Production of quasicrystals and crystalline approximants in the Al–Pd–(Fe,Ru,Os) systems
Introduction
The discovery of the thermodynamically stable icosahedral (i) quasicrystals such as i–Al–Cu–(Fe,Ru,Os) [1], [2] and i–Al–Pd–(Mn,Re) [3] has made it possible to obtain a phason-free single i-phase sample with high degree of quasicrystallinity. As a consequence, the quality of the i-phases was found to play a crucial role upon physical properties, especially on electrical properties, and disorder was claimed to obscure the subtle features introduced by quasiperiodicity [4], [5]. It is now a well-known fact that reducing structural disorder by high-temperature annealing significantly raises the electrical resistivity of the sample. Conversely, the resistivity is sometimes regarded as a sensitive measure of the sample quality.
Since the first systematic search for quasicrystals in the Al–Pd–transition metal systems by Tsai et al. [6], not much attention has been paid to Al–Pd–(Fe,Ru,Os) systems, partly because the i-phases are produced there only in a metastable state. However, in view of the physical properties, it is still of a great interest to produce the single i-phases of high structural quality also in the Al–Pd–(Fe,Ru,Os) systems and to study their properties in comparison with those of Al–Pd–(Mn,Re) and Al–Cu–(Fe,Ru,Os) i-phases. Our recent preliminary investigations have shown the formation of structurally ordered i-phases in the Al–Pd–(Ru,Os) systems when annealed at high temperature [7]. Therefore, in this paper the single i-phase region in the Al–Pd–(Fe,Ru,Os) systems are closely investigated by annealing melt-quenched ribbons at substantially high temperatures for 1 h.
As far as the published ternary equilibrium phase diagrams are concerned, no ternary phases have been observed in the Al-rich part of Al–Pd–Fe alloy [8], [9], while there are no diagrams available so far for Al–Pd–Ru and Al–Pd–Os systems. However, recently several cubic approximants have been found in the Al–Pd–(Fe,Ru) systems. For the Al–Pd–Fe system, the existence of a (2/1)-cubic approximant, Al70Pd10Fe20, was first reported by Tsai et al. [10]. Recently, Edler et al. [11] have found a (1/0)-cubic approximant, Al39Pd21Fe2, which has a face-centered unit cell with . For the Al–Pd–Ru system, Mahne and Steurer [12] have reported the formation of a (1/0)-cubic approximant, Al68Pd20Ru12, which, in this case, has a primitive unit cell with . The (1/0)- and (2/1)-cubic approximants correspond to the first and the third lowest-order crystalline analogs to the i-phase, and hence, the Al–Pd–(Fe,Ru,Os) systems would be ideal systems for the comparison of physical properties between the approximants and i-phases as a function of the approximation degree and the lattice parameter.
In this paper, we present the formation regions of the single i-phases and related approximants for Al–Pd–Fe, Al–Pd–Ru and Al–Pd–Os systems based on the result of powder X-ray diffraction measurement and also report on the production of the single i-phase by annealing of an as-cast Al–Pd–Ru alloy.
Section snippets
Experimental
Pure elements (Al 99.99%, Pd 99.9%, Fe 99.9%, Ru 99.9%, Os 99.9%) were melted and alloyed under an argon atmosphere in an arc furnace. The alloys were melt-spun and then subjected to heat treatment at various temperatures for 1–24 h in sealed quartz tubes under an argon atmosphere. The characterization of the samples was performed by means of powder X-ray diffraction using radiation and by electron diffraction measurements. For the Al–Pd–Ru system, some samples were cut out from ingots and
Al–Pd–Fe alloys
Fig. 1 illustrates the results of the X-ray diffraction measurement of the melt-spun Al–Pd–Fe alloys. In this and the subsequent ternary diagrams, a line indicating e/a (electrons per atom) ratio of 1.75 is drawn, around which the i-phase is considered to be most stable [13]. Here the effective valences for Al, Pd, Fe, Ru and Os are assumed to be 3, 0, −2.66,−2.66 and −2.66, respectively [14]. In the Al–Pd–Fe system, the single i-phase samples are obtained at very limited compositions around Al
Conclusions
Formation regions of a single i-phase in the Al–Pd–(Fe,Ru,Os) ternary systems are closely investigated by means of powder X-ray diffraction measurement for melt-spun and annealed ribbons. A single i-phase is formed around the composition Al71Pd18Fe11 in the melt-spun Al–Pd–Fe alloys, but it transforms to a (1/0)-cubic approximant when annealed at a low temperature of 600°C. Structurally ordered i-phases have been successfully obtained in the Al–Pd–(Ru,Os) alloys in the composition regions along
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