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Microstructure of the complex metallic β-Al3Mg2 phase

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Aluminium and magnesium are fairly simple metals with close packed lattices: Mg is hcp, whereas Al is fcc. The phase diagram of Al-Mg system contains intermetallic compounds. The mixture of Al/Mg ratio 3:2 atoms yields an extremely complex crystal structure. The β-Al3Mg2, called also Samson phase (Fd-3mS: ICSD 57964, cF 1168), has cubic lattice with a = 28.242 Å and a unit cell containing 1168 atoms [1]. It is decorated by a number of large clusters of atoms, with near icosahedral atoms symmetry, leaving few atoms in between to fill-in space [2]. The Samson phase has hardness about 350 HV, Young’s modulus 68 GPa and low density 2.3 g/cm3. This phase is very brittle at room temperature (fracture toughness K1c is below 0.5 MPa⋅m1/2). Under compressive stress and temperatures higher than 250°C, β-phase become plastic [3]. Comparison with Al shows the best compromise between strength and plastic deformation - strength 3 times higher than pure Al and plastic strain of about 40%. It means that application fields for this phase are in aeronautic and automotive industry. Different stability range of the β-Al3Mg2 in Al-Mg phase diagram is proposed in literature [4, 5]. According to the phase diagram published by Murray [4], the β-phase has a congruent point at 451°C and 38.5% Mg. The stability range of this phase extends from 38.5 up to 40.3% Mg in room temperature. On the Al-rich side, β-Al3Mg2 coexists with the α-phase (which is a solid solution of Mg in Al). On the Mg-rich side, the β-phase coexists with γ-Al12Mg17 and with R-phase that forms below 350°C by [...]

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