5. Summary and conclusions
TEM and SANS were used to characterise the microstructures
of EM10 and T91 martensitic steel samples
implanted with 0.5 at.% helium at 250 and 550 _C, respectively.
This microstructural analysis revealed the
presence of small defect clusters in the steels implanted
at 250 _C, as well as helium bubbles following implantation
at both temperatures. Furthermore, the SANS
experiments showed that these bubbles are close to
thermodynamic equilibrium, i.e. their internal gas pressure
is balanced by the surface tension.
The TEM study was performed on discs punched out
from tensile specimens following the tests, which indicated
for the samples implanted at 250 _C a very large
hardening and a total loss of ductility associated with a
predominantly intergranular fracture mode [1].
It was shown that the observed hardening is not due
to the small defect clusters but to the high density of
small He bubbles detected by the microstructural analysis.
It is furthermore suggested that the brittle, intergranular
fracture mode results from the combination of
pronounced hardening and weakening of PAG boundaries
due to helium.
A typical spallation environment is characterised by
a lower He creation rate and a higher damage creation
rate than in the case of the simulation experiments
mentioned above. It is therefore expected that the helium
distribution in the steels implanted at 250 _C be
representative of that obtained after exposure in a
spallation radiation field at lower temperature. Consequently,
the severe embrittlement observed after implantation
at 250 _C raises the question of the suitability
of martensitic steels as structural materials for the ESS
container window, which will operate below 250 _C. The
results obtained in the present study do not, however,
rule out the use of 9Cr martensitic steels for the Megapie
or ADS proton beam windows since in both cases the
operating temperature will be significantly higher than
250 _C. Nevertheless, additional data, both from simulation
experiments and irradiation tests in a spallation
spectrum, are obviously needed.
Appendix A. Intensity scattered by a population of small
dislocation loops
The intensity scattered by a population of small
dislocation loops of radius r with an isotropic distribution
of orientations is given by [8]