Structural Change from Amorphous to Nano Icosahedral Quasicrystalline Phase with Quenching Rate in Zr-Pt Binary Alloy

Abstract

Structural change with quenching rate in the Zr₈₀Pt₂₀ alloy was studied by controlling the roll speed in the melt-spinning technique. The as-quenched ribbon had an amorphous structure at a roll speed of 50 ms⁻¹. An icosahedral quasicrystalline phase was directly formed at a roll speed in the range of 40 to 30 ms⁻¹. The critical roll speed for the formation of the icosahedral phase was 25 ms⁻¹, where the volume fraction of the icosahedral phase was very small. Several crystalline phases precipitated in the melt-spun ribbon at roll speeds below 20 ms⁻¹. The DSC curve of the sample prepared at 50 ms⁻¹ clearly exhibited two exothermic peaks. The first exothermic peak at 717 K, corresponded to the transformation from the amorphous to the icosahedral phase and the second peak was due to the decomposition reaction of the icosahedral phase. The size of the icosahedral phase in the as-quenched and annealed samples lay in the diameter range of 5 to 20 nm and the particles were distributed homogeneously. Thus, it was clarified that the nano icosahedral phase can be formed in the as-quenched state from the melt and in the annealed state from the amorphous phase in the Zr₈₀Pt₂₀ binary alloy. The icosahedral phase transformed to Zr, ZrPt, Zr₉Pt₁₁ and Zr₅Pt₃ phases at a temperature of approximately 900 K . The formation of the nano scale icosahedral phase indicated the possibility that icosahedral short-range order exists in the melted state of the Zr–Pt binary alloy. The strong chemical affinity between Zr and Pt probably contributed to the stabilization of the icosahedral short-range order and restraining the long-range rearrangement of constitutional elements to form a stable crystalline phase, which is an important factor in the formation of an icosahedral phase.