Early-stage microstructural evolution and phase stability in neutron-irradiated ferritic-martensitic steel T91

A Fe-9Cr ferritic martensitic (F/M) steel T91 was neutron-irradiated in the Advanced Test Reactor up to 3.96 dpa in two temperature ranges, 466 °C to 534 °C and 571 °C to 632 °C. The microstructure evolution including dislocation loops, precipitation, segregation of elements and phase stability were studied using analytical scanning-transmission electron microscopy and atom probe tomography. The hardening induced by irradiation was measured by nanoindentation. Ni/Si/Mn clusters were identified in all conditions except the one irradiated around 600 °C to 3.23 dpa. The compositions of Ni/Si/Mn clusters were found to be converging to G phase stoichiometrically with increasing dose, with Mn partially substituted by Cu. Significant coarsening of this phase was observed in high temperature cases, with total dissolution of intragranular G phase after prolonged irradiation. A similar trend was also identified for dislocation loops. The results obtained in this experiment provide evidences that the absence of dislocation loops in some specimens irradiated to high dose level under high irradiation temperature range (typically above 500 °C) is not due to the suppression of nucleation and growth by high point defect recombination rate, but rather fast coalescence of dislocation loops. Hardness measurements show different dose dependences for two temperature ranges. For specimen irradiated around 600 °C significant hardening before 0.5 dpa followed by softening process was observed, while in lower temperature range (450 °C ~ 500 °C) the normal hardening pattern with increasing dose was observed.