Irradiation and inhomogeneity effects on ductility and toughness of (ODS)-7 -13Cr steels

Full text of publication follows: The superimposed effect of irradiation defect and structural inhomogeneity formation on tensile ductility and dynamic toughness of ferritic-martensitic 7-13CrW(Mo)VTa(Nb) and oxide dispersion-strengthened (ODS)-7-13CrWVTa(Ti)- RAFM steels has been examined by work hardening and local stress/strain-induced ductile fracture models. Structural inhomogeneities which strongly promoting plastic instability and localized flow might be formed by the applied fabrication process, high dose irradiation and additionally further during deformation by enhanced local dislocation generation around fine particles or due to slip band formation with localized heating at high impact strain rates ε'. The work hardening model takes into account superimposed dislocation multiplication from stored dislocations, dispersions and also grain boundaries as well as annihilation by cross-slip. Analytical relations have been deduced from the model describing uniform ductility and ductile upper shelf energy (USE) observed from Charpy-impact testes. Especially, the influence of different irradiation defects like atomic clusters, dislocation loops and coherent chromium-rich α'- precipitates have been considered together with effects from strain rate as well as irradiation (TI) and test temperature TT. Strengthening by clusters and more pronounced by dislocation loops formed at higher TI>250 deg. C reduces uniform ductility and also distinctly stronger dynamic toughness USE. A superimposed hardening by the α'- formation in higher Cr containing 9-13Cr steels strongly reduces toughness assisted by a combined grain-boundary embrittlement with reduction of the ductile fracture stress. But that improves work hardening and uniform ductility as observed particularly due to nano-scale Y2O3- dispersions in ODS-RAFM steels. For ODS- steels additionally the strength-induced reduction of toughness is diminished by a combined microstructural-induced increase of the ductile fracture stress involving dispersions. That reduction, however, enhances by localized flow due to strain-induced dislocation inhomogeneity formations. As shown experimentally in agreement with predictions at high yield strengths above a critical value σc* uniform ductility generally disappear. This critical strength increases by dispersion hardening and increasing strain rate sensitivity of strength 0 ≥ m δlnσy=/δlnε' ≤ 1 but reduces due to pronounced loop strengthening at higher TI and preexisted inhomogeneities. An additional strain-induced inhomogeneity formation otherwise reduces uniform ductility and toughness more pronounced at lower yield strengths. Thus, the strength-induced reduction of uniform ductility diminishes in contrast to an increase of strain rate sensitivity m as expected with increasing irradiation hardening and strain rate. As experimentally observed in 7-13Cr steels irradiated at 100-550 deg. C to the high lattice damage <60 dpa, increasing irradiation and test temperature reduces critical strength σc* c and uniform ductility and additionally strongly enhances also their strength-induced reduction. The deduced analytical and numerical results are especially used for the analyses of experimental results of tensile ductility and dynamic toughness obtained from conventional 9- 12CrMoVNb and (ODS)-RAFM steels with 0- 0.5wt.%Y2O3 irradiated to <60 dpa at 100- 550 deg. C including particularly (ODS)-Eurofer'97. (authors)