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.     

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.     

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.     

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.     

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.     

Formation of the icosahedral quasicrystalline phase in Zr70Pd30 binary glassy alloy

It is found that a single icosahedral quasicrystalline phase is formed as a primary precipitation phase in the melt-spun Zr₇₀Pd₃₀ binary glassy alloy with a two-stage crystallization process. The onset temperature of the transformation from the amorphous to the icosahedral phase is 701 K at the heating rate of 0.67 K s-1. The size of the icosahedral particles lies in the diameter range below 10 nm and the particles are distributed homogeneously. The second-stage crystallization reaction results in the formation of a Zr₂Pd phase through a single exothermic reaction. The formation of the nanoscale icosahedral phase indicates the possibility that icosahedral short-range order exists in the Zr-Pd binary glassy state. Comparison with the thermal stability of an icosahedral phase in the Zr-Ni-Pd system shows that the icosahedral phase is stabilized by the addition of Ni. The stabilization is due to the restraint of the long-range rearrangement of the constitutional elements resulting from the strong chemical affinity between Zr and Ni.     

Misorientation characteristics of penetrating morphologies at the growth front of abnormally growing grains in aluminum alloy

The initial stage of abnormal grain growth of the aluminum alloy 5052 has been investigated using electron back-scattered diffraction to analyze the characteristic of misorientations of the penetrating morphology at the growth front. Among the 84 penetrating morphologies examined, none of the penetrated grain boundaries has low angles or coincidence site lattice (CSL) relations, whereas 66 penetrating grain boundaries have low angles or CSL relations. These results strongly suggest that the penetrating morphologies should result from triple-junction wetting.     

Deformation strengthening of grain-boundary sliding in a Pb-62wt% Sn alloy

Abstract

Direct observation in a scanning electron microscope of the evolution of the grain-boundary sliding (GBS) process in a Pb-62wt%Sn eutectic alloy during superplastic deformation in shear is reported. The distribution of GBS along the shear surface is found to be inhomogeneous and there is evidence of coupling of the processes of GBS and grain-boundary migration. The rate of GBS increases at small stages of strain (up to about 0·8) and decreases thereafter, indicating that the GBS process is eventually accompanied by strain hardening. The observations are compatible with the dislocation model for GBS.     

New understanding of the role of coincidence site lattice boundaries in abnormal grain growth of aluminium alloy

The sequential microstructure evolution of abnormal grain growth (AGG) in the aluminium alloy (AA5052) was investigated to analyse the migration behaviour of coincidence site lattice (CSL) boundaries, which are known to play an important role in inducing AGG. The sequential evolution showed that CSL boundaries tend to disappear more slowly than general boundaries at the growth front of abnormally growing grains. Especially, the migration rate of Σ9 boundaries was noticeably low, which is contrary to the previous suggestions.