Characterisation of the early stages of solute clustering in 1Ni-1.3Mn welds containing Cu

Microstructural characterisation of neutron irradiated low alloy steels is important for developing mechanistic understanding of irradiation embrittlement. This work is focused on the early stages of irradiation-induced clustering in a low Cu (0.03wt%), high Ni ( approximately 1wt%) weld. The weld was irradiated at a very high dose rate and then examined by atom probe (energy-compensated position-sensitive atom probe (ECOPoSAP) and local electrode atom probe (LEAP)) with supporting microstructural information obtained by small angle neutron scattering (SANS) and positron annihilation (PALA). It was demonstrated that extreme care must be taken optimising parameters used to characterise the extent of clustering. This is particularly important during the early stages of irradiation-damage when the clusters are poorly defined and significant compositional variations are present in what is traditionally described as matrix. Analysis of the irradiated materials showed increasing clustering of Cu, Mn, Ni and Si with dose. In the low Cu steel the results showed that initially the irradiation damage results in clustering of Mn, Ni and Si, but at very high doses, at very high dose rates, redistribution of Si is significantly more advanced than that for Mn and Ni.