CHARACTERIZATION OF HIGH-CYCLE BENDING FATIGUE BEHAVIORS FOR PISTON ALUMINUM MATRIX SiO 2 NANO-COMPOSITES IN COMPARISON WITH ALUMINUM–SILICON ALLOYS Mehrdad Zolfaghari and Mohammad Azadi Faculty of Mechanical Engineering, Semnan University, Semnan, Iran Mahboobeh Azadi Faculty of Materials and Metallurgical Engineering, Semnan University, Semnan, Iran Copyright Ó 2020 American Foundry Society https://doi.org/10.1007/s40962-020-00437-y Abstract In automotive industries, one failure mechanism in engine pistons is due to the fatigue phenomenon. Therefore, to enhance fatigue properties of piston aluminum alloys is a major concern for designers. One reinforcement method could be the addition of nano-particles in the aluminum matrix. In this article, high-cycle fatigue properties of the aluminum matrix nano-composite were characterized under bending loadings and then compared to those of the aluminum–silicon alloy. For this objective, fully reversed bending fatigue tests were performed on standard speci- mens, based on the ISO-1143:2010 standard. Before test- ing, nano-composite samples were stir-casted by the addition of 1 wt% SiO 2 nano-particles, and aluminum specimens were gravity-casted in a cast-iron mold. The microstructure of materials and the distribution of nano- particles in the aluminum matrix were evaluated by the optical microscopy and the field emission scanning elec- tron microscopy. Experimental data indicated that nano- particles had a significant effect on the high-cycle fatigue lifetime. The reason for this improvement in high-cycle fatigue properties could be finer grains, higher hardness, the proper distribution of nano-particles in the aluminum matrix and stronger bonding strength at the Al/Si interface. However, based on fracture surfaces, all samples had the brittle behavior due to cleavage and quasi-cleavage marks. Keywords: nano-composite, aluminum–silicon alloy, SiO 2 nano-particles, high-cycle fatigue, fracture behavior Introduction Aluminum alloys have several applications, especially in automotive industries, due to their unique properties, such as low density, high ratio of the strength to the weight and suitable wear resistance. 1 In order to improve properties of these alloys, different reinforcing particles, such as TiO 2 , 1 Al 2 O 3 , 2–4 MgO, 5,6 TiB 2 ,7,8 and TiCN 9 or a combination of these particles, such as Al 2 O 3 /SiC, 10 have been used. By such a process, a metal matrix composite could be made with enhanced properties. These particles have their special effects on mechanical and metallurgical properties of aluminum alloys. Ceramic reinforcement particles, such as SiC 11,12 and SiO 2 , 13 were utilized to strengthen aluminum alloys at high temperatures. 14,15 A uniform distribution would be led to a finer, evenly dispersed, homogenous microstructure, which causes to decrease the amount of degenerate particles and improve mechanical properties (the strength or the elon- gation) of the material. Therefore, adding nano-particles in the matrix, in comparison with the addition of micro-par- ticles, is more effective on the improvement of mechanical and metallurgical properties. 7,16,17 Several methods have been used to produce aluminum matrix composites, including conventional casting, 18 the powder metallurgy, 3 forging 19 and the extrusion. 6,20 The most suitable method for adding reinforcing particles to the aluminum matrix is the stir-casting process. The ultrasonic method 21,22 produces a better distribution of reinforce- ments, in comparison with that of stir-casting, without the International Journal of Metalcasting