Formation of Nano Icosahedral Quasicrystalline Phase in Zr-based Binary and Ternary Glassy Alloys

摘要

It is found that a nano icosahedral phase with diameters below 50 nm is formed as a primary phase in the Zr_(70)Ni_(10)M_(20), Zr_(70)TM_(10)Pd_(20), Zr_(70)Au_(10)Pd_(20) and Zr_(75)Pt_(10)Pd_(15) ternary and Zr_(70)Pd_(30) binary glassy alloys. The nanoscale icosahedral phase in the diameter range below 10 nm was also found to be formed directly in the melt-spun Zr_(80)Pt_(20) binary alloy. These icosahedral phases transform to the crystalline phase(s) at the higher annealing temperature. The nucleation kinetics for the precipitation of the icosahedral phase from supercooled liquid were examined in the Zr_(70)Pd_(30) and Zr_(70)Ni_(10)Pd_(20) glassy alloys. It was clarified that the transformation of both alloys proceeds in the diffusion-controlled growth mode with increasing nucleation rate. The formation of the nanometer-scale icosahedral phase is due to the transformation mode. The activation energy of nucleation is evaluated to be 267 kJmol~(-1) for the binary alloy and 311 kJmol~(-1) for the ternary alloy. The difference between the two alloy systems seems to originate from the difference in the number of atoms for rearrangements in the nucleation mode. The short-range ordering is observed in the as-quenched Zr_(70)Pd_(30) glassy alloy, which is indicative of the icosahedral structure. The formation of the nano-scale icosahedral phase in the Zr-based binary and ternary alloys is due to the existence of an icosahedral short-range order in the glassy or liquid state. It is suggested that the icosahedral short-range order is stabilized by the restraint of the long-range atomic rearrangements that lead to the transition to a periodic structure by the strong chemical affinities of Pd or Pt with Zr.