• Vrellou, Maria

Abstract

Irradiation is well known to alter the Reactor Pressure Vessel’s (RPV) microstructure, inducing embrittlement, and thus threatening the integrity and safety of the Nuclear Power Plants. In spite of the many research studies carried out on the correlation of the microstructure with irradiation hardening of the RPV steel, the underlying mechanisms driving this process are still unclear, mainly because irradiation evolution of microstructure is very sensitive to many parameters concerning the irradiation conditions (as temperature, flux, fluence) and also the material (chemical composition, its fabrication conditions and even its thermomechanical history).In this work, the way that the typical RPV alloying elements Mn and Ni are involved in the formation and evolution of the irradiation induced microstructure and its consequential effects on the mechanical properties, in the absence of the well-studied impurity Cu, was studied using two ferritic binary model alloys (Fe-Ni and Fe-Mn) and a ternary (Fe-Mn-Ni).APT microstructural study of the alloys revealed the formation of nano-sized clusters under neutron irradiation. The clusters formed in the Fe-Mn alloy outnumbered those of the other two alloys, producing the highest number density, almost an order of magnitude higher that the density of the other binary alloy (Fe-Ni), the clusters of which were more solute enriched. Therefore, it appears that there is a synergistic effect between Mn and Ni. The Mn contributes by producing solute clusters in high number density and while Ni increases their solute enrichment, which combined together led to the significant increased volume fraction of the clusters developed in the Fe-Mn-Ni alloy.The mechanical properties of the three alloys before and after irradiation were assessed by in-situ SEM micro-compression of FIB fabricated single crystal micro-pillars, inside grains having favorable crystal orientation. The irradiation hardening was calculated higher in the ternary alloy, followed by the Fe-Ni and the Fe-Mn.The ability of the irradiation induced clusters to hinder the motion of the dislocations, was evaluated using the obstacle strength, provided from two theoretical models. The obtained specific resistance and obstacle strength values, suggest that the presence of Ni in the solute clusters leads to an enhanced resistance to dislocation motion, while the Mn produces clusters, that cannot effectively hinder the dislocations which pass through them relatively easily and hence the Mn-enriched clusters, mainly contribute to irradiation hardening due to their highly increased number density.The study of the energy dissipated by the dislocations for overcoming the solute clusters during the plastic deformation of the micropillars is in good agreement between the strength of the clusters as suggested by the theoretical models.To study the effect of the deformation on the microstructure, APT tips and TEM lamellas were fabricated from lifted-out pillars. The TEM calculated dislocation density of a non-compressed pillar of the non-irradiated Fe-Mn-Ni model alloy, verified the literature suggested value.APT analysis of the compressed neutron irradiated Fe-Mn pillars, indicated that the characteristics of the clusters were similar with those of the bulk ones, with the exception that an increased number density of ‘large’ clusters was detected. These ‘large’ clusters were assumed to be partially sheared from the dislocations that passed through them and, to our knowledge, their presence is reported for the first time.

Topics

• density
• impedance spectroscopy
• cluster
• polymer
• single crystal
• grain
• scanning electron microscopy
• strength
• steel
• chemical composition
• positron annihilation lifetime spectroscopy
• Photoacoustic spectroscopy
• transmission electron microscopy
• dislocation
• atom probe tomography
• lamellae