Microstructural effects of neutron irradiation on ferritic/martensitic stainless steels

A commercial grade 12%CrMo VNb ferritic/martensitic stainless steel in the form of parent plate and high-nickel off-normal weld material has been fast neutron irradiated to equivalent damage levels of 33 and 50 dpa at 400 and 465°C respectively. The microstructural and microchemical changes induced in the irradiated material, together with as-tempered and thermal control material, have been determined to high resolution by conventional transmission electron microscopy and the use of a field emission gun scanning transmission electron microscope (FEGSTEM). Equilibrium (co )segregation of chromium, molybdenum and phosphorus was detected at boundary planes in thermally aged material, with greater enrichment at the higher ageing temperature. The relative magnitudes of apparent phosphorus segregation at the two temperatures were in accordance with McLean's model governing the kinetic approach to equilibrium. The electron probe I segregant interaction was modelled in an attempt to deconvolute true segregant concentrations from derived concentration profiles: these 'deconvoluted' concentrations approximated those predicted by McLean's model. The net effects of irradiation on parent plate interfacial microchemistry were found to be to: (i) inhibit the (co )segregation of chromium, molybdenum and phosphorus, (ii) cause chromium depletion from adjacent to boundary planes, (iii) cause enrichment of silicon at prior austenite and lath boundaries during irradiation at 400°C and (iv) cause enrichment of nickel at lath boundary planes only, at both temperatures. The radiationinduced precipitates ~C and G phase, both nickel- and silicon-rich, were observed. The fully martensitic off-normal weld metal transformed to a duplex austenite!ferrite structure during irradiation at 465°C; in contrast the thermal control was at least metastable. The transformation was thought to be a martensitic reversion, facilitated by radiation-generated dislocation loops acting as nucleation sites. The austenite was heavily voided (-15 vol.%); the ferrite was relatively void-free. Depletion of the oversized solutes chromium, manganese and molybdenum and enrichment of nickel, silicon, aluminium and traces of titanium were detected at void interfaces in the austenite: little segregation could be discerned at voids in the ferrite. Overall, the results within this work and in comparison to previous studies highlight the sensitivity to initial composition, microstructure and heat treatment that the 12%Cr ferritic/martensitic steels display in their response to irradiation.