Influence of oxygen impurity on containerless solidification of quasicrystalline-forming Zr 80 Pt 20 alloy
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Influence of oxygen impurity on containerless solidification of quasicrystalline-forming Zr80Pt20 alloy Takeshi Harada1, Akitoshi Mizuno1, Masahito Watanabe1 1 Gakushuin university, 1-5-1 Mejiro, Toshima, Tokyo, Japan ABSTRACT The influence of oxygen content on containerless solidification of Zr80Pt20 alloy has been studied by using conical nozzle levitation (CNL) technique. The doping level of oxygen from 41 to 5450 ppm mass oxygen (PMO) affects the undercooling of the liquid Zr80Pt20 alloy. Timeresolved synchrotron x-ray diffraction revealed that the quasicrystalline (QC) phase precipitated as a primary phase during solidification of the Zr80Pt20 alloy. The amount of the QC phase depends on the oxygen content in the alloy. This indicates that the doping level of oxygen in Zr80Pt20 alloy can be related to the metastable phase formation as well as the glass-formation ability. INTRODUCTION Bulk metallic glasses (BMGs) are regarded as functional materials with excellent mechanical properties such as large ductility, high-toughness and high-strength [1]. While it was empirically derived that multicomponent alloys with three or more elements should be prepared to yield the BMGs, recent studies have shown that a careful selection of the composition can form the BMGs even from binary alloys such as Zr-Cu, Ni-Nb and Cu-Hf systems [2,3]. On the other hand, it has been reported that the glass-forming ability can be improved by controlling oxygen content in several Zr-based [4-6] and Fe-based [7] alloys. In the case of Zr-Cu alloys, Wang et al. proposed that high-solubility of oxygen in zirconium contributes to the uniform dispersion in liquids and avoiding the formation of oxides rather than inducing the inhomogeneous nucleation sites [4]. In addition, they concluded that the oxygen can be considered as the alloying element rather than impurity on the glass formation of the Zr-Cu alloys. As for Zr80Pt20 alloy, the oxygen level directly affects the as-quenched structure (equilibrium crystal, quasicrystal (QC), or amorphous) of melt-spun ribbons produced at constant cooling rate [6], whereas it was previously reported that the as-quenched structure was affected by the cooling rate in melt-spinning [8]. These results suggest that the glass-forming ability and the phase selection depend on the oxygen content in the Zr-based alloys. Besides, there have been many suggestions that the QC phase forms as a metastable phase because of the similarity of its structure to icosahedral short-range order in the liquid as well as in the glass [6]. Therefore, it is essential to investigate the influence of the oxygen on structure variation during the solidification of the QC-forming alloys in relation to the glass-forming ability. For the last decade, it has been well known that a containerless levitation technique is useful to study solidification processes especially for high-temperature materials [9]. In addition, a combination of the containerless technique and synchrotron radiation is quite effective to investigate structure of high-temp
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