Neutron irradiated ferritic-martensitic 9Cr steels: Effect of helium and nanoscaled ODS particles on microstructure, tensile and fatigue properties

A good understanding of structure-property correlations in neutron irradiated advanced
steels is indispensable for design, lifetime prediction and economical operation of fast
breeder and future fusion power reactors. Irradiation campaigns on ferritic-martensitic
9CrlWTaV steels (mostly EUROFER) have been performed at the HFR reactor in The
Netherlands up to an accumulated damage dose of 16.3 dpa at irradiation temperatures
between 250 and 450°C. In the first campaign 80-5100 appm He has been produced by
10B doping to analyze after irradiation systematically Helium embrittlement at low and
high strain rates. While tensile and fatigue tests at low stain rates show no Helium effect
up to ~400 appm, Charpy tests at high strain rates reveal a helium related pronounced
increase in DBTT of more than 100°C already at 80 appm He. In the second campaign
50 kg EUROFER-ODS was fabricated by powder metallurgy. Compared to EUROFER
and other 9CrlWTaV steels the ODS-steel shows in the entire temperature range
convincing tensile and fatigue properties after neutron irradiation: much less irradiation
hardening, unprecedented uniform elongation and no dislocation channeling induced
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strain localization. It is to our knowledge the first time that results on neutron irradiated
ODS-steels are presented in such a wide temperature range. Correlations are made
between strengthening, plasticity, cyclic softening, fatigue lifetime and fracture behavior
on the one hand and microstructure evolution on the other hand. Finally conclusions are
drawn regarding the microstructural engineering of irradiation tolerant materials.