Mechanical Properties and Microstrutures of Nano-Sized SiC Particles Reinforced AZ91D Nanocomposites Fabricated by High Intensity

Ultrasonic Assisted Casting

Liu Shiying1,2,a, Gao Feipeng1,2, Zhang Qiongyuan1,2 and Li Wenzhen1,2

1 Dept. of Mechanical Engineering, Tsinghua University, Beijing, 100084, China 2 Key Laboratory for Advanced Materials Processing Technology, Ministry of Education, Beijing,

100084, China a zqqlwz@tsinghua.edu.cn

Keywords: magnesium matrix composite, nano particle reinforcement, ultrasonic assisted casting

Abstract

A high intensity ultrasonic assisted casting method was used to fabricate SiC nanoparticles reinforced magnesium matrix nanocomposites (n-SiCp/AZ91D). The microstructures and mechanical properties of the nanocomposites were investigated. The results show that n-SiCp are well dispersed in the matrix and the grain size was refined. A HRTEM study of the interface between n-SiCp and the matrix suggests that SiC bonds well with matrix without forming an intermediate phase. With the lower addition of n-SiCp, the mechanical properties of nanocomposites are greatly improved. As compared to an unreinforced magnesium alloy matrix, the tensile and yield strength were improved by 43.6% and 117% respectively.

Introduction

Due to the increasing demand on lightweight materials in automotive and aerospace applications, magnesium and its alloys have gained widespread attention in scientific research and commercial application. But the mechanical properties of magnesium limit its application at elevated temperatures. In order to cover the deficiencies in mechanical properties of magnesium, efforts have been made to develop Mg metal matrix composites (MMCs) due to their promising advanced properties [1-6].

Nanoparticle reinforcements can significantly improve the mechanical properties of magnesium alloys and even a small volume fraction of nanoparticles (less than 5vol%) can attain reinforced effects[1]. But it is very difficult to uniformly disperse nanoparticles in metal melts due to their much higher specific surface area. Powder metallurgical techniques [2], disintegrated melt deposition [3], friction stir processing [4] and ultrasonic fabrication methods [5] have been used in fabricating magnesium nanocomposites. The ultrasonic fabrication method was thought to be the most promising.

In this paper, magnesium matrix nanocomposites, namely, AZ91D reinforced by n-SiCp are fabricated using the high-intensity ultrasonic method. The tensile properties and microstructure of nanocomposites are then investigated.

Experiment

Commercially used AZ91D alloy was selected as matrix. β-SiC particles with an average diameter of 40nm were used as reinforcements. An electric resistance-heating furnace was used to melt AZ91D alloy in a stainless steel crucible and the melt temperature was measured and controlled by thermocouple. A high-intensity ultrasonic wave with a 20kHz, a maximum 4.0kW power output and a titanium waveguide of 45 mm in diameter was used for processing AZ91D alloy melts. The melt was protected with N2+0.2%SF6 mixed gas. The ingots of AZ91D were melted at 650ºC and hold

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for 20 minutes. Then the pre-processed n-SiCp, which were packed with aluminium foil, were added into melts. The melts were processed for 600s and the temperature was controlled at 650~680 ºC. After processing, the melts were cast into a permanent steel mould (preheated to 300~350 ºC). Magnesium matrix nanocomposites containing various weight fractions of n-SiCp were fabricated including 0.5, 1.0, 1.5 and 2.0wt.%. For comparison, samples of 0wt.% n-SiCp were prepared without ultrasonic processing.

The as-cast nanocomposite test bars were cut into various samples for the following analyses. For metallographic and SEM analysis the specimens were polished and etched with a 2.0 vol.% solution of nitric acid in ethanol for 5s at room temperature. The metallographic microstructures were observed using a Zeiss Axio Imager A1m microscope. The distribution of the nanoparticles was investigated using scanning electron microscopy (FEI Siron200 SEM). The interface between the nanoparticles and the matrix was analysed with a transmission electric microscope (FEI Sirion 200 TEM). Nanocomposite samples were also machined as standard test bars for tensile testing using a SANS universal electronic material tester. The hardness test was conducted with a HV-120 tester (load 30kg).

Results and Discussion

Figure 1 shows the optical microstructures of AZ91D matrix and the nanocomposites. It can be seen that the grain size of nanocomposites was refined with increasing n-SiCp addition. Compared with the unreinforced alloy, the average grain size is reduced by 30% and 45% for 1.0wt%n-SiCp/AZ91D and 2.0wt%n-SiCp/AZ91D respectively. The microstructure of the as-cast AZ91D alloy consists of primary α-Mg and eutectic phase β-Mg17Al12, in which the big-sized α-Mg dendrite will reduce mechanical properties of alloys. After adding n-SiCp, however, the big-sized dendritic structure was obviously refined.

Figure 1. Optical microstructures of nanocomposites 200X (a) 0.5wt%n-SiCp/AZ91D; (b) 1.0wt%n-SiCp/AZ91D; (c) 1.5wt%n-SiCp/AZ91D; (d) 2.0wt%n-SiCp/AZ91D.

Figure 2 shows the distribution of n-SiCp in the matrix. In low magnification, n-SiCp are uniform with some clusters in the matrix, but the cluster size is less than about 500nm. In high magnification, n-SiCp shows a single granulated distribution, but locally pellets show blank regions.

450 Light Metals Technology 2009

High intensity ultrasonic waves can disperse n-SiCp in the matrix. But to optimise parameters to disperse nanoparticles more uniformly it is essential. In Figure 2 (b), there are some nanopores in the scanning area.

The uniform distribution of n-SiCp may be attributed to the function of the ultrasonic wave. High-intensity ultrasonic waves can generate non-linear effects in melts, namely, acoustic cavitation and acoustic streaming. Acoustic streaming, i.e., a liquid flow due to the acoustic pressure gradient, is very effective for stirring [5] Acoustic cavitations can produce an implosive impact strong enough to destroy the solid surface [6]. Acoustic cavitations can break up the clustered particles and disperse them more uniformly in the melts. So nanocomposites with n-SiCp homogeneously dispersed in the matrix are the result of the cooperative effects of acoustic cavitations and high speed acoustic streaming in the AZ91D alloy melt.

Figure 2. Analysis results of SEM and EDS of 2.0wt.%n-SiC/AZ91D nanocomposite (a) Low magnification; (b) High magnification; (c) EDS of position1 and (d) EDS of position2.

Figure 3 shows The HRTEM micrographs of the SiC/AZ91D interface. It can be seen that SiC bonds well with the matrix without forming an intermediate phase at the interface.

Figure 3. HRTEM analysis results of the interface between nanoparticle and matrix.

Matrix

SiC

Matrix

SiC

SiC

Matrix

Materials Science Forum Vols. 618-619 451

Table 1. Tensile properties of n-SiCp/AZ91D at ambient temperature. Materials Tensile stress

Rm/Mpa Yield stress Rp0.2/Mpa

Elastic modulus E/Gpa

Elongation δ/%

AZ91D 133 65.3 37.1 2.2 0.5SiCp/AZ91D 180 140 37.7 2.1 1.0SiCp/AZ91D 191 141 38.0 2.4 1.5SiCp/AZ91D 146 98 39.0 1.5 2.0SiCp/AZ91D 137 105 42.3 1.3

The ambient temperature mechanical properties of the unreinforced alloys and the nanocomposites are listed in Table 1. As compared with unreinforced alloy, the test results for all composites show the increase in the tensile strength, yield strength and elastic modulus. The increase in tensile strength and the yield strength is as high as 43.6% and 117% for 1.0wt%SiC/AZ91D respectively. However, the total elongation of the nanocomposites is decreased.

Conclusion

n-SiCp/AZ91D composites were fabricated using a high-intensity ultrasonic assisted casting method. The nano-sized SiC particles were well dispersed in the magnesium alloy matrix by ultrasonic processing. As compared with unreinforced magnesium alloy, the average grain size is reduced by 30% and 45% for 1.0wt%n-SiCp/AZ91D and 2.0wt%n-SiCp/AZ91D respectively. HRTEM analysis showed that SiC nano particles bond well with the matrix without forming an intermediate phase. Compared with the unreinforced AZ91D matrix, the tensile strength, the yield strengths and the elastic modulus were increased by 43.6%, 117% and 9% respectively for 1% SiC addition.

References

[1] Srikanth, N., Zhong, X. L. and Gupta, M. (2005) Enhancing damping of pure magnesium using nano-size alumina particulates, Materials Letters 59, 3851 – 3855

[2] Ferkel, H. and Mordike, B. L.(2001)Magnesium strengthened by SiC nanoparticles ,Materials Science and Engineering A,298, 193-199

[3] Hassan, S.F. and Gupta, M. (2004) Development of high performance magnesium nanocomposites using solidification processing route, Materials Science and Technology20, 1383-1388

[4] Lee, C.J., Huang, J.C. and Hsieh, P.J. (2006) Mg based nano-composites fabricated by friction stir processing, Scripta Materialia 54, 1415–1420

[5] Lan, J., Yang, Y. and Li, X. C. (2004) Microstructure and microhardness of SiC nanoparticles reinforced magnesium composites fabricated by ultrasonic method, Materials Science and Engineering A 386, 284–290

[6] Pan, L., Tao, J. and Liu, Z. L. (2005) Zinc matrix composites prepared by ultrasonic assisted casting method, The Chinese Journal of Nonferrous Metals 15(3),104-109

452 Light Metals Technology 2009

Light Metals Technology 2009 10.4028/www.scientific.net/MSF.618-619 Mechanical Properties and Microstrutures of Nano-Sized SiC Particles Reinforced AZ91D

Nanocomposites Fabricated by High Intensity Ultrasonic Assisted Casting 10.4028/www.scientific.net/MSF.618-619.449

DOI References

[2] Ferkel, H. and Mordike, B. L.(2001)Magnesium strengthened by SiC nanoparticles, Materials Science and

Engineering A,298, 193-199

doi:10.1016/S0921-5093(00)01283-1 [1] Srikanth, N., Zhong, X. L. and Gupta, M. (2005) Enhancing damping of pure magnesium using ano-size

alumina particulates, Materials Letters 59, 3851 – 3855

doi:10.1016/j.matlet.2005.07.029 [2] Ferkel, H. and Mordike, B. L.(2001)Magnesium strengthened by SiC nanoparticles ,Materials cience and

Engineering A,298, 193-199

doi:10.1016/S0921-5093(00)01283-1 [3] Hassan, S.F. and Gupta, M. (2004) Development of high performance magnesium anocomposites using

solidification processing route, Materials Science and Technology20, 383-1388

doi:10.1179/026708304X3980 [4] Lee, C.J., Huang, J.C. and Hsieh, P.J. (2006) Mg based nano-composites fabricated by friction tir

processing, Scripta Materialia 54, 1415–1420

doi:10.1016/j.scriptamat.2005.11.056 [5] Lan, J., Yang, Y. and Li, X. C. (2004) Microstructure and microhardness of SiC nanoparticles einforced

magnesium composites fabricated by ultrasonic method, Materials Science and ngineering A 386, 284–290

doi:10.1016/j.msea.2004.07.024 [1] Srikanth, N., Zhong, X. L. and Gupta, M. (2005) Enhancing damping of pure magnesium using nano-size

alumina particulates, Materials Letters 59, 3851 – 3855

doi:10.1016/j.matlet.2005.07.029 [2] Ferkel, H. and Mordike, B. L.(2001)Magnesium strengthened by SiC nanoparticles ,Materials Science and

Engineering A,298, 193-199

doi:10.1016/S0921-5093(00)01283-1 [3] Hassan, S.F. and Gupta, M. (2004) Development of high performance magnesium nanocomposites using

solidification processing route, Materials Science and Technology20, 1383-1388

doi:10.1179/026708304X3980 [4] Lee, C.J., Huang, J.C. and Hsieh, P.J. (2006) Mg based nano-composites fabricated by friction stir

processing, Scripta Materialia 54, 1415–1420

doi:10.1016/j.scriptamat.2005.11.056 [5] Lan, J., Yang, Y. and Li, X. C. (2004) Microstructure and microhardness of SiC nanoparticles reinforced

magnesium composites fabricated by ultrasonic method, Materials Science and Engineering A 386, 284–290

doi:10.1016/S0921-5093(04)00936-0