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 ternar y 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, previo usly identified as a periodic approximant of Al-Mn and Al-Cr quasicrystalli ne 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 P si is also found and characterised. Its structure is monoclinic with cell p arameters 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 symmet ry. But as peculiar structural faults are observed in this monoclinic Psi-p hase, its formation is assumed to be the result of an allotropic transforma tion 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 arr angement of icosahedral clusters as, for instance, those defined in both th e mu Al4Mn and lambda Al4.5Mn structures. (C) 1998 Elsevier Science S.A. Al l rights reserved.