Arrangement of atomic clusters in a 2/1 cubic approximant in the Al-Zn-Mg alloy system

An arrangement of the so-called Bergman clusters of atoms in a 2/1 cubic crystalline approximant phase, which is closely related to Frank-Kasper-type icosahedral quasicrystals, in an Al-Zn-Mg alloy system is discussed in detail. The 2/1 cubic structure has eight Bergman clusters in its unit cell and the positions of the clusters can be understood as twelvefold sites in the 2/1 cubic approximant of the three-dimensional Penrose lattice. The atomic clusters are located at vertices of two types of rhombohedron, which are formed by six rhombic planes with an edge length of 3 1/2 a 2 /( 2 + 1) 1/2 and a shorter diagonal of 2 a 2 /( 2 + 1) 1/2 ( is the golden ratio and a is a quasilattice constant), which correspond to half a body diagonal and an edge length respectively of the 1/1 cubic approximant unit cell. The present results provide an important key for understanding the arrangements of atomic clusters in icosahedral quasicrystals.     

A new well-ordered simple icosahedral quasicrystalline phase in the Zn-Mg-Er system

A well-ordered simple icosahedral quasicrystalline phase has been found in the Zn-Mg-Er system. The structure of a splat-cooled and annealed sample was investigated by electron and X-ray powder diffraction. A quasilattice constant of 0.513nm was determined by the Elser method.     

New 1/1 cubic approximant phases in the Al-Cu-Ru-Si system studied by electron diffraction and high-resolution electron microscopy

Two types of new Al-Cu-Ru-Si 1/1 cubic approximant phases, both of which have cell parameters a=12.68A have been found in an as-cast Al_58.5Cu₁₈Ru_13.5Si₁₀ alloy. The two phases have sc and bcc structures respectively and are finely mixed in transmission electron microscopy observations. It is proposed that the sc structure is caused by ordering of atoms in the bcc structure.     

A new stacking motif in the decagonal approximant Mn3Ga5

The structure of the decagonal approximant Mn₃Ga₅     

Formation of decagonal and approximant phases in the Al-Ni-Ru system

Al-Ni-Ru alloys with a wide composition range have been synthesized and examined by X-ray and electron diffraction experiments. In this system, two types of decagonal (d) phases are formed, namely basic-type and superlattice-type d phases. Annealing experiments have shown that the basic-type d phase is stable at high temperatures and that its single-phase region is located in a small composition range around Al 75 Ni 15 Ru 10 . On the other hand, the superlattice-type d phase is metastable and has been found to form only in the samples before annealing. Besides the d phases, a high-order crystal approximant phase with lattice parameters a = 99.5 Å and b = 84.6 Å has been found. The structural relation between the approximant phase and the d phase is discussed in terms of a phason tensor.     

Bending experiments on the xi'-(Al-Pd-Mn) quasicrystal approximant

Single crystals of the quasicrystal approximant phase xi'1-(Al-Pd-Mn) were deformed at a high temperature in three-point bending geometry. Two different mechanisms of plastic deformation were observed in this phase: one based on the motion of phason lines and the other based on dislocations. Line directions and Burgers vector directions of the dislocations were determined. The relative importances of the two mechanisms are discussed as a function of the sample orientation with respect to the bending geometry.     

Hydrogen diffusion in ultrafine-grained iron with the body-centered cubic crystal structure

The role of grain boundaries in Fe on hydrogen diffusion has been investigated by electrochemical permeation tests using ultrafine-grained Fe produced by high-pressure torsion (HPT) processing. Permeation tests were also conducted on cold-rolled and water-quenched Fe to understand the trapping effect of dislocations and vacancies. Hydrogen diffusion was delayed in all these discs. However, the delay mechanism in the HPT-processed disc was different from that in rolled and water-quenched Fe. Grain boundaries do not act as trapping sites but slow the diffusion. The diffusion coefficients of hydrogen were significantly decreased by HPT processing on account of the high activation energy for hydrogen diffusion in grain boundaries.     

A new high-pressure phase transition in a zirconium-niobium alloy

It is reported for the first time that a g M y transformation can be induced in a g -stabilized zirconium alloy subjected to shock pressure. The y phase formed in the g matrix has been found to have a plate shape akin to martensitic plates. The lattice correspondence between the g and y structures has been found to be the same as that produced by thermal treatment. The formation of the plate-shaped y phase is explained in terms of a mechanism involving shear on <112> planes of the bcc lattice and the mechanical instability of the g phase.     

Tensile strength and fracture of a tilt grain boundary in cubic SiC: A first-principles study

The ab initio tensile test has been applied to the non-polar interface of the {122}, Sigma = 9 tilt boundary in cubic SiC, where the tensile strength and mechanical behaviour at zero temperature are examined using the ab initio pseudopotential method based on the local density-functional theory. This interface is strong because of the reconstruction of interfacial bonds. The maximum tensile stress in the unaxial extension normal to the interface is about 42 GPa, which is about 80% of the theoretical and experimental values of the strength of bulk crystal along the <111> direction. Young's modulus and the fracture toughness are also comparable with the values of the bulk crystal. The back Si-C bond of the interfacial C-C bond is broken first because the C-C bond has a high strength and a short length like a diamond bond. Then the interfacial Si-C bonds are broken, and finally the Si-Si bond. The Si-C bonds are rapidly stretched and broken if the bond stretching exceeds about 20%, and the bond charge clearly disappears when the bond stretching exceeds about 30%. Changes in the electronic structure associated with the bond breaking are analysed.     

Evidence of carbon ordering and morphology change in a cubic carbide phase

Abstract

The crystallographic and microstructural changes of the ordered cubic carbide (K) phase on ageing have been studied in rapidly solidified Fe-30.9 wt% Mn-8.9 wt% AI-2.8 wt%C ((Fe_0.65, Mn_0.35)_0.83 Al_0.17–12 at.%C) alloy. In the as-rapidly solidified condition, an austenitic phase coexists with the carbide phase which is composed of antiphase domains in the interior of the solidification cell. The antiphase domain boundaries were aligned nearly parallel to the {100} planes. By visually comparing the intensity ratio (I₁₁₀/I₁₀₀) of the selected-area diffraction pattern in the transmission electron microscope and by quantitatively comparing the same X-ray diffraction intensity ratio, we have deduced that the K phase tends to consolidate into the L'1₂ structure on ageing at 450°C but into the L1₂ structure on ageing at 700°C.     

Stacking-fault formation in [001] small-angle symmetric tilt grain boundaries in cubic zirconia bicrystals

Small-angle symmetric [001] tilt grain boundaries in cubic zirconia bicrystals with misorientation angles 2θ =1.0° and 2θ =5.0° have been fabricated by diffusion bonding. High-resolution electron microscopy observations revealed that the 1.0° boundary consists of a periodic array of mixed dislocations with Burgers vector b =( a /2)[101] or b = ( a /2)[101], while the 5.0° boundary consists of a periodic array of edge dislocations with Burgers vector b = ( a /2)[100], associated with stacking faults at alternate intervals. This suggests that there is a critical angle for structural transitions in the series of the [001] small-angle tilt grain boundaries.     

Investigation of the interaction of He and D in FeBSi alloy

He and H can be produced by nuclear reaction. In addition, energetic particles of He and T and D, which are isotopes of H, in the plasma of fusion reactor induce the damage in the surface of materials, such as erosion, sputtering and blistering. To investigate the interaction of He and D, amorphous and crystalline FeBSi alloys were irradiated by He or D₂ or He?+?D₂ ions with 5?keV. The results of thermal desorption indicated that more He atoms were trapped in both the amorphous and crystalline alloy irradiated by He ions than D atoms in those alloys irradiated by D₂ ions. Although He and D atoms were trapped in FeBSi alloy irradiated by He?+?D₂ ions, desorption peaks of D₂ and He were separated. Absorption of D in an amorphous alloy was influenced by the presence of He; however, absorption of He was independent of D₂ irradiation in both alloys.     

Ductile fracture mechanism in fine-grained magnesium alloy

The ductile fracture mechanism in a fine-grained magnesium alloy has been investigated by transmission electron microscopy-focused ion beam techniques. In coarse-grained or conventional magnesium alloys, twins form at the very beginning of the deformation process, and crack propagation occurs through the twin boundaries. However, in the alloy used in this study, subgrain structures were found instead of twins at the crack tip. Nanoscale twins formed subsequently owing to large stress in the crack propagation route. The fine-grained alloys showed high fracture toughness resulting from resistance to the twins at the beginning of the deformation.     

Atomic diffusion by the dumb-bell mechanism in a concentrated random bcc alloy

Matter transport by <110> dumb-bell interstitials in the bcc random alloy is considered in an analytical treatment based on linear response theory. Four different jump mechanisms of the dumb-bell are dealt with. It was found that at a low level of approximation relations can be deduced which give two of the three phenomenological coefficients in terms of the third one. (These relations are analogous to exact ones deduced for vacancy diffusion in the random alloy by Moleko, L. K., and Allnatt, A. R., 1998, Phil. Mag. A, 58, 666). Monte Carlo simulation shows that the relations are quite accurate, suggesting that they may well be exact.     

Effects of load direction on the mechanical properties of open-cellular epoxy with a cubic prism structure

The mechanical properties of open-cellular epoxies with a cubic prism structure have been investigated by compressive tests with angles between the beam and the load direction of 0^o (90^o) and 45^o. When the angle was 45^o, the stress fluctuated in a plateau region about a roughly constant flow up to a strain of 77%. However, when the angle was 0^o (90^o), the flow stress changed significantly with the strain passing through a series of peaks. The absorption energy per unit volume was found to be essentially independent of the angle between the beam and the load direction.     

On convergence of the generalized Maxwell scheme: conductivity of composites containing cubic arrays of spherical particles

The Maxwell concept of equivalent inhomogeneity generalized to account for the interactions between the particles in the cluster and combined with recently reported results on the polarizability of a cube is used to evaluate the effective conductivities of the materials reinforced by cubic arrays of spherical particles. New numerical results demonstrate that the estimates of the effective properties based on the generalized Maxwell scheme with the equivalent inhomogeneity of cubic shape converge to the accurate periodic benchmark solutions, unlike spherical shape-based estimates.     

The formation of cubic and monoclinic Y2O3 nanoparticles in a gas-phase flame process

Synthesis of phase-pure cubic and monoclinic Y₂O₃ nanoparticles (16–90?nm) was achieved in a gas-phase flame process. The effect of process parameters on the crystal structure of the Y₂O₃ nanoparticles was systematically investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Fuel gases, H₂ or C₂H₄, at various flow rates, two yttrium precursors, an oxidant stream with varying degrees of N₂ or Ar dilution and different burner diameters were used to examine the effects of chemical atmosphere, flame temperature, residence time and precursor concentration on the crystal structure of the synthesized Y₂O₃ nanoparticles. Regardless of the other process parameters, at diluent/O₂ ratios of 0.25 or lower, monoclinic Y₂O₃ nanoparticles were obtained, whereas at diluent/O₂ ratios of 1 or higher, cubic Y₂O₃ nanoparticles were obtained. A lower diluent/O₂ ratio was related to higher flame temperature. Thermodynamic analyses suggest that high temperatures likely favour the formation of monoclinic Y₂O₃ nanoparticles in this flame process.     

Cluster formulae for alloy phases

Composition formulae for alloy phases are developed using first-neighbour coordination polyhedra plus their connections. The resultant cluster formulae [cluster](glue atom) _x , similar to molecular formulae for chemicals, contain key structural and composition information on the alloy phases. As examples, Al–Ni–Zr alloy phases are analysed with the objective of revealing cluster formula properties such as the principal cluster, the cluster phase and the definition of complex alloy phases.