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The Institute of Nuclear Physics

Ministry of Energy of the Republic of Kazakhstan

КМУ1
Implementation period: 01.2020 – 10.2022
АР08052488 Control of reactor materials plasticity after high-dose neutron irradiation

Relevance

Austenitic steels are the main structural material in the industrial light water nuclear reactors and the perspective fast reactors with sodium coolant. The defects of radiation nature prevent the movement of dislocations, causing premature localization of deformation and destruction. The decrease in plasticity after neutron irradiation is one of the three main problems of modern reactors and the reason for forced limitation of the service life of products. This reduces the economic efficiency of nuclear facilities.

To solve the problem, it is proposed to provide additional local hardening of the material, which would compensate for insufficient strain hardening in the highly irradiated materials and prevent premature localization. Local hardening can be achieved as a result of: martensitic γ→α’-transformation, dynamic strain aging and twinning. The efforts of the group will be focused on establishing the complex relationships between the parameters of material, deformation, irradiation; the strengthening processes and plasticity of highly irradiated steels.

The obtained results will make it possible to better understand the nature of structural-phase transformations under neutron irradiation and deformation of meta-stable austenitic steels, to evaluate their effect on the physical and mechanical properties of materials and stability of plastic flow, and to more accurately predict the performance of structural steels of the existing reactor facilities with a service life extension by 20 years and more.

Goal

To study the physical-mechanical properties and structural changes that determine the plasticity of highly irradiated steels in the temperature range -60÷650⁰С.

Expected results

Study of austenitic steels AISI304, 12Cr18Ni10Ti, AISI316LN and armco-iron. Determination of mechanical characteristics in the process of uniaxial tension with registration of local deformations using the digital image correlation method. Identification of structural features using the optical and transmission electron microscopy, magnetometry, microhardness and hydrostatic weighing.

Identification of the relationship between mechanical characteristics and radiation dose, test temperature and chemical composition. Analysis of the effect of twinning, strain-induced martensitic transformation, dynamic strain aging, and alloying on the strength and ductility of austenitic steels over a wide temperature range. As a result, assessment of the ability of each mechanism to provide stable deformation and high plasticity.

Achieved results

Neutron irradiation of 180 samples of AISI 304 and 12Cr18Ni10T steels was performed in the reactor WWR-K.

The TEM and SEM study showed that pure austenite prevails in the structure of the modified steels based on AISI 316LN, and a small amount of χ-phase and MnS is noted. Irradiation with neutrons in the reactor WWR-K does not lead to significant structural changes. Transmission microscopy in 12Cr18Ni10Ti steel irradiated in the reactor BN-350 to a dose of 57.6 dpa at 304°C revealed the bcc-inclusions located between austenite grains and characterized by low corrosion resistance. The resulting bcc phase leads to the growth of steel magnetization from 0.05 vol% in the non-irradiated state to 3–9 vol% after neutron irradiation at the temperature of 300–400°C. The fractographic and TEM study of the structure of irradiated AISI 304 steel after mechanical tests was performed. It is shown that stacking/packaging faults are not distinguishable due to the developed dislocation structure, the dislocation density increased by several orders of magnitude compared to the non-deformed material. The type of dislocation structure was determined.

Mechanical tensile tests were performed with registration of local deformations magnetization of the irradiated samples of the studied steels. The mechanical characteristics and kinetic parameters of the martensitic γ→α’-transformation were determined, and the connection with the initial structure of materials was revealed. It was shown that the parameter α increases with the growth of the damaging dose, which may indicate a decrease in the stacking fault energy as a result of neutron irradiation.

To study the process of dynamic strain aging, mechanical tests were performed in the temperature range from room temperature to 650°C with registration of local deformations, irradiated with neutrons to various fluences of the samples of armco-iron and AISI 304 steel. Mechanical characteristics were determined and their dependence on the irradiation dose and test temperature was established. Using the example of armco-iron, it was shown that neutron irradiation suppresses the process of dynamic strain aging. A similar effect was observed for AISI 304 steel, but its degree was lower.

The effect of alloying with manganese and nitrogen on ductility of AISI 316 steel, which is stable to martensitic transformation, was studied. Mechanical tests were performed with registration of local magnetization, the maps of local deformations distribution were prepared, and mechanical characteristics were determined. It has been established that alloying with nitrogen and manganese strengthens the austenitic matrix.

Names of the research team members (position, degree) with their identifiers (ScopusAuthor ID, Researcher ID, ORCID, if any) and links to relevant profiles

Merezhko D.А. – Candidate of Physics and Math Sciences, Senior Researcher, Laboratory of Radiation Materials Science

ORCIDID:0000-0002-5885-6464;

Rofman О.V., PhD, Senior Researcher, Laboratory of Radiation Materials Science;

ORCIDID:0000-0001-5466-9402;

ResearcherID: M-7412-2015

Merezhko М.S., Master of Engineering and Technologies, Acting Head of the Laboratory of Radiation Materials Science; ORCIDID:0000-0002-8727-4404

Zakharov М.А., Master of Technical Sciences, Senior Researcher, Laboratory of Radiation Materials Science;

Takieva А.М., Bachelor, Engineer;

Kireev А.V., Engineer, Head of the accelerator UKP 2-1;

Vasiltsov A.G., Engineer, undergraduate.

List of publications (with links) and patents

  1. Rofman O.V., Prosviryakov A.S., Kotov A.D., Bazlov A.L., milovich P.O., Karunakaran G., Mikhaylovskaya A.V. Fabrication of AA2024/SiCp Metal Matrix Composite by Mechanical Alloying // Metals and Materials International. – 2021 (DOI: 10.1007/s12540-020-00924-1, Q1/Q2, IF=3,642)
  2. Maksimkin O.P., Merezhko D.A., Merezhko M.S., Sarsenbaeva K.B. Formation of plastic properties of 12Cr18Ni10T steel highly irradiated with neutrons // Issues of atomic science and technology. – №2(132). – С.66-73. (doi:10.46813/2021-132-066 (Nuclearscience&technology – Q4, IF=0,136)
  3. Merezhko M.S., Merezhko D.A. Maksimkin O.P. Localization of plastic deformation in cylindrical samples of copper and austenitic steels 12Cr18Ni10Ti and AISI 304 irradiated with neutrons // III International Scientific Forum “Nuclear Science and Technologies” dedicated to the 30th anniversary of the independence of the Republic of Kazakhstan, p.127.
  4. Merezhko D.A., Merezhko M.S., Rofman O.V., Gussev M.N., Garner F.A. “Phase instability in steel 12Cr18Ni10Ti irradiated with high-energy particles // III International Scientific Forum “Nuclear Science and Technologies” dedicated to the 30th anniversary of the independence of the Republic of Kazakhstan, p.139.
  5. Merezhko M.S., Merezhko D.A., Tsay K.V. “Features of mechanical properties formation in the process of medium-temperature plastic deformation of armco-iron irradiated with neutrons” // PhММ. 2022 – V. 123, № 2, P. 207-213
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