A. F. Prekul

High-temperature heat capacity of the Al63Cu25Fe12 quasicrystal

The mechanisms of energy absorption by metallic alloys with long-range aperiodic lattice order and electronic properties of marginal metals are studied. The heat capacity and linear expansion coefficient of the Al63Cu25Fe12 icosahedral phase in the temperature range 300–1000 K are measured for the first time. Disagreement between the measured heat capacity and predictions made from the Debye model is found and analyzed. It is shown that the excess heat capacity observed at the temperatures of the experiment is fitted by Einstein’s function in the approximation T ΘE.

Calorimetric Investigation of Electronic and Lattice Excitations of the Icosahedral Quasicrystal in the Range of Moderate Temperatures

The precise measurements of the specific heat and the linear expansion coefficient of polygrain samples of the ordered icosahedral phase Al63Cu25Fe12 have been performed in the temperature range 1.8–400.0 K. The deviations from the Grüneisen law, according to which the temperature dependences of the lattice specific heat at constant volume and the linear expansion coefficient are identical to each other, have been analyzed. The proofs that the specific heat of the quasicrystals contains latent electronic and lattice contributions of the Schottky type have been obtained. The revealed contributions can be thermodynamic consequences of the fractal energy spectra.

Nonmetallic nature of the interrelation of thermal, magnetic, and electrical properties in icosahedral quasicrystals at high temperatures

In the icosahedral phase of Al-Cu-Fe alloys in the temperature range above 300 K there has been found an interrelation between the heat capacity, magnetic susceptibility, and conductivity not characteristic of typical metals. It is supposed that these phases are systems whose properties at finite temperatures are predominantly determined by excited electrons rather than by electrons with a Fermi energy EF.

Comparative study of the heat capacity of icosahedral quasicrystals in solid and liquid states

The heat capacity of icosahedral quasicrystals Al63Cu25Fe12 and Al62Cu25.5Fe12.5 has been studied at high temperatures up to 1700 K, which is by almost 400 K higher than the melting point of the material. It has been shown that the melt exhibits an excess heat capacity with respect to that determined by the Dulong-Petit law and that is a direct extension of the excess heat capacity of the solid state. It has been concluded that the excess heat capacity is related, as a whole, to the short-range order in the quasicrystal structure. This circumstance allows the identification of the orbital hybridization as the most probable mechanism of formation of the pseudogap in the electronic structure of the quasicrystals.

On nature of the excess heat capacity in icosahedral quasicrystals

Experimental evidence is presented which indicates that the total heat capacity in the Al63Cu25Fe12 icosahedral phase contains a contribution associated with inconstancy of the charge carrier concentration. The contribution has an oscillating temperature dependence and is supposedly traceable to two Schottky-like heat anomalies.

Structural state of β-solid solution in rapidly quenched alloys of Al50Cu50―xFex (x = 17, 13, 6) and ordered η1-AlCu phase formation

Conventional transmission and scanning electron microscopy were employed to investigate the structural state and phase transformations of the β(CsCl)-solid solution in rapidly quenched alloys of Al50Cu50− x Fe x with respect to the Cu/Fe ratio. We studied the alloys from the central (x = 17 and 13) and border areas (x = 6) of the β-solid solution homogeneity region. The structural state of the β-solid solution was characterized by premartensitic structural instability of the bcc lattice for x = 17 and 13, and the presence of combined short-range order accompanied by the intense diffuse scattering for x = 6. This short-range order can be described by the ordering of atoms and vacancies in the planes (111)β and ω-like atomic displacements (longitudinally polarized waves of displacement in the direction of the [111]β). The structural state of the β-solid solution with the combined short-range order was regarded as a pretransition state for the revealed transformation with homogeneous precipitation of the nanodispersed phase. The precipitation phase was attributed to an orthorhombic Al(Cu, Fe) η1-phase, belonging to the family of ordered β-based phases with orientation relationships of [100]η1 ||[110]β, [010]η1 ||[110]β, [001]η1 ||[001]β. We concluded that the atomic structure of the η1-phase is characterized by ordering, accompanied by ω-like atomic displacements of the adjacent layers in 3d metals (Cu, Fe) and aluminum.

Electric, magnetic, and thermal properties of quasicrystal-forming melts

The electric resistance, magnetic susceptibility, and specific heat of the icosahedral phases of the Al-Cu-Fe system have been examined in the melt region. It has been shown that the features of the properties of a homogeneous solid state, as well as correlations between these features, hold in melts up to temperatures above the melting point by several hundreds of degrees. The results indicate that the short-range order and orbital hybridization determine the mechanism responsible for the electronic spectrum and ultrahigh-resistance state of quasicrystals.

Structural phase transformations in quasicrystal-forming Al-Cu-Fe alloys and defects of the icosahedral phase

Transmission electron microscopy was used to investigate structural and phase transformations and defects of the icosahedral (ι) phase that is formed upon isothermal annealings (Tann = 550–700°C) of quenched quasicrystal-forming alloys Al61Cu26Fe13 and Al63Cu25Fe12 (β solid solution + ι phase). It has been established that in the Al63Cu25Fe12 alloy there occurs a reversible ι-R-approximant transformation, whereas in the Al61Cu26Fe13 alloy there is formed a single-phase ι structure with regions with a high density of randomly distributed planar defects (Tann = 550°C), which are partially annealed at 650°C. The observed defects are, mainly, ultrathin interlayers (“intergrowths” to 3–5 nm in thickness) on quasicrystal planes with A5ι axes with an imperfect decagonal structure. As the basic mechanism, the growth mechanism of the formation of defects during the β → i transformation is proposed. The role of the alloy composition and low-temperature β → 3C-phase transformation in the realization of this mechanism is discussed.

Short-range order and singularities of the electronic structure of icosahedral quasicrystals

For icosahedral phases of the Al-Cu-Fe system, components of the electrical conductivity, magnetic susceptibility, Hall effect, and heat capacity associated with thermally induced charge carriers for the first time have been considered jointly over a wide temperature range. It has been shown that the full range of thermal effects can be understood in the framework of the unified concept, which is based on an inhomogeneous system of two-level electronic excitations. A model of the inhomogeneous electronic state and the mechanism of its formation with the dominant role of short-range order have been proposed.

Planar defects in the icosahedral phase in quasicrystal-forming AlCuFe alloys

Electron microscopy has revealed that, upon the formation of a stable icosahedral (i) AlCuFe phase, there arise a large number of planar defects. The revealed defects are nanometer-sized coherent intergrowths of the P1-pentagonal approximant. It has been found that the formation of these defects is the result of a nonequilibrium intermediate transformation. The intergrowths are located in the planes with the fivefold symmetry axis and give rise to an elastic-strain state of the i-phase. Analysis of the diffraction contrast has revealed the presence of phason and phonon atomic displacements due to the mismatch between the quasiperiodic (i-phase) and periodic (P1-phase) lattices. The formed structure exhibits a higher electrical resistance as compared to the i-phase and has been considered as a model state of the imperfect icosahedral phase with preferred phonon displacements.