New Al3Si7 phase with tetragonal silicon structure in quasicrystal-forming near-eutectic Al-Cu-Fe-Si alloys

Highlights: • FCI quasicrystal forms as primary phase for 7 at% Si:Cu/Fe substitution. • The Al₃Si₇ (Al₄Si₁₉) phase was synthesized for the first time. • Al₃Si₇ coexists with ω-CuAl₂ forming an eutectic. • The melting point of Al₃Si₇ was found to be 514 °C. • Eutectic-containing alloys were 43% less hard than undoped base alloy. -- Abstract: A range of aluminum-based quaternary alloys was prepared by arc melting, followed by suction casting. Silicon doping was proven to exhibit the high potential of stabilization of face-centered icosahedral (FCI) quasicrystal. The FCI phase stability region in pseudoternary Al-Cu-Fe-(Si) diagram agrees with compositional limitations imposed by Hume-Rothery electron-counting rules. It was found that FCI forms as primary phase for 7 at% Si:Cu/Fe substitution. The finding might allow crystallization of the FCI phase monocrystals directly from the melt. The study revealed the presence of eutectic in Al-Cu-Fe-Si system. It is composed of tetragonal Al₃Si₇ that form between CuAl₂ orthorhombic dendrites and isolated needles of hexagonal Al-Fe-Si intermetallic. The stability of Al₃Si₇ phase was proven experimentally for the first time, but was previously predicted by a number of ab-initio studies. The overall Vickers hardness of the studied alloys was not significantly affected by Si content if doping did not exceed 7 at%. For 10 at% Si:Fe or Si:Cu doping, the hardness dropped by as much as 330 HV1 or 43% with respect to the undoped base alloy. This could be explained by the formation of soft eutectic.