The mechanical properties and microstructure of commercial Al-15Si-2.5Cu-0.5Mg (Alumix-231 (R)) alloy was subjected to extrusion (EXTR), hot extrusion into rods at 565 degrees C at an extrusion ratio of 4:1 and high-pressure torsion (HPT) processing, one of the most severe plastic deformation (SPD) techniques, with an anvil rotation speed of 0.5 rpm under a quasi-hydrostatic pressure of 5 GPa. Afterwards, the microstructures of specimens were systematically investigated using XRD, SEM, EBSD, and TEM. Also, tensile properties of Alumix-231 processed by EXTR and HPT were determined at room temperature. The HPT specimen shows refined microstructure (around average 165 nm) having more spherical and homogeneously distributed eutectic silicon second phases in Al metal matrix as compared to EXTR specimen. The hardness values are greater on the surface of the HPT specimen (239.1 Hv) as compared to that EXTR specimen (71.2 Hv) and increase with distance from the center of HET specimen (198.3 Hv). Tensile strength and ductility of HPT specimen increased approximately 2 times than that of EXTR sample, respectively. The results indicate that the morphological changes in eutectic silicon especially after HET play a critical role in enhancing or limiting the overall properties of the Alumix-231 material. (C) 2011 Elsevier B.V. All rights reserved.