Formation of icosahedral quasicrystalline phase in Zr-Al-Ni-Cu-M (M = Ag, Pd, Au or Pt) systems

Icosahedral (I) particles with spherical morphology were found to appear as a primary precipitation phase in amorphous Zr65Al7.5Ni10Cu12.5M5 (M = Ag, Pd, Au or Pt) alloys with two-stage crystallization process. The element M was selected with the aim of deviating the three empirical rules for stabilization of supercooled liquid, i.e., multicomponent of more than three elements, atomic size mismatch over 12%, and negative enthalpy of mixing. The I phase in an amorphous matrix has a small size of 25 to 40 nm. The second crystallization reaction results in the formation of Zr2Cu, Zr2Ni and Zr2Al3 phases for the alloys containing Ag, Pd or Au, and Zr2Cu, Zr2Ni, Zr2Al3 and ZrPt phases for the Pt-containing alloy. The Zr65Al7.5Ni10Cu17.5 amorphous alloy crystallizes directly to a mixed structure of Zr,Ni, Zr2Cu, Zr2Al3 and ZrNi phases through a single exothermic reaction. The addition of the element M is essential for the precipitation of the metastable I phase. The reason is that the structure changes from a long-range homogeneous configuration to an inhomogeneous one by the addition of the element M.