Object kinetic Monte Carlo study of the effect of grain boundaries in martensitic Fe-Cr-C alloys

Fe-Cr-C alloys with chromium concentrations in the range from similar to 2 to 12 wt.% form ferritic-martensitic structures by rapid cooling from the austenite state already in the presence of relatively low carbon concentrations. In this process it is possible to obtain different ratios of ferrite and martensite, as well as formation of carbides, by varying the thermal treatment. The presence of ferrite or martensite might have an influence on the nanostructural evolution under irradiation of these alloys. Here, considering a tempered martensite reference alloy with 9% Cr, we make use of an already validated object kinetic Monte Carlo (OKMC) model in order to study the possible effect of the formation of martensite laths on the material nanostructural evolution under neutron irradiation, assuming that the relevant boundaries act as sinks for radiation defects. The results show that the reduction of the grain size (including in this definition the average size of prior austenite grains, packets, blocks, and laths) does not play any relevant role until sizes of the order of similar to 0.5 mu m are reached: for smaller grains, the number of defects being absorbed by the boundaries becomes dominant. However, this threshold is lower than the experimentally observed martensite lath dimensions, thereby suggesting that what makes the difference in martensitic Fe-Cr-C alloys with respect to ferrite, concerning events and mechanisms taking place during irradiation, are not the lath boundaries as sinks. Differences between the nanostructural evolution under neutron irradiation in ferrite and martensite should therefore be ascribed to other factors. (C) 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei

The nanostructure evolution in Fe–C systems under irradiation at 560 K

Peer reviewe

Computer simulation of the nanostructural evolution under irradiation in ferritic alloys

Dans ce travail nous avons développé des modèles de Monte Carlo cinétique d’objets (OKMC) qui ont permis de prédire l'évolution nano-structurelle des amas de lacunes et des auto-interstitiels sous irradiation neutronique, à la température de fonctionnement des réacteurs de génération II dans les alliages Fe-C-MnNi (alliages modèles pour les aciers de cuve) et Fe-Cr-C (matériaux envisagés pour les réacteurs de génération IV). Un véritable acier de cuve venant du programme de surveillance de la centrale nucléaire suédoise de Ringhals a aussi été modélisé. Pour ce faire nous avons développé deux modèles OKMC fondés sur les données les plus actuelles concernant la mobilité et la stabilité des amas de défauts. Les effets des solutés d'intérêt ont été introduits dans nos modèles dans l’hypothèse simplifiée ‘‘d’alliage gris’’, c'est-à-dire que les solutés ne sont pas explicitement introduits dans le modèle, qui ne peut donc pas décrire leur redistribution, mais leur effet est introduit dans les paramètres liés à la mobilité des amas de défauts. A l’aide de cette approche nous avons modélisé diverses conditions de température et de débit de dose ainsi que des études de recuits isochrones d’alliages Fe-C-MnNi. L'origine du durcissement par irradiation neutronique à basse température a également été étudiée et les modèles ont fortement soutenu l'hypothèse selon laquelle les solutés ségrégent sur des boucles interstitielles immobiles, qui agissent donc comme des sites de nucléation hétérogène pour la formation d’amas enrichis en NiSiPCr et MnNi. A chaque fois nos modèles ont été validés par comparaison des résultats obtenus avec les observations expérimentales disponibles dans la littérature.We developed object kinetic Monte Carlo (OKMC) models that proved able to predict the nanostructure evolution under neutron irradiation in both RPV and F/M steels. These were modelled, respectively, in terms of Fe-C-MnNi and Fe-C-Cr alloys, but the model was also validated against data obtained on a real RPV steel coming from the surveillance programme of the Ringhals Swedish nuclear power plant. The effects of the substitutional solutes of interest were introduced in our OKMC model under the simplifying assumptions of ‘‘grey alloy’’ scheme, i.e. they were not explicitly introduced in the model, which therefore cannot describe their redistribution under irradiation, but their effect was translated into modified parameters for the mobility of defect clusters. The possible origin of low temperature radiation hardening (and subsequent embrittlement) was also investigated and the models strongly supported the hypothesis that solute clusters segregate on immobile interstitial loops, which act therefore as heterogeneous nucleation sites for the formation of the NiSiPCr- and MnNi-enriched cluster populations experimentally, as observed with atom probe tomography in, respectively, F/M and RPV steels. In other words, the so-called matrix damage would be intimately associated with solute atom clusters and precipitates which increase their stability and reduce their mobility: their ultimate effect is reflected in an alteration of the macroscopic mechanical properties of the investigated alloys. Throughout all our work the obtained results have been systematically validated on existing experimental data, in a process of continuous improvement of the physical hypotheses adopted

Población y migraciones rurales en México: Hipótesis para otro siglo

This article approaches the issue on territorial population allocation in Mexico and its concentration in the cities, understood as a specific dimension of the countryís economic development. Specifically, two strong trends, pointed out in some population projections for the year 2000, are analyzed. These trends are: the decrease on rural-urban migration and the decrease on rural population in total terms. The first hypothesis comes to contradict, what has been called for about a century, one of the ìdemographic lawsî: the steady increase of rural-urban migration. The second hypothesis demands for a detailed analysis because at first glance it would seem opposed to the former, but the contradiction vanishes if the phenomenon is considered from the reclassification perspective of rural localities into urban ones. Ultimately, both hypothesis represent important warnings for the design of population politics

DISTRIBUCIÓN DEL INGRESO DE LA POBLACIÓN ECONÓMICAMENTE ACTIVA EN LAS GRANDES CIUDADES DE MÉXICO: 1989

En años recientes la Encuesta Nacional de Empleo Urbano (ENEU) ha ofrecido un buen número de ta­bulaciones referidas a la distribución del ingreso. Con base en ellas, este documento intenta examinar angi­nas diferencias en la distribución del ingreso de los hombres y las mujeres en las ciudades de México, Guadalajara y Monterrey en 1989

La economía y las modalidades de la urbanización en México: 1940-1990

El objetivo de este artículo es mostrar cómo las diversas formas de urbanización en México, en los últimos 50 años, están asociadas -en mayor o menor grado- con los altibajos de la economía mexicana. Los cambios en la economía y urbanización parecen señalar que en el periodo 1970-1990, la descentralización de la población empezó con el periodo de auge petrolero y continuó con la crisis económica de los años ochenta

Simulation of nanostructural evolution under irradiation in Fe-9%CrC alloys: An object kinetic Monte Carlo study of the effect of temperature and dose-rate

This work explores the effects of both temperature and dose-rate on the nanostructural evolution under irradiation of the Fe-9%CrC alloy, model material for high-Cr ferritic/martensitic steels. Starting from an object kinetic Monte Carlo model validated at 563K, we investigate here the accumulation of radiation damage as a function of temperature and dose-rate, attempting to highlight its connection with low-temperature radiation-induced hardening. The results show that the defect cluster mobility becomes high enough to partially counteract the material hardening process only above ∼290°C, while high fluxes are responsible for higher densities of defects, so that an increase of the hardening process with increasing dose-rates may be expected