Heat Capacity of Decagonal and Icosahedral Quasicrystalline Phases at High Temperatures

The paper deals with the calculations of heat capacity of quasicrystalline decagonal Al69Co21Ni10 and icosahedral Al63Cu25Fe12 quasicrystalline phases of Al–Co–Ni and Al–Cu–Fe alloys, respectively. According to the Gruneisen law, heat capacity is an energy characteristic, which reflects the phases’ resistance to failure. For calculations of the heat capacity, structure of quasicrystalline phases is considered in the model representation of anisotropic crystals. As a result, it is found that the heat capacity of quasicrystalline phases at high temperatures is the excessive one, i.e. it exceeds the Dulong-Petit value. Therefore, quasicrystalline phases at high temperatures are more stable, than the crystalline phase. For the decagonal quasicrystalline phase, heat capacity is more than 3R in the temperature range of ~480–1500 К, and for the icosahedral quasicrystalline phase – in the temperature range of ~380–1120 К. It follows that decagonal phases remain stable at high temperatures at which the icosahedral phases are destroyed.

Atomic clusters and atomic surfaces in icosahedral quasicrystals

This paper presents the basic tools commonly used to describe the atomic structures of quasicrystals with a specific focus on the icosahedral phases. After a brief recall of the main properties of quasiperiodic objects, two simple physical rules are discussed that lead one to eventually obtain a surprisingly small number of atomic structures as ideal quasiperiodic models for real quasicrystals. This is due to the fact that the atomic surfaces (ASs) used to describe all known icosahedral phases are located on high-symmetry special points in six-dimensional space. The first rule ismaximizing the densityusing simple polyhedral ASs that leads to two possible sets of ASs according to the value of the six-dimensional lattice parameterAbetween 0.63 and 0.79 nm. The second rule ismaximizing the number of complete orbits of high symmetryto construct as large as possible atomic clusters similar to those observed in complex intermetallic structures and approximant phases. The practical use of these two rules together is demonstrated on two typical examples of icosahedral phases,i-AlMnSi andi-CdRE (RE = Gd, Ho, Tm).

Features of the Mackay Clusters with and without a Center Atom in Al Based Approximants of Icosahedral Quasicrystals

The Packing Features of Mackay and Pseudo Mackay Clusters in Al Based Approximants to Icosahedral Phases Are Surveyed. The Cubic Approximants of α-AlMnSi, AlReSi, and α-(Al,Si)CuFe Indicate Typical Mackay Clusters Composed by the Dense Icosahedral Shell Packing. On the other Hand, the Cubic Approximants of α-Alcuru and α-(Al,Si)CuFe and the Non-Cubic Approximants of Ir9Al28, χ-AlPdRe, and R-AlPdCo Shows so-Called Pseudo Mackay Clusters in which the Icosahedral Packing Feature Is Disappeared at their First Shells. The Distorted First Shells Represent a Variation of Pseudo Mackay Clusters, which Could Be Classified into Three Subgroups Based on the Symmetry.