Microstructures and mechanical properties of the Al2014 composites reinforced with bimodal sized SiC particles

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Microstructures and mechanical properties of the Al2014 composites reinforced with bimodal sized SiC particles Long-Jiang Zhang, Feng Qiu, Jin-Guo Wang, Hui-Yuan Wang, Qi-Chuan Jiang

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S0921-5093(15)00409-8 http://dx.doi.org/10.1016/j.msea.2015.04.012 MSA32235

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Materials Science & Engineering A

Received date: 28 February 2015 Revised date: 30 March 2015 Accepted date: 3 April 2015 Cite this article as: Long-Jiang Zhang, Feng Qiu, Jin-Guo Wang, Hui-Yuan Wang, Qi-Chuan Jiang, Microstructures and mechanical properties of the Al2014 composites reinforced with bimodal sized SiC particles, Materials Science & Engineering A, http://dx.doi.org/10.1016/j.msea.2015.04.012 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.

Microstructures and mechanical properties of the Al2014 composites reinforced with bimodal sized SiC particles Long-Jiang Zhang, Feng Qiu‡, Jin-Guo Wang, Hui-Yuan Wang, Qi-Chuan Jiang† Key Laboratory of Automobile Materials, Ministry of Education, and Department of Materials Science and Engineering, Jilin University, No. 5988 Renmin Street, Changchun, 130025, P.R. China. Abstract The (micron + nano) bimodal sized SiCp/Al2014 composites fabricated by semi-solid stirring exhibited fine Į-Al grains, well-dispersed bimodal sized SiCp and well-bonded interface between SiCp and matrix. The yield and ultimate tensile strength of the extruded bimodal sized SiCp/Al2014 composites were significantly enhanced, comparing to the extruded single-sized SiCp/Al2014 composites.

Keywords: Composites; Bimodal sized SiC particles; Mechanical properties;

Corresponding author. Tel./fax:+86 431 85094699. E-mail address: [email protected] (Feng Qiu). † Corresponding author. Tel./fax:+86 431 85094699. E-mail address: [email protected] (Qi-Chuan Jiang). ‡

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1. Introduction It is well established that the reinforcement size has significant impacts on the mechanical properties of particles reinforced metal matrix composites (PMMCs) [1,2]. Commonly the micron-sized particles are used to improve the strength and elastic modulus of the metal-matrix, while the ductility decreases severely. On the other hand, the nano-sized particles can obviously strengthen the metal-matrix without sacrificing the ductility. However, the agglomerations appear when the nano-particles content is higher than 1 vol.% [3,4]. Then the enhancement of modulus is restricted. Therefore, the mixture of micron-sized and a little amount nano-sized particles might have significant influence on the mechanical properties of the composites. Previous studies primarily focused on the thermal properties of bimodal sized PMMCs [5-7], but little attention had been paid to their mechanical properties. Tabandeh Khorshid et al. [8] studied the effect of the ratio of nano- to submicron-sized Al2O3 particles on the mechanical properties of Al matrix composites, and suggested that by increasing the nano-particles content, the hardness and strength of the composites first increase and then decrease when the amount of the nano-particles exceeds 4 wt.%. Shen

et al. [9] investigated the influence of bimodal sized SiC particles (micron + nano) on the mechanical properties of AZ31B matrix alloy, and found that the yield and ultimate tensile strength of bimodal sized SiCp/AZ31B composites were enhanced, comparing with single-sized SiCp reinforced composites. The researches by Deng et al. [10] also revealed that the yield strength of bimodal sized particles (10 µm + 0.2 µm)

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reinforced composites was higher than that of single-sized particles reinforced composites. But few works have been done about the mechanical properties of the (micron + nano) bimodal sized SiCp/Al composites at present. Besides, a lot of works have been done about the strengthening mechanisms of single-sized particles reinforced aluminum matrix composites [11-15]. However, very limited strengthening mechanisms of the bimodal sized particles reinforced aluminum matrix composites were given at present. Therefore, in this paper, the mechanical properties of bimodal sized (micron + nano) SiCp/Al2014 composites were investigated, and the strengthening mechanisms were discussed. 2. Experimental procedures The bimodal sized SiCp/Al2014 composites containing 1 vol.% of 40 nm nano-SiCp and 4 vol.% of 15 µm micron-SiCp (denote as “N-1 + M-4”) was fabricated by semi-solid stirring. The Al2014 matrix alloy was melted at 1023 K in a graphite crucible. As the bimodal sized SiCp mixed with Al2014 alloy powders by ball milling were all added into the melt, the melt was cooled to 873 K at which the alloy was in semi-solid condition. Then the melt were cast into a preheated cylindrical steel mould as soon as possible after semi-solid stirring for 30 min. Finally, the cast cylindrical rods were extruded with an extrusion ratio of 18:1 to get sheet samples. To identify the effect of bimodal sized SiC particles on the Al2014 matrix alloy, 15 µm 4 vol.% (denote as “M-4”) as well as 40 nm 1 vol.% (denote as “N-1”) SiCp/Al2014 composites were fabricated by using the same processing.

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The microstructures were detected by optical microscope (OM, Axio. Image. A2m, Carl Zeiss, Germany), scanning electron microscopy (SEM, Evo18, Carl Zeiss, Germany) and

high-resolution transmission electron

microscopy

(HRTEM,

JEM-2100F, Japan). The tensile tests referring to the National Standards of People’s Republic of China (GB/T228-2002) [16] were carried out under a servo-hydraulic materials testing system (MTS, MTS810, USA) with the strain rate of 3×10-4 s-1. The elastic modulus tests referring to the National Standards of People’s Republic of China (GB/T22315-2008) [17] were detected by ultrasonic etching thickness gauge (Olympus-NDT, 38DLP-XT). The TEM specimens, 0.5 mm in thickness, were cut by spark erosion and slowly thinned to 30 µm in thickness by 600 and 2000 grit abrasive papers, followed by punching 3 mm diameter discs. Finally, the discs were ion beam thinned. According to the quadrat method [18,19], the distribution of SiC particles in the composites was evaluated by formula (1):

β=

q

×(

( q − 1)( q − 2 )

Ni − N

σ

)3

(1)

q

¦ (N − N ) i

σ=

2

i =1

(2)

q

Where q is the total number of quadrats, Ni is the number of SiC particles in the

ith quadrat, N is mean number of each quadrat, σ is the standard deviation, the uniform distribution of SiC particles in the composites decreases with the increase of

ȕ. The quadrat method was performed on 50 SEM micrographs within square fields. Each field was divided into 36 quadrats with the same sizes. 4

3. Results and discussion Fig. 1 gives the engineering stress-strain curves of the extruded Al2014 alloy and SiCp/Al2014 composites. The yield strength (ı0.2), ultimate tensile strength (ıUTS), fracture strain (İf) and modulus (E) of them are summarized in Table 1. It can be seen that the tensile strengths and modulus of N-1 + M-4 SiCp/Al2014 composites were higher than those of the Al2014 alloy and single-sized SiCp reinforced composites. This is consistent with the results in reference [9]. Firstly, the ı0.2, ıUTS and E of the N-1 + M-4 SiCp/Al2014 composites are remarkably improved from 312 MPa, 512 MPa and 72.8 GPa to 358 MPa, 585 MPa and 80.8 GPa, respectively, comparing with the N-1 SiCp/Al2014 composites. However, the İf of the N-1 + M-4 SiCp/Al2014 composites slightly decreases from 10.4% to 9.9%. Besides, in contrast to the M-4 SiCp/Al2014 composites, the addition of 1 vol.% nano-SiC particles obviously enhances its ı0.2, ıUTS, İf and E from 302 MPa, 503 MPa, 4.9% and 75.5 GPa to 358 MPa, 585 MPa, 9.9% and 80.8 GPa, respectively. Fig. 2 shows the optical microstructures of as-cast Al2014 alloy and SiCp/Al2014 composites. As indicated, in the Al2014 alloy (Fig. 2(a)), the average size of Į-Al grains is 122 µm. In the N-1 SiCp/Al2014 composites, the average size of Į-Al grains is 67 µm (Fig. 2(b)). While in M-4 SiCp/Al2014 composites, the Į-Al grains change from cystiform structure to dendrite with little change of grain size. This is because the “push” effect of the solid-liquid interface on the micron-SiC particles, resulting in the growth of Į-Al dendrites as been explained in our previous study [20]. Furthermore, the average size of Į-Al grains in the N-1 + M-4 SiCp/Al2014

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composites is 50 µm as shown in Figs. 2(d). It illustrates that the bimodal sized SiC particles have significantly effect on refining the grain size of as-cast composites, which was also shown in previous studies [8-10]. According to the classic Hall-Petch equation: ıy = ı0 + Kyd-1/2, where ıy is the yield strength, ı0 and Ky are material constants, and d is the mean grain size. The classic Hall-Petch equation illustrates that the ıy of the composites increases as the decrease of grain size. Fig. 3 shows the SEM micrographs of extruded SiCp/Al2014 composites perpendicular to the extrusion direction. The ȕ values of the composites are summarized in Table 1. It can be seen that the nano-SiCp distributed well in the N-1 SiCp/Al2014 composites, corresponding to a ȕ value of 0.38. The morphology at higher magnification of N-1 + M-4 SiCp/Al2014 composites is shown in Fig. 3(d). It can be found that the nano-SiCp distributed relatively uniformly in the matrix, corresponding to a ȕ value of 0.25. In addition, one can see that the micron-SiCp distributed relatively homogeneous in the M-4 SiCp/Al2014 composites (Fig. 3(b)) and N-1 + M-4 SiCp/Al2014 composites (Fig. 3 (c)), corresponding to a ȕ value of 0.32 and 0.21, respectively. It suggests that the distribution of the micron-SiCp and nano-SiCp in N-1 + M-4 SiCp/Al2014 composites are more homogeneous, as compared to the single-sized SiCp/Al2014 composites. It was indicated that the strength of the composites was significantly enhanced by the improvement of particles distribution [9,21]. Fig. 4 shows the TEM microstructures of the extruded N-1 + M-4 SiCp/Al2014 composites. By the TEM observation of nano-SiCp in Fig. 4(b), the nano-SiCp bond

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well with the matrix, the interface between nano-SiCp and Al2014 matrix is clean. The micron-SiCp also bond well with the matrix. But the interfacial reaction products (about 3 nm in thickness) exist at the interface. It is identified as Al2O3 phase according to the TEM-EDS analysis and selected area electron diffraction (SAED) patterns of the interface area in Fig. 4(c). The well bonded interfaces between bimodal sized SiCp and matrix can benefit to the effective transfer of tensile load from the matrix to the particles, and thus improving the tensile strength of the composites. Moreover, the mismatch of coefficient of thermal expansion (CTE) between SiC particles and Al2014 matrix leads to geometrically necessary dislocation generation in the vicinity of particle during cooling process after hot deformation, which may cause an increase of yield strength [10]. Based on above analysis, the significant increase of the tensile strengths of bimodal sized SiCp/Al2014 composites may be attributed to grain refinement, uniform particles distribution, load transfer effect and CTE mismatch strengthening.

4. Conclusion The (micron + nano) bimodal sized SiCp/Al2014 composites fabricated by semi-solid stirring exhibited fine Į-Al grains, homogeneous bimodal sized SiCp distribution and well bonded interface between bimodal sized SiCp and matrix. Compared with the extruded single-sized SiCp/Al2014 composites, the yield and ultimate tensile strength of bimodal sized SiCp/Al2014 composites were significantly enhanced. The improvements may be attributed to grain refinement, uniform bimodal sized SiCp distribution, load transfer effect and CTE mismatch strengthening.

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Acknowledgements This work is supported by the National Basic Research Program of China (973 Program, No. 2012CB619600), the Research Fund for the Doctoral Program of Higher Education of China (No. 20130061110037) and the Project 985–High Performance Materials of Jilin University.

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(2010) 3880-3884. [9] M.J. Shen, X.J.Wang, M.F. Zhang, X.S. Hu, M.Y. Zheng, K. Wu, Mater. Sci. Eng. A 601 (2014) 58-64. [10] K.K. Deng, C.J. Wang, X.J. Wang, Y.W. Wu, K.B. Nie, K. Wu, Compos. Part A 43 (2012) 1280-1284. [11] G. Ramu, B. Bauri, Mater. Des. 30 (2009) 3554-3559. [12] Z.P. Luo, Acta mater. 54 (2006) 47-58. [13] B.L. Bramfitt, Metall. Trans. 7 (1970) 1987-1995. [14] Z.P. Luo, Y.G. Song, S.Q. Zhang, Scripta Mater. 45 (2001) 1183-1189. [15] A. Luo, Scripta Metall. Mater. 31 (1993) 1253-1258. [16] GB/T228-2002. Metallic materials–Tensile testing at ambient temperature. Standardization Administration of the People’s Republic of China; 2002. [17] GB/T22315-2008. Metallic materials–Determination of modulus of elasticity and Poission's ratio test. Standardization Administration of the People’s Republic of China; 2008. [18] P.A. Karnezis, G. Durrant, B. Cantor, Mater. Character. 40 (1998) 97-109. [19] I. Sabirov, O. Kolednik, R.Z. Valiev, R. Pippan, Acta Mater. 53 (2005) 4919-4930. [20] L.J. Zhang, F. Qiu, J.G. Wang, Q.C. Jiang, Mater. Sci. Eng. A 626 (2015) 338-341. [21] Z. Zhang, D.L. Chen, Mater. Sci. Eng. A 483-484 (2008) 148-152.

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Table 1 The elastic modulus, tensile properties and ȕ of the extruded Al2014 alloy and SiCp/Al2014 composites. Samples

Ε (GPa)

σ0.2 (MPa)

σUTS (MPa)

εf (%)

ȕ

Al2014

77.8

242 -+98

460 -23

+12

1.0 17.1 -+2.0

-

N-1 composites

80.2

312 -6+8

512 -6+15

+1.5 10.4 -0.5

0.38

M-4 composites

83.4

302 -9+12

+10 503 -11

+1.0 4.9 -0.5

0.32

N-1+M-4 composites

88

+14 358 -14

+20 585 -20

+1.0 9.9 -1.0

N-1:0.25 M-4:0.21

Fig. 1 The tensile engineering stress-strain curves of Al2014 alloy and SiCp/Al2014 composites after hot extrusion. Fig. 2 The OM micrographs of as-cast (a) Al2014 alloy, (b) N-1, (c) M-4 and (d) N-1+M-4 SiCp/Al2014 composites. Fig. 3 The SEM micrographs of extruded (a) N-1, (b) M-4 and (c) N-1 + M-4 SiCp/Al2014 composites, (d) is a higher magnification of (c). Fig. 4 (a) The TEM micrographs of N-1 + M-4 SiCp/Al2014 composites after hot extrusion, (b) the interface between nano-SiCp and matrix, (c) the interface between micron-SiCp and matrix and (d) the TEM-EDS of the interface between micron-SiCp and matrix.

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Figure(s)

Fig. 1. The tensile engineering stress-strain curves of Al2014 alloy and SiCp/Al2014 composites after hot extrusion.

Figure(s)

Fig. 2. The OM micrographs of as-cast (a) Al2014 alloy, (b) N-1, (c) M-4 and (d) N-1 + M-4 SiCp/Al2014 composites.

Figure(s)

Fig. 3. The SEM micrographs of extruded (a) N-1, (b) M-4 and (c) N-1 + M-4 SiCp/Al2014 composites, (d) is a higher magnification of (c).

Figure(s)

Fig. 4. (a) The TEM micrographs of N-1 + M-4 SiCp/Al2014 composites after hot extrusion, (b) the interface between nano-SiCp and matrix, (c) the interface between micron-SiCp and matrix and (d) the TEM-EDS of the interface between micron-SiCp and matrix.