Atomic structure of grain boundaries in iron modeled using the atomic density function

A model based on the continuous atomic density function (ADF) approach is applied to predict the atomic structure of grain boundaries (GBs) in iron. Symmetrical [100] and [110] tilt GBs in bcc iron are modeled with the ADF method and relaxed afterwards in molecular dynamics (MD) simulations. The shape of the GB energy curve obtained in the ADF model reproduces well the peculiarities of the angles of 70.53 deg. [$\Sigma$ 3(112)] and 129.52 deg. [$\Sigma$ 11(332)] for [110] tilt GBs. The results of MD relaxation with an embedded-atom method potential for iron confirm that the atomic GB configurations obtained in ADF modeling are very close to equilibrium ones. The developed model provides well-localized atomic positions for GBs of various geometries.Comment: 8 pages, 8 figures, revised versio

Ultrafast laser-triggered field ion emission from semiconductor tip

International audienceWe study experimentally and theoretically the controlled field evaporation of single atoms from a semiconductor surface by ultrafast laser-assisted atom probe tomography. The conventional physical mechanisms of field evaporation cannot explain the experimental results recently reported for such materials. A new model is presented in which the positive dc field leads to band bending with a high density of laser-generated holes near the surface of the sample. The laser energy absorption by these holes and the subsequent energy transfer to the lattice considerably increase the tip temperature. We show that this heating plays an important role in the field ion emission process. In addition, experiments are carried out for germanium and silicon tips to check the role of the dc field in the absorption processes, as well as the heating of the tip and the following evaporation. Good agreement between the predictions of our model and the experimental data is found

THERMAL STABILITY AND EXTRA-STRENGTH OF AN ULTRAFINE GRAINED STAINLESS STEEL PRODUCED BY HIGH PRESSURE TORSION

International audienceInvestigations of an ultrafine-grained (UFG) Cr-Ni austenitic stainless steel produced by high pressure torsion (HPT) at room and elevated (400 C) temperatures followed by series of annealing up to 700 C are reported. The grain size of the alloy processed at room temperature (55 nm) was found to be about twice lower than the grain size of the alloy (90 nm) processed at elevated temperature. Besides, both as-processed states demonstrated a very high value of microhardness (~590 Hv) , while the steel in initial quenched state had the microhardness about 155 Hv. It is shown that the hardness of the steel in both UFG states does not decrease with annealing up to 650 C, and even a certain increase in hardness was observed for the steel produced at room temperature. At higher temperature (700 C), the recrystallization starts, and precipitation was observed

Перераспределение примесных и легирующих элементов в стали корпуса реактора ВВЭР-440 под действием эксплуатационных факторов

Материал, вырезанный из темплетов сварного металла ВВЕР-440, был исследован при помощи APFIM в трех состояниях: облученном, облученном и отожженном, облученном, отожженном и повторно облученном. Были получены данные по химическому составу ферритной матрицы, а также 3D изображения структурных особенностей (кластеров, мелких карбидов и т.д.). Эксперименты показали формирование мелких (примерно 2 нм) кластеров, обогащенных Cu, Si, Ni, Mn, P в процессе облучения. распад этих кластеров происходит в процессе отжига с формированием новых, более крупных (примерно 5 нм) кластеров из чистой Сu с последующим формированием Р-обогащенных зон на второй стадии облучения с незначительными изменениями в более крупных Cu-кластерных структурах в рамках изучаемого интервала флюенса повторного облучения. Предполагается, что Р-обогащенные зоны играют важную роль в упрочнении ферритной матрицы в процессе повторного облучения. Результаты демонстрируют существенное обеднение матрицы Р под облучением. Этот процесс может оказывать влияние на радиационно-обусловленную сегрегацию границ зерна Р в основном материале бака ВВЕР-440. Уровень сегрегации был вычислен при помощи модифицированной модели McLean. Результаты демонстрируют хорошее соответствие с экспериментальными данными, полученными оже-электронной спектроскопией.Матеріал, вирізаний з темплетів зварного металу ВВЕР-440, було досліджено за допомогою APFIM у трьох станах: опроміненому, опроміненому та відпаленому, опроміненому, відпаленому та вторинно опроміненому. Були отримані дані по хімічному складу ферітної матриці, а також 3D зображення структурних особливостей (кластерів, малих карбідів та інших). Експерименти показали формування малих (біля 2 нм) кластерів, збагачених Cu, Si, Ni, Mn, P під дією опромінення. розпад цих кластерів при відпалі з формуванням нових, більших за розміром (приблизно 5нм) кластерів практично із чистої Cu з подальшим формуванням Р-збагачених зон на другому етапі опромінення з незначними змінами в структурі крупніших Cu-кластерів у діапазоні флюенсу вторинного опромінення. Р-збагачені зони відіграють важливу роль у зміцненні ферітної матриці під дією повторного опромінення. Отримані результати демонструють суттєве збідніння матриці на Р під впливом опромінення. Цей процес може впливати на радіаційно-індуковану сегрегацію границь зерен Р в основному матеріалі баку реактора ВВЕР-440. Рівень сегрегації було розраховано за допомогою модифікованої моделі МсLеan. Отримані результати показують відповідність з експериментальними даними, отриманими оже- електронною спектроскопією.The material cut out of templets of WWER-440 weld metal was investigated by APFIM in three conditions: irradiated; irradiated and annealed; irradiated, annealed and re-irradiated. Data on the chemical content of ferrite matrix as well as 3D images of structural features (clusters, small carbides etc) were obtained. The experiments showed the formation of small (about 2 nm) clusters enriched by Cu, Si, Ni, Mn, P during irradiation, the decay of these clusters during annealing with the formation of new bigger (about 5 nm) almost pure Cu clusters and subsequent formation of P-enriched zones during the second stage of irradiation with no remarkable changes in the bigger Cu-clusters structure within the considered re-irradiation fluence range. The P-enriched zones are supposed to play an important role in the ferrite matrix hardening during re-irradiation. The results demonstrated the significant exhaustion of matrix by P under irradiation. This process could have an influence of radiation-induced grain boundary segregation of P in the base material of WWER-440 pressure vessel. The level of the segregation was calculated using the modified McLean model. The results showed the qualitative agreement with experimental data obtained by Auger electron spectroscopy

Review on the EFDA work programme on nano-structured ODS RAF steels

This proceeding is: The 14th International Conference on Fusion Reactor Materials (ICFRM-14) was held at the Sapporo Convention Center in Sapporo, Japan from 7 to 12 September 2009.The 2008─2009 work programme of the European research project on nano-structured oxide dispersion strengthened (ODS) reduced activation ferritic (RAF) steels is being organized along the four following programmatic lines: (1) improve the present generation of nano-structured ODS RAF steels; (2) start the industrial fabrication of the present generation of nano-structured ODS RAF steels; (3) develop an optimised generation of nano-structured and nano-grained ODS RAF steels; (4) investigate the stability of present and optimised generation of nano-structured ODS RAF steels under creep and irradiation. This paper presents the main objectives of current R&D activities being performed within the European research project on nano-structured ODS RAF steels, the main obtained results and the main future activities in the case of the four programmatic lines mentioned just above.This work, supported by the European Communities, was carried out within the framework of the European Fusion Development Agreement.Publicad

Triggering of the 2014 M_w7.3 Papanoa earthquake by a slow slip event in Guerrero, Mexico

Since their discovery two decades ago, slow slip events have been shown to play an important role in accommodating strain in subduction zones. However, the physical mechanisms that generate slow slip and the relationships with earthquakes are unclear. Slow slip events have been recorded in the Guerrero segment of the Cocos–North America subduction zone. Here we use inversion of position time series recorded by a continuous GPS network to reconstruct the evolution of aseismic slip on the subduction interface of the Guerrero segment. We find that a slow slip event began in February 2014, two months before the magnitude (M_w) 7.3 Papanoa earthquake on 18 April. The slow slip event initiated in a region adjacent to the earthquake hypocentre and extended into the vicinity of the seismogenic zone. This spatio-temporal proximity strongly suggests that the Papanoa earthquake was triggered by the ongoing slow slip event. We demonstrate that the triggering mechanism could be either static stress increases in the hypocentral region, as revealed by Coulomb stress modelling, or enhanced weakening of the earthquake hypocentral area by the slow slip. We also show that the plate interface in the Guerrero area is highly coupled between slow slip events, and that most of the accumulated strain is released aseismically during the slow slip episodes

Co‐location of the Downdip End of Seismic Coupling and the Continental Shelf Break

International audienceAlong subduction margins, the morphology of the near shore domain records the combined action of erosion from ocean waves and permanent tectonic deformation from the convergence of plates. We observe that at subduction margins around the globe, the edge of continental shelves tends to be located above the downdip end of seismic coupling on the megathrust. Coastlines lie farther landward at variable distances. This observation stems from a compilation of well-resolved coseismic and interseismic coupling data sets. The permanent interseismic uplift component of the total tectonic deformation can explain the localization of the shelf break. It contributes a short wave-length gradient in vertical deformation on top of the structural and isostatic deformation of the margin. This places a hinge line between seaward subsidence and landward uplift above the downdip end of high coupling. Landward of the hinge line, rocks are uplifted in the domain of wave-base erosion and a shelf is maintained by the competition of rock uplift and wave erosion. Wave erosion then sets the coastline back from the tectonically meaningful shelf break. We combine a wave erosion model with an elastic deformation model to illustrate how the downdip end of high coupling pins the location of the shelf break. In areas where the shelf is wide, onshore geodetic constraints on seismic coupling are limited and could be advantageously complemented by considering the location of the shelf break. Subduction margin morphology integrates hundreds of seismic cycles and could inform the persistence of seismic coupling patterns through time

Superficial simplicity of the 2010 El Mayor–Cucapah earthquake of Baja California in Mexico

The geometry of faults is usually thought to be more complicated at the surface than at depth and to control the initiation, propagation and arrest of seismic ruptures. The fault system that runs from southern California into Mexico is a simple strike-slip boundary: the west side of California and Mexico moves northwards with respect to the east. However, the M_w 7.2 2010 El Mayor–Cucapah earthquake on this fault system produced a pattern of seismic waves that indicates a far more complex source than slip on a planar strike-slip fault. Here we use geodetic, remote-sensing and seismological data to reconstruct the fault geometry and history of slip during this earthquake. We find that the earthquake produced a straight 120-km-long fault trace that cut through the Cucapah mountain range and across the Colorado River delta. However, at depth, the fault is made up of two different segments connected by a small extensional fault. Both segments strike N130° E, but dip in opposite directions. The earthquake was initiated on the connecting extensional fault and 15 s later ruptured the two main segments with dominantly strike-slip motion. We show that complexities in the fault geometry at depth explain well the complex pattern of radiated seismic waves. We conclude that the location and detailed characteristics of the earthquake could not have been anticipated on the basis of observations of surface geology alone