Correlation functions in the Calogero-Sutherland model with open boundaries

Calogero-Sutherland models of type BC_N are known to be relevant to the physics of one-dimensional quantum impurity effects. Here we represent certain correlation functions of these models in terms of generalized hypergeometric functions. Their asymptotic behaviour supports the predictions of (boundary) conformal field theory for the orthogonality catastrophy and Friedel oscillations. Received: 12 February 1998 / Accepted: 17 March 1998     

Modelling the kinetics of solute segregation to partial dislocations for isothermal microcalorimetric evaluations

A model is proposed to describe the kinetics of solute segregation to partial dislocations in solid solutions of cold-rolled alloys. The case when half edge and half screw dislocations are present is considered. The model gives account of the kinetic behaviour observed in a deformed Cu-19 at% Al alloy where two unknown processes could be assessed during calorimetric isothermal experiments. The faster process corresponds to segregation to screw dissociated dislocations while the slower one corresponds to segregation to edge dissociated dislocations. Experimental activation energies, larger for edge dislocations, are close to that for pipe diffusion along the partials corrected by pinner binding energy terms. It is also predicted that segregation occurs faster as the dislocation density is increased. A quantitative comparison of experimental results with model predictions is given.The authors would like to thank the Fondo de Desarrollo Científico y Tecnológico (FONDECYT) for financial support through Project 1950566, and the Institute de Investigaciones y Ensayes de Materiales (IDIEM), Facultad de Ciencias Físicas y Matemticas, Universidad de Chile, for the facilities provided for this research.     

Transmission electron microscopy of dislocations in cementite deformed at high pressure and high temperature

Polycrystalline aggregates of cementite (Fe₃C) and (Fe,Ni)₃C have been synthesised at 10 GPa and 1250 °C in the multianvil apparatus. Further, deformation of the carbides by stress relaxation has been carried out at temperature of 1250 °C and for 8 h at the same pressure. Dislocations have been characterised by transmission electron microscopy. They are of the [1?0?0] and [0?0?1] type, [1?0?0] being the most frequent. [1?0?0] dislocations are dissociated and glide in the (0?1?0) plane. [0?0?1] dislocations glide in (1?0?0) and (0?1?0). Given the plastic anisotropy of cementite, the morphology of the lamellae in pearlitic steels appears to have a major role in the strengthening role played by this phase, since activation of easy slip systems is geometrically inhibited in most cases.     

Curvature from spin

We show that, considering the dislocation defect induced by torsion in spacetime, which behaves like a string with tension, we are lead also to defect angle and then to curvature of spacetime. The space with torsion and curvature is then equivalent to an elastic continuum which has undergone plastic deformations and, following Sakharov idea of the spacetime as a elastic continuum, we are lead to a gravitational constant, which occurs in the Einstein action, as the metrical elasticity of spacetime with the exact value without introducing any arbitrary cutoff, when also torsion is considered.     

Causal Stroh formalism for uniformly-moving dislocations in anisotropic media: Somigliana dislocations and Mach cones

In this work, Stroh’s formalism is endowed with causal properties on the basis of an analysis of the radiation condition in the Green tensor of the elastodynamic wave equation. The modified formalism is applied to dislocations moving uniformly in an anisotropic medium. In practice, accounting for causality amounts to a simple analytic continuation procedure whereby to the dislocation velocity is added an infinitesimal positive imaginary part. This device allows for a straightforward computation of velocity-dependent field expressions that are valid whatever the dislocation velocity–including supersonic regimes–without needing to consider subsonic and supersonic cases separately. As an illustration, the distortion field of a Somigliana dislocation of the Peierls–Nabarro–Eshelby-type with finite-width core is computed analytically, starting from the Green’s tensor of elastodynamics. To obtain the result in the form of a single compact expression, use of the modified Stroh formalism requires splitting the Green’s function into its reactive and radiative parts. In supersonic regimes, the solution obtained displays Mach cones, which are supported by Dirac measures in the Volterra limit. From these results, an explanation of Payton’s ‘backward’ Mach cones (Payton, 1995) is given in terms of slowness surfaces, and a simple criterion for their existence is derived. The findings are illustrated by full-field calculations from analytical formulas for a dislocation of finite width in iron, and by Huygens-type geometric constructions of Mach cones from ray surfaces.     

Computer simulation of the vacancy defects interaction with shuffle dislocation in silicon

The interactions between the 60^° shuffle dislocation and two different types of vacancy defects in silicon are separately studied via the molecular dynamics simulation method. The Stillinger–Weber potential is used to describe the atomic interactions. The results show that the dislocation slip velocity will decrease due to the interaction with the vacancy cluster (V ₆). The simulation also reveals that the divacancy will be absorbed by the dislocation. Meanwhile, a climbing of the dislocation occurs during their interactions. However, the divacancy has little effect on the dislocation slip velocity. Based on the above results, the decrease in threading dislocation density in SiGe/Si heterostructures with the use of low-temperature Si buffer layer may be explained.     

Stress‐enhanced dislocation density reduction in multicrystalline silicon

Stress is generally perceived to be detrimental for multicrystalline silicon (mc‐Si), leading to dislocation multiplication during crystal growth and processing. Herein, we evaluate the role of stress as a driving force for dislocation density reduction in mc‐Si. At high temperatures, close to the melting point (>0.8T_m), we observe that the application of stress as well as the relief of residual stress, can modify the density of pre‐existing dislocations in as‐grown mc‐Si under certain conditions, leading to a net local reduction of dislocation density. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Is the brittle–ductile transition preceding melting related to 5- and 7-membered ring defects in graphite?

The brittle–ductile transition (BDT) is a very general phenomenon in materials science. The temperature T _BDT in numerous materials correlates somewhat with the higher melting temperature T _m. We have earlier proposed the disclination (or rotation-dislocation) as a universal ingredient in the BDT. The present study is exclusively concerned with graphite, where T _m?≈?4000?K. Our conclusion is that the energetics of 5- and 7-membered rings play a crucial role in determining these temperatures. However, experiment is to be invoked, should T _BDT eventually be measured for graphite, so as to decide between various mechanisms, all of which here depend on 5- and 7-membered rings. One mechanism involves interlayer van der Waals coupling, whence a pressure experiment may hold the key.