Stability and phase transformations of icosahedral phase in a 41.5Zr41.5Ti17Ni alloy
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Phase stability and transformations of the icosahedral phase (I-phase) in a 41.5Zr41.5Ti17Ni alloy were investigated using melt-spun ribbons and arc-melted bulk samples. A perfect I-phase can be formed directly from liquid through the melt-spinning technique. The I-phase formed in the ribbon is thermodynamically stable and transforms to W-phase, a 1/1 rational approximant above 565 °C. Formation of the perfect I-phase during annealing treatment of the arc-melted sample is very sluggish. Various types of approximants exist as intermediate states for the transformation of crystalline phases to a perfect I-phase.
I. INTRODUCTION
Icosahedral phase (I-phase) with quasiperiodic order has been found in a number of alloy systems.1–3 Since I-phase has a new type of crystal structure that is forbidden in the classical crystallography, the physical/ mechanical properties of I-phase would be very unique in comparison with those of normal crystalline phases. Most of the I-phases reported are metastable phases that can be formed by rapid quenching techniques. However, thermodynamically stable I-phases were found in several alloy systems including the Zr–Ti–Ni alloy system.2–4 The discovery of the stable I-phases makes it possible to study the physical/mechanical properties of the bulk I-phases. In many cases, I-phases have good oxidation/ corrosion resistance, low friction coefficients, but very poor ductility.5 Therefore, the practical applications of I-phase materials have been limited. However, I-phase can be considered as a microstructural option for highstrength multiphase alloys or composites. Excellent examples are surgical tools that have exceptionally high strength due to I-phase precipitates in the maraging steels.6 By controlling the multiphase microstructure including I-phase, the material properties can be improved or optimized. Studies on the phase equilibria and phase transformations of the stable I-phase alloy systems are essential for microstructural control. In the Zr–Ti–Ni system, metastable I-phase forms in a broad composition range,7 while the stable I-phase forms only in a narrow composition range near 41.5Zr41.5Ti17Ni. Extensive studies on the crystallography for the unstable and stable I-phases have been
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II. EXPERIMENTAL
An arc-melted ingot (30 g) of 41.5Zr41.5Ti17Ni composition was prepared under an Ar atmosphere using high-purity elements (Ti: 99.95%, Zr: 99.8%, Ni: 99.95%). The ingot was sliced into pieces by low-speed diamond saw. For melt-spinning experiments, the pieces were then inductively remelted in fused silica tube and quenched onto a rotating copper wheel (wheel surface velocity ∼30 m/s). The ribbon was approximately 15–50 m in thickness and 1–4 mm in width. The annealing experiments were carried out in a high-vacuum furnace (
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