Effect of Plastic Instability on Ductile Crack Resistance Behaviour of 9Cr-ferritic-martensitic Steels

R. Chaouadi, SCK-CEN, Mol (Belgium) 
Plastic instability is becoming an important research topic as many structural materials exhibit such a behavior after neutron irradiation at low temperature. This phenomenon is usually experimentally observed on the tensile test records when necking occurs shortly after yielding. As a result, the uniform elongation is reduced to zero and consequently the strain hardening capacity vanishes. This phenomenon is also occurring when testing at sufficiently high temperatures.
In this work, the plastic instability phenomenon is analyzed on a 9%Cr-ferritic/martensitic steel, Eurofer-97, that is extensively investigated within the European R&D program. This material exhibits localization deformation not only when irradiated at low temperature but also when tested in the unirradiated condition at high temperature (550°C). In our case, Eurofer-97 was irradiated at 300°C to a neutron dose greater than about 1 dpa. Tensile and crack resistance measurements were performed at 550°C in the unirradiated condition. In the irradiated condition, tests were performed at 25 and 300°C.
The results will be presented and discussed in terms of the effect of localized deformation on the ductile crack initiation and propagation (crack resistance).

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