Review on the EFDA work programme on nano-structured ODS RAF steels
Introduction
As the upper temperature for use of reduced activation ferritic/martensitic (RAFM) steels for structural applications in fusion power reactors, such as the European EUROFER 97 RAFM steel, is presently limited by a drop in mechanical strength at about 550 °C (see e.g. [1]), Europe, Japan and the US are actively researching steels with high strength at higher operating temperatures, mainly using stable oxide dispersion. In addition, the numerous interfaces between the matrix and the oxide particles are expected to act as sinks for the irradiation-induced defects and transmutation elements such as He and H [2]. Main R&D activities aim at finding a compromise between good tensile and creep strength and sufficient ductility, especially in terms of fracture toughness. It has been recently found that the oxide dispersion strengthened (ODS) RAFM steels, such as the ODS EUROFER steel, could be used for structural applications in fusion power reactors up to about 650 °C (see e.g. [3]). The nano-structured ODS reduced activation ferritic (RAF) steels appear as very promising materials as they are expected to be used for structural applications in fusion power reactors up to about 750 °C (see e.g. [4]).
The plate supporting the tungsten tiles in the European dual-coolant lithium–lead (DCLL) breeding blanket concept [5] and the cartridge within the finger-like parts of the European He-cooled divertor concept [5] are presently foreseen to be made of ODS EUROFER [6], [7]. The use of ODS RAF steels with a higher creep strength up to about 750 °C and a reasonable fracture toughness at ambient and intermediate temperatures will provide these components with additional integrity margin and lifetime.
The 2008–2009 work programme of the European research project on nano-structured ODS RAF steels is being organized along the four following programmatic lines: (1) improve the present generation of nano-structured ODS RAF steels; (2) start the industrial fabrication of the present generation of nano-structured ODS RAF steels; (3) develop an optimised generation of nano-structured and nano-grained ODS RAF steels; (4) investigate the stability of present and optimised generation of nano-structured ODS RAF steels under creep and irradiation.
This paper presents the main objectives of current R&D activities being performed within the European research project on nano-structured ODS RAF steels, the main obtained results and the main future activities in the case of the four programmatic lines mentioned just above.
Section snippets
Main objectives
The main objectives of this programmatic line are (i) to optimize parameters of manufacturing by powder metallurgy, including mechanical alloying followed by either hot isostatic pressing (HIPping) or hot extrusion and thermal–mechanical treatments, in order to obtain a dense population of small nano-clusters and sub-micrometre grains, both conditions being required for high creep strength and reasonable fracture toughness after irradiation, and (ii) to fabricate small batches of about 1 kg at
Main objectives
Both presently identified applications require quantities of materials that are orders of magnitude larger than those currently manufactured at the laboratory scale and, hence, will have to be fabricated using industrial-scale methods. The main objective of this programmatic line is to assess the possibility of fabrication of semi-industrial batches of ODS RAF steels of about 10 kg in Europe, on the basis of the optimized manufacturing route that is being defined within the programmatic line
Main objectives
The main objectives of this programmatic line are (1) to develop the dynamical phase diagram of the driven Fe–Cr–W–Ti–Y2O3 system under mechanical alloying and subsequent thermal–mechanical treatments (see e.g. [10]), in the aim to classify the various mechanical alloying processes, compaction methods and thermal–mechanical treatments required for obtaining a dense population of small nano-clusters, and (2) to determine the optimum grain size and work-hardening level for achieving the best
Main objectives
The main objectives of this programmatic line are (1) to perform post-irradiation examination of the ODS ferritic steels previously irradiated in fission reactors or in spallation neutron sources, and (2) to define the characterisation programme of laboratory scale and semi-industrial European batches of ODS ferritic steel, in terms of long term annealing experiments, creep and fatigue tests, and irradiation experiments. In what concerns the investigation of radiation effects, many-beam ion
Summary
European activities in the field of ODS RAF steels focus on the development of materials combining high strength and creep behaviours at elevated temperatures with reasonable ductility and fracture toughness at low and intermediate temperatures and good radiation resistance under fusion power reactor irradiation conditions. Such R&D activities are strongly supported by fundamental investigations, in order to achieve a detailed understanding of the effects of the chemical composition and
Acknowledgement
This work, supported by the European Communities, was carried out within the framework of the European Fusion Development Agreement. The views and opinions expressed herein do not necessarily reflect those of the European Commission.
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