摘要:

Phase transformations in the Al70Co15Ni15decagonal quasicrystal (DQC), induced by 120 keV Ar+ion irradiation at room temperature, have been studied by means of transmission electron microscopy. The transformation behaviour is dependent on the dose, the dose rate and the thickness of the specimen. In the Ar+-ion-affected depth the ordered DQC phase transformed into disordered DQC and a b.c.c. phase when the dose is equal to or less than 5 × 1013Ar+cm−2. Then the b.c.c. phase was ordered to a CsCl-type structure when the dose was increased to 1014-5 × 1015Ar+cm−2. However, the ordered CsCl structure transformed back to disordered b.c.c. phase with further increase in the irradiation dose and dose rate. This disordering-ordering-disordering transformation sequence is explained by the Dienes-Potter ordering rate expressions.

ISSN:0950-0839    

DOI:10.1080/09500839508240999

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Plastic deformation experiments were performed on icosahedral Al-Pd-Mn single-quasicrystals to determine the thermodynamic activation parameters of the deformation process. The stress exponent and the strain-rate sensitivity of the flow stress were obtained by means of stress relaxation experiments. The activation enthalpy of the deformation process was measured by temperature change experiments. In the range from 730 to 800°C a nearly constant value of about 7 eV was determined.

ISSN:0950-0839    

DOI:10.1080/09500839508241000

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The plastic deformation of icosahedral Al-Pd-Mn single quasicrystals has been studied byin situstraining experiments in a high-voltage electron microscope at elevated temperatures. The results provide the first direct evidence for dislocation motion in quasicrystals. The dislocation velocity, for an applied stress of 390 MPa, was determined as 7×10−7ms−1. It was found that the dislocation motion takes place in planes which are perpendicular to threefold and fivefold lattice directions.

ISSN:0950-0839    

DOI:10.1080/09500839508241001

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The variation in the d.c. magnetic susceptibility for periodic and quasiperiodic antiferromagnetichon-magnetic Cr/Cu multilayers has been investigated in the temperature range 10–300 K. The compositionally quasiperiodic multilayers were made using the Fibonacci series. The temperature dependence of magnetization was found to exhibit hysteretic behaviour at low temperatures in both types of multilayers. The irreversible-to-reversible transition temperature Ttrof the quasiperiodic layers was about 1.6Ttrof the periodic layers. The hysteretic behaviour of the magnetization is similar to that observed in Fe-rare-earth amorphous alloys where the paramagnetic-to-spin-glass phase transition takes place owing to freezing of the spin components at two different temperatures.

ISSN:0950-0839    

DOI:10.1080/09500839508241002

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The second-electron affinity of the oxygen atom is often required in modelling the behaviour of ceramics. It is usually taken to be about 8 eV, independent of the structure of the oxide being modelled. We show that this is not so.

ISSN:0950-0839    

DOI:10.1080/09500839508241003

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

A structure model of octagonal quasicrystals has been obtained using electron microscopy and crystallographic image processing. Octagonal quasicrystals in a rapidly solidified Mn-Si-Al alloy were studied by high-resolution electron microscopy (HREM). HREM images taken along the main symmetry directions, that is the eightfold and the two twofold axes were combined into a density map. The symmetry of the model is 84/mmc. Within each unit-cell repeat of 0.63 nm along the eightfold direction there are four layers. Alternating layers have eightfold and fourfold symmetry. Projections of the model match the HREM images. The shortest interatomic distances, 0.24 nm, and the density of 6.9 g cm−3for the model are in good agreement with expected values.

ISSN:0950-0839    

DOI:10.1080/09500839508241004

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Through-focus electron holography and quantitative electron diffraction have been applied to determine the structure of the 21 K superconductor ThPdxB6−2xwithx≈ 0.65. This compound adopts a cubic structure (a= 0.42nm; space group,Pm3m), which is an intermediate between the structures of the CaB6and Cu3Au. Short-range ordering results in a superstructure as is evident from diffuse streaks in the electron diffraction pattern.

ISSN:0950-0839    

DOI:10.1080/09500839508241005

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

An orthorhombic phase with a = 1.23 nm,b= 1.24 nm andc= 3.07nm has been found in the Al12Fe2Cr alloy. The distribution of strong diffraction spots in the [010], [100] and [001] electron diffraction patterns (EDPs) corresponds to the tenfold, the 2D twofold, and the 2P twofold EDPs respectively of a decagonal quasicrystal. Therefore this orthorhombic phase is called a decagonal approximant. However, unlike other orthorhombic approximants, its parameterc= 3.07 nm does not agree with any of the values obtained by substituting a rational ratio of two consecutive Fibonacci numbers, such as 1/1, 2/1, 3/2, 5/3, etc., for the irrational τ in one of the quasiperiodic directions in a decagonal quasicrystal. Its [010] high-resolution electron microscopy image consists of a network of 72° and 36° rhombi whose vertices are surrounded by a decagon of image points. The ratio of the thick to thin rhombi is 4 to 1 rather than a Fibonacci ratio. This is a new type of decagonal approximant. On ordering, a C-centred monoclinic superlattice withaM=c/sin β = 3.31 nm, bM= a= 1.23nm,cM= 2b= 2.48 nm and β = 112° results.

ISSN:0950-0839    

DOI:10.1080/09500839508241006

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

We show that samples of the decagonal Al-Co-Cu(-Si) phase produced by different slow solidification methods are in the microcrystalline and not in the quasicrystalline state. Parameters for the underlying unit cell area = b ≈51.3 Å,c≈ 4.1 Å and γ = 108°. Experiments on differently produced samples show the same microcrystalline state and that the (5,7) approximant seems to be highly favoured, compared with the set of possible high-order approximant states.

ISSN:0950-0839    

DOI:10.1080/09500839508241007

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The first report of an oxygen-stabilized quasicrystal is presented. Oxygen is shown to be essential for the formation of the icosahedral and 1/1 approximant phases in Ti-Mn-Si and Ti-Cr-Si alloys. Recent X-ray and neutron diffraction structural studies by us of the 1/1 approximant phase reveal specific sites for the oxygen in the Mackay icosahedral clusters. On the basis of these data, possible explanations for the role of oxygen in quasicrystal formation are considered. These include firstly the creation of a nucleation barrier for competing crystalline phases, secondly the stabilization of the fundamental Mackay icosahedral cluster and thirdly an adjustment of the electron-to-atom ratio to shift the Fermi surface into a minimum in the density of states.

ISSN:0950-0839    

DOI:10.1080/09500839508241008

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor