Eiji Abe

Long-period ordered structure in a high-strength nanocrystalline Mg-1 at% Zn-2 at% Y alloy studied by atomic-resolution Z-contrast STEM

The microstructure of a nanocrystalline Mg97Zn1Y2 (at%) bulk alloy prepared by warm extrusion of rapidly solidified powders has been investigated by a combination of techniques, such as conventional high-resolution transmission electron microscopy (HRTEM), atomic-resolution high-angle annular dark field scanning-TEM (HAADF-STEM) with Z-contrast and energy-dispersive X-ray spectroscopy (EDS) with a sub-nanometer electron probe. We show that a novel long-period ordered structure is formed in the alloy, whose unit cell is composed of six close-packed planes of the magnesium crystal with a stacking sequence of ABCBCB′ where A and B′ layers are significantly enriched by Zn and Y. The lattice is distorted from an ideal hexagonal lattice of 6H-type (ABCBCB), which is probably due to an asymmetry of the chemical order with respect to the 6H-type stacking order. The present results demonstrate that the additional elements of a few atomic percent to Mg lead to formation of a long-period chemical-ordered as well as stacking-ordered structure, as directly revealed by a unique Z-contrast method.

A stable binary quasicrystal.

All stable quasicrystals known so far are composed of at least three metallic elements. Sixteen years after the discovery of the quasicrystal, we describe a stable binary quasicrystalline alloy in a cadmium–ytterbium (Cd–Yb) system. The structure of this alloy represents a new class of packing of 66-atom icosahedral clusters whose internal structure breaks the icosahedral symmetry. The binary quasicrystal offers a new opportunity to investigate the relation between thermodynamic stability and quasiperiodic structure, as well as providing a basis for the construction of crystallographic models.

Transmission electron microscopy study of the evolution of precipitates in aged Al–Li–Cu alloys: the θ′ and T1 phases

Abstract We have investigated the structures of the θ′ and T1 precipitates in Al–1.6wt%Li–3.2wt%Cu and Al–2.4wt%Li–3.2wt%Cu alloys aged at 220 °C. The θ′ precipitates in the 1.6 wt%-Li alloy are those known for the Al–Cu binary system (a=0.40 and c=0.58 nm); whereas those in the 2.4 wt%-Li alloy exhibited two atypical structures. One, named a type I TB′ plate in this study, is isostructural to the known θ phase with a large c value of about 0.64 nm, having a habit plane parallel to the matrix {1u20080u20080}α; the other, type II TB′, is characterized by a=0.41 and c=0.61 nm, having a habit plane inclined at about 20° with {1u20080u20080}α, while maintaining a coherent interface. Also images of {1u20081u20081} precipitates in the 1.6 wt%-Li alloy revealed a continuous change from the T1 phase (c=0.935 nm), to a structure with c=0.87–0.90 nm. The image and small lattice parameter suggest that this {1u20081u20081} precipitate is likely to be the Ω phase.

Direct observation of a local thermal vibration anomaly in a quasicrystal

Quasicrystals have long-range order with symmetries that are incompatible with periodicity, and are often described with reference to a higher-dimensional analogue of a periodic lattice. Within the context of this ‘hyperspace’ crystallography, lattice dynamics of quasicrystals can be described by a combination of lattice vibrations and atomic fluctuations—phonons and phasons. However, it is difficult to see localized fluctuations in a real-space quasicrystal structure, and so the nature of phason-related fluctuations and their contribution to thermodynamic stability are still not fully understood. Here we use atomic-resolution annular dark-field scanning transmission electron microscopy to map directly the change in thermal diffuse scattering intensity distribution in the quasicrystal, through in situ high-temperature observation of decagonal Al72Ni20Co8. We find that, at 1,100u2009K, a local anomaly of atomic vibrations becomes significant at specific atomic sites in the structure. The distribution of these localized vibrations is not random but well-correlated, with a quasiperiodic length scale of 2u2009nm. We are able to explain this feature by an anomalous temperature (Debye–Waller) factor for the Al atoms that sit at the phason-related sites defined within the framework of hyperspace crystallography. The present results therefore provide a direct observation of local thermal vibration anomalies in a solid.

Transmission electron microscopy study of the early stage of precipitates in aged Al–Li–Cu alloys

We report here on the early stage of the precipitation behavior of aged Al–Li–Cu alloys using transmission electron microscopy. The δ′ phase (Al3Li, L12 structure (Structurbericht)) was found to exist in the as-quenched specimen containing 2.4 wt%-Li, whereas no precipitation was observed in the as-quenched specimen with 1.6 wt%-Li. After aging at 100 °C for 3 h, GP-I zones nucleated homogeneously in both specimens; but no δ′ phase was detected in the 1.6 wt%-Li alloy at that stage. As the aging proceeds, the δ′ phase nucleates and grows on the GP-I zones. The composite structure, in which the GP-I zone is flanked by a pair of lenticular δ′ particles, is stable in 180–200 °C. The facing lenticular δ′ particles on a GP-I zone were systematically found to be anti-phase with respect to each other.