Fabrication and properties of mechanically milled alumina/aluminum nanocomposites

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Abstract

The reinforcement agglomeration in nanocomposites is a key issue that needs to be solved in order to fully benefit of the gain in strength and ductility associated with the decrease in reinforcement size from microscale to nanoscale. In this study, mechanical milling has been used successfully to disperse nanometric alumina (n-Al2O3) in an aluminum matrix. Al2O3/Al nanocomposite powders have been produced for various alumina sizes and concentrations. The 10 vol% n-Al2O3/Al powders display hardness values near five times higher than pure unmilled Al. A decrease in the Al2O3 particle size from 400 to 4 nm has increased the nanocomposite powder hardness by 11%. The microhardness and compression properties of an Al2O3/Al nanocomposite compact consolidated by hot pressing were measured. Comparison with modeled values and literature results indicates that the higher experimental yield strength obtained with the addition of n-Al2O3 versus micron size Al2O3 is due to in situ matrix strengthening.

Research highlights

▶ Mechanical milling can successfully disperse nano-alumina in an aluminum matrix. ▶ Al2O3/Al nanocomposite powder hardness is five times higher than pure unmilled Al. ▶ A drop in Al2O3 size from 400 to 4 nm increases composite powder hardness of 11%. ▶ Addition of nano-Al2O3 particle promotes in situ matrix strengthening. ▶ Milling strengthens the composite through grain refinement and dispersoid formation.

Introduction

Due to their lightweight and high specific strength, particulate reinforced aluminum composites are attractive structural materials for various domains such as automotive and aerospace applications. They also offer moderate fabrication cost, easier manufacturing than continuous fiber composites with reproducible properties and the possibility to use standard or near standard metal working methods [1].

The most common reinforcements for aluminum are silicon carbide (SiC) and alumina (Al2O3), due to high availability, low cost and overall good properties [2]. In general, a gain in stiffness of 50% can be obtained with up to 30 vol% of SiC or Al2O3 [1]. Strength values reported are more scattered, but an increase of up to 60% can be obtained for the yield and ultimate tensile strength. However, ductility was always found to be reduced with reinforcement addition, as well as toughness [2].

Recently, Al2O3/Al nanocomposites have shown great potential with their unique mechanical properties. In one study [3], the tensile strength obtained with 1 vol% n-Al2O3 was found to be equivalent to that of the 10 vol% SiC (13 μm)/Al composite produced in the same conditions. If compared with pure Al, 2 vol% n-Al2O3 addition improves yield strength of around 66%, hardness of around 50% and tensile strength of around 80%. In another study [4], ultrason casting method was used to dispersed 2 wt% Al2O3 (10 nm) in aluminum. Compared with pure Al casted following the same method, composite hardness was increased by 92% and the yield strength by 56%. Research studies on other nanocomposites have shown an increase in strength combined with an improvement of ductility [5], [6], [7].

However, the processing of composites with nanoreinforcement is still an issue. In the two papers cited above involving Al2O3/Al [3], [4], it has not been possible to obtain the homogeneous distribution needed for optimized properties.

High energy mechanical milling was found to eliminate clustering issue for several nanocomposites [8], [9], [10], [11]. It leads to uniform distribution of the reinforcements as well as inducing a significant grain size reduction and lattice strains, also beneficial to strength properties. Furthermore, the reinforcing phase helps to maintain the submicron grain size obtained from milling by pinning grain boundaries during subsequent processing, such as extrusion. However, only one work [8] was found on the possibility of using high energy mechanical milling for dispersion of nanoscale alumina particles in aluminum. The resulting strengthening of the materials was not evaluated.

This paper first intends to evaluate mechanical milling as a possible dispersion method for Al2O3/Al nanocomposites. The extent of Al2O3 dispersion is evaluated, together with the effect of milling on Al matrix microstructure. The effect of Al2O3 size on composite strengthening is also studied through comparison of the measurements obtained in this study with previous experimental results and with modeling.

Section snippets

Experimental procedures

Three different sizes of Al2O3 powder were tested. Spherical Al2O3 of 4 nm nominal size was bought from Aldrich. From observations with a FE-SEM Hitachi S-4700, the average particle size was measured to be 25 nm. Spherical Al2O3 of 80 nm average particle size was produced by combustion synthesis [12]. Finally, calcined equiaxed Al2O3 was bought from Whittaker, Clark and Daniels (now MPSI) with a nominal particle size of 400 nm but a measured average particle size closer to 310 nm. The size

Results and discussion

After milling of the Al2O3 powder with the aluminum for 5 h, equiaxed Al powders are obtained where Al2O3 particles seem to be well distributed. Fig. 4 shows the 80 nm Al2O3 particles distributed at the surface of the Al powder. A low amount is observed because the remaining Al2O3 is now embedded in the Al particles.

The dispersion of the Al2O3 within the Al was observed in the hot pressed compacts for the three Al2O3 size. From the SEM micrographs shown in Fig. 5, the 4 and 80 nm Al2O3 powders

Conclusions

In this paper, mechanical milling was investigated as a possible way to disperse uniformly n-Al2O3 in Al powder. It was found that the Al2O3 composites resulting from milling display a uniform dispersion of the second phase with few agglomerates of around 1 micron in size. The Al2O3/Al nanocomposite powders hardness is near five times higher than pure unmilled Al. A decrease in the Al2O3 particle size from 400 to 4 nm has increased the nanocomposite powder hardness of 11%. Presence of Al2O3 also

Acknowledgements

This work is financially supported by the National Research Council of Canada (NSERC), “Fonds québécois de la recherche sur la nature et les technologies” (FQRNT) and Nanoquebec. The 80 nm size Al2O3 powder was provided by David Frost, from McGill. The authors would like to thank Dr. Xue Dong Liu, Slavek Poplawski and Ronald Sheppard from McGill University for TEM operations, XRD measurements and compression testing and René Veillette from IREQ for the FIB sample preparation.

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