Microstructural evolution in high nickel submerged arc welds

The effect of nickel on irradiation sensitivity in ferritic steels is substantial, complex, and not yet fully understood. In this study, the evolution of irradiation induced microstructures in high nickel submerged arc welds has been examined in a series of small angle neutron scattering (SANS), field emission gun scanning transmission electron microscopy (FEGSTEM) and optical position sensitive atom probe (OPoSAP) experiments. For a low (similar to0.05%) copper content weld. SANS measurements showed that a high density of small features had been formed during irradiation. It was expected that clustering would be evident from visual examination of the OPoSAP elemental maps, but this was not the case. However, a statistical assessment of the data revealed that the manganese atoms were non-randomly distributed. For a medium copper (0.15%) weld, clusters were again not visible in OPoSAP, but non-randomness of both manganese and nickel could be detected at a low dose; at higher doses copper was non-random also. At high levels of copper (0.24% and 0.56%), clusters microalloyed with nickel, manganese, copper, silicon and iron were visible. The SANS data showed that the feature size tended to increase with copper and dose. but the number density remained fairly constant. The paper discusses how the differences between the SANS and OPoSAP data might be reconciled, and the implications in relation to the mechanisms of irradiation damage.