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

The aspect of the Al-Mn-Cr phase diagram related to liquid-solid equilibria in the subsection Al4Cr-Al4Mn-Al was investigated. It is shown that ternary Al4MnxCr1-x compounds (0 less than or equal to x less than or equal to 1) whose structure correspond to that of the hexagonal mu Al4Mn 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 Psi is also found and characterised. Its structure is monoclinic with cell parameters a(Psi) = 17.48 Angstrom, b(Psi) = 30.31 Angstrom, c(Psi) = 24.695 Angstrom, beta = 135 degrees and a space group of either C2/c or Cc symmetry. But as peculiar structural faults are observed in this monoclinic Psi-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 mu and Psi structures, it is suggested that the atomic structure of Psi could also be typical of an arrangement of icosahedral clusters as, for instance, those defined in both the mu Al4Mn and lambda Al4.5Mn structures. (C) 1998 Elsevier Science S.A. All rights reserved.