Critical assessment of Cr-rich precipitates in neutron-irradiated Fe-12at%Cr

The composition of solute-enriched clusters and precipitates formed in Fe-Cr alloys as the result of neutron irradiation is an unsolved issue. It is an important issue for several reasons, namely:

• to reach a complete and consistent description of the nanoscale features derived from the application of necessarily several complementary techniques,
• to correctly design and calibrate models addressing the long-term evolution of the nanoscale features,
• to correctly draw conclusions and configure models on the hardening effect of those nanoscale features.

Three sets of data separately reported in the published literature [1-3] have been selected for a critical consideration of the cluster composition in commercial-purity Fe-12at%Cr irradiated at 300°C up to a neutron exposure of 0.6 dpa. The first set of data was derived from the nuclear component of small-angle neutron scattering (SANS) [1]. The second set is based on an atom probe tomography (APT) study [2]. The APT needles were prepared from the bulk of the SANS sample. The third piece of information is adopted from an investigation of the same material by means of positron annihilation spectroscopy (PAS) [3]. The SANS results [1] were found to be consistent with the assumption that the dominant scatterers are α’-phase particles near thermodynamic equilibrium. A composition far from equilibrium was deduced from the APT data [2]. Similar apparent discrepancies were reported in the literature for other systems. In the presentation an effort to overcome the apparent discrepancy will be reported in detail.
The basic weakness of SANS is the integrating and one-parametric nature of the composition-related information hidden in the nuclear scattering contrast. Weaknesses of APT are a possible overestimation of Fe in clusters due to trajectory overlap and the insensitivity to vacancies. In the latter respect, PAS data add a value to the comparison. Other factors will be considered as well. The approach is based on the idea that the measured value of the Porod invariant of nuclear SANS can be directly compared with the corresponding quantity calculated solely from the APT data, namely volume fraction and cluster composition. Careful treatment of all potential factors of uncertainty allows the Fe fraction in the clusters to be estimated.
[1] F. Bergner, A. Ulbricht, C. Heintze, Estimation of the solubility limit of Cr in Fe at 300 °C from small-angle neutron scattering in neutron-irradiated Fe–Cr alloys, Scripta Materialia 61 (2009) 1060–1063.
[2] V. Kuksenko, C. Pareige, C. Genevois, F. Cuvilly, M. Roussel, P. Pareige, Effect of neutron-irradiation on the microstructure of a Fe–12at.%Cr alloy, Journal of Nuclear Materials 415 (2011) 61–66.
[3] M. Lambrecht, L. Malerba, Positron annihilation spectroscopy on binary Fe–Cr alloys and ferritic/martensitic steels after neutron irradiation, Acta Materialia 59 (2011) 6547–6555.