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 Zr₆₅Al_7.5Ni₁₀Cu_12.5M₅ (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 Zr₆₅Al_7.5Ni₁₀Cu_17.5 amorphous alloy crystallizes directly to a mixed structure of Zr₂Ni, 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.