SANS investigation of neutron-irradiated Fe-Cr alloys


SANS investigation of neutron-irradiated Fe-Cr alloys

Ulbricht, A.; Heintze, C.; Bergner, F.; Eckerlebe, H.

Ferritic-martensitic chromium steels are candidate materials for future applications in both Gen-IV fission and fusion technology. Experimental investigation of Fe-Cr alloys is important for the understanding of the effect of chromium on the irradiation behaviour of more complex alloys.
The materials investigated in this work are four binary Fe-Cr alloys of commercial purity with chromium contents between 2.5 and 12.5at%. Neutron irradiation was performed in the reactor BR2 at Mol (Belgium) at a temperature of 300°C and neutron flux of 9x1013 cm-2s-1 (E > 1 MeV). The neutron exposures expressed in units of displacements per atom correspond to 0.6 and 1.5 dpa.
In the SANS experiments carried out at the SANS-2 facility of GKSS, a wavelength of 0.58 nm and three detector-sample distances of 1, 4 and 16 m were used. The samples were placed in a saturation magnetic field. Special details related to the analysis of the SANS results (e.g. interference for concentrated solutions, bimodal distribution of scatterers) will be discussed in the full paper. The scattering curves obtained for the unirradiated conditions of the four Fe-Cr alloys were taken as reference in order to calculate difference scattering curves for the irradiated conditions.
We have found that (1) the volume fraction of scatterers slightly increases with neutron exposure (Fe-9at%Cr) or exhibits a saturation (Fe-2.5, 5 and 12.5at%Cr), (2) the volume fraction at 1.5 dpa is an increasing function of the chromium level with a slight increase up to Fe-9at%Cr and an abrupt increase between 9 and 12.5at%Cr, (3) the radii of irradiation-induced scatterers are essentially less than 8 nm and the size distributions can be reasonably described by a monomodal (Fe-2.5, 5 and 12.5at%Cr) or a bimodal (Fe-9at%Cr) distribution, (4) the A-ratio of 2.05 obtained for Fe-12.5at%Cr corresponds with the expectation for the alpha’-phase, (5) for the lower chromium levels the A-ratio can be explained neither as alpha’ nor as vacancy clusters alone, clusters composed of chromium and impurity carbon are possible candidates.
Conclusions will be drawn on the effect of the chromium content and the neutron exposure on the concentration and nature of the irradiation-induced scatterers. For the special case of Fe-12.5at%Cr the volume fraction of alpha’ and the solubility limit for chromium in iron at the irradiation temperature can be estimated.

Keywords: SANS; Fe-Cr alloys; neutron irradiation

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Publ.-Id: 13159