Mechanical and Tribological Characterization of Al-Mg 2 Si Composites After Yttrium Addition and Heat Treatment H.R. Jafari Nodooshan, Wencai Liu, Guohua Wu, A. Bahrami, M.I. Pech-Canul, and M. Emamy (Submitted September 20, 2013; in revised form December 30, 2013) In this study, the effect of heat treatment and yttrium additions on the microstructure, mechanical prop- erties, and tribological behavior of Al-15% Mg 2 Si cast composites was investigated. The microstructural study revealed the presence of both primary and secondary Mg 2 Si phases in all composite specimens and also Y-containing intermetallics (Al 2 Y phases) at higher concentrations. It was also found that Y addition does not change the size and morphology of primary Mg 2 Si particles considerably, but the pseudo-eutectic Mg 2 Si changed from a flake-like morphology to fine fibrous or rod-like one. The results show that proper content of Y additions can reduce the amount of Mg 2 Si phase through dissolving it into the matrix, lead to the precipitation of Al 2 Y phase and improve the mechanical properties. Modified composites with 0.5% Y exhibited an ultimate tensile strength (UTS) of 290 MPa with an elongation of 4.3%. After exposing the composite to solution treatment at 520 °C for 4 h, the tensile strength of the composite continuously increased with the increase of Y content, and reached the maximum at 1% Y. The maximum UTS and elongation at room temperature for the heat-treated composites are 294 MPa and 7.4%, respectively. In the cast specimen, fracture surfaces are covered by packets with coarse steps, suggesting a brittle mode of failure. Modified composites with 0.5 wt.% Y contain several cracked particles together with a few deco- hered primary Mg 2 Si particles. In solution heat-treated state, dimples present at the fracture surface are rather coarse but homogenous, showing a semi-ductile mode of fracture. Wear test results showed that the wear resistance of all specimens increases with the addition of Y up to 0.3 wt.%. Scanning electron microscopic observations of the worn surfaces revealed that the dominant wear mechanism was abrasive wear accompanied by some delamination wear mode. Keywords casting, mechanical properties, metal-matrix compos- ites (MMCs), wear 1. Introduction In order to meet the requirements of modern industry for producing light materials with high strength, aluminum matrix composites (AMCs) have been developed as a class of advanced engineering materials for high performance applica- tions. This is on account of their combination of physical and mechanical properties such as excellent castability, low density, and good wear resistance, as well as good elevated-temperature resistance (Ref 1, 2). Moreover, aluminum can accommodate a wide variety of reinforcing agents (Ref 3, 4). Recently, particulate reinforced metal-matrix composites (PMMCs) have attracted considerable attention due to their relatively low cost and inherent isotropic properties (Ref 5, 6). Among the particulate reinforcing materials that are commonly used, Al-Si-Mg alloys are of special interest. These materials are heat treatable and their mechanical properties can be improved by performing different heat treatment operations (Ref 3, 7, 8). Within these materials, Mg 2 Si has the lowest density and therefore, it has the greatest potential as a reinforcing phase for Al- and Mg-based metal-matrix compos- ites (MMCs) for weight-saving applications (Ref 9). The Mg 2 Si phase can also impede grain boundary sliding at elevated temperatures (Ref 10). Furthermore, the intermetallic com- pound Mg 2 Si exhibits many excellent properties such as high melting temperature, high hardness, and low thermal expansion coefficient (Ref 11-14). However, under normal cast conditions, the primary Mg 2 Si phase is prone to form coarse Chinese script or dendritic morphology, which is a brittle phase, resulting in composites with low ductility and strength (Ref 15, 16). The coarse Chinese script morphology can be modified in the cast condition to obtain a finer structure and improve the mechanical properties of the alloys (Ref 17). Therefore, it is very important to choose an appropriate modifier for the modification of the Mg 2 Si phase in Al-Mg-Si alloys. Recent studies (Ref 14, 18) show that the rare earth Yelement can be used to modify the Mg 2 Si phase and alter the mechanical properties in those alloys. The UTS and elongation values reported in a previous work (Ref 14) are 290 MPa and 4.3%, respectively, with an optimum H.R. Jafari Nodooshan, Young Researcher Club, Islamic Azad University, South Tehran Branch, 1988875361 Tehran, Iran; and National Engineering Research Center of Light Alloy Net Forming, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China; Wencai Liu and Guohua Wu, National Engineering Research Center of Light Alloy Net Forming, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China; A. Bahrami and M.I. Pech- Canul, Centro de Investigacio ´n y de Estudios Avanzados del IPN Unidad Saltillo, Av. Industria Metalu ´ rgica, No. 1062, Parque Industrial, 25900 Ramos Arizpe, Coahuila, Mexico; and M. Emamy, School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, Tehran, Iran. Contact e-mail: hamidj_63@sjtu.edu.cn. JMEPEG ÓASM International DOI: 10.1007/s11665-014-0900-4 1059-9495/$19.00 Journal of Materials Engineering and Performance