Microstructure and fracture toughness characterization of three 9Cr ODS EUROFER steels with different thermo-mechanical treatments

• Published in: Journal of nuclear materials 2020-12, Vol.542, p.152464, Article 152464
• Main Authors: Das, A., Chekhonin, P., Altstadt, E., McClintock, D., Bergner, F., Heintze, C., Lindau, R.
• Format: Article
• Language: English
• Subjects: Crack initiation; Dispersion hardening alloys; Dispersion hardening steels; Ductile fracture; Ductility; Electron backscatter diffraction; Electron microscopy; Ferritic martensitic alloys; Ferritic stainless steels; Fracture toughness; Fusion reactors; Grain boundaries; Heat treating; High strength alloys; Martensitic stainless steels; MATERIALS SCIENCE; Microscopy; Microstructure; Microstructure characterization; Nuclear fission; Nuclear fusion; Nuclear reactors; ODS steel; Scanning electron microscopy; Steel; Structure-property relationship; Thermomechanical treatment; Transmission electron microscopy
• ISSN: 0022-3115; 1873-4820
• DOI: 10.1016/j.jnucmat.2020.152464

•Three 9Cr ODS EUROFER steels are compared with respect to microstructure and fracture toughness.•Secondary cracking induced by residual ferrite improves fracture toughness at lower temperatures.•Matrix ductility predominantly controls the fracture toughness at temperatures between 100 ˚C and 500 ˚C.•Above 500 ˚C, the grain boundary strength predominantly controls the fracture toughness.•Variations in thermo-mechanical history lead to significant differences in microstructure and fracture behavior. Ferritic martensitic ODS steels are one of the candidate structural materials for future Gen-IV nuclear fission and fusion reactors. The dependence of fracture toughness on microstructure was investigated by comparing three 9Cr ODS EUROFER steels manufactured through different thermo-mechanical processing routes. Quasi-static fracture toughness testing was performed with sub-sized C(T) specimens and microstructural characterization was carried out using scanning electron microscopy, electron backscatter diffraction, and transmission electron microscopy. It was found that at lower test temperatures (−100 – 22 °C), the fracture toughness was primarily controlled by crack initiation at sub-micron particles and by production of secondary cracks during fracture. At higher temperatures (above 100 °C), fracture toughness was predominantly controlled by the matrix ductility and the grain boundary strength with a relatively ductile coarse-grained alloy demonstrating higher fracture toughness compared to high-strength fine-grained alloys. These results and discussion show that variations in thermomechanical treatments can produce significant differences in microstructure and fracture toughness behavior of ferritic martensitic ODS steels. [Display omitted]