| Quasicrystals exhibit long-range quasi-periodic translational order and non-crystallographic rotational symmetries, which gives rise to various unique physical, chemical and mechanical properties. The formation and stabilization mechanism of quasicrystals have long been the important issues of condensed matter physics. In our previous work, an atomic-cluster-plus-glue-atom model has been proposed for describing the structures of quasicrystals, in which the atomic cluster is defined as the nearest neighbor coordination polyhedron around a given atom, and the next nearest neighbor atoms are treated as a glue structure. With this model, the composition of a quasicrystal can be expressed by a universal formula,[cluster](glue)x, where x is the number of the glue atoms. In the present work, the composition character of Zr-Ti-Ni icosahedral quasicrystals (IQCs) is considered within the framework of the atomic-cluster-plus-glue-atom model. The formation, thermal stability and phase transformation of Zr-Ti-Ni QCs have been investigated over a wide range of compositions.The experimental results are summarized as follows:In the Zr-Ti-Ni system, the IQCs are structurally very closely related with three crystalline phases:θ-Zr2Ni, η-Ti2Ni and λ-TiZrNi (MgZn2-type C14-Laves phase). The atomic structures of these crystalline phase have been analyzed using the Carine Crystallography3.1software. Consider also the closest packing and the cluster isolation criteria. Two Ni-centered clusters, namely, Ni3Zr8and Ni4Ti9, and a Ti/Ni-centered cluster, Ti4.488Ni3.192Zr5.292, are determined for the three crystalline phases. The cluster formulas of Zr-Ti-Ni IQCs are derived with these atomic clusters to explain the composition zones of IQC formation. The Zr-rich and Ti-rich compositions are separately considered:In the Zr-rich region of the Zr-Ti-Ni phase diagram, the eutectic composition, Zr76Ni24, can be expressed by a cluster formula [Ni3Zr8]Zr (=Zr75Ni25). Replacing the glue atom Zr by Ti works out a cluster formula [Ni3Zr8]Ti=Zr66.67Ti8.33Ni25. The Ti-rich eutectic point Ti76Ni24corresponds to a formula [Ni4Ti9]Ti3(=Ti75Ni25). In a similar way, three cluster formulas in the exact forms of [Ni4Ti9]Ti2Zr,[Ni4Ti9]TiZr2and [Ni4Ti9]Zr3are constructed by replacing one, two or three Ti atoms with Zr for the formula [Ni4Ti9]Ti3, the corresponding compositions being Ti68.75Zr6.25Ni25,Ti62.5Zr12.5Ni25and Ti56.25Zr18.75Ni25, respectively. Similarly, a cluster formula [Ti4.488Ni3.192Zr5.29]Ti2Zr(=Ti39.39Zr40.62Ni19.98)is proposed for Ti39.5Zr39.5Ni21eutectic, where [Ti4.488Ni3.192Zr5.29]stands for the principal cluster of λ phase. And four cluster formulas, namely,[T14.488NZ3.192Zr5.29]Ti3,[Ti4.488Ni3.192Zr5.29]TiZr2,[Ti4.488Ni3.192Zr5.29]Ti2Zr and are constructed by substitution alloying the glue structure (replacement of Ti/Ni atoms by Ti or Zr), the corresponding chemical compositions are Ti46.88Zr33.13Ni19.98、Ti34.36Zr45.64Ni19.98,Ti39.39Zr40.62Ni19.98and Ti28.10Zr51.92Ni19.98, respectively.The microstructures and phase transformation behaviors of two series of Zr-Ti-Ni alloys containing20and25at.%Ni have been thoroughly studied. It has been found that a single phase IQC microstructure can be reached by annealing melt-spun amorphous samples with the composition [Ni3Zr8]Ti. At the formula compositions derived from the λ orηphase, single phase IQC can be made by means of mould casting or melt-spinning.The formation and high phase stability of IQC are most favored at the X-phase derived formula compositons. In particular,2mm diameter IQC rods have been successfully prepared at [T14.488NZ3.192Zr5.29]Ti3(Ti46.88Zr33.13Ni19.98), the structure of which is stable up to a temperature TIQC=712℃... |
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