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Abstract: The rapid quenching processes of a larger-scale Al50Mg50 alloy system consisting of 100,000 atoms have been simulated by using molecular dynamics method. The formation and structure of short-range order (SRO) and medium-range order (MRO) in the rapidly quenched Al50Mg50 alloy are investigated by means of several structural analysis methods. It is found that the massive icosahedra in the Al50Mg50 supercooled liquid prevent it from crystallizing and play a critical role in the formation of metallic glass. The SRO in Al50Mg50 metallic glass cannot be modeled by a uniquely prescribed stereo-chemical structure or five Bernal polyhedra, but various types of basic clusters in which the icosahedron is dominant. The MRO is characterized by some certain types of extended icosahedral clusters combined by intercross-sharing atoms in the form of chains or rings, which is different from the FCC and icosahedral building schemes for the MROs in metallic glasses with significant chemical SRO. The size distributions of these MRO clusters exhibit a magic number sequence of 19, 23, 25, 27, 29, 31, 33, 35, 37, 41…. [Copyright &y& Elsevier]