A. Niikura

Highly ordered structure of icosahedral quasicrystals in Zn-Mg-RE (RE ≡ rare earth metals) systems

Abstract A new group of stable icosahedral phases (i-phases) in the Zn-Mg-RE system were found to have an ideal composition close to Zn50 Mg42RE8 (RE ≡ Y, Gd, Tb, Dy, Ho or Er). The new i-phases exhibit a highly ordered and nearly perfect face-centred icosahedral lattice as revealed by electron and X-ray diffraction and high-resolution electron microscopy. Powder X-ray diffraction indicates that the i-phase has long-range structural order with a correlation length over 1000 A, which is the largest found in the Frank-Kasper group. These new i-phases have the common valence concentration (about 2.08) and could be regarded as a new class of Hume-Rothery alloys.

Stable Zn[sbnd]Mg[sbnd]rare[sbnd]earth face-centred icosahedral alloys with pentagonal dodecahedral solidification morphology

Abstract A thermodynamically stable face-centred icosahedral phase with a pentagonal dodecahedral solidification morphology has been found in Zn[sbnd]Mg[sbnd]RE (RE [tbnd] Y, Tb, Dy, Ho or Er) alloys. We report here the structure, stability and morphology in this new group of icosahedral alloys on the basis of results for the Zn[sbnd]Mg[sbnd]Y system.

Chemical structural relaxation-induced embrittlement in amorphous MgCuY alloys

Abstract Annealing-induced embrittlement of MgCuY amorphous ribbons was evaluated by bend testing and differential scanning calorimetry. Two embrittlement parameters, i.e., the onset temperature of embrittlement, TB, and the minimum amount of enthalpy relaxation leading to embrittlement, ΔHB are sensitive to the alloy composition. TB and ΔHB decrease with increasing Cu and Y content. It is presumed that relaxation-induced embrittlement was a consequence of the chemical short-range ordering between Mg and Cu or Y atoms.

New Class of Amorphous and Icosahedral Phases in Zn-Mg-Rare-Earth Metal Alloys.

A new class of amorphous and icosahedral phases has been obtained in rapidly solidified Zn-Mg-Rare-Earth Metal (RE) ternary alloys. The rapidly solidified alloy structure exhibits the amorphous phase when it contains the RE elements of La, Ce, Pr or Eu with large atomic size. On the other hand, it exhibits the icosahedral phase when it contains Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu or Y. The tendency for the formation of amorphous or icosahedral phase is mainly dominated by the atomic size of RE elements.

Electronic specific heat coefficient and magnetic entropy of icosahedral Mg-RE-Zn (RE=Gd, Tb and Y) quasicrystals

The low-temperature specific heat for non-magnetic Mg42Y8Zn50 quasicrystal has been investigated to get information on the electronic structure. The electronic specific heat coefficient gamma is estimated to be 0.63 mJ mol-1 K-2, which is about 70% of the value expected from the free-electron model. This small gamma value strongly suggests the existence of a pseudogap of the electronic density of states in the vicinity of the Fermi level. The low-temperature specific heats of icosahedral Mg42Gd8Zn50 and Mg42Tb8Zn50 quasicrystals, which exhibit spin-glass behaviour, have also been investigated. The specific heat of Mg42Gd8Zn50 exhibits a broad maximum at a temperature that is a few kelvins above the spin freezing temperature Tf determined by AC susceptibility measurements. The magnetic entropy at Tf for the Mg42Gd8Zn50 quasicrystal reaches about 30% of the theoretical value of R ln 8, being comparable to those of crystalline spin-glass systems such as magnetically dilute AuFe and CuMn alloys. The magnetic specific heat of Mg42Tb8Zn50 quasicrystal is smaller than that of Mg42Gd8Zn50, suggesting the splitting of the ground state due to the local electrostatic field.

Amorphous and quasi-crystalline phases in rapidly solidified MgAlZn alloys

Abstract The formation of amorphous and icosahedral phases and approximants has been identified in rapidly solidified MgAlZn alloys. The icosahedral phase is located at the composition range between those of the amorphous phase and approximants. Mg40Al40Zn20 is a cubic 1 1 approximant in the equilibrium state and a cubic 5 3 rational approximant in the rapidly solidified state. Rapidly solidified Mg50Al30Zn20 and Mg50Al20Zn30 have a disordered icosahedral structure with a high density of random phason strain. The valence concentration e/a plays an important role in the selections of rapidly solidified structure between the amorphous and the icosahedral phases and of phason strains between isotropic and anisotropic.

Hydrogen absorption in an icosahedral ZnMgY alloy

Abstract Hydrogen absorption in a stable icosahedral (i-) Zn 50 Mg 42 Y 8 alloy has been studied by X-ray diffraction, transmission electron microscopy and differential scanning calorimetry. The i-phase decomposed to a hexagonal Zn 2 Mg Laves phase, YH 3 and MgH 2 after hydrogenation under 3 MPa hydrogen pressure at 573 K for 259.2 ks. Two stages of hydrogenation for the i-phase were identified; i-ZnMgY+ x H 2 →i-ZnMgYH 2 x in the initial stage and then i-ZnMgYH 2 x +H 2 →Zn 2 Mg+YH 2 +MgH 2 . DSC curve of the fully hydrogenated (3 MPa, 573 K, 259.2 ks) sample revealed an endothermic peak around 730 K corresponding to the hydrogen desorption. The transformations between the icosahedral and crystalline phases due to hydrogen absorption and desorption are reversible.

Structural model of a face-centred icosahedral phase in Al[sbnd]Mg[sbnd]Li alloys

Abstract A structural model of a face-centred icosahedral quasicrystal in Al[sbnd]Mg[sbnd]Li alloys is proposed. It has an ordered arrangement of two icosahedral atom clusters. One consists of 20 Mg and 24Al and the other contains 20 Li and 24 Al. The clusters are situated at even- and odd-parity twelvefold vertices of the three-dimensional Penrose pattern, forming a face-centred icosahedral structure. The centres of the clusters are linked by three structural units: a prolate rhombohedron with two body diagonal positions occupied by Mg and Li atoms and two rhombic dodecahedra accommodating eight Mg atoms and eight Li atoms. The corresponding 1/1 crystal approximant has the Al24Mg20 cluster at the origin and the Al24Li20 cluster at the body centre and its space group is Pm3.

Novel face-centered icosahedral phase in Al-Mg-Li system

A new class of the icosahedral phase is found for the first time in a melt-quenched Al50Mg25Li25 alloy. The icosahedral phase reveals a face-centered icosahedral lattice analogous to the structure of icosahedral AlCuFe.

Icosahedron and icosahedral order in rapidly solidified Al-Ni-Mo alloys

The quasicrystalline state characterized by long range icosahedral order was firstly reported by Shechtman et al. Icosahedral bond-orientational order has ben observed in a molecular dynamics simulation of an under-cooled Lennard-Jones liquid. A correlation between the structure factors of metallic glasses and the icosahedral solid was also noted. Al[sub 12]Mo(or Al[sub 12]W) is a compound with a structure consisting of a bcc array of Al icosahedron centered by Mo(or W); thus it contains 13 atoms per primitive unit cell and local icosahedral packing which is in favor of the formation of an icosahedral phase in a quenched state. Meanwhile Ni is known as a promotion element for the creation of an Al based amorphous alloy. The authors have observed that the icosahedral phase is created in AlNiMo alloys and transited to an amorphous phase with increasing Ni. Transformation from amorphous via i-phase and finally to Al[sub 12]Mo was observed in Al[sub 85Ni][sub 10Mo][sub 5]. In this paper, the authors focus on the icosahedron and icosahedral order in correlation with composition in the Al-Ni-Mo system.