An experimental investigation of the effect of casting parameters during the fabrication of functionally graded Al-B-Mg (Al-2 wt. % B-6 wt. % Mg) composites via centrifugal casting method has been conducted. Particular interest was dedicated to the transformation of the dodecaboride particles into complex aluminum diboride particles and the redistribution of these particles on the matrix of the composites. Pertinent image analysis permitted characterizing the resulting composite microstructures

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... both quantitatively and qualitatively. Our results revealed that as casting time, rotational speed and pouring temperature of the melt increased, the concentration of the reinforcement particles becomes denser at the outer periphery of the casting. The data obtained from both superficial Rockwell hardness and Vickers microhardness analysis demonstrated that both mechanical properties bore similar trend as the gradient in particles distribution. Additionally, backscattered electron imaging helped differentiate the morphology and apparent composition of the aluminum boride particles with different levels of boron. The interaction of the magnesium levels in the composite matrix and the reinforcement particles via energy dispersive spectroscopy is also discussed. aluminum matrix composites (AMCs) Ω.Ι. In this method, the difference in densities between the reinforcing particles and the melt controls the segregation via differential centrifugal forces /3/. Additional advantages attainable with this technique are good mold filling combined with proper microstructure control, which furnishes excellent mechanical properties /4/. Among the most relevant reinforcement particles in AMCs there are A1 2 O 3 , graphite, and SiC p 151. It is a known issue that whenever particles are used in aluminum melt, interfacial reaction may occur. For example, aluminum melt in contact with SiC p can cause a deleterious reaction product A1 4 C 3 161. The aforementioned reaction products weaken the particle-matrix interface and can affect the proper load transfer between matrix and reinforcement. However, this problem and those due to poor wettability between the matrix and the reinforcements are tackled in two ways: via the addition of alloying elements, such as magnesium, calcium, titanium or zirconia ΠI and, secondly, by the use of A1B 2 and/or AIB, 2 particles in aluminum matrix composites /3, 8, and 9/. The crystal structure of AlB 2 -type transition metal diborides is hp3 with the space group symmetry p6/mm with a hexagonal lattice in which close-packed metal layers alternate with graphene-like (honeycomb) boron layers /10/. On the other hand, crystal structure of A1B[ 2 has a much more complex orthorhombic arrangement /I I/. The two reinforcement particles possess different melting temperatures and chemical stability. The study of the Al-B system has been hindered by controversy surrounding the different thermodynamic states claimed in the phase diagram /12/. It is worthy to note that the hardness value and melting point of A1B 2 is much lower than any of the dodccaborides 712, 137. In addition, it is believed that upon manufacturing these Al-B alloys, alloying elements such as copper /14/ and magnesium /15/ could bring about changes in the thermodynamic stability of the reinforcements, which might result in subsequent phase transformations. The present investigation focuses on the fabrication of functionally graded Al-Mg-B composites by centrifugal casting and the resulting phase transformation of the reinforcement phases (A1B X ) caused by the addition of magnesium. The fabrication parameters considered in this study are casting time, rotational speed [expressed in terms of G number] and pouring temperature of the aluminum composite melt. Their influence on the compositional gradient is also assessed. As aforementioned, density difference between the reinforcement particles and the molten matrix should give rise to the redistribution of the dispersoids in the matrix, with the heavier particles moving faster towards the periphery along the longitudinal section of casting /16/. The density of the aluminum borides are approximately 3,190 kg/m 3 for A1B 2 /17/ and 2,600 kg/m 3 for A1B, 2 /18/, which are in both cases higher than the liquid AI (estimated at 2,400 kg/m 3 ) /19/ for the manufacturing temperatures used /9/. EXPERIMENTAL PROCEDURE The Al-B-Mg composites were prepared by mixing 99% pure aluminum with two master alloys: Al-Mg (10 wt. % Mg), Al-B (7.2 wt. % B), where boron was present in the form of AIB )2 particles. The target chemical composition of the manufactured Al-B-Mg composite was 2 wt. % B and 6 wt. % Mg. The required amount of the materials was then melted in a graphite crucible and held for 25 minutes at 850°C for homogenization purposes. A graphite mold was used because of its high machinability. This cylindrical mold was preheated in an electrical furnace at 500°C for 50 minutes to prevent "frozen skin" formation, which would have constricted the molten composite flow into the mold /20/. A centrifugal caster was then employed for the fabrication of specimens with the required specifications. Table 1 shows the experimental conditions. In the first set of experiments, casting times of 1 to 3 minutes were used while the rotational speed and pouring temperature were kept at 32G and 820°C respectively. In a second set, the rotational speeds were varied from 32 to 57G at a fixed casting time of 3 minutes and a pouring temperature of 820°C. Finally, in the