- Email: [email protected]
Al composites at cryogenic temperatures
Author’s Accepted Manuscript Enhanced mechanical properties of composites at cryogenic temperatures
SiC/Al
Qiuyuan Liu, Feng Wang, Weiwei Wu, Di An, Zhiyong He, Yanpeng Xue, Qifu Zhang, Zhipeng Xie www.elsevier.com/locate/ceri
PII: DOI: Reference:
S0272-8842(18)33051-7 https://doi.org/10.1016/j.ceramint.2018.10.233 CERI19945
To appear in: Ceramics International Received date: 8 October 2018 Revised date: 24 October 2018 Accepted date: 29 October 2018 Cite this article as: Qiuyuan Liu, Feng Wang, Weiwei Wu, Di An, Zhiyong He, Yanpeng Xue, Qifu Zhang and Zhipeng Xie, Enhanced mechanical properties of SiC/Al composites at cryogenic temperatures, Ceramics International, https://doi.org/10.1016/j.ceramint.2018.10.233 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Enhanced mechanical properties of SiC/Al composites at cryogenic temperatures
Qiuyuan Liu a,b, Feng Wang b,Weiwei Wuc, Di Ana, Zhiyong He b, Yanpeng Xued, Qifu Zhang b, Zhipeng Xiea,* a
State Key Laboratory of New Ceramics and Fine Processing, Tsinghua University,
Beijing 100084, China b
c
China Iron & Steel Research Institute Group, Beijing 100081, China
Sinoma Wind Power Blade Co., Ltd, Beijing 100192, China
d
National Center for Materials Service Safety, University of Science and Technology
Beijing, 100083, Beijing, China *
Corresponding Zhipeng Xie, Tel/Fax: +86 1062794603, [email protected]
Abstract: In this study, silicon carbide particulates reinforced Al matrix (SiC/Al) composites was prepared by the pressureless infiltration technique. The phase transformation and mechanical properties of SiC/Al compositess were observed at different cryogenic temperatures. With the temperature decreasing, the phase composition retained the same and no new diffraction peaks were detected. The mechanical properties of the samples were improved at cryogenic temperatures. Upon decreasing the temperature from 293 K to 77 K, the flexural strength increased from 289.08±21.58 MPa to 353.46±18.75 MPa, and fracture toughness went up from 7.52±0.72 MPa·m1/2 to
9.26±0.53 MPa·m1/2, respectively. The enhancement of mechanical properties was due to the elastic modulus increasing. Possessing the excellent mechanical properties at cryogenic temperatures, SiC/Al composites could have potential applications in cryogenic environment.
Keywords: SiC/Al composites; Mechanical properties; Cryogenic temperatures; Strengthening mechanism
1. Introduction Recently, the requirements for reducing the quality of spacecraft structures and improving the payload capacity of spacecraft have been becoming more and more urgent due to the development of aerospace science and engineering. In order to solve this problem, the best way is to select the materials with specific strength, thermal stability and better comprehensive performance for the design of spacecraft structure. SiC/Al composites has attracted more and more attentions because of its relatively good comprehensive properties, such as high specific modulus, hardness and corrosion resistance
[1-3]
. Many literatures have reported that SiC/Al is a potential
material which could be used for the cryogenic application[4-20]. For example, SiC/Al composites could be used as the structure material in spacecraft for remote sensing satellites with long duty cycles. But few researches about mechanical properties of SiC/Al composites under cryogenic environment are reported.
In this work, the SiC/Al composites prepared by pressureless infiltration technique was used as the experimental material. The effects of different temperatures on the phase evolution, flexural strength and fracture toughness of SiC/Al composites were investigated. The strengthening mechanism was also discussed as well. 2. Experimental 2.1. Fabrication of the SiC/Al composites α-SiC powders with diameter of 10 µm and 90 µm (purity of 99.7%, Qingzhou micro powder Co., Ltd, Shandong province, China) and aluminum alloy (Self-made, Al-8%Si-5%Mg) were used as starting materials to prepare SiC/Al composites. Firstly, SiC powders were blended with the PVA (Polyvinyl alcohol with average degree of polymerization of 1750, Sinopharm Chemical Reagent Co., Ltd., Beijing, China), then pressed to cylindrical pellets (60 mm) at a pressure of 20 MPa. The obtained samples were heated at 500 ºC for 2 h to burn out the binder, then sintered at 1100 ºC for 4 h to form a SiO2 layer on the particle surfaces which was used to enhance the wetting properties of SiC by aluminum alloy. The aluminum alloy and sintered SiC samples were put into a graphite furnace, then sintered at 930 ºC for 2 hours in the flowing pure nitrogen atmosphere. Finally, the SiC/Al composites was obtained when the temperature cooled down. 2.2. Characterization The density of the obtained samples was evaluated according to Archimedes’ method, by using deionized water as the medium. The microstructures of the samples were determined by scanning electron microscope (SEM, Merlin Compact, Zeiss,
Germany). The phase composition of the samples at the temperature of 293 K, 195 K and 77 K was characterized by In-situ x-ray diffraction (XRD, XRD-6000, SHIMADZU, Japan), respectively. The cryogenic temperature was achieved by a test chamber filled with liquid nitrogen. The flexural strength (σf) was tested by a three-point bending method on 4 mm × 3 mm × 36 mm specimens in a universal testing machine (AG-IC, Shimadzu, Japan) with a span of 30 mm and a crosshead speed of 0.5 mm/min. The fracture toughness (K1C) was evaluated by a single-edge notched beam test with a span of 30 mm using 4 mm × 6 mm × 36 mm test bars and the crosshead speed of 0.05 mm/min. The test temperature of the flexural strength and fracture toughness was 293 K, 195 K and 77 K, respectively. The Raman spectra were measured by a Raman spectroscopic apparatus (Lab RAM HR Evolution, HORIBA Jobin Yvon, Paris, France). Its excitation source as an argon-ion laser that was operated at a wavelength of 473 nm and 532 nm with a power of 300 mW at the laser head. 3. Results and discussion The bulk density of the SiC/Al composites is 2.90 g/cm3. The morphology of the as-prepared samples is shown in Fig. 1. It could be observed from the image that SiC particles with diameter of 10 µm and 90 µm distributed uniformly in the SiC/Al composites. In addition, no big pores were observed. The cross section reveals that the interface between second-phase SiC particles and Al matrix is visible and organizational structure is even.
Fig. 1. SEM images of the samples (a) surface topography of and (b) cross-section topography. Fig. 2 displayed the XRD patterns of the samples at 77 K, 195 K and 293 K. At 293 K, the major phases of the composites were SiC and Al. Other weak diffraction peaks, such as Mg2Si, were also detected. With the temperature decreasing, the phase composition retained the same and only the diffraction peaks shifted towards low theta value. The lattice constants of SiC and Al of the composites were exhibited in Table 1. When the temperature reduced the from 293 K to 77 K, lattice constant of SiC went down from 15.1437 Å to15.1351 Å according to the diffraction peaks of (006) and (112). Similarly, the calculated lattice constant of Al decreased from 4.0496 Å to 4.0411 Å in view of the diffraction peaks (111) position. Moreover, the cell volume of Al matrix reduced from 66.4104 Å3 to 65.9882 Å3. The results above indicate that lattice of SiC/Al composites would shrink at cryogenic temperatures.
Fig. 2. XRD patterns of the samples at different temperatures.
Table 1. Lattice constants of the SiC and Al phases lattice constant
crystal phase
SiC Al
indices
293 K
77 K
(006)、(112)
15.1437 Å
15.1351 Å
(111)
4.0496 Å
4.0411 Å
The flexural strength of the composites was tested by three-point bending method at 293 K, 195 K and 77 K, which were displayed in Fig. 3. It could be observed that the temperature went down from 293 K to 77 K, the flexural strength increased from 289.08±21.58 MPa to 353.46±18.75 MPa.
Fig. 3. Flexural strength of the sample at different temperatures.
Under the cryogenic environment, Griffth equation is used to describe the enhanced strength qualitatively[21], 1 2 E f Y c0
1
2
(1)
where, Eγ, γ,c0 and Y is elastic modulus, fracture surface energy ,original crack size and crack shape parameter, respectively. The elastic modulus E and fracture surface energy γ will increase by reducing the temperature which can be attributed to the stronger bonding force between atoms, while c0 and Y remain constant. The lattice constants of SiC and Al of the composites were shrunk with the temperature decreasing (as seen in Table 1). It was suggested that the distance between atoms was reduced, then the bonding force was enhanced which would lead to the elastic modulus E increasing. This is the strength improvement mechanism of the sample at cryogenic temperature. Fig. 4 presents the fracture toughness of the composites measured at different
temperatures. It was easily observed that the fracture toughness was 9.26±0.53 MPa·m1/2 at 77 K, increasing by 23.1% compared with that of the sample at 273 K. According to the literature
[22]
, fracture toughness KIC could be calculated by the
following equation,
K1C 2 E
1
2
(2)
where E is the elastic modulus and γ is the fracture surface energy. According to the analysis of the flexural strength, the elastic modulus E and fracture surface energy γ increased with the the temperature reducing. Therefore the fracture toughness would improve due to the enhancement of E and γ under cryogenic temperatures. Moreover, according to the literature
[23-26]
the residual stress will generate with the temperature
decreasing due to the thermal expansion mismatch of second-phase SiC particles (αSiC=4.2×10-6 K-1) compared with Al matrix (αAl=23×10-6 K-1). The induced residual stress could be expressed by the following formula [27],
1 m
T 2 Em 1 2 p E p
(3)
where σ is the residual stress, ΔT is the difference between sintering and testing temperature, Δα is the difference in coefficients of thermal expansion, Em ( Ep) and νm (νp) is the Young's modulus of the matrix(the particle) and the Poisson's rations of the matrix( the particle), respectively.
Fig. 4. Fracture toughness of the samples at different temperatures. According to the formula, the residual stress σ is proportional to ΔT. Therefore, σ will increase by reducing the temperature down to 77 K. Due to the smaller linear coefficient of thermal expansion of the SiC compared to the SiC/Al matrix, free SiC particles are subjected to a radial compression field which creates extra resistance to the approaching crack front. For illustrating this, in-situ raman spectroscopy of SiC/Al composites at selected temperatures was recorded (Fig.5). During cooling, the SiC characteristic peak located at 790 cm-1 went red shift, indicating that residual stresses increased as a result of the difference of thermal expansion coefficient with the temperature decreasing[28]. Hence, resulting from higher residual stress, stronger resistance to crack propagation at 77 K could augment the fracture toughness of SiC/Al composites than that at room temperature.
Fig. 5 In-situ Raman spectrum of the samples at various temperatures
4. Conclusions In summary, SiC/Al composites were fabricated by the pressureless infiltration technique. Mechanical properties of the composites at cryogenic temperatures were investigated. At 77 K, the flexural strength and fracture toughness was 353.46±18.75 MPa and 9.26±0.53 MPa·m1/2 respectively, increasing by 22.3% and 23.1%, as compared with that of the sample at 273 K. The mechanical properties of the samples were enhanced with the temperature decreasing. With the temperature decreasing, the elastic modulus was increasing due to binding energy enhancement among atoms under cryogenic environment which was the reason for the enhancement of mechanical properties. Moreover, under the condition of cryogenic temperatures, the improvement of fracture toughness could be attributed to the residual stress originating from the thermal expansion mismatch between the second-phase SiC particles and Al matrix. Based on the favorable performances, the as-prepared SiC/Al
composites would have potential applications in cryogenic environment.
Acknowledgments The National Science and Technology Major Project (Grant no. 2013ZX02104) and CAS Key Laboratory of Cryogenics, TIPC (CRYO201804) were sincerely acknowledged by the authors for the support.
References: [1] A. Shaga, P. Shen, R. F. Guo, Effects of oxide addition on the microstructure and mechanical properties of lamellar SiC scaffolds and Al-Si-Mg/SiC composites prepared by freeze casting and pressureless infiltration, Ceram. Int.42 (2016) 9653–9659. [2] G. Chen, W. Yang, R.Dong, Interfacial microstructure and its effect on thermal conductivity of SiCp/Cu composites, Mater. Des. 63 (2014) 109–114. [3] L. Wang, Q. Fan, G. Li, Experimental observation and numerical simulation of SiC 3D/Al interpenetrating phase composite material subjected to a three-point bending load, Comp. Mater. Sci. 95 (2014) 408–413. [4] L. Guo, X. Zhang, Q. Fan, Simulation of damage and failure processes of interpenetrating SiC/Al composites subjected to dynamic compressive loading, Acta Mater. 78 (2014) 190–202. [5] P. Shen, Y. Wang, L. Ren, Influence of SiC surface polarity on the wettability and reactivity in an Al/SiC system, Appl. Surf. Sci. 355 (2015) 930–938. [6] P. Feng, G. Liang, J. Zhang, Ultrasonic vibration-assisted scratch characteristics of
silicon carbide-reinforced aluminum matrix composites. Ceram. Int. 40 (2014) 10817–10823. [7] M.K. Aghajanian, M. A. Rocazella, J.T. Burke, The fabrication of metal matrix composites by a pressureless infiltration technique, Mater. Sci. 26 (1991) 447–454. [8] B.Srinivasa Rao, V.Jayaram, Pressureless infiltration of Al-Mg based alloys into Al2O3 preforms: mechanisms and phenomenology, Acta Mater. 49 (2001) 2373–2385. [9] M. Rodrýìguez-Reyes, M.I. Pech-Canul, E.E. Parras-Medécigo, Effect of Mg loss on the kinetics of pressureless infiltration in the processing of Al-Si-Mg/SiCp composites, Mater. Lett. 57 (2003) 2081–2089. [10] M.I. Pech-Canul, R.N. Katz, M.M. Makhlouf, Optimum conditions for pressureless infiltration of SiCp preforms by aluminum alloys, Mater. Process. Technol. 108 (2000) 68–77. [11] J. Liu, Z. Zheng, J. Wang, Y. Wu, Pressureless infiltration of liquid aluminum alloy into SiC preforms to form near-net-shape SiC/Al composites, Alloy. Compd. 465 (2008) 239–243. [12] H.S. Lee, K.Y. Jeon, H.Y. Kim, Fabrication process and thermal properties of SiCp/Al metal matrix composites for electronic packaging applications, Mater. Sci. 35 (2000) 6231–6236. [13] S. Ren, X. He, X. Qu, Effect of Si addition to Al-8Mg alloy on the microstructure and thermo-physical properties of SiCp/Al composites prepared by pressureless
infiltration, Mater. Sci. Eng. B: Adv. 138 (2007) 263–270. [14] S. Ren, X. He, X. Qu, Effect of Mg and Si in the aluminum on the thermo-mechanical properties of pressureless infiltrated SiCp/Al composites, Compos. Sci. Technol. 67 (2007) 2103–2113. [15] S.P. Rawal, Metal-matrix composites for space applications, JOM. 53 (2001) 14-17. [16] X. Shen, S. Ren, X. He, Study on methods to strengthen SiC preforms for SiCp/Al composites by pressureless infiltration, J. Alloy. Compd. 468 (2009) 158-163. [17] Y. Cui, L.F. Wang, J.Y. Ren, Multifunctional SiC/Al Composites for Aerospace Applications, Chin. J. Aeronaut, 21 (2008) 578-584. [18] M.K. Aghajanian, M.A. Rocazella, J.T. Burke, The fabrication of metal matrix composites by a pressureless infiltration technique, Mater. Sci. 26 (1991) 447-454. [19] H.S. Lee, K.Y. Jeon, H.Y. Kim, Fabrication process and thermal properties of SiCp/Al metal matrix composites for electronic packaging applications, Mater. Sci. 35 (2000) 6231-6236. [20] A. Zulfia, R.J. Hand, The production of Al-Mg alloy/SiC metal matrix composites by pressureless infiltration, Mater. Sci. 37 (2002) 955-961. [21] D.R. Clarke, K.T. Faber, Fracture of ceramics and glasses, J. Phys. Chem. Solids. 48 (1987) 1115-1157. [22] R.W. Davidge, Mechanical Behavior of Ceramics, Cambridge University Press, 1979. [23] D. Sciti, S. Guicciardi, M. Nygren. Spark plasma sintering and mechanical
behavior of ZrC-based composites, Scr. Mater. 59 (2008) 638-641. [24] A. Mukhopadhyay, D. Chakravarty, B. Basu, Spark Plasma-Sintered WC-ZrO2-Co Nano composites with High Fracture Toughness and Strength, J. Am. Ceram. Soc. 93 (2012) 1754-1763. [25] J. Gong, Z. Zhao, H. Miao, R-curve behavior of TiC particle reinforced Al2O3 composites, Scr. Mater. 43 (2000) 27-31. [26] M. Taya, S. Hayashi, A.S. Kobayashi, Toughening of a Particulate‐Reinforced Ceramic-Matrix Composite by Thermal Residual Stress, Am. Ceram. Soc. 73 (1990) 1382-1391. [27] J. Selsing, Internal stresses in ceramics, J. Am. Ceram. Soc. 44 (1961) 419. [28] J.F. DiGregorio, T.E. Furtak, Analysis of residual stress in 6H-SiC particles with in Al2O3/SiC composites through raman spectroscopy, J. Am. Ceram. Soc.75 (1992) 1854–1857.
SiC/Al
Qiuyuan Liu, Feng Wang, Weiwei Wu, Di An, Zhiyong He, Yanpeng Xue, Qifu Zhang, Zhipeng Xie www.elsevier.com/locate/ceri
PII: DOI: Reference:
S0272-8842(18)33051-7 https://doi.org/10.1016/j.ceramint.2018.10.233 CERI19945
To appear in: Ceramics International Received date: 8 October 2018 Revised date: 24 October 2018 Accepted date: 29 October 2018 Cite this article as: Qiuyuan Liu, Feng Wang, Weiwei Wu, Di An, Zhiyong He, Yanpeng Xue, Qifu Zhang and Zhipeng Xie, Enhanced mechanical properties of SiC/Al composites at cryogenic temperatures, Ceramics International, https://doi.org/10.1016/j.ceramint.2018.10.233 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Enhanced mechanical properties of SiC/Al composites at cryogenic temperatures
Qiuyuan Liu a,b, Feng Wang b,Weiwei Wuc, Di Ana, Zhiyong He b, Yanpeng Xued, Qifu Zhang b, Zhipeng Xiea,* a
State Key Laboratory of New Ceramics and Fine Processing, Tsinghua University,
Beijing 100084, China b
c
China Iron & Steel Research Institute Group, Beijing 100081, China
Sinoma Wind Power Blade Co., Ltd, Beijing 100192, China
d
National Center for Materials Service Safety, University of Science and Technology
Beijing, 100083, Beijing, China *
Corresponding Zhipeng Xie, Tel/Fax: +86 1062794603, [email protected]
Abstract: In this study, silicon carbide particulates reinforced Al matrix (SiC/Al) composites was prepared by the pressureless infiltration technique. The phase transformation and mechanical properties of SiC/Al compositess were observed at different cryogenic temperatures. With the temperature decreasing, the phase composition retained the same and no new diffraction peaks were detected. The mechanical properties of the samples were improved at cryogenic temperatures. Upon decreasing the temperature from 293 K to 77 K, the flexural strength increased from 289.08±21.58 MPa to 353.46±18.75 MPa, and fracture toughness went up from 7.52±0.72 MPa·m1/2 to
9.26±0.53 MPa·m1/2, respectively. The enhancement of mechanical properties was due to the elastic modulus increasing. Possessing the excellent mechanical properties at cryogenic temperatures, SiC/Al composites could have potential applications in cryogenic environment.
Keywords: SiC/Al composites; Mechanical properties; Cryogenic temperatures; Strengthening mechanism
1. Introduction Recently, the requirements for reducing the quality of spacecraft structures and improving the payload capacity of spacecraft have been becoming more and more urgent due to the development of aerospace science and engineering. In order to solve this problem, the best way is to select the materials with specific strength, thermal stability and better comprehensive performance for the design of spacecraft structure. SiC/Al composites has attracted more and more attentions because of its relatively good comprehensive properties, such as high specific modulus, hardness and corrosion resistance
[1-3]
. Many literatures have reported that SiC/Al is a potential
material which could be used for the cryogenic application[4-20]. For example, SiC/Al composites could be used as the structure material in spacecraft for remote sensing satellites with long duty cycles. But few researches about mechanical properties of SiC/Al composites under cryogenic environment are reported.
In this work, the SiC/Al composites prepared by pressureless infiltration technique was used as the experimental material. The effects of different temperatures on the phase evolution, flexural strength and fracture toughness of SiC/Al composites were investigated. The strengthening mechanism was also discussed as well. 2. Experimental 2.1. Fabrication of the SiC/Al composites α-SiC powders with diameter of 10 µm and 90 µm (purity of 99.7%, Qingzhou micro powder Co., Ltd, Shandong province, China) and aluminum alloy (Self-made, Al-8%Si-5%Mg) were used as starting materials to prepare SiC/Al composites. Firstly, SiC powders were blended with the PVA (Polyvinyl alcohol with average degree of polymerization of 1750, Sinopharm Chemical Reagent Co., Ltd., Beijing, China), then pressed to cylindrical pellets (60 mm) at a pressure of 20 MPa. The obtained samples were heated at 500 ºC for 2 h to burn out the binder, then sintered at 1100 ºC for 4 h to form a SiO2 layer on the particle surfaces which was used to enhance the wetting properties of SiC by aluminum alloy. The aluminum alloy and sintered SiC samples were put into a graphite furnace, then sintered at 930 ºC for 2 hours in the flowing pure nitrogen atmosphere. Finally, the SiC/Al composites was obtained when the temperature cooled down. 2.2. Characterization The density of the obtained samples was evaluated according to Archimedes’ method, by using deionized water as the medium. The microstructures of the samples were determined by scanning electron microscope (SEM, Merlin Compact, Zeiss,
Germany). The phase composition of the samples at the temperature of 293 K, 195 K and 77 K was characterized by In-situ x-ray diffraction (XRD, XRD-6000, SHIMADZU, Japan), respectively. The cryogenic temperature was achieved by a test chamber filled with liquid nitrogen. The flexural strength (σf) was tested by a three-point bending method on 4 mm × 3 mm × 36 mm specimens in a universal testing machine (AG-IC, Shimadzu, Japan) with a span of 30 mm and a crosshead speed of 0.5 mm/min. The fracture toughness (K1C) was evaluated by a single-edge notched beam test with a span of 30 mm using 4 mm × 6 mm × 36 mm test bars and the crosshead speed of 0.05 mm/min. The test temperature of the flexural strength and fracture toughness was 293 K, 195 K and 77 K, respectively. The Raman spectra were measured by a Raman spectroscopic apparatus (Lab RAM HR Evolution, HORIBA Jobin Yvon, Paris, France). Its excitation source as an argon-ion laser that was operated at a wavelength of 473 nm and 532 nm with a power of 300 mW at the laser head. 3. Results and discussion The bulk density of the SiC/Al composites is 2.90 g/cm3. The morphology of the as-prepared samples is shown in Fig. 1. It could be observed from the image that SiC particles with diameter of 10 µm and 90 µm distributed uniformly in the SiC/Al composites. In addition, no big pores were observed. The cross section reveals that the interface between second-phase SiC particles and Al matrix is visible and organizational structure is even.
Fig. 1. SEM images of the samples (a) surface topography of and (b) cross-section topography. Fig. 2 displayed the XRD patterns of the samples at 77 K, 195 K and 293 K. At 293 K, the major phases of the composites were SiC and Al. Other weak diffraction peaks, such as Mg2Si, were also detected. With the temperature decreasing, the phase composition retained the same and only the diffraction peaks shifted towards low theta value. The lattice constants of SiC and Al of the composites were exhibited in Table 1. When the temperature reduced the from 293 K to 77 K, lattice constant of SiC went down from 15.1437 Å to15.1351 Å according to the diffraction peaks of (006) and (112). Similarly, the calculated lattice constant of Al decreased from 4.0496 Å to 4.0411 Å in view of the diffraction peaks (111) position. Moreover, the cell volume of Al matrix reduced from 66.4104 Å3 to 65.9882 Å3. The results above indicate that lattice of SiC/Al composites would shrink at cryogenic temperatures.
Fig. 2. XRD patterns of the samples at different temperatures.
Table 1. Lattice constants of the SiC and Al phases lattice constant
crystal phase
SiC Al
indices
293 K
77 K
(006)、(112)
15.1437 Å
15.1351 Å
(111)
4.0496 Å
4.0411 Å
The flexural strength of the composites was tested by three-point bending method at 293 K, 195 K and 77 K, which were displayed in Fig. 3. It could be observed that the temperature went down from 293 K to 77 K, the flexural strength increased from 289.08±21.58 MPa to 353.46±18.75 MPa.
Fig. 3. Flexural strength of the sample at different temperatures.
Under the cryogenic environment, Griffth equation is used to describe the enhanced strength qualitatively[21], 1 2 E f Y c0
1
2
(1)
where, Eγ, γ,c0 and Y is elastic modulus, fracture surface energy ,original crack size and crack shape parameter, respectively. The elastic modulus E and fracture surface energy γ will increase by reducing the temperature which can be attributed to the stronger bonding force between atoms, while c0 and Y remain constant. The lattice constants of SiC and Al of the composites were shrunk with the temperature decreasing (as seen in Table 1). It was suggested that the distance between atoms was reduced, then the bonding force was enhanced which would lead to the elastic modulus E increasing. This is the strength improvement mechanism of the sample at cryogenic temperature. Fig. 4 presents the fracture toughness of the composites measured at different
temperatures. It was easily observed that the fracture toughness was 9.26±0.53 MPa·m1/2 at 77 K, increasing by 23.1% compared with that of the sample at 273 K. According to the literature
[22]
, fracture toughness KIC could be calculated by the
following equation,
K1C 2 E
1
2
(2)
where E is the elastic modulus and γ is the fracture surface energy. According to the analysis of the flexural strength, the elastic modulus E and fracture surface energy γ increased with the the temperature reducing. Therefore the fracture toughness would improve due to the enhancement of E and γ under cryogenic temperatures. Moreover, according to the literature
[23-26]
the residual stress will generate with the temperature
decreasing due to the thermal expansion mismatch of second-phase SiC particles (αSiC=4.2×10-6 K-1) compared with Al matrix (αAl=23×10-6 K-1). The induced residual stress could be expressed by the following formula [27],
1 m
T 2 Em 1 2 p E p
(3)
where σ is the residual stress, ΔT is the difference between sintering and testing temperature, Δα is the difference in coefficients of thermal expansion, Em ( Ep) and νm (νp) is the Young's modulus of the matrix(the particle) and the Poisson's rations of the matrix( the particle), respectively.
Fig. 4. Fracture toughness of the samples at different temperatures. According to the formula, the residual stress σ is proportional to ΔT. Therefore, σ will increase by reducing the temperature down to 77 K. Due to the smaller linear coefficient of thermal expansion of the SiC compared to the SiC/Al matrix, free SiC particles are subjected to a radial compression field which creates extra resistance to the approaching crack front. For illustrating this, in-situ raman spectroscopy of SiC/Al composites at selected temperatures was recorded (Fig.5). During cooling, the SiC characteristic peak located at 790 cm-1 went red shift, indicating that residual stresses increased as a result of the difference of thermal expansion coefficient with the temperature decreasing[28]. Hence, resulting from higher residual stress, stronger resistance to crack propagation at 77 K could augment the fracture toughness of SiC/Al composites than that at room temperature.
Fig. 5 In-situ Raman spectrum of the samples at various temperatures
4. Conclusions In summary, SiC/Al composites were fabricated by the pressureless infiltration technique. Mechanical properties of the composites at cryogenic temperatures were investigated. At 77 K, the flexural strength and fracture toughness was 353.46±18.75 MPa and 9.26±0.53 MPa·m1/2 respectively, increasing by 22.3% and 23.1%, as compared with that of the sample at 273 K. The mechanical properties of the samples were enhanced with the temperature decreasing. With the temperature decreasing, the elastic modulus was increasing due to binding energy enhancement among atoms under cryogenic environment which was the reason for the enhancement of mechanical properties. Moreover, under the condition of cryogenic temperatures, the improvement of fracture toughness could be attributed to the residual stress originating from the thermal expansion mismatch between the second-phase SiC particles and Al matrix. Based on the favorable performances, the as-prepared SiC/Al
composites would have potential applications in cryogenic environment.
Acknowledgments The National Science and Technology Major Project (Grant no. 2013ZX02104) and CAS Key Laboratory of Cryogenics, TIPC (CRYO201804) were sincerely acknowledged by the authors for the support.
References: [1] A. Shaga, P. Shen, R. F. Guo, Effects of oxide addition on the microstructure and mechanical properties of lamellar SiC scaffolds and Al-Si-Mg/SiC composites prepared by freeze casting and pressureless infiltration, Ceram. Int.42 (2016) 9653–9659. [2] G. Chen, W. Yang, R.Dong, Interfacial microstructure and its effect on thermal conductivity of SiCp/Cu composites, Mater. Des. 63 (2014) 109–114. [3] L. Wang, Q. Fan, G. Li, Experimental observation and numerical simulation of SiC 3D/Al interpenetrating phase composite material subjected to a three-point bending load, Comp. Mater. Sci. 95 (2014) 408–413. [4] L. Guo, X. Zhang, Q. Fan, Simulation of damage and failure processes of interpenetrating SiC/Al composites subjected to dynamic compressive loading, Acta Mater. 78 (2014) 190–202. [5] P. Shen, Y. Wang, L. Ren, Influence of SiC surface polarity on the wettability and reactivity in an Al/SiC system, Appl. Surf. Sci. 355 (2015) 930–938. [6] P. Feng, G. Liang, J. Zhang, Ultrasonic vibration-assisted scratch characteristics of
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