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Origin of irradiation creep - studied by in-beam creep testing, ex-situ TEM observation and SIMS trace diffusion measurement

Applicant Chen Jia-Chao
Number 192130
Funding scheme Project funding (Div. I-III)
Research institution Nukleare Energie und Sicherheit Paul Scherrer Institut
Institution of higher education Paul Scherrer Institute - PSI
Main discipline Material Sciences
Start/End 01.02.2021 - 31.01.2025
Approved amount 290'359.00
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Keywords (4)

SIMS trace diffusion measurement; in-beam creep testing; ex-situ TEM observation; irradiation creep mechanism

Lay Summary (German)

Lead
Es besteht eine weltweite Notwendigkeit zur Entwicklung von sicheren, unabhängigen und nachhaltigen Energiequelle. Die entsprechend Forschung schliesst auch fortgeschrittene nukleare Technologien ein. Hierbei handelt es sich um Systeme, welche einerseits eine erhöhte Sicherheit bieten, und andererseits die Effizienz wesentlich steigern sollen. Eine Erhöhung dieser beiden Kenngrössen stellt andererseits aber auch eine hohe Anforderung an die Materialien dar, und es kann mit Sicherheit gesagt werden, dass die Materialien sogar den limitierenden Faktor darstellen. Die Entwicklung neuartiger Kernbrennstoffe und Strukturmaterialien ist daher unabdingbar. Temperatur und Bestrahlung erzeugen Kriechen, eine plastische Verformung bei vergleichsweise kleinen Spannungen. Das Kriechen unter Bestrahlung ist nicht vollumfänglich verstanden und Gegenstand von dieser Arbeit.
Lay summary

Inhalt und Zweck des Forschungsprogrammes

Das vorliegende Projekt soll den Mechanismus, welcher dem Kriechen unter Bestrahlung zugrunde liegt, beleuchten. Die Untersuchungsmethoden sind: Dehnungs-Versuche von Einkristallen unter ausgewählten Spannungsrichtungen, während die Probe bestrahlt wird, Sekundärionenmassenspektrometrie (SIMS) basierte Messung der Diffusion, um die Bestrahlungs-induzierte Selbstdiffusion unter den verschiedenen Spannungs- und Verformungsgegebenheiten zu determinieren, und Transmissionselektronenmikroskopie (TEM) um die Grössen- und Dichte-Verteilung der Versetzungsringe, als Funktion der angelegten Spannung zu bestimmen. All diese Information ist bis heute nicht bekannt, ist aber essentiell für das Verständnis des Bestrahlungskriechens.

 

Wissenschaftliche und Gesellschaftliche Aspekte des Antrages

Die mit Bedacht ausgelegten Experimente dieses Antrages, werden neue wesentliche Informationen liefern, wie beispielsweise die Auswirkung der kristallbezogenen Spannungsrichtung auf das Kriechen, den Einfluss der Spannung/Dehnung auf die strahlungsinduzierte Eigendiffusion, und den Einfluss der angelegten Spannung auf die Dichte und Grössenverteilung der Versetzungsringe. Diese völlig neuen Erkenntnisse werden helfen, die Quelle des Bestrahlungskriechens fundamental zu verstehen. Die Information kann auch einer entsprechenden Computersimulation zugrunde gelegt werden, um das Potential neuer Materialkandidaten schneller eruieren zu können. Damit wird es möglich sein, bessere und sicherere Materialien für die künftige Generation von Kernreaktoren zu Entwickeln.

Direct link to Lay Summary Last update: 21.04.2020

Responsible applicant and co-applicants

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Abstract

The objective of this proposal is aimed to understand the mechanism of irradiation creep in nuclear environments. The investigation methods are in-beam creep testing of single crystal samples under selected loading direction during irradiation, SIMS (Secondary Ion Mass Spectrometry) trace diffusion measurement for determining the irradiation-enhanced self-diffusion coefficient at various stress/strain conditions after irradiation creep tests and ex-situ TEM (Transmission Electron Microscopy) observation to obtain density and size distribution of dislocation loops regarding to applied stress. All of that information are still missing in literature.Under irradiation, the damages in materials can be attributed to the fates of two types of point defects (and their small clusters) created during colliding process: vacancies and self-interstitials (also called Frenkel pairs). Temperature is obviously one factor affecting the fate of Frenkel pairs, which has been studied from the beginning of the nuclear reactors development. Besides temperature, another important factor affecting their fates and the related mechanical property is stress. Additional to the internal stresses, external stresses result from deformation induced by nearby reactor components or from radiation environment, especially the thermal stresses induced by the coolants. The stress/strain fields could cause the bias transport and absorption of various defects into sinks which leads to plastic deformation under stress below yield strength of materials at lower temperatures where thermal creep is not activated. Such phenomenon is called irradiation creep. The irradiation creep has been intensively studied for decades, both experimentally and theoretically, but a detail understanding has not yet been achieved due to lack of the elaborately designed experiments and the analytical methods to handle the huge inelastic distortion in the lattice. Previous experimental studies demonstrate that, dose rate, stress, temperature, composition and lattice structure of materials are all possible factors that affect irradiation creep compliance. According to the current understanding, three promising mechanisms are mainly considered to explain irradiation creep, which are: (1) stress induced preferential absorption (SIPA), (2) stress induced preferential nucleation (SIPN) and (3) climb controlled glide mechanisms (CCG). The important predictions of the most prominent irradiation creep models, i.e. SIPN and SIPA are unequal distribution (loop orientation with respect to the applied stress) of loops. While SIPN should show up in an unequal distribution of loop densities, SIPA should have such an effect mainly on loop sizes. Irradiation creep compliance of a single crystal sample is also dependent on loading direction with respect to lattice orientation. However, based on an extensive review on the status of irradiation creep studies, proposed models are not clearly and largely verified by experimental results. Further, there are some contradictory experimental evidences in literatures. It should be emphasized that irradiation-enhanced self-diffusion under applied stress/strain has never been considered in proposed irradiation creep mechanisms and has never been investigated as well according to the applicant’s knowledge. Therefore, there is a need for systematic reliable experimental results including the parametric dependence of the irradiation creep compliance, microstructure observation and irradiation-enhanced self-diffusion under stress/strain to verify existing models and new description in the irradiation creep study. Such a study is essential and important not only for the basic understanding of structural materials in nuclear environment but also for their reliable safe applications in future reactors including fusion reactors and high energy accelerator applications. The proposed study consists of in-beam creep testing, ex-situ TEM observations and SIMS trace diffusion measurements. The selected representative material will be single crystals of Ni (fcc lattice structure) and/or Fe (bcc lattice structure). By using the unique cyclotron based in situ irradiation device installed at CEMHTI/CNRS in Orleans, France (PSI-CEMHTI/CNRS cooperation facility of EMIR - Réseau national d’accélérateurs pour les Etudes des Matériaux sous Irradiation, see website “http://emir.in2p3.fr/”), irradiation creep strain vs dose and dislocation loop imprints under different stress direction with respect to lattice orientation will be investigated. Additionally, irradiation-enhanced self-diffusion with and without applied stress/strain will be studied by SIMS, a stable isotope trace diffusion technique, after irradiation creep tests. The details of irradiation-enhanced diffusion under stress/strain, parametric dependence and density and size distribution of dislocation loops in respect to the applied stress, those completely new evidences will help us to understand what the origin of irradiation creep phenomenon is: preferential nucleation and biased absorption or volume diffusion under stress similar like Harper-Dorn creep, which is still a mystery. The work will form a PhD thesis.
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