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  MECHANICAL ENGINEERING Microstructural and corrosion behavior of Al/SiCmetal matrix composites H.M. Zakaria  * Mechanical Engineering Department, Shoubra Faculty of Engineering, Benha University, Cairo, Egypt Received 12 June 2013; revised 16 February 2014; accepted 16 March 2014 KEYWORDS Corrosion;Al/SiC;Matrix;Composites;NaCl Abstract  Several Al/SiC MMCs having several volume fractions up to 15 vol.% and different SiCparticulates average sizes, typically, 11, 6 and 3  l m were fabricated using conventional powdermetallurgy (PM) route. The effect of the size and volume fraction of SiC particulates on themicrostructural and corrosion behavior of Al/SiC metal matrix composites (MMCs) were studied.The results revealed that the Al/SiC MMCs exhibited higher density than pure Al matrix. The staticimmersion corrosion tests of Al/SiC MMCs in 3.5 wt.% NaCl aqueous solution at several temper-atures showed that, at ambient temperature, the Al/SiC MMCs have better corrosion resistancethan the pure Al matrix. Reducing the SiC particles size and/or increasing the volume fraction of the SiC particulates reduce(s) the corrosion rate of the Al/SiC MMCs. In contrast, the Al/SiCcomposites exhibited higher corrosion rates at 50   C and 75   C than the pure Al matrix.   2014 Production and hosting by Elsevier B.V. on behalf of Ain Shams University. 1. Introduction Aluminum alloys reinforced with ceramic particulates havesignificant potential for structural applications due to theirhigh specific strength and stiffness as well as low density [1– 3]. These properties have made particle-reinforced metal ma-trix composites (MMCs) an attractive candidate for the usein weight-sensitive and stiffness-critical components in aero-space, transportation and industrial sectors [4]. Corrosionbehavior is very important parameter for assessing the applica-tion potential of composites as structural materials [5].While considerable work has been done on the physical,mechanicalandtribologicalcharacteristicsofAlMMCs,verylit-tle systematic studies have been done to study the corrosionbehavior of Al MMCs [6–10]. Reinforcement particulates mayinteract electrochemically, chemically, or physically with the ma-trix leading to accelerated corrosion [4–6]. In addition, galvanicinteractionsbetweenthereinforcementandmatrixcanalsoaccel-erate corrosion. Preferential corrosion along a particle matrixinterface can lead to rapid penetration along the large interfacialareas in composites. This can result in enhanced corrosion of MMCsincomparisonwiththecorrosionoftherespectivemono-lithic matrix alloys. Crevice attack at the metal/reinforcementinterfaces and preferred localized attack on structural and com-positional inhomogeneities can occur within the matrix. Sincecorrosion decreases the load-bearing capacity resulting in cata-strophic failures, corrosion can limit the application of MMCsin corrosive environments especially in the presence of stresses.Previous corrosion studies conducted of Al matrix compos-ites have been focused on the corrosion susceptibility in NaCl *Tel.: +20 111 000 7708.E-mail address: zhossam@hotmail.com.Peer review under responsibility of Ain Shams University. Production and hosting by Elsevier  Ain Shams Engineering Journal (2014)  xxx , xxx  –  xxx Ain Shams University Ain Shams Engineering Journal www.elsevier.com/locate/asejwww.sciencedirect.com2090-4479    2014 Production and hosting by Elsevier B.V. on behalf of Ain Shams University.http://dx.doi.org/10.1016/j.asej.2014.03.003 Please cite this article in press as: Zakaria HM, Microstructural and corrosion behavior of Al/SiC metal matrix composites, Ain Shams Eng J(2014), http://dx.doi.org/10.1016/j.asej.2014.03.003  solution, as well as pitting potential and pit morphology [11– 15]. Researchers reported that pits initiate at the secondaryparticles within the matrix; therefore, composites generallyhave more pits than the monolithic matrix. For example,improvement in corrosion resistance has been declared withdecreasing volume fraction of Al 2 O 3  particles in Al–4 wt.%Mg alloy matrix composites [11]. Kiourtsidis and Skolianos[12,13], explained the progress of corrosion by two anodicreactions; namely corrosion of the  a -phase adjacent to inter-metallic regions, and pitting of the dendrite cores, instead of galvanic corrosion developed between the reinforcing particlesand the matrix. Additionally, they supported this argument bya further report that the pitting potential is unaffected by theSiC particles. Since the processing method can heavily alterthe microstructure, the contradictory results of corrosion testsconducted on Al MMCs, may arise from processing methods[14,15].The aim of the current investigation is to study the staticimmersion corrosion behavior of Al/SiC MMCs in 3.5 wt.%NaCl solution at both ambient and elevated temperatures.The Al/SiC MMCs were fabricated using the conventionalpowder metallurgy (PM) route. The effect of the SiC particu-lates size and volume fraction on the corrosion characteristicswas extensively studied. 2. Experimental procedures Commercially pure aluminum powder having minimum purityof 99.8% was used as a matrix material. The aluminum pow-ders have an average size of    60  l m. The SiC ceramic partic-ulates were used as reinforcement. The SiC particulates havethree different average sizes, typically, 11, 6 and 3  l m. TheSiC particulates were dispersed in the Al matrix with 5, 10and 15 vol.% using conventional PM route as follows: BothAl powder and the SiC particulates in addition to 1– 1.5 wt.% paraffin lubricant wax were placed into a blender,mechanically mixed until a homogeneous mixture is achieved,and then placed into containers. The mixed Al/SiC powderswere cold compacted in a tool steel die shown schematicallyin Fig. 1. The powders were then pressed using a hydraulicpress having a capacity of 500 kN. The compaction pressureapplied was about 400 MPa. The Al/SiC composites producedfrom the cold compaction step were subjected to sintering at600   C for 120 min. The sintering process was performed un-der argon inert gas atmosphere. After sintering, the Al/SiCcomposites were subjected to hot extrusion. The Al/SiC com-posites billets were extruded at 490   C. The extrusion reduc-tion ratio was 2:1 by area. The final Al/SiC compositesamples had cylindrical shape of 8 mm diameter and about12 mm length. The Al/SiC composites cylindrical extrudedrods were cut in the transverse directions for microstructuralexaminations using optical and scanning electron microscopes(SEM).Specimens were ground under water on a rotating diskusing abrasive disks of increasing grade up to 1000 grit. Thenthey were polished using 3  l m alumina paste and 1  l m dia-mond paste, then cleaned with acetone. The density of theAl/SiC composites was calculated using water displacementapproach (Archimedean density) according to ASTM B311-08. The theoretic density of Al/SiC composites was calculatedusing the rule of mixtures. The cylindrical sample was weighedin air ( W  a ), then suspended in distilled water and weighedagain ( W  w ). The actual density was calculated according thefollowing equation: q a  ¼  W  a ð W  a  W  w Þ q w  ð 1 Þ where  q a  is the actual density,  W  a  is the mass of the cylindricalsample in air,  W  w  is the mass in distilled water and  q w  is thedensity of distilled water. The sample was weighed using a dig-ital balance with an accuracy of 0.1 mg. Vicker’s hardness testmeasurements were carried out using a load of 10 kg. A mini-mum of ten readings were taken for each sample and the aver-age value was determined.Static immersion corrosion tests were carried out at threedifferent temperatures, typically, room temperature, 50 and75   C. Weight loss was measured to determine the corrosionrate of Al/SiC composites using a digital accuracy with anaccuracy of 0.1 mg. Each specimen was first weighed beforebeing immersed in 3.5 wt.% NaCl solution and later takenout after 24, 48, 72, 96 and 120 h, respectively. After dryingthoroughly, the specimens were weighted again. The weightloss was measured and converted into corrosion rate expressedin mm penetration per year (mm/year). The corroded surfaceswere examined using SEM. Corrosion tests were carried out bysuspending the Al/SiC composite samples in a still solution of 3.5 wt.% NaCl aqueous solution. To avoid crevice corrosion,the specimens were suspended in the solution with a plasticstring. The results of corrosion tests were evaluated usingweight loss measurements, performed following the ASTM-G31 recommended practice [16]. Before immersing in3.5 wt.% NaCl aqueous solution, the Al/SiC composite sam-ples were ground to 1000 grit and then cleaned with deionizedwater followed by rinsing with methanol and dried. For theelevated temperatures corrosion tests (i.e. accelerated tests), a3.5 wt.% NaCl solution was prepared, and heated to 50 ± 1and/or 75 ± 1   C using an electric heater. The specimens wereput into the warm solution and a glass cover was put on thetop of the vessel to prevent evaporation.The corrosion rate CR (from the mass loss) was calculatedusing the following equation [5]: Figure 1  A schematic illustration of the cold compaction dieused for preparation of Al/SiC composites. 2 H.M. Zakaria Please cite this article in press as: Zakaria HM, Microstructural and corrosion behavior of Al/SiC metal matrix composites, Ain Shams Eng J(2014), http://dx.doi.org/10.1016/j.asej.2014.03.003  CR ¼  K   W A  D  T   ð 2 Þ where CR is the corrosion rate (mm/year),  K   is a constant(8.766  ·  10 4 ),  T   is the time of exposure (h) to the nearest0.01 h,  A  is the area (cm 2 ),  W   is the weight loss in the nearest1 mg and  D  is the density of the material (g/cm 3 ). 3. Results and discussion 3.1. Microstructural characteristics of Al/SiC composites Fig. 2 shows SEM micrographs of the fabricated Al/SiC com-posites having a constant volume fraction of 10 vol.% but withdifferent sizes of the SiC particulates. Although some agglom-eration of SiC particulates could be observed, the distributiongenerally appeared to be fairly homogeneous throughout thealuminum matrix. Fig. 3 shows typical optical micrographsof Al/SiC composites having a constant SiC particulates sizeof 11  l m but with different volume fractions of the SiC partic-ulates. It has been observed that increasing the volume fractionincreases the agglomerations of the SiC particulates. Suchobservation has been reported also by many workers [2–4].The agglomerations size was found varying between 10 and35  l m. 3.2. Density of the Al/SiC composites The variation in the measured (actual) density of the Al/SiCcomposites with the volume fraction at different SiC particlessize is illustrated in Fig. 4. The Al/SiC composites exhibitedhigher densities than the pure Al matrix. The Al/SiC compos-ites exhibited actual densities of about 97–98% of the theoret-ical density. The Al/SiC composites (6  l m) contain 5, 10, and15 vol.% of SiC particulates exhibited densities 2.682, 2.695,and 2.7195 g/cm 3 , respectively. The Al matrix alloy has density2.685 g/cm 3 . It has been found that increasing the volume frac-tion of SiC particulates increases the density of the composites.The increase in the density of aluminum alloys due to the addi-tion of ceramic particulates was reported by many investiga-tors [17,18]. The results revealed that the reinforcementsenhance the density of the MMCs. Moreover, the density of the composites increased with the increase in particulate vol-ume fraction. The increase in the density can be attributed tothe higher density of the reinforcement particulates. 3.3. Corrosion behavior of the Al/SiC composites Fig. 5 shows the variation in the corrosion rate of Al/SiC com-posites with the exposure duration in 3.5 wt.% NaCl solutionat room temperature. Generally, it has been found that theAl/SiC composites showed better corrosion resistance whencompared with the pure Al matrix. Increasing the volume frac-tion of the SiC particulates increasing the corrosion resistanceof the Al/SiC composites. Moreover, reducing the SiC particlessize improved significantly the corrosion resistance of the SiCcomposites. It has been found that the increasing the durationexposure reduces the corrosion rate. Such observation imply-ing that the corrosion resistance of the materials under inves-tigation increases as the exposure duration is increased. The Figure 2  SEM micrographs of Al/10 wt.% SiC composites reinforced with SiC particulates have size of (a) 11  l m; (b) 6  l m; (c) 3  l m;and (d) Higher magnification micrograph of (c) showing clearly the SiC particulates. Microstructural and corrosion behavior of Al/SiC metal matrix composites 3 Please cite this article in press as: Zakaria HM, Microstructural and corrosion behavior of Al/SiC metal matrix composites, Ain Shams Eng J(2014), http://dx.doi.org/10.1016/j.asej.2014.03.003  phenomenon of decreasing the corrosion rate with respect tothe exposure duration indicates some passivation of the matrixalloy.TheimprovementofthecorrosionresistanceoftheAl-MMCsdue to the increase in the SiC volume fraction was reported bymany workers [19,20]. For example, Feng et al. [19] examined the effects of the volume fraction of SiC particulate reinforce-ments and the concentration of chloride ions in solution on thelocalized corrosion characteristics of SiC  p /2024 Al-MMCs. Theyreported that the increase in the volume fraction of SiC  p  rein-forcementintheSiC  p /2024Alcompositesresultedinasignificantdecreaseinpittingpotential.Candan[20]studiedtheeffectofSiCparticle size on corrosion behavior of Al–60 vol.% SiC particlecomposites. Experimental results showed that the weight loss of the composites increased with increasing particle size and expo-sure time. The results showed also that intermetallics as a resultof reaction between Al and SiC particle have a beneficial effecton corrosion resistance of the composites due to interruption of the continuity of the matrix channels within the pressure infil-trated composites. Moreover, the weight loss of the compositesinstill3.5 wt.%NaClsolutionsincreasedwithincreasingparticlesize.Figs. 6 and 7 show typical variation in the corrosion rate of the Al/SiC composites with the temperature after exposure in3.5 wt.% NaCl solution for 24 and 120 h. In contract to the re-sults obtained at room temperature, it has been found that theAl/SiC composites have higher corrosion rates when comparedwith the pure Al matrix at elevated temperatures. Increasingthe volume fraction and/or the SiC particles size reduce(s)the corrosion rates of the Al/SiC composites. The corrosionrates of the pure Al as well as the Al/SiC composites werefound to increase linearly with the temperature. At fixed expo-sure duration, the Al/SiC composites always exhibited highercorrosion rates at 75   C than at 50   C.Fig. 8 shows typical SEM micrographs of the corroded sur-face for both pure Al matrix and Al/15 vol.% SiC (3  l m)composites after exposure in 3.5 wt.% NaCl solution for96 h at room temperature. It is clear that severe damage isfound on the surface of the pure Al matrix. Large pits were Figure 3  Optical micrographs of Al/SiC composites reinforced with SiC particulates having size of 11  l m and (a) 5 vol.%, (b) 10 vol.%and (c) 15 vol.% of SiC partculates. Figure 4  Variation in the density with the volume fraction of theSiC particulates having different average sizes. 4 H.M. Zakaria Please cite this article in press as: Zakaria HM, Microstructural and corrosion behavior of Al/SiC metal matrix composites, Ain Shams Eng J(2014), http://dx.doi.org/10.1016/j.asej.2014.03.003
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