Molecular dynamics study on compressive yield strength in Ni3Al micro-pillars

Recent nanoindentation experiments on bulk samples as well as compression tests on micro-pillars indicate that the micron-sized material volumes exhibit a first yield point that strongly depends on the sample size. In this work, molecular dynamics (MD) simulations are carried out to investigate the onset of yielding in Ni₃Al nano-sized pillars. The MD results show that dislocation generation is from the free surfaces of the micro-pillars, when thermal vibration induces too large a local interatomic displacement. The statistical distributions of the near-surface thermal atomic displacements gathered from the MD simulations are used in conjunction with a survival probability model to predict the yield conditions of real-sized micro-pillars in real time scales. The predicted results agree fairly well with experimental results in the literature.     

Frank's 'cubic' hexagonal phase: An intermetallic cluster compound as an example

Frank introduced in 1965 the novel idea of projection from a four-dimensional cube to recover the points of a hexagonal lattice with a special c / a ratio of ( 3 - 2 ½. This was called a cubic hexagonal crystal, as there was a similarity to the conventional cubic crystals in that directions were perpendicular to planes with the same Miller-Bravais indices. While a number of crystals in the NiAs-Ni 2 In family have been reported with this special axial ratio, the number of atoms in the unit cell is small. As the first example of an intermetallic cluster compound, we identify µ-Al 4 Mn, µ-Al 4 Cr, Zn-Mg-Sm and a host of related intermetallics featuring a special aggregate of icosahedra as Frank's 'cubic' hexagonal phase or its variant. The metric is generated by the Friauf polyhedra and the icosahedral linkages and leads to a multimetric crystal and several interesting connections with hexagonal quasicrystals, hexagonal phases and derivative orthorhombic and lower-symmetry structures.     

Use of electron channelling contrast imaging to assess near-surface mechanical damage

The crystal structure of a cubic phase in the system Zn-Mg-Er has been solved by a combination of high-resolution electron microscopy and X-ray powder diffraction. This phase is considered to be related to that of the quasicrystalline phase. The structure consists of 448 atoms in the unit cell with lattice constant of a ₀ = 20.20Å and the space group is F43m. Important structural elements in the cubic structure are interpenetrating icosahedral units around Zn and Mg atoms and Frank-Kasper polyhedra around the Mg atoms. No giant icosahedral atomic cluster, such as the 136-atom Bergman cluster, was found in the stucture.     

Crystal structure of a hexagonal phase and its relation to a quasicrystalline phase in Zn-Mg-Y alloy

The crystal structure of a Zn-Mg-Y hexagonal phase, considered to be related to that of the quasicrystalline phase, has been determined by single-crystal X-ray diffraction. The atomic model, refined to a final R value of 0.027, has the composition Zn65.22Mg27.92Y6.86 and 92 atoms in a unit cell with lattice constants a = 14.579(2) A and c = 8.687(1) A and the space group P63/mmc. The Y atoms are situated at two different sites; one is fully occupied by Y atoms, while the other is shared with Mg atoms. The structure can be characterized by a layer structure stacked along the c axis, and also viewed as a columnar structure composed of fused Friauf polyhedras sharing hexagonal rings. The similarity of this hexagonal phase to the MgZn2 phase is shown. The structural relationship between this hexagonal phase and the icosahedral phase is also discussed.     

Muscle dysmorphia: Current research and potential classification as a disorder

ObjectiveTo review the literature for scientific evidence in support of inclusion of Muscle Dysmorphia (MD) in the fifth edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5).DesignThe criteria proposed by Blashfield, Sprock, and Fuller (1990) were used for determining whether scientific evidence supports the introduction of MD as a new disorder into a disease classification system.MethodPeer-reviewed journal articles were identified by searching databases for articles published (in print and electronically) from 2001 to 2011.ResultsThe search identified 59 journal articles that specifically focused on MD, of which 39 were empirical journal articles. There is ample literature on MD, including a common set of diagnostic criteria and assessment instruments to measure MD. However, questions remain about the diagnostic reliability and validity, including inter-rater reliability, and whether MD represents a disorder that consists of symptoms that frequently co-occur. Also, evidence of syndrome differentiation is lacking. Only two of the five criteria proposed by Blashfield et al. have been met.ConclusionLiterature suggests that MD is associated with several indicators of clinical significance and distinctiveness. However the current review has found significant limitations and gaps in the scientific literature on MD. Possible options regarding the status of MD in the DSM-5 are proposed, including introducing MD as an example of an eating disorder not otherwise specified, retaining MD as a body dysmorphic disorder, introducing MD as a new disorder, or introducing MD as a provisional diagnosis in need of further study.     

Stacking fault and dislocation glide on the basal plane of graphite

Generalized stacking-fault energies for the basal plane of graphite are calculated from first principles for slip along two high-symmetry directions. The rhombohedral fault energy compares well with experiment and the anisotropy in behaviour is consistent with observed dislocation network geometry. Utilizing these calculated fault energies within a modified Peierls-Nabarro model, we estimate the barrier for basal dislocation motion based on lattice friction. This is found to be extremely small, from which we conclude that dislocation network interaction and pinning, rather than the Peierls barrier, must determine the practical shear strength of graphite. However, at low dislocation densities or over small crystallite regions, the shear strength should tend to this lower limit. We discuss the relevance of this to the mechanism of lubrication.     

Atomistic simulations of cross-slip of jogged screw dislocations in copper

We have performed atomic-scale simulations of cross-slip processes of screw dislocations in copper, simulating jog-free dislocations as well as different types of jogged screw dislocations. Minimum-energy paths and corresponding transition state energies are obtained using the nudged-elastic-band path technique. We find low barriers and effective masses for the conservative motion along the dislocations of elementary jogs on both ordinary {111}<110> and nonoctahedral {110}<110> slip systems. The jogs are found to be constricted and therefore effectively act as pre-existing constrictions; the cross-slip activation energy is thereby dramatically reduced, yielding values in agreement with experiment.     

Deformation mechanism crossover and mechanical behaviour in nanocrystalline materials

We use molecular dynamics simulations to elucidate the transition with decreasing grain size from a dislocation- to a grain-boundary-based deformation mechanism in nanocrystalline fcc metals. Our simulations reveal that this crossover is accompanied by a pronounced transition in the mechanical behaviour of the material; namely, at the grain size where the crossover occurs (the 'strongest size'), the strain rate under tensile elongation goes through a minimum. This simultaneous transition in both the deformation mechanism and the corresponding mechanical behaviour offers an explanation for the experimentally observed crossover in the yield strength of nanocrystalline materials, from Hall-Petch hardening to 'inverse Hall-Petch' softening.     

The flow stress contribution of the dislocation 'forest' in bcc lattices

The athermal flow stress contribution by junction reactions of a glide dislocation with a 'forest' of density rho is usually written as tau = alpha mu b rho ^1/2. The strength coefficient alpha has been computed for the bcc lattice by virtual displacement of triple nodes following the method of Puschl et al. (1982, Physica status solidi (a), 74, 211). In the isotropic approximation, the values for glide dislocations with 0, 30, 60 and 90^o character are as alpha = 0.18, 0.21, 0.19 and 0.22 respectively. A comparison with values calculated previously by Frydman is made, and good agreement is found when differences in the cut-off radii of the linear elastic solution are accounted for.     

Decagonal quasicrystal with ordered body-centred (CsCl-type) hypercubic lattice

An ordered structure, which was found in a decagonal quasicrystal with 0.4 nm periodicity in a conventionally solidified Al₇₀Ni₂₀Ru₁₀ alloy, has been examined theoretically by the projection of an ordered body-centred (CsCl-type) hypercubic lattice. The ordered structure can be characterized as an ordered arrangement of two kinds of atom columnar cluster with different directions of pentagonal symmetry. Two lattices, which are constructed by connecting the atom clusters in the same directions, can be interpreted as ordered sublattices formed by each of vertex and body-centred positions of the CsCl-type hypercubic lattice. Diffraction patterns calculated from the ordered and disordered body-centred hypercubic lattices replicate well the electron diffraction patterns observed from the Al-Ni-Ru and other decagonal quasicrystals.     

A new modulated structure in GaN nanoparticles

A new modulated structure with a superlattice having parameters a = 2.209nm, b = 3.826nm, c = 1.037nm and f = g = n = 90 has been found in GaN nanoparticles synthesized by a dc arc plasma method. The nanoparticles transformed further into particles with holes at their centres under electron-beam irradiation during high-resolution electron microscopy observations. At the same time, Ga atoms were extruded on to the surface of the nanoparticles and formed an amorphous layer. A series of simulations of high-resolution images and electron diffraction patterns revealed that the modulation could be attributed to aggregations of N vacancies founded during the electron bombardment. Molecular mechanics calculations show that the aggregation of N vacancies is far more energetically favourable than that of Ga vacancies. The stability of the GaN particles is discussed.     

An ordered arrangement of atom columnar clusters in a pentagonal quasiperiodic lattice of an Al-Ni-Co decagonal quasicrystal

The structure of an Al-Ni-Co decagonal quasicrystal (called the S1-type superstructure), which shows diffraction patterns with superlattice reflections, has been studied by atomic-scale observations of electron microscopy. The structure of the decagonal quasicrystal can be characterized as an ordered arrangement of two kinds of atom columnar cluster with different directions of pentagonal symmetry. A fundamental lattice, which is constructed by connecting all the atom clusters, shows a pentagonal quasiperiodic lattice with a bond length of about 2 nm. The pentagonal lattice is divided into two superlattices, which are constructed by connecting atom clusters with the same directions of pentagonal symmetry, with a bond length of 2     

Diffusion kinetics for divacancies in the bcc lattice

In this letter we employ high-precision Monte Carlo simulations to investigate tracer diffusion kinetics for diffusion via divacancies in the bcc lattice. We utilize the mechanisms originally identified by Mehrer in which the divacancy can move by nearest-neighbour jumps with stable first-nearest-neighbour, second-nearest-neighbour and fourth-nearest-neighbour configurations of the divacancy. The tracer correlation factor and the 'impurity-form' correlation factor were found to be some 4-6% lower than those found by the matrix method. The tracer diffusion data and isotope effect for sodium were revisited in terms of monovacancy and divacancy contributions. The main change is a revision upwards from 0.55 to 0.61 of the j K kinetic factor for divacancies.     

Preparation of decagonal Zn-Mg-Y by melt spinning

It is shown, for the first time, how pure decagonal Zn-Mg-Y can be prepared by melt spinning and annealing. Study of the phase formation was performed by differential scanning calorimetry and the sample was further investigated by X-ray and electron diffraction. The lattice constants of the decagonal phase are a q =4.592Å and c =5.198Å calculated from a physical space model, or a q =4.485Å and c =5.193Å from a hyperspace model.     

A molecular dynamics study of atom mobility on steps of a Si(100) surface

Thermally activated motions of atoms on steps of a Si(100) surface have been studied using molecular dynamics simulations with standard three-body potentials. In the temperature interval 100-450K, these motions have two salient characteristics: firstly, a growth rate t^m leading to displacements of 1A in some tens of picoseconds and secondly, a marked dependence on the step morphology. These results are discussed in the light of experimental observations of step dynamics.     

Core Energetic Couples Therapy

SUMMARY

Core energetics is a method of psychotherapy that seeks the integration of all aspects of humanity: the emotional, physical, mental, and spiritual. Based on a deep understanding of the ways in which energy and consciousness work together, the therapy, developed by John C. Pierrakos, MD, invites couples to experience their inner truth and to identify their universal life goals and core feelings. Core energetics emphasizes wholeness and unification without loss of individuation for the couple. This article describes core energetics as it is used in couples work; the theory, therapeutic stance, methods of diagnosis, and energetic techniques are discussed, as well as the focus on sexuality and Spirituality.     

The inherent tensile strength of iron

The cohesive energy of Fe as a function of structure, strain and magnetic state has been computed using the full potential linearized augmented-plane-wave method within the framework of density functional theory and the generalized gradient approximation. Calculations corresponding to uniaxial stress in the <100> direction reveal that the ideal tensile strength of bcc Fe is about 14.2GPa and is determined by instability with respect to transformation into an unstable ferromagnetic fcc structure. The low-energy fcc phase is a modulatedantiferromagnetic fcc structure that is connected to the bcc phase via a first-order magnetic transformation and does not compromise its ideal strength.     

纯小数加工的心理机制:选择通达还是平行通达?

采用数字线索提示的目标觉察范式,以60名在校大学生与研究生为被试,设计3个实验探讨纯小数(整数部分是零的小数,例如0.2)的加工及其与空间表征的联系。实验1探讨纯小数作为线索时是否能引起空间注意的空间-数字反应编码联合效应(Spatial Numerical Association of Response Codes,SNARC),结果发现,纯小数数量大小的加工可以引起空间注意的SNARC效应;实验2探讨纯小数的加工是否会同时激活小数点后对应的自然数,结果发现,对纯小数数量大小相同、小数点后对应的自然数是否有0(例如0.2和0.20,0.4和0.40)的加工能引起空间注意的转移;实验3比较纯小数的加工对纯小数本身及小数点后对应的自然数激活强度,结果发现,在纯小数数量大小判断和纯小数小数点后对应的自然数数量大小判断冲突的条件下,纯小数的加工未能引起注意的SNARC效应。该研究结果表明,在目标觉察范式中,纯小数的加工采取了平行通达的方式,引发了注意的SNARC效应,并且纯小数空间注意的转移受到纯小数本身以及对应的自然数的影响。     

Structure images of unaged martensite containing a high carbon content

Abstract

A highly tetragonal (large c/a ratio) martensite in a Fe-4 wt% Al-1·6 wt% Mn-2 wt% Calloy, which is not aged and considered to be as ‘fresh’ as possible, has been observed by high-resolution electron microscope. From the arrangement of atom rows in the [010] structure image and the corresponding electron diffraction pattern, it is concluded that carbon atoms are clustered on the (305) plane. The unit structure of these carbon-clustered regions is similar to that of the modulated structure observed in Fe-C alloys, although a periodic layer distribution of carbon atoms is not observed.     

Twinning in the structure of the ‘decagonal’ phase Al65Cu20Co15

Abstract

The existence of twinning in the pseudo-decagonal phase Al₆₅Cu₂₀Co₁₅ has been established through TEM studies. From electron diffraction patterns, a rhombic crystalline lattice with a very large unit cell is easily identified. Diffraction patterns for microtwinned regions exhibit splitting and distortion of the reflections which is a result of the overlap of lattices which are rotated by 36° with respect to one another. While patterns from single components of the twins exhibit only pseudo-tenfold symmetry, those from the microtwinned regions exhibit nearly perfect tenfold symmetry.