Liquid–solid equilibria in the Al-rich corner of the Al–Mn–Cr system

Abstract

The aspect of the Al–Mn–Cr phase diagram related to liquid–solid equilibria in the subsection Al 4Cr–Al 4Mn–Al was investigated. It is shown that ternary Al 4Mn x Cr 1− x compounds (0≤ x≤1) whose structure correspond to that of the hexagonal μAl 4Mn phase, previously identified as a periodic approximant of Al–Mn and Al–Cr quasicrystalline phases, are in equilibrium with a liquidus phase field extending between both binary Al–Mn and Al–Cr limits. A new ternary Al–Mn–Cr phase, called Ψ, is also found and characterised. Its structure is monoclinic with cell parameters a Ψ=17.48 Å, b Ψ=30.31 Å, c Ψ=24.695 Å, β=135° and a space group of either C 2/ c or Cc symmetry . But as peculiar structural faults are observed in this monoclinic Ψ-phase, its formation is assumed to be the result of an allotropic transformation of an orthorhombic structure which would first solidify. Besides, from structural relationships observed between the μ and Ψ structures, it is suggested that the atomic structure of Ψ could also be typical of an arrangement of icosahedral clusters as, for instance, those defined in both the μAl 4Mn and λAl 4.5Mn structures.