en گروه ریخته گری و انجماد Casting Processes and Solidification Phenomena Research Paper Research Paper This study deals with the effects of erbium (Er) addition on the microstructural evolution and tensile properties of Al-Mg₂Si in-situ metal matrix composites. The morphology of primary Mg₂Si and eutectic phases were observed in details using optical microscope and scanning electron microscopy (SEM). The results showed that the increase of Er content has a slight effect on the size and morphology of primary Mg₂Si phases, but the eutectic structure evolves from the coarse structure into the fine one. Also, with Er addition the eutectic mixtures of Al and Mg₂Si with fibrous morphology has been developed instead of the flake like Al-Mg2Si eutectic microstructure. Meanwhile, Al₃Er phase was observed in the samples containing Er. The ultimate tensile strength (UTS) of the composite changes under the various content of Er. The maximum strength was found at the 0.6 wt% Er with the fine eutectic microstructure. The study of SEM micrographs from the fracture surface of composites revealed that Er addition changes the fracture mode from brittle to ductile one with fine dimples. The mechanism of microstructural evolution was discussed in details. In-situ composites,Al-Mg2Si metal matrix composite,Microstructure,Tensile properties. 71 79 http://ijmse.iust.ac.ir/browse.php?a_code=A-10-2489-2&slc_lang=en&sid=1 Zahra Rousta zahraroosta2009@gmail.com 1800319475328460011235 1800319475328460011235 No Department of Materials Engineering, Faculty of Engineering, University of Sistan and Baluchestan, Zahedan, Iran Esmaeil Tohidlou etohidlou@eng.usb.ac.ir 1800319475328460011236 1800319475328460011236 Yes Department of Materials Engineering, Faculty of Engineering, University of Sistan and Baluchestan, P.O. Box 98135-987, Zahedan, Iran Hamed Khosravi hkhosravi@eng.usb.ac.ir 1800319475328460011237 1800319475328460011237 No Department of Materials Engineering, Faculty of Engineering, University of Sistan and Baluchestan, Zahedan, Iran