.6 ⎛ ⎞ + 21.8 ⎛ ⎞ − 37.6 ⎛ ⎞ + 38.7 ⎛ ⎞ W W W W W ⎝ ⎠ ⎝ ⎠ ⎝ ⎠ ⎝ ⎠ ⎝ ⎠ (8) 1/2
Here KIC is the fracture toughness (MPa·m ), PC is the ultimate load of fracture (N), S is the span length (mm), B is the specimen width 7
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Fig. 17. Fracture morphology of the swaged W-0.25 wt% Al2O3 alloy annealed at different temperatures. (a) 1300 °C, (b) 1400 °C, (c) 1500 °C, (d) 1600 °C.
cohesion strength of grain boundary. The SEM micrographs of fracture surfaces for the swaged alloys with different Al2O3 contents are shown in Fig. 15. The fracture mode is predominant cleavage fracture which is characterized by river pattern. The fracture of the swaged alloys exhibit entirely transgranular fracture. This is also one of the main reasons for the great improvement of the fracture toughness. From the above analysis, it has been found that the swaged W0.25 wt% Al2O3 alloy possesses the higher miro-hardness and fracture toughness in comparison with pure W. Therefore, only the fracture morphology of the swaged W-0.25 wt% Al2O3 and pure W alloys annealed at high temperature are shown to analyze the effect of annealing temperature on fracture toughness and recrystallization temperature. Figs. 16 and 17 show the fracture morphology of the swaged W-0.25 wt % Al2O3 and pure W alloys annealed at different temperatures, respectively. For the swaged pure W alloy annealed at 1300 °C, the dominating kind of fracture is transcrystalline and some regions of the fracture surface show intercrystalline fracture, as shown in Fig. 16(a). After annealing at 1400 °C, the main fracture mode of the swaged pure W alloy is intercrystalline fracture and transgranular fracture occurs in only a small part of the fracture surface region. When annealing temperature is above 1500 °C, the fracture morphology of the swaged pure W alloy presents all intergranular fracture. Moreover, the swaged W0.25 wt% Al2O3 alloys annealed at different temperatures exhibit a similar morphology. The whole transgranular fracture appeared on the fracture surface of the annealed alloys, as shown in Fig. 17. From the above analysis, the recrystallization behavior and grain growth occurred in the swaged pure W alloy at or above 1400 °C. These behaviors lead to recrystallization brittleness, which is the primary reason why the fracture toughness of the swaged pure W alloy decreases sharply after annealing above 1400 °C. Nevertheless, the microstructure of the
(mm), W is the specimen height (mm) and a is the specimen notch depth (mm). Fig. 12 shows the values of the fracture toughness (KIC) for the sintered and swaged alloys with different Al2O3 contents. Either the sintered or swaged alloys, the Al2O3 reinforced tungsten alloys display excellent fracture toughness compared with pure tungsten. In addition, the fracture toughness of the alloys was highly improved by the swaging process. The W-0.25 wt% Al2O3 alloy has the highest fracture toughness value of 21 MPa·m1/2, which is 52% higher than that of the swaged pure tungsten. Nevertheless, the fracture toughnesses are comparable with increasing Al2O3 content. This can be caused by the agglomeration of Al2O3, weakening the barrier effect of Al2O3 on crack propagation. The average values of fracture toughness for the swaged alloys with different Al2O3 contents annealed at variant temperatures are shown in Fig. 13. The values of fracture toughness for all alloys decreased slightly after annealing at 1300 °C. Presumably, the residual stress is released by the hot-swaging. After annealing above 1400 °C, the fracture toughness of the swaged Al2O3/W alloys tends to be stable. On the contrary, the fracture toughness of the swaged pure W decreases. Fig. 14 shows the fracture morphology of the sintered alloys with different Al2O3 contents. The addition of Al2O3 has a significant influence on the refinement of tungsten grains. Moreover, the change in tungsten grain size is remarkably consistent with that the counted values from the SEM images of the etched samples (see Section 3.1). The fracture surface presents brittle facture, and the fracture mode is typical intergranular fracture for the all tested samples. It is worth noting that a small amount of transgranular fracture can be observed from Fig. 14(c) and (d). This presumably attributes to the addition of Al2O3 by improving sintering activity of tungsten alloy and hence resulting in better density in comparison with pure tungsten, which enhancing the 8
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Technology Project of Henan Province (No.152102410037).
swaged W-0.25 wt% Al2O3 alloy did not change apparently, which also leads to a relatively stable fracture toughness. These results indicate that the swaged W-0.25 wt% Al2O3 alloy possess better thermal stability in comparison with the swaged pure W.
References [1] K. Unocic, B. Pint, D. Hoelzer, Advanced TEM characterization of oxide nanoparticles in ODS Fe-12Cr-5Al alloys, J. Mater. Sci. 51 (2016) 9190–9206. [2] A. Certain, S. Kuchibhatla, V. Shutthanandan, et al., Radiation stability of nanoclusters in nano-structured oxide dispersion strengthened (ODS) steels, J. Nucl. Mater. 434 (1) (2013) 311–321. [3] Y. Kim, K.H. Lee, E.-P. Kim, et al., Fabrication of high temperature oxides dispersion strengthened tungsten composites by spark plasma sintering process, Int. J. Refract. Met. Hard Mater. 27 (5) (2009) 842–846. [4] R. Liu, Z.M. Xie, Q.F. Fang, et al., Nanostructured yttria dispersion-strengthened tungsten synthesized by sol–gel method, J. Alloys Compd. 657 (2016) 73–80. [5] N. Liu, Z. Dong, Z. Ma, et al., Eliminating bimodal structures of W-Y2O3 composite nanopowders synthesized by wet chemical method via controlling reaction conditions, J. Alloys. Compd. 774 (2019) 122–128. [6] C. Wang, L. Zhang, S. Wei, et al., Effect of ZrO2 content on microstructure and mechanical properties of W alloys fabricated by spark plasma sintering, Int. J. Refract. Met. Hard Mater. 79 (2019) 79–89. [7] Y. Zhang, A.V. Ganeev, J.T. Wang, et al., Observations on the ductile-to-brittle transition in ultrafine-grained tungsten of commercial purity, Mater. Sci. Eng. A 503 (2009) 37–40. [8] Y. Kim, M.-H. Hong, S. Ho Lee, et al., The effect of yttrium oxide on the sintering behavior and hardness of tungsten, Met. Mater. Int. 12 (2006) 245–248. [9] C.J. Wang, L.Q. Zhang, S.Z. Wei, et al., Preparation, microstructure, and constitutive equation of W-0.25 wt% Al2O3 alloy, Mater. Sci. Eng. A 744 (2019) 79–85. [10] I. Wesemann, W. Spielmann, P. Heel, et al., Fracture strength and microstructure of ODS tungsten alloys, Int. J. Refract. Met. Hard Mater. 28 (6) (2010) 687–691. [11] Z. Dong, N. Liu, Z.Q. Ma, et al., Synthesis of nanosized composite powders via a wet chemical process for sintering high performance W-Y2O3 alloy, Int. J. Refract. Met. Hard Mater. 69 (2017) 266–272. [12] X.Y. Tan, L.M. Luo, H.Y. Chen, et al., Mechanical properties and microstructural change of W-Y2O3 alloy under helium irradiation, Sci. Pep. 5 (2015) 18. [13] M. Battabyal, R. Schaublin, P. Spatig, et al., W-2 wt.%Y2O3 composite: microstructure and mechanical properties, Mater. Sci. Eng. A 538 (2012) 53–57. [14] Z.M. Xie, R. Liu, S. Miao, et al., Effect of high temperature swaging and annealing on the mechanical properties and thermal conductivity of W-Y2O3, J. Nucl. Mater. 464 (2015) 193–199. [15] Y. Pan, S.Q. Xiao, X. Lu, et al., Fabrication, mechanical properties and electrical conductivity of Al2O3 reinforced Cu/CNTs composites, J. Alloys Compd. 782 (2019) 1015–1023. [16] K.H. Yeon, H.S. Park, M.S. Kim, et al., Characterization of hot deformation behavior of nanosized Al2O3 reinforced Al6061 composites, J. Nanosci. Nanotechnol. 19 (7) (2019) 3929–3934. [17] X.X. Pang, Y.J. Xian, W. Wang, et al., Tensile properties and strengthening effects of 6061Al/12 wt%B4C composites reinforced with nano-Al2O3 particles, J. Alloys Compd. 768 (2018) 476–484. [18] A.C. Fischer-Cripps, A simple phenomenological approach to nanoindentation creep, Mater. Sci. Eng. A 385 (1) (2004) 74–82. [19] W.C. Oliver, G.M. Pharr, An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments, J. Mater. Res. 7 (6) (1992) 1564–1583. [20] M. Faleschini, H. Kreuzer, D. Kiener, et al., Fracture toughness investigations of tungsten alloys and SPD tungsten alloys, J. Nucl. Mater. 367-370 ( (2007) 800–805. [21] L. Zhang, K. Pan, J. Lin, Fracture toughness and fracture mechanisms in Mo5SiB2 at ambient to elevated temperatures, Intermetallics 38 (2013) 49–54.
4. Conclusion In this work, the consolidated Al2O3-reinfored tungsten alloys were fabricated by powder metallurgy and hot swaging. The results can be summarized as following: (1) The addition of Al2O3 leads to a refinement of the W grains in the sintered alloys. The dispersed Al2O3 particles in the tungsten matrix can effectively prevent the migration of tungsten grain boundaries during high temperature sintering process, thus inhibit the growth of tungsten grains. (2) After swaging, the micro-hardness of the Al2O3/W alloys increases in comparison with the sintered alloys. The micro-hardnesss of the swaged W-0.25 wt% Al2O3 alloy is 559 HV, which is 30% higher that of the swaged pure tungsten. The swaging process can lead to a fully density and grain refinement strengthening, resulting in a prominent micro-hardness. (3) For the swaged W-0.25 wt% Al2O3 alloy, the highest nano-hardness is 7 GPa and the greatest modulus is 435 GPa. Moreover, both the hardness and modulus of the Al2O3 doped tungsten alloys are higher than those of pure tungsten. (4) The value of fracture toughness for the swaged W-0.25 wt% Al2O3 alloy was 21 MPa·m1/2, which is 52% higher than that of pure W. This is probably because of the dispersed Al2O3 particles refine the W grains and improve the resistance to crack propagation. After annealing above 1400 °C, the fracture toughness of the swaged Al2O3/W alloys tends to be stable. On the contrary, the fracture toughness of the swaged pure W decreases. (5) Above 1400 °C, the recrystallization and grain growth occurred in the swaged pure W alloy. Nevertheless, the microstructures of the swaged W-0.25 wt% Al2O3 alloys annealed at different temperatures were almost same. These results indicate that the swaged W0.25 wt% Al2O3 alloy has better thermal stability in comparison with the swaged pure W. Acknowledgement The research is supported by the National Natural Science Foundation of China (No. 51672070), Key Projects of the State Key Research and Development Plan (No. 2017YFB0306000) and Science &
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