Chromium-vacancy clusters in dilute bcc Fe-Cr alloys: an ab initio study

Using an ab initio approach, we explore the stability of small vacancy and vacancy-chromium clusters in dilute body-centred cubic Fe-Cr alloys. To explain experimental observations described in C.D. Hardie et al., J. Nucl. Mater. 439, 33 (2013) and showing the occurrence of Cr segregation in low-Cr alloys, we investigate if chromium can form stable bound configurations with vacancies in alloys with chromium concentration below the low-temperature chromium solubility limit of 10-11 at. %. We find that a single vacancy can attract up to four Cr atoms in the most energetically favourable cluster configuration. The binding energy of a cluster containing a single vacancy and from one to eight Cr atoms can be well described by a linear function of the number of chromium atoms in the second, third and fifth nearest neighbour coordination. The magnetic origin of the binding energy trend is confirmed by a correlation between the average value of the magnetic moment of a Cr atom and the binding energy. Similar trends are also found for di-vacancy-Cr clusters, confirming that they likely also characterise larger systems not yet accessible to ab initio calculations. The ratio of the binding energy to the number of Cr atoms increased more than twice in the di-vacancy case in comparison with a single vacancy case.Comment: 25 pages, 9 figure

Are there stable long-range ordered Fe(1-x)Cr(x) compounds?

The heat of formation of Fe-Cr alloys undergoes an anomalous change of sign at small Cr concentrations. This observation raises the question whether there are intermetallic phases present in this composition range. Here we report the discovery of several long-range ordered structures that represent ground state phases at zero Kelvin. In particular we have identified a structure at 3.7% Cr with an embedding energy which is 49 meV/Cr atom below the solid solution. This implies there is an effective long-range attractive interaction between Cr atoms. We propose that the structures found in this study complete the low temperature-low Cr region of the phase diagram.Comment: 3 pages, 2 figure

Neutron-induced dpa, transmutations, gas production, and helium embrittlement of fusion materials

In a fusion reactor materials will be subjected to significant fluxes of high-energy neutrons. As well as causing radiation damage, the neutrons also initiate nuclear reactions leading to changes in the chemical composition of materials (transmutation). Many of these reactions produce gases, particularly helium, which cause additional swelling and embrittlement of materials. This paper investigates, using a combination of neutron-transport and inventory calculations, the variation in displacements per atom (dpa) and helium production levels as a function of position within the high flux regions of a recent conceptual model for the "next-step" fusion device DEMO. Subsequently, the gas production rates are used to provide revised estimates, based on new density-functional-theory results, for the critical component lifetimes associated with the helium-induced grain-boundary embrittlement of materials. The revised estimates give more optimistic projections for the lifetimes of materials in a fusion power plant compared to a previous study, while at the same time indicating that helium embrittlement remains one of the most significant factors controlling the structural integrity of fusion power plant components.Comment: 9 pages, 9 figure

The Effect of Electronic Structure on the Phases Present in High Entropy Alloys

Multicomponent systems, termed High Entropy Alloys (HEAs), with predominantly single solid solution phases are a current area of focus in alloy development. Although different empirical rules have been introduced to understand phase formation and determine what the dominant phases may be in these systems, experimental investigation has revealed that in many cases their structure is not a single solid solution phase, and that the rules may not accurately distinguish the stability of the phase boundaries. Here, a combined modelling and experimental approach that looks into the electronic structure is proposed to improve accuracy of the predictions of the majority phase. To do this, the Rigid Band model is generalised for magnetic systems in prediction of the majority phase most likely to be found. Good agreement is found when the predictions are confronted with data from experiments, including a new magnetic HEA system (CoFeNiV). This also includes predicting the structural transition with varying levels of constituent elements, as a function of the valence electron concentration, n, obtained from the integrated spin-polarised density of states. This method is suitable as a new predictive technique to identify compositions for further screening, in particular for magnetic HEAs

Иноязычная коммуникативная компетенция современного преподавателя технического вуза

В статье рассматриваются организационно-педагогические и методические условия формирования иноязычной коммуникативной компетенции преподавателя технического вуза на примере реализации программы повышения квалификации "Формирование профессиональной дидактической компетенции средствами английского языка". Иностранный язык становится инструментом для выполнения профессиональной деятельности современного преподавателя технического вуза

A simple environment-dependent overlap potential and Cauchy violation in solid argon

We develop an analytic and environment-dependent interatomic potential for the overlap repulsion in solid argon, based on an approximate treatment of the non-orthogonal Tight-Binding theory for the closed-shell systems. The present model can well reproduce the observed elastic properties of solid argon including Cauchy violation at high pressures, yet very simple. A useful and novel analysis is given to show how the elastic properties are related to the environment-dependence incorporated into a generic pairwise potential. The present study has a close link to the broad field of computational materials science, in which the inclusion of environment dependence in short-ranged repulsive part of a potential model is sometimes crucial in predicting the elastic properties correctly.Comment: 10 pages, 3 figure