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Mechanical behavior of α-Al2O3-coated SiC particle reinforced nickel matrix composites
Journal of Alloys and Compounds 570 (2013) 81–85
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Mechanical behavior of a-Al2O3-coated SiC particle reinforced nickel matrix composites Zhong Wu, Lei Liu ⇑, Bin Shen, Yating Wu, Yida Deng, Cheng Zhong, Wenbin Hu State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200040, China
a r t i c l e
i n f o
Article history: Received 5 February 2013 Accepted 12 March 2013 Available online 27 March 2013 Keywords: Metal matrix composites Sol–gel processes Mechanical properties Microstructure Dislocations
a b s t r a c t The influence of particle reinforcement on the mechanical behavior of nickel matrix composites with heat treatment at 600 °C was studied. The Ni/SiC composite, with SiC particles coated with sol–gel a-Al2O3, was fabricated by electrodeposition. Through the material design of the introduction of the a-Al2O3 coatings, the coated SiC particles still remain in the composite without reacting with nickel, so that the true effect of SiC particles on the mechanical behavior of the nickel matrix composite with heat treatment can be revealed. The result showed that the presence of coated SiC particles enhanced strength by inhibiting grain growth, while simultaneously interacting with dislocations. The yield strength of 338 MPa was achieved in the composite, increased by more than six times compared to pure nickel. Furthermore, the fracture mechanisms were discussed. Cracking of the coated SiC particles was observed in the composite, resulting in load transfer strengthening. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Nickel, being an engineering material, has been used in numerous applications for many years. Ni/SiC composites in particular, have been commercialized for the protection of friction parts, combustion engines and casting molds [1–3]. The presence of SiC particles in the composites will significantly affect the mechanical properties (e.g., hardness, tensile strength and wear behavior). Research into the effect of SiC particles given to the microstructure and properties of the composites has been investigated by numerous investigators [4–7]. However, little study has been focused on these composites with heat treatment at high temperature (above 450 °C). It is because that SiC begins to react with nickel above 450 °C [8], thus it is difficult to reveal the true effect of the SiC particles given to the composites on the mechanical behavior. Moreover, SiC reacts with nickel to produce brittle nickel silicide in the metal–ceramic interface which degrades the material, especially in high-temperature environments, such as aero-engine bearings (about 600 °C) [9]. To avoid the consumption of SiC particles, the use of coating on the reinforcements is now the leading investigation procedure [10–13]. Among them, alumina coatings obtained from sol–gel technology are the main ones used. From our previous study [14], Ni/a-Al2O3-coated SiC (Ni/CSp) composite shows excellent thermal stability without interfacial reaction and the coated SiC particles still remain in the nickel matrix with heat treatment at 600 °C. On the contrary, the uncoated SiC particles ⇑ Corresponding author. Tel.: +86 021 34202554; fax: +86 021 34202749. E-mail address: [email protected] (L. Liu). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.03.125
have reacted completely with nickel matrix, left many defects (pores and microcracks). Not surprisingly, the Ni/CSp composite exhibit much better mechanical properties (hardness and tensile properties) than the Ni/uncoated SiC (Ni/UCSp) composite [15]. The yield strength improvement of Ni/CSp composite with heat treatment at 600 °C is much better than that of Ni/UCSp composite. However, it is still unclear of the strengthening mechanisms of SiC particles for the nickel matrix composite, especially with heat treatment at 600 °C. In this study, through the material design of the introduction of the a-Al2O3 coatings, the coated SiC particles still remain in the Ni/ CSp composite, so that the true effect of SiC particles on the mechanical behavior of the nickel matrix composite with heat treatment can be revealed. The tensile properties of the Ni/CSp composite and pure nickel were tested and the relevant strengthening mechanisms of SiC particles for the composite with heat treatment at 600 °C, discussed based on the experimental evidences.
2. Experiment procedures 2.1. Coatings of SiC particles The a-Al2O3 coatings were made by the sol–gel route from aluminum nitrate (1 mol/L) and ammonia solution (1 mol/L) diluted in distilled water. Hydrolysis was left in acid conditions (nitric acid) for 24 h at 90 °C to obtain a homogeneous transparent sol. And then, SiC particles (99% purity), with a mean diameter of 2 lm, were submerged in the sol with an ultrasonic dispersion for 30 min. After that, the particles were filtered and cleaned to avoid their joining. Finally, they were calcined at about 1200 °C to obtain a-Al2O3 coatings on the surfaces of SiC particles.
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2.2. Composite material fabrication The Ni/CSp composite was fabricated by direct-current electrodeposition from a base electrolyte consisting of 300 g/L Ni(NH2SO3)24H2O, 40 g/L H3BO3, 10 g/L NiCl26H2O, 0.1 g/L cationic surfactant cetyltrimethylammonium bromide (CTAB) and 10 g/L a-Al2O3-coated SiC particles. The solution temperature was kept at 50 ± 1 °C and pH value was adjusted to 4.0 ± 0.2 with amine sulfonate. The electrolyte was stirred continuously by a magnet pump to maintain the particles in suspension. As a comparison, pure nickel sample was also prepared at the same condition. In order to investigate the effect of coated SiC particles given to the composite with heat treatment, all of the samples were heated at 600 °C for 24 h.
2.3. Composite material characterization Studies by field emission scanning electron microscope (FESEM) were held with a Philips SEM 515. The microstructure of the Ni/CSp composite was examined using a transmission electron microscope (TEM, JEOL-2100F). Grain size of the samples were examined with the digital microscope (VHX-1000E). Tensile specimens with a gauge width length of 4.0 mm 16.0 mm and a thickness of 0.55 mm were cut using a wire electrodischarging machine and then polished to a mirror-like finish surface. Tensile tests were carried out on a BTCT1-FR020TN.A50 system under a strain rate of 103 s1 at room temperature [15]. The yield strength (0.2% off-set) and elongation were determined from the load/ extension data.
the stress–strain curves of Ni/CSp composite and pure nickel with heat treatment at 600 °C. The yield strength increases from 43 MPa to 338 MPa by the addition of 10 vol.% coated SiC particles and a uniform elongation of 6.76% is retained, which shows that the coated SiC particles are the promising reinforcement in the nickel matrix composite in high-temperature conditions. The strengthening mechanisms of particles that are operative in the metal matrix composites can be mainly attributed to three factors: (i) the load-bearing effect from the reinforcement (DrL–T), in which the reinforcement can share the applied stress directly by stress transfer from the matrix [16], (ii) the grain refinement strengthening according to the Hall-Petch relationship (DrH–P) [17], and (iii) the dislocation strengthening in the matrix (Drdis), which is derived from the nucleation of additional dislocations in the matrix due to the introduction of the reinforcement particles [18,19]. These three factors are interdependent and act simultaneously, resulting in a combined effect, hence it is possible to estimate the yield strength of the composite (ryc) by assuming a linear superimposition of these mechanisms by following equations: [20]:
rym ¼ r0 þ DrHP þ Drdis
ð1Þ
3. Results and discussion
ryc ¼ rym þ DrLT
ð2Þ
The a-Al2O3-coated SiC particles have an average cross-sectional size of 2 lm with a standard deviation of 0.1 lm, as shown in Fig. 1a. A SEM micrograph of the cross-section of the Ni/CSp composite with heat treatment at 600 °C is shown in Fig. 1b. The image clearly indicates that the coated SiC particles (dark particles) are homogeneously dispersed in the nickel matrix. Fig. 1c shows
where r0 = 43 MPa is the yield strength of pure nickel, and rym is the yield strength of the nickel matrix. Grain size in the Ni/CSp composite and pure nickel was examined, as shown in Fig. 2. Optical microscope images show that the average grain size of pure nickel is 20 lm but around 5 lm in the matrix of the Ni/CSp composite with the same heat
Fig. 1. SEM image of a-Al2O3-coated SiC particles (a), cross-sectional micrograph of the Ni/CSp composite (b) and tensile properties of the Ni/CSp composite and pure nickel with heat treatment at 600 °C (c).
Z. Wu et al. / Journal of Alloys and Compounds 570 (2013) 81–85
83
Fig. 2. Optical microscope images of samples with heat treatment at 600 °C for pure nickel (a) and the Ni/CSp composite (b).
treatment at 600 °C. It is concluded that the codeposition of SiC particles inhibits grain growth of crystallites during the heat treatment, resulting in grain size refinement. The grain refinement strengthening is calculated with Eq. (3) [21]:
pffiffiffi DrHP ¼ ky = d
ð3Þ
where ky is a material constant and d is the mean grain size. Using ky = 26281 MPa nm1/2 for the pure nickel [22], DrH–P is calculated to be 185 MPa. The increase of the yield strength caused by dislocation strengthening is expressed via a quadratic relationship [23]:
Drdis ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðDror Þ2 þ ðDrthe Þ2 þ ðDrgeo Þ2
ð4Þ
Dror is the contribution of the Orowan strengthening from the presence of coated SiC particles and it can be given in its simplest form by Eq. (5) [24]:
Dror ¼ 2Gb=k
ð5Þ
where G is the shear modulus of the pure nickel matrix (76 GPa), b is the Burgers vector (2.49 1010 m) and k is the interparticle spacing. The k can be calculated for an ideally distributed two phase system by a mathematical mean analysis with the following equation:
k ¼ 2Dð1 V R Þ=3V R
ð6Þ
where D is the average diameter of SiC particle and VR is the volume fraction of the reinforcing particles. If the Ni/CSp composite is considered (VR = 0.10, D = 2 lm), the Dror is calculated to be only 3 MPa. For micrometric particle, dislocations can easily pass
through an array of impeding particles so that not leads to the increase of stress. Therefore the Orowan strengthening only contributes a very small part of strength and can be neglected here [25]. Drthe accounts for the stress contribution due to geometry necessary dislocations induced by the difference in the coefficient of thermal expansion (CTE) between the matrix and reinforcement. In other words, because the coefficient of thermal expansion of pure nickel matrix (13.4 106 K1) is much higher than that of SiC reinforcement particles (4.7 106 K1), there can be relatively high residual stress around SiC particles at ambient temperature after high temperature processing of such composites [26]. The equation proposed for prediction of yield strength increase by DCTE is, as follows [27]:
1=2 1=2 V R Be 1 Drthe ¼ aGb D 1 VR b
ð7Þ
where a is a constant that is equal to 1.25, G is the shear modulus of the pure nickel matrix (76 GPa), b is the Burgers vector (2.49 1010 m), VR is the volume fraction of reinforcement (0.10), B is 12 for spherical particle reinforcement, e the misfit strain that is a function of processing and test temperature due to the DCTE, and D is the average diameter of SiC particle (2 lm). The processing temperature is 600 °C and, assuming room temperature is 25 °C. Then, misfit strain e can be calculated by the following equation:
e ¼ DCTE DT
ð8Þ
The result of Drthe calculated with Eq. (7) is 87 MPa. Drgeo is the stress contribution due to strain gradient effect associated with geometrically necessary dislocations caused by
Fig. 3. Dislocation distribution adjacent to the coated SiC particles in the Ni/CSp composite with heat treatment at 600 °C before tensile test (a) and with 0.2% strain (b).
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Z. Wu et al. / Journal of Alloys and Compounds 570 (2013) 81–85
Fig. 4. Fracture surfaces of tensile samples with heat treatment at 600 °C for pure nickel (a) and the Ni/CSp composite (b) with the inset image showing the cracking of the coated SiC particles.
plastic deformation mismatch between the matrix and particles and it can be calculated by the following equation [28]:
Drgeo
sffiffiffiffiffiffiffiffiffiffiffiffiffi 8V R ey ¼ aGb bDL
ð9Þ
where a is a constant that is equal to 1.25, G is the shear modulus of the pure nickel matrix (76 GPa), b is the Burgers vector (2.49 1010 m), VR is the volume fraction of reinforcement (0.10), DL the diameter of the prismatic dislocation loop around the reinforcement particles, and ey the yielding strain. By using an approximate DL value of 2 lm, which is the average diameter of SiC particles [29] and ey value of 0.2%, Drgeo is calculated to be 42 MPa. According to the Eq. (4), the yield strength increase from dislocation strengthening (Drdis) is then calculated to be 97 MPa. TEM result (Fig. 3) also shows that higher density dislocations exist in the nickel matrix adjacent to the SiC particle before tensile test and with 0.2% strain, which is the qualitative evidence of dislocation strengthening. The load-bearing effect from the reinforcement (DrL–T) is evaluated using the modified shear lag model [16] and is equal to 16 MPa:
DrLT ¼
1 rym V R s 2
ð10Þ
where s is the aspect ratio of the reinforcement and taken to be 1, and VR represents the volume fraction of reinforcement (0.10). Thus the yield strength of the Ni/CSp composite is estimated to be ryc = 341 MPa by the Eq. (2). This result is quite coincided with the experimental value (338 MPa). Grain refinement and dislocation strengthening contributed much more in strengthening the Ni/CSp composite, while load transfer only contributed a small part, thus it is concluded that the first two factors play major roles in strengthening the composite. The load-bearing effect of the SiC particles is also evidenced by the fracture morphology of Ni/CSp composite, which displays larger dimples and some SiC particles cracking at the edges of the dimples (Fig. 4b and inset). The reason for load transfer is mainly the mismatch strain between SiC particles and nickel matrix produced during the tensile test owing to great difference in elastic modulus of them. It also means that the SiC particles are bonded well with nickel matrix and the external load can transfer efficiently from matrix to reinforcements, finally leading to the cracking of the SiC particles. Slight inter-diffusion at the particle-matrix interface may occur during the heat treatment, which improves the interfacial bonding strength to accommodate the mismatch strain. Whereas for pure nickel, typical microvoid coalescence fracture is
observed in Fig. 4a. Without SiC reinforcement sharing the load, only dimples and microcracks is revealed. 4. Conclusions The present results suggest that a-Al2O3-coated SiC particles can actually act as effective reinforcements in nickel matrix composites in high temperature conditions. With heat treatment at 600 °C, the Ni/CSp composite exhibits a yield strength improvement of more than six times of pure nickel, as well as a uniform elongation (6.76%) beyond the 5% standard for engineering applications. Our results show that coated SiC particles enhance strength by inhibiting grain growth, while simultaneously interacting with dislocations. Furthermore, cracking of the coated SiC particles is observed in the Ni/CSp composite, resulting in load transfer strengthening. Acknowledgements This research was supported by the National Key Technology R&D Program (No. 2011BAE13B08). The authors also like to acknowledge Instrumental Analysis Center of Shanghai Jiao Tong University for sample characterization. References [1] F. Hu, K.C. Chan, Appl. Surf. Sci. 243 (2005) 251–258. [2] C.M. Das, P.K. Limaye, A.K. Grover, A.K. Suri, J. Alloys Comp. 436 (2007) 328– 334. [3] T. Lampke, A. Leopold, D. Dietrich, G. Alisch, B. Wielage, Surf. Coat. Technol. 201 (2006) 3510–3517. [4] P. Ari-Gur, J. Sariel, S. Vemuganti, J. Alloys Comp. 434–435 (2007) 704–706. [5] A.F. Zimmerman, G. Palumbo, K.T. Aust, U. Erb, Mater. Sci. Eng. A. 328 (2002) 137–146. [6] M. Srivastava, V.K. William Grips, K.S. Rajam, Mater. Lett. 62 (2008) 3487– 3489. [7] K.C. Chan, G.F. Wang, C.L. Wang, K.F. Zhang, Mater. Sci. Eng. A. 404 (2005) 108– 116. [8] A. Bächli, M.A. Nicolet, L. Baud, C. Jaussaud, R. Madar, Mater. Sci. Eng. B. 56 (1998) 11–23. [9] F. Michael, Aerosp. Sci. Technol. 10 (2006) 611–617. [10] B. Kindl, Y.L. Liu, E. Nyberg, N. Hansen, Compos. Sci. Technol. 43 (1992) 85–93. [11] D.Y. Ding, J.C. Rao, D.Z. Wang, Z.Y. Ma, L. Geng, C.K. Yao, Mater. Sci. Eng. A. 279 (2000) 138–141. [12] H. Fritze, J. Jojic, T. Witke, C. Rüscher, S. Weber, S. Scherrer, R. Weiß, B. Schultrich, G. Borchardt, J. Eur. Ceram. Soc. 18 (1998) 2351–2364. [13] A. Ureña, P. Escalera, J.L. Rodrigo, L. Baldonedo, Gil, J. Microsc. 201 (2001) 122– 136. [14] Z. Wu, B. Shen, L. Liu, Surf. Coat. Technol. 206 (2012) 3173–3178. [15] Z. Wu, L. Liu, B. Shen, C. Zhong, W. Hu, Mater. Sci. Eng. A. 556 (2012) 767–774. [16] V.C. Nardone, K.M. Prewo, Scripta. Metal. 20 (1986) 43–48. [17] M.J. Bermingham, S.D. McDonald, M.S. Dargusch, D.H. StJohn, Scripta. Metal. 58 (2008) 1050–1053. [18] M. Vogelsang, R.J. Arsenault, R.M. Fisher, Metall. Trans. A 17A (1986) 379–389. [19] N. Shi, B. Wilner, R.J. Arsenault, Acta. Metal. Mater. 40 (1992) 2841–2854.
Z. Wu et al. / Journal of Alloys and Compounds 570 (2013) 81–85 [20] S. Joshi, C. Eberl, B. Cao, K. Ramesh, K. Hemker, Exp. Mech. 49 (2009) 207–218. [21] J.P. Hirth, in: W.C. Robert, H. Peter (Eds.), Physical Metallurgy, fourth ed., Elsevier Science B.V, North-Holland, 1996, pp. 1831–1875. [22] A.M. El-Sherik, U. Erb, G. Palumbo, K.T. Aust, Scripta, Metal. Mater. 27 (1992) 1185–1188. [23] S. Scudino, G. Liu, M. Sakaliyska, K.B. Surreddi, J. Eckert, Acta. Mater. 57 (2009) 4529–4538. [24] F.J. Humphreys, A. Basu, M.R. Djazeb, in: Hansen N (Eds.), Metal Matrix Composites-Processing, Microstructure and Properties: Proceedings of the
[25] [26] [27] [28] [29]
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12th Risø international symposium on material science, Risø National Laboratory, Roskilde, Denmark, 1991, pp. 51–66. B. Zong, F. Zhang, G. Wang, L. Zuo, J. Mater. Sci. 42 (2007) 4215–4226. R.J. Arsenault, N. Shi, Mater. Sci. Eng. 81 (1986) 175–187. F. Tang, I.E. Anderson, T. Gnaupel-Herold, H. Prask, Mater. Sci. Eng. A. 383 (2004) 362–373. M.A. Meyers, K.K. Chawla, Mechanical Metallurgy-Principles and Applications, Prentice-Hall, New Jersey, 1984. F. Tang, I.E. Anderson, S.B. Biner, J. Light. Met. 2 (2002) 201–214.
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Mechanical behavior of a-Al2O3-coated SiC particle reinforced nickel matrix composites Zhong Wu, Lei Liu ⇑, Bin Shen, Yating Wu, Yida Deng, Cheng Zhong, Wenbin Hu State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200040, China
a r t i c l e
i n f o
Article history: Received 5 February 2013 Accepted 12 March 2013 Available online 27 March 2013 Keywords: Metal matrix composites Sol–gel processes Mechanical properties Microstructure Dislocations
a b s t r a c t The influence of particle reinforcement on the mechanical behavior of nickel matrix composites with heat treatment at 600 °C was studied. The Ni/SiC composite, with SiC particles coated with sol–gel a-Al2O3, was fabricated by electrodeposition. Through the material design of the introduction of the a-Al2O3 coatings, the coated SiC particles still remain in the composite without reacting with nickel, so that the true effect of SiC particles on the mechanical behavior of the nickel matrix composite with heat treatment can be revealed. The result showed that the presence of coated SiC particles enhanced strength by inhibiting grain growth, while simultaneously interacting with dislocations. The yield strength of 338 MPa was achieved in the composite, increased by more than six times compared to pure nickel. Furthermore, the fracture mechanisms were discussed. Cracking of the coated SiC particles was observed in the composite, resulting in load transfer strengthening. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Nickel, being an engineering material, has been used in numerous applications for many years. Ni/SiC composites in particular, have been commercialized for the protection of friction parts, combustion engines and casting molds [1–3]. The presence of SiC particles in the composites will significantly affect the mechanical properties (e.g., hardness, tensile strength and wear behavior). Research into the effect of SiC particles given to the microstructure and properties of the composites has been investigated by numerous investigators [4–7]. However, little study has been focused on these composites with heat treatment at high temperature (above 450 °C). It is because that SiC begins to react with nickel above 450 °C [8], thus it is difficult to reveal the true effect of the SiC particles given to the composites on the mechanical behavior. Moreover, SiC reacts with nickel to produce brittle nickel silicide in the metal–ceramic interface which degrades the material, especially in high-temperature environments, such as aero-engine bearings (about 600 °C) [9]. To avoid the consumption of SiC particles, the use of coating on the reinforcements is now the leading investigation procedure [10–13]. Among them, alumina coatings obtained from sol–gel technology are the main ones used. From our previous study [14], Ni/a-Al2O3-coated SiC (Ni/CSp) composite shows excellent thermal stability without interfacial reaction and the coated SiC particles still remain in the nickel matrix with heat treatment at 600 °C. On the contrary, the uncoated SiC particles ⇑ Corresponding author. Tel.: +86 021 34202554; fax: +86 021 34202749. E-mail address: [email protected] (L. Liu). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.03.125
have reacted completely with nickel matrix, left many defects (pores and microcracks). Not surprisingly, the Ni/CSp composite exhibit much better mechanical properties (hardness and tensile properties) than the Ni/uncoated SiC (Ni/UCSp) composite [15]. The yield strength improvement of Ni/CSp composite with heat treatment at 600 °C is much better than that of Ni/UCSp composite. However, it is still unclear of the strengthening mechanisms of SiC particles for the nickel matrix composite, especially with heat treatment at 600 °C. In this study, through the material design of the introduction of the a-Al2O3 coatings, the coated SiC particles still remain in the Ni/ CSp composite, so that the true effect of SiC particles on the mechanical behavior of the nickel matrix composite with heat treatment can be revealed. The tensile properties of the Ni/CSp composite and pure nickel were tested and the relevant strengthening mechanisms of SiC particles for the composite with heat treatment at 600 °C, discussed based on the experimental evidences.
2. Experiment procedures 2.1. Coatings of SiC particles The a-Al2O3 coatings were made by the sol–gel route from aluminum nitrate (1 mol/L) and ammonia solution (1 mol/L) diluted in distilled water. Hydrolysis was left in acid conditions (nitric acid) for 24 h at 90 °C to obtain a homogeneous transparent sol. And then, SiC particles (99% purity), with a mean diameter of 2 lm, were submerged in the sol with an ultrasonic dispersion for 30 min. After that, the particles were filtered and cleaned to avoid their joining. Finally, they were calcined at about 1200 °C to obtain a-Al2O3 coatings on the surfaces of SiC particles.
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Z. Wu et al. / Journal of Alloys and Compounds 570 (2013) 81–85
2.2. Composite material fabrication The Ni/CSp composite was fabricated by direct-current electrodeposition from a base electrolyte consisting of 300 g/L Ni(NH2SO3)24H2O, 40 g/L H3BO3, 10 g/L NiCl26H2O, 0.1 g/L cationic surfactant cetyltrimethylammonium bromide (CTAB) and 10 g/L a-Al2O3-coated SiC particles. The solution temperature was kept at 50 ± 1 °C and pH value was adjusted to 4.0 ± 0.2 with amine sulfonate. The electrolyte was stirred continuously by a magnet pump to maintain the particles in suspension. As a comparison, pure nickel sample was also prepared at the same condition. In order to investigate the effect of coated SiC particles given to the composite with heat treatment, all of the samples were heated at 600 °C for 24 h.
2.3. Composite material characterization Studies by field emission scanning electron microscope (FESEM) were held with a Philips SEM 515. The microstructure of the Ni/CSp composite was examined using a transmission electron microscope (TEM, JEOL-2100F). Grain size of the samples were examined with the digital microscope (VHX-1000E). Tensile specimens with a gauge width length of 4.0 mm 16.0 mm and a thickness of 0.55 mm were cut using a wire electrodischarging machine and then polished to a mirror-like finish surface. Tensile tests were carried out on a BTCT1-FR020TN.A50 system under a strain rate of 103 s1 at room temperature [15]. The yield strength (0.2% off-set) and elongation were determined from the load/ extension data.
the stress–strain curves of Ni/CSp composite and pure nickel with heat treatment at 600 °C. The yield strength increases from 43 MPa to 338 MPa by the addition of 10 vol.% coated SiC particles and a uniform elongation of 6.76% is retained, which shows that the coated SiC particles are the promising reinforcement in the nickel matrix composite in high-temperature conditions. The strengthening mechanisms of particles that are operative in the metal matrix composites can be mainly attributed to three factors: (i) the load-bearing effect from the reinforcement (DrL–T), in which the reinforcement can share the applied stress directly by stress transfer from the matrix [16], (ii) the grain refinement strengthening according to the Hall-Petch relationship (DrH–P) [17], and (iii) the dislocation strengthening in the matrix (Drdis), which is derived from the nucleation of additional dislocations in the matrix due to the introduction of the reinforcement particles [18,19]. These three factors are interdependent and act simultaneously, resulting in a combined effect, hence it is possible to estimate the yield strength of the composite (ryc) by assuming a linear superimposition of these mechanisms by following equations: [20]:
rym ¼ r0 þ DrHP þ Drdis
ð1Þ
3. Results and discussion
ryc ¼ rym þ DrLT
ð2Þ
The a-Al2O3-coated SiC particles have an average cross-sectional size of 2 lm with a standard deviation of 0.1 lm, as shown in Fig. 1a. A SEM micrograph of the cross-section of the Ni/CSp composite with heat treatment at 600 °C is shown in Fig. 1b. The image clearly indicates that the coated SiC particles (dark particles) are homogeneously dispersed in the nickel matrix. Fig. 1c shows
where r0 = 43 MPa is the yield strength of pure nickel, and rym is the yield strength of the nickel matrix. Grain size in the Ni/CSp composite and pure nickel was examined, as shown in Fig. 2. Optical microscope images show that the average grain size of pure nickel is 20 lm but around 5 lm in the matrix of the Ni/CSp composite with the same heat
Fig. 1. SEM image of a-Al2O3-coated SiC particles (a), cross-sectional micrograph of the Ni/CSp composite (b) and tensile properties of the Ni/CSp composite and pure nickel with heat treatment at 600 °C (c).
Z. Wu et al. / Journal of Alloys and Compounds 570 (2013) 81–85
83
Fig. 2. Optical microscope images of samples with heat treatment at 600 °C for pure nickel (a) and the Ni/CSp composite (b).
treatment at 600 °C. It is concluded that the codeposition of SiC particles inhibits grain growth of crystallites during the heat treatment, resulting in grain size refinement. The grain refinement strengthening is calculated with Eq. (3) [21]:
pffiffiffi DrHP ¼ ky = d
ð3Þ
where ky is a material constant and d is the mean grain size. Using ky = 26281 MPa nm1/2 for the pure nickel [22], DrH–P is calculated to be 185 MPa. The increase of the yield strength caused by dislocation strengthening is expressed via a quadratic relationship [23]:
Drdis ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðDror Þ2 þ ðDrthe Þ2 þ ðDrgeo Þ2
ð4Þ
Dror is the contribution of the Orowan strengthening from the presence of coated SiC particles and it can be given in its simplest form by Eq. (5) [24]:
Dror ¼ 2Gb=k
ð5Þ
where G is the shear modulus of the pure nickel matrix (76 GPa), b is the Burgers vector (2.49 1010 m) and k is the interparticle spacing. The k can be calculated for an ideally distributed two phase system by a mathematical mean analysis with the following equation:
k ¼ 2Dð1 V R Þ=3V R
ð6Þ
where D is the average diameter of SiC particle and VR is the volume fraction of the reinforcing particles. If the Ni/CSp composite is considered (VR = 0.10, D = 2 lm), the Dror is calculated to be only 3 MPa. For micrometric particle, dislocations can easily pass
through an array of impeding particles so that not leads to the increase of stress. Therefore the Orowan strengthening only contributes a very small part of strength and can be neglected here [25]. Drthe accounts for the stress contribution due to geometry necessary dislocations induced by the difference in the coefficient of thermal expansion (CTE) between the matrix and reinforcement. In other words, because the coefficient of thermal expansion of pure nickel matrix (13.4 106 K1) is much higher than that of SiC reinforcement particles (4.7 106 K1), there can be relatively high residual stress around SiC particles at ambient temperature after high temperature processing of such composites [26]. The equation proposed for prediction of yield strength increase by DCTE is, as follows [27]:
1=2 1=2 V R Be 1 Drthe ¼ aGb D 1 VR b
ð7Þ
where a is a constant that is equal to 1.25, G is the shear modulus of the pure nickel matrix (76 GPa), b is the Burgers vector (2.49 1010 m), VR is the volume fraction of reinforcement (0.10), B is 12 for spherical particle reinforcement, e the misfit strain that is a function of processing and test temperature due to the DCTE, and D is the average diameter of SiC particle (2 lm). The processing temperature is 600 °C and, assuming room temperature is 25 °C. Then, misfit strain e can be calculated by the following equation:
e ¼ DCTE DT
ð8Þ
The result of Drthe calculated with Eq. (7) is 87 MPa. Drgeo is the stress contribution due to strain gradient effect associated with geometrically necessary dislocations caused by
Fig. 3. Dislocation distribution adjacent to the coated SiC particles in the Ni/CSp composite with heat treatment at 600 °C before tensile test (a) and with 0.2% strain (b).
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Fig. 4. Fracture surfaces of tensile samples with heat treatment at 600 °C for pure nickel (a) and the Ni/CSp composite (b) with the inset image showing the cracking of the coated SiC particles.
plastic deformation mismatch between the matrix and particles and it can be calculated by the following equation [28]:
Drgeo
sffiffiffiffiffiffiffiffiffiffiffiffiffi 8V R ey ¼ aGb bDL
ð9Þ
where a is a constant that is equal to 1.25, G is the shear modulus of the pure nickel matrix (76 GPa), b is the Burgers vector (2.49 1010 m), VR is the volume fraction of reinforcement (0.10), DL the diameter of the prismatic dislocation loop around the reinforcement particles, and ey the yielding strain. By using an approximate DL value of 2 lm, which is the average diameter of SiC particles [29] and ey value of 0.2%, Drgeo is calculated to be 42 MPa. According to the Eq. (4), the yield strength increase from dislocation strengthening (Drdis) is then calculated to be 97 MPa. TEM result (Fig. 3) also shows that higher density dislocations exist in the nickel matrix adjacent to the SiC particle before tensile test and with 0.2% strain, which is the qualitative evidence of dislocation strengthening. The load-bearing effect from the reinforcement (DrL–T) is evaluated using the modified shear lag model [16] and is equal to 16 MPa:
DrLT ¼
1 rym V R s 2
ð10Þ
where s is the aspect ratio of the reinforcement and taken to be 1, and VR represents the volume fraction of reinforcement (0.10). Thus the yield strength of the Ni/CSp composite is estimated to be ryc = 341 MPa by the Eq. (2). This result is quite coincided with the experimental value (338 MPa). Grain refinement and dislocation strengthening contributed much more in strengthening the Ni/CSp composite, while load transfer only contributed a small part, thus it is concluded that the first two factors play major roles in strengthening the composite. The load-bearing effect of the SiC particles is also evidenced by the fracture morphology of Ni/CSp composite, which displays larger dimples and some SiC particles cracking at the edges of the dimples (Fig. 4b and inset). The reason for load transfer is mainly the mismatch strain between SiC particles and nickel matrix produced during the tensile test owing to great difference in elastic modulus of them. It also means that the SiC particles are bonded well with nickel matrix and the external load can transfer efficiently from matrix to reinforcements, finally leading to the cracking of the SiC particles. Slight inter-diffusion at the particle-matrix interface may occur during the heat treatment, which improves the interfacial bonding strength to accommodate the mismatch strain. Whereas for pure nickel, typical microvoid coalescence fracture is
observed in Fig. 4a. Without SiC reinforcement sharing the load, only dimples and microcracks is revealed. 4. Conclusions The present results suggest that a-Al2O3-coated SiC particles can actually act as effective reinforcements in nickel matrix composites in high temperature conditions. With heat treatment at 600 °C, the Ni/CSp composite exhibits a yield strength improvement of more than six times of pure nickel, as well as a uniform elongation (6.76%) beyond the 5% standard for engineering applications. Our results show that coated SiC particles enhance strength by inhibiting grain growth, while simultaneously interacting with dislocations. Furthermore, cracking of the coated SiC particles is observed in the Ni/CSp composite, resulting in load transfer strengthening. Acknowledgements This research was supported by the National Key Technology R&D Program (No. 2011BAE13B08). The authors also like to acknowledge Instrumental Analysis Center of Shanghai Jiao Tong University for sample characterization. References [1] F. Hu, K.C. Chan, Appl. Surf. Sci. 243 (2005) 251–258. [2] C.M. Das, P.K. Limaye, A.K. Grover, A.K. Suri, J. Alloys Comp. 436 (2007) 328– 334. [3] T. Lampke, A. Leopold, D. Dietrich, G. Alisch, B. Wielage, Surf. Coat. Technol. 201 (2006) 3510–3517. [4] P. Ari-Gur, J. Sariel, S. Vemuganti, J. Alloys Comp. 434–435 (2007) 704–706. [5] A.F. Zimmerman, G. Palumbo, K.T. Aust, U. Erb, Mater. Sci. Eng. A. 328 (2002) 137–146. [6] M. Srivastava, V.K. William Grips, K.S. Rajam, Mater. Lett. 62 (2008) 3487– 3489. [7] K.C. Chan, G.F. Wang, C.L. Wang, K.F. Zhang, Mater. Sci. Eng. A. 404 (2005) 108– 116. [8] A. Bächli, M.A. Nicolet, L. Baud, C. Jaussaud, R. Madar, Mater. Sci. Eng. B. 56 (1998) 11–23. [9] F. Michael, Aerosp. Sci. Technol. 10 (2006) 611–617. [10] B. Kindl, Y.L. Liu, E. Nyberg, N. Hansen, Compos. Sci. Technol. 43 (1992) 85–93. [11] D.Y. Ding, J.C. Rao, D.Z. Wang, Z.Y. Ma, L. Geng, C.K. Yao, Mater. Sci. Eng. A. 279 (2000) 138–141. [12] H. Fritze, J. Jojic, T. Witke, C. Rüscher, S. Weber, S. Scherrer, R. Weiß, B. Schultrich, G. Borchardt, J. Eur. Ceram. Soc. 18 (1998) 2351–2364. [13] A. Ureña, P. Escalera, J.L. Rodrigo, L. Baldonedo, Gil, J. Microsc. 201 (2001) 122– 136. [14] Z. Wu, B. Shen, L. Liu, Surf. Coat. Technol. 206 (2012) 3173–3178. [15] Z. Wu, L. Liu, B. Shen, C. Zhong, W. Hu, Mater. Sci. Eng. A. 556 (2012) 767–774. [16] V.C. Nardone, K.M. Prewo, Scripta. Metal. 20 (1986) 43–48. [17] M.J. Bermingham, S.D. McDonald, M.S. Dargusch, D.H. StJohn, Scripta. Metal. 58 (2008) 1050–1053. [18] M. Vogelsang, R.J. Arsenault, R.M. Fisher, Metall. Trans. A 17A (1986) 379–389. [19] N. Shi, B. Wilner, R.J. Arsenault, Acta. Metal. Mater. 40 (1992) 2841–2854.
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