An Al–Mg–Si alloy matrixcomposite reinforced with 10 vol.% of alumina whiskers (Al2O3w) has been processed by powder metallurgy and investigated. The Al2O3w were produced as single crystal c-axis alpha-alumina fibres at pre-pilot scale via vapour–liquid–solid (VLS) deposition in a cold-wall air-tight furnace with alumina linings. As far as we know, this is the first report of the utilization of whiskers of Al2O3 as reinforcing elements for Al alloys. Tensile tests have been performed on the composite at room and high temperatures. Results show that the AA6061 alloy reinforced with the as-produced Al2O3whiskers has remarkably high mechanical properties at room temperature. This is attributed to the high quality of the Al2O3 single crystals and to the strong bonding attained between them and the 6061 alloy matrixFinancial support of Spanish MEC, project MAT2006-01251, and regional government, Xunta de Galicia, PGIDT05TMT23902CTPeer reviewe

Carrochano, J.

Cerecedo, C.

Guitián, Francisco

Lieblich, Marcela

Valcárcel, V.

Materials Letters

Digital.CSIC

 1Whiskers of Al2O3 as reinforcement of a powder metallurgical 6061 1 aluminium matrix composite  2  3 J. Corrochanoa, C. Cerecedob, V. Valcárcelb, M. Lieblicha,*, F. Guitiánb 4  5 a Physical Metallurgy Department, National Centre for Metallurgical Research, CENIM - 6 CSIC, Avda. Gregorio del Amo 8, 28040 Madrid, Spain. 7  8 b Instituto de Cerámica de Galicia, ICG, Avda. Mestre Mateo, S.N, Campus Sur, University of 9 Santiago de Compostela, 15706 Spain. 10  11 * Corresponding author. Tel.: +34915538900; fax: +34915347425. E-mail address: 12 marcela@cenim.csic.es (M. Lieblich). 13  14 Abstract 15 An Al-Mg-Si alloy matrix composite reinforced with 10% volume of alumina whiskers 16 (Al2O3w) has been processed by powder metallurgy and investigated. The Al2O3w were 17 produced as single crystal c-axis alpha alumina fibres at pre-pilot scale via Vapour Liquid Solid 18 (VLS) deposition in a cold-wall air-tight furnace with alumina linings. As far as we know, this 19 is the first report of the utilization of whiskers of Al2O3 as reinforcing elements for Al alloys. 20 Tensile tests have been performed on the composite at room and high temperature. Results show 21 that the AA6061 alloy reinforced with the as-produced Al2O3 whiskers has remarkably high 22 mechanical properties at room temperature. This is attributed to the high quality of the Al2O3 23 single crystals and to the strong bonding attained between them and the 6061 alloy matrix. 24  25 Keywords: Composite materials; Microstructure; Mechanical properties; Aluminium, Alumina 26 whiskers; Powder metallurgy. 27  2 28 1. Introduction 29 Particle-, whisker- and fibre-reinforced metal matrix composites can sustain higher loadings 30 than the unreinforced equivalent matrixes. Whiskers are defect-free, very thin single crystal 31 fibres, which are among the most resistant materials known. Among ceramic whiskers, -Al2O3 32 grown with c-axis orientation are good candidates for use as strengtheners in composites 33 because of their high elastic modulus, thermal and chemical stability, fracture strength, and 34 creep resistance even at high temperatures [1-3]. However, the use of Al2O3 whiskers has been 35 hindered up to now by its extremely high production cost. Accordingly, studies on metal matrix 36 composites reinforced with Al2O3 has been limited to particles, see for example ref. [6, 7], and 37 short fibres [8-11]. 38 In this work, a novel method has been used for producing enough amount of c-axis -Al2O3 39 whiskers, based on vapour-liquid-solid deposition (VLS) in Ar atmospheres containing metal 40 vapours [12-14]. To explore the reinforcing potential of these whiskers, an AA6061 alloy 41 powder has been blended with 10% volume of Al2O3 whiskers and consolidated by extrusion. 42 Powder metallurgy is especially suitable for producing aluminium alloy matrix composites 43 (AMCs) because it prevents some wettability problems of Al2O3 in molten aluminium and 44 deleterious reactions with the Mg atoms of the matrix that may appear during casting routes 45 [15]. The microstructure and tensile properties of the 6061/Al2O3w composite have been 46 evaluated at room and high temperature. 47  48 2. Experiments  49  The c-axis -Al2O3 whiskers were obtained employing a cold-wall air-tight chamber 50 furnace with alumina linings using 450 g of quartz sand blended with 45 g of Ni powder. A 51 complete explanation of the experimental procedure for the obtaining of the alumina whiskers 52 can be found in (14). VLS Al2O3 whiskers morphology was examined by scanning electron 53  3microscopy (SEM) and their composition systematically controlled via energy-dispersive X-ray 54 spectroscopy and X-ray diffractometry. 55 Gas atomised 6061 aluminium alloy powder (particle diameter < 50 μm) was blended with 56 10% volume of Al2O3w in a planetary ball mill at 100 rpm for 2.5 hours with a ratio of balls to 57 material of 7:1. The blend was consolidated by extrusion at 723 K, ram speed of 0.3 mm/s and 58 extrusion ratio of 27:1. A monolith 6061 alloy bar was also processed from the alloy powder for 59 comparison purposes. 60 The consolidated materials were solution heat treated at 793 K for 0.5 hours and water 61 quenched. Cylindrical tensile specimens of 3 mm diameter and 20 mm gauge length were 62 employed to perform tensile tests on T6 specimens (maximum hardness). Yield stress (0.2), 63 ultimate tensile strength (UTS) and elongation to fracture () were determined at room 64 temperature (RT) and at 373 K, 473 K and 573 K at a strain rate of 5x10−4 s−1. Elastic modulus 65 was measured on cylindrical samples of 6 mm in diameter and 40 mm in length according to 66 ASTM E1876 and C1259. Microstructural characterization was performed by SEM with a FEG 67 JEOL 6500. Image analysis of the composite was performed on backscattered electron images 68 and data treated with Image-Pro Plus software. 69  70 3. Results and discussion 71 Figure 1 shows a SEM micrograph of a bundle of the as-produced VLS c-axis -Al2O3 72 whiskers that can exhibit lengths of hundreds of microns with aspect ratios that can be larger 73 than 100 and a high level of size dispersion. In the bundles, apart from alpha-alumina, traces of 74 Ni, Fe, Si and SiO2 were detected as extrinsic contamination. 75  76  4 77 Fig. 1. VLS Al2O3 whiskers. 78  79 The 6061/Al2O3w extruded composite show no agglomerates and no band structure, 80 typically found in extrusion of powders. The Al2O3 whiskers appear shorter in average 81 indicating that they became broken during the processing. Image analysis revealed that the 82 composite presents about 15% volume of Al2O3 whiskers, i.e. aspect ratio above 5, which are 83 typically between 10 and 20 μm long and 2 to 4 μm in diameter. The remaining Al2O3 show an 84 aparent aspect ratio below 5 and diameter of about 2 μm. Whiskers are aligned along the 85 extrusion direction, so that 90% of them have their major axis inside an angle of less than 20º 86 around this direction. This alignment occurred during the extrusion [16]. Apart from the Al2O3 87 whiskers, three types of particles appear as contamination from the alumina whiskers production 88 process: Ni(Fe)-rich, Si-rich and Si- and O-rich particles. These will be described below in more 89 detail. 90 In comparison with the unreinforced alloy, the 6061/Al2O3w composite presents 91 significantly higher tensile (and specific properties) for the whole temperature range, Figure 2. 92 Comparison with 6061 matrix composites reinforced with Al2O3 particles [5-7] shows the 93 considerable advantage of Al2O3w, even when comparing with composites reinforced with 20% 94 volume of particles [5, 7], while keeping an  of 4%. In contrast, when test temperature is 95 increased, tensile strength diminished more abruptly than in the other composites. On the other 96 hand, elastic modulus of 6061/10%Al2O3w resulted in 90 GPa, higher than those of other 10% 97  5volume Al2O3 reinforced composites [4, 7]. Moreover, specific elastic modulus of the composite 98 is about 20% higher than that of the alloy matrix, i.e. 32x106 against 26x106 Nm/kg. 99  100  101 Fig. 2. UTS and 0.2 of 6061/10%Al2O3w, 6061 and comparable composites at different temperatures. 102  103 Figure 3a and b show backscattered images of longitudinal sections close to the fracture 104 surface of specimens tested at RT. In addition to Al2O3, three types of particles are present. A 105 first one appears brighter and is Ni-rich, with some Fe occasionally also being found. These 106 particles are quite spherical in shape and of less than 5 μm in diameter. A second type, arrowed 107 in Figure 3a, contains only Si, is bright-grey, irregular in shape and up to 30 μm in size. Finally, 108 a third type, arrowed in Figure 3b, corresponds to SiO2. These particles are very large 109 polyhedrals (typically of about 100 μm in diameter) and are surrounded by a Mg and O-rich 110 phase with Al2O3 whiskers sometimes embedded. Under RT tensile conditions the Al2O3 111 whiskers become mainly cracked, which indicates that the shear strength at the 6061 112 matrix/Al2O3 interface was higher than the Al2O3 whisker fracture strength, and thus, that a 113 strong mechanical bond between matrix and reinforcement has been generated during the 114 extrusion process [17]. With regards to the contaminating particles, those that are Ni-rich seem 115 not to be affected during the tensile test so that most of them appear neither cracked nor 116 debonded. In contrast, the Si particles appear mostly debonded from the matrix, whereas the 117 SiO2 particles appear catastrophically broken, with cracks running through them perpendicular 118  6to the loading direction. The fracture mechanism clearly changes at high temperature, as it is 119 shown in the micrograph of Figure 3c. In this case, decohesion becomes the main cause of 120 damage, so that void nucleation can clearly be observed at whisker and contaminant 121 particle/matrix interfaces. 122  123    124 Fig. 3. Longitudinal sections close to the fracture surface of specimens tested at (a) and (b) RT, (c) 573 K. 125  126 The high RT tensile and specific tensile properties of the 6061/Al2O3w composite, even 127 though it contains contaminating particles and even though the Al2O3w became broken during 128 processing, validates the quality of the VLS alumina whiskers and the suitability of the 129 fabrication route that promotes a strong bonding between matrix and reinforcement. 130 Comparison with similar composites reinforced with Al2O3 short fibres or SiC whiskers is not 131 straightforward because few results are available with the same matrix in T6. Composites with 132 higher volume fraction of fibres or whiskers present, obviously, higher properties. For example, 133 RT 0.2 and UTS of a 6061 alloy reinforced with 18% volume of short Al2O3 fibres [8] were 400 134 MPa and 470 MPa, respectively, and for 6061 reinforced with 20%SiCw [18, 19] were 440 MPa 135 and 500 MPa, respectively, with similar UTS reported for a 6061/22%SiCw composite [20]. In 136 all these materials, not only was reinforcement volume fraction about two times larger than in 137 our composite material, but also median Al2O3w aspect ratios were as high as 100. This 138 indicates that improved properties can be achieved for the 6061/Al2O3w composite by 139 optimising the blending step, so that the large aspect ratio of the as-produced VLS Al2O3 140 whiskers can be preserved. Moreover, further improvement should be obtained if a cleaning 141  7protocol is applied to the Al2O3 bundles in order to eliminate the weakly adhered Si particles 142 and the brittle SiO2 ones. 143  144 4. Conclusions  145 VLS-Al2O3 whiskers have been investigated as reinforcement in a powder metallurgy 146 6061/10% vol. Al2O3w composite. In comparison with the unreinforced alloy, the 6061/Al2O3w 147 composite presents significantly higher mechanical properties and specific properties at room 148 and high temperatures. At RT, comparison with 6061 matrix composites reinforced with Al2O3 149 particles shows the advantage of Al2O3w reinforcement, even when comparing with composites 150 reinforced with 20% volume of particles. This is attributed to the high quality of the Al2O3 151 single crystals obtained by the VLS process and to the strong bonding between them and the 152 6061 alloy matrix attained during consolidation. Breaking of the whiskers occurred during 153 processing. Three types of contaminating particles are present, coming from the VLS process: 154 Ni(Fe)-rich, Si and SiO2 particles. The first type did not affect tensile properties, whereas the 155 other two clearly produced a deleterious effect. Improved properties are expected by enhancing 156 the purity of the Al2O3w bundles and by preserving the high whiskers aspect ratio through 157 optimisation of the blending parameters. 158  159 Acknowledgements 160 Financial support of Spanish MEC, project MAT2006-01251, and regional government, 161 Xunta de Galicia, PGIDT05TMT23902CT, is gratefully acknowledged. 162  163 References 164 [1] V. Lavaste, J. Besson, M. Berger, A.R. Bunsel, J. Am. Ceram. Soc. 78 (1995) 3081. 165 [2] G. Das, Ceram. Eng. Sci. Proc. 16 (1995) 977. 166 [3] T.F. Cooke, J. Am. Ceram. Soc. 74 (1991) 2959. 167  8[4] B.G. Park, A.G. Crosky, A.K. Hellier, J. Mater. Sci. 36 (2001) 2417. 168 [5] L. Ceschini, A. Morri, R. Cocomazzi, E. Troiani, Mat –wiss U Werkstofftech. 34 (2003) 370. 169 [6] Chia-Chaw Perng, Jiun-Ren Hwang, Ji-Liang Doong, Mater. Sci. Eng. 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Whiskers of Al2O3 as reinforcement of a powder metallurgical 6061 aluminium matrix composite

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