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Plasticity of Fe-Ni-Cr alloys under irradiation

Date: 10/10/2014

Author: Bakaev, A.

Subject: Plasticity of Fe-Ni-Cr alloys under irradiation

University: UGent

Promotor: Van Neck, D.

SCK CEN Mentor: Terentyev, D.

The safe exploitation of current and future fission nuclear reactors and construction of future thermonuclear fusion reactors requires a deep understanding of the processes leading to the radiation-induced degradation of the structural reactor materials. In this PhD thesis we consider radiation damage in three groups of materials: austenitic stainless steels used in pressurized water reactor (PWR) internals, ferritic-martensitic (FM) steels to be used in next generation fission and in future fusion reactors, and reactor pressure vessel (RPV) steels currently used in PWRs.

The radiation-induced degradation of these materials initiates in a similar way. Firstly, the neutron radiation causes the generation of atomic displacement cascades, which lead to the creation of lattice point defects such as vacancies and interstitials. Eventually, these defects agglomerate in the form of dislocation loops and voids which act as obstacles for dislocation movement during plastic deformation. This effect causes hardening, which is the increase of the yield stress usually measured experimentally in the mechanical tests. Also, the dislocations can absorb the radiation defects and this process can lead to the formation of the so called `dislocation channels' --- zones free from radiation defects, which under stress can lead to  localized plastic deformation and, consequently, to stress concentration and initiation of a crack. Secondly, the point defects created due to neutron radiation are responsible for the increased mass transport of the solute atoms which start diffusing faster and migrate to sinks, namely the grain boundaries and dislocations. The segregation of the solute atoms to the dislocation lines in the form of loops may lead to the formation of solute clusters, possibly making the loops stronger obstacles for the dislocation motion, leading to enhanced hardening.

This PhD thesis is devoted to the study of the nano-scale dislocation-defect interaction mechanisms, at the origin of the hardening at the atomic scale, which may also provide an explanation for the formation of dislocation channels. The atomistic calculations (ab initio simulations) are also applied to identify the mechanisms of nucleation of nanometer-scale solute-rich clusters under irradiation in the above mentioned structural and RPV steels, and to explore their contribution to hardening (molecular dynamics simulations).

The performed work clarifies the nucleation mechanisms of solute-rich clusters in the FM and RPV steels, and contributes to the numerical evaluation of the hardening due to their presence in association with the agglomeration of self-interstitial atom (SIA)-clusters --- dislocation loops. The dislocation-defect interaction mechanisms leading to hardening and dislocation channel - related embrittlement in austenitic steels have been also investigated. The results obtained are planned to be incorporated in upper scale models, such as dislocation dynamics, in order to obtain the stress-strain relationships for a further comparison with the experimental data and to study the formation and the pattern of dislocation channels in the austenitic steels with different stacking fault energy. The nucleation mechanisms of the solute-rich clusters together with the database of the corresponding binding energies can be used as input for kinetic Monte Carlo or mean field rate theory methods, in order to model the evolution of the solute-rich defects in FM and RPV steels under irradiation.

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