JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS Vol. 13, No. 9, September 2011, p. 1172 - 1175 Characterization of Al matrix composites reinforced with alumina nanoparticles obtained by PM method V. TSAKIRIS, W. KAPPEL, E. ENESCU, G. ALECU * , F. ALBU, F. GRIGORE, V. MARINESCU, M. LUNGU National Institute for Research and Development in Electrical Engineering ICPE-CA, Splaiul Unirii 313, Bucharest, Romania Al-Al2O3 metal matrix composite powders with 2%vol.Al2O3 nanoparticles were synthesized by high-energy milling of the blended component powders and consolidated via traditional powder metallurgy (PM) route. The evaluation of the milling time from 2 h until 8 h on the microstructures, electrical and mechanical properties was analyzed. Mechanical properties like: hardness, strength and ductility were determined on the specimens with high densification obtained by hot extrusion. The optimum milling time which assured uniform distribution of nano-size alumina particles in the aluminum matrix and best values for the mechanical properties was emphasized. (Received June 14, 2011; accepted September 15, 2011) Keywords: Nanometric particles, Aluminum composites, PM method 1. Introduction Discontinuously reinforced aluminum matrix composites (DRA) have been attracting attention because of their amenability to undergo deformation processing by conventional metalworking techniques. Extrusion is used in processing of DRA composites for consolidation, redistribution of reinforcements, and shape forming. Use of nanoparticles to reinforce metallic materials lead to the development of novel composites with unique mechanical and physical properties. In order to achieve desired mechanical properties of composites, reinforcing nanoparticles must be distributed uniformly within metal matrix of the composites. One of the main challenges towards a homogeneous distribution of ceramic phase in the metal matrix is the selection of the appropriate processing technique. Mechanical milling (MM) technique was employed in the synthesis of composite powders because of its simplicity, low cost operation and for dispersion nanoparticles more uniformly in metal matrix. MM involves repeated cold welding, fracturing and re-welding of powder particles. This is achieved by repeated collisions between the grinding medium in the milling container. A balance is achieved between the rate of welding that increase the average composite particle size and the rate of fracturing that decreases the average composite particle size [1]. This leads to a steady-state particle size distribution of the composite metal particles. The continuous interaction between the fracture and welding mechanisms tend to refine the grain structure resulting in uniformly distributed particles in the metal matrix. Lately, literature [2, 3] revealed that size of the particles have a strong effect on the failure mode, strength, and ductility of the Al-based composites. The smallest particles (< 200 nm) usually well bonds to the matrix and do not initiate cavities in the particle. For optimum strength, the second-phase dispersion strengthened particles must be fine and the interparticular spacing small. Tensile strength and ductility decrease with increasing particle size. The mechanical properties [4] of metal-matrix composites can be further enhanced by decreasing not only the sizes of ceramic particulates but also the matrix grains from micro to nanometer level. In the present work, the fabrication of nanometric particulates reinforced aluminum matrix composite via traditional powder metallurgy (PM) method was performed. The aim has been to study the effect of nanometric reinforcement of Al 2 O 3 and the influence of the processing parameters on the microstructure, electrical and mechanical properties. 2. Experimental procedure Pure aluminum Al 2 O 3 and stearic acid powders were obtained from commercial vendors. Pure Al powder of 99.4% purity was obtained from Alba Aluminum Slatna, Romania and Al 2 O 3 and stearic acid powders were provided from Alfa Aesar, Germany. The used raw materials for the obtaining of the homogeneous powders mixtures from the Al-Al 2 O 3 system are shown in Table 1. The following technological flux was used in order to elaborate reinforced Al-2%vol.Al 2 O 3 composites of 65 g each charge: powder mixing-homogenizing by MM (2 h, 4 h, 6 h, 8 h) - pressing-sintering-extrusion. The MM process took place into a mill of Fritsch-Pulverisette 6 type, in the following conditions: balls/powders ratio (BPR) of 10:1, milling speed of 250 rot/min, Ar atmosphere. A combination of different sizes of balls (diameter of 19 mm, 14 mm, 10 mm and 5 mm), was used to obtain higher collision energies.The milling times were 2, 4, 6 and 8 h. To avoid excessive cold welding of Al particles during milling, 1.5% gr. stearic acid powders have been used as a process-control agent (PCA) [5] The PCA gets adsorbed onto the surface of the powders and minimizes the effect of cold welding and thus inhibits agglomeration.