Atom probe tomographic analysis of high dose oxide-dispersion strengthened steel (alloy MA957) at selected irradiation conditions
In an effort to understand the effect of high dose neutron irradiation on fast reactor cladding candidate materials, oxide-dispersion strengthened (ODS) alloy MA957 was irradiated to doses exceeding 100 displacements per atom (dpa) at various irradiation temperatures. The finely distributed Y-Ti-O particles, which provide MA957 its attractive properties, were examined by atom probe tomography (APT). Significant increases in oxide cluster number density and reductions in oxide cluster size were observed in specimens irradiated at 412 °C and below. A substantial hardness increase, measured by nanoindentation, was also observed at these low irradiation temperatures. It was found that the increase in oxide cluster number density, reduction in oxide cluster size, and associated increase in hardness is due to the inhibition of reformation processes of the Y-Ti-O particles following ballistic dissolution by incident radiation. Redistribution of oxide particle material along the grain boundaries is also observed at the low irradiation temperatures. The intermetallic phase α’ was observed in the low temperature samples. This observation of this phase provides additional experimental evidence for the location of the phase boundary for this low temperature precipitate. The conclusion of this work is that MA957 is microstructurally stable under neutron irradiation at and above 495 °C.