On the effects of irradiation and helium on the yield stress changes and hardening and non-hardening embrittlement of ∼8Cr tempered martensitic steels: Compilation and analysis of existing data

Abstract

Data on irradiation hardening and embrittlement of 8–10Cr normalized and tempered martensitic steel (TMS) alloys has been compiled from the literature, including results from neutron, spallation proton (SP) and He-ion (HI) irradiations. Limitations of this database are briefly described. Simple, phenomenological–empirical fitting models were used to assess the dose (displacement-per-atom, dpa), irradiation temperature ( T i) and test temperature ( T t) dependence of yield stress changes (Δ σ y), as well as the corresponding dependence of sub-sized Charpy V-notch impact test transition temperature shifts (Δ T c). The Δ σ y are generally similar for SP and neutron irradiations, with very high and low helium to dpa ratios, respectively. Further, the Δ σ y trends were found to be remarkably consistent with the T i and dpa hardening-dependence of low alloy steels irradiated at much lower doses. The similar T i and (low) dose dependence of Δ σ y and Δ T c, as well as an analysis of paired Δ T c–Δ σ y datasets, show that embrittlement is typically dominated by a hardening mechanism below about 400 °C. However, the corresponding hardening-Charpy shift coefficient, C c = Δ T c/Δ σ y ≈ 0.38 ± 0.18 °C/MPa is lower than that for the fracture toughness reference temperature, T 0, with Δ T 0/Δ σ y ≈ 0.58 ± 0.1 °C/MPa, indicating that sub-sized Charpy tests provide non-conservative estimates of embrittlement. The C c increases at T i > 400 °C, and Δ T c > 0 are sometimes observed in association with Δ σ y ⩽ 0, indicative of a non-hardening embrittlement (NHE) contribution. Analysis of limited data on embrittlement due to thermal aging supports this conclusion, and we hypothesize that the NHE regime may be shifted to lower temperatures by radiation enhanced diffusion. Possible effects of helium on embrittlement for T i between 300 and 400 °C are also assessed based on observed trends in C c. The available data is limited, scattered, and potentially confounded. However, collectively the database suggests that there is a minimal NHE due to helium up to several hundred appm. However, a contribution of helium to NHE appears to emerge at higher helium concentrations, estimated to be more than 400–600 appm. This is accompanied by a transition from transgranular cleavage (TGC) to intergranular fracture (IGF). IGF generally occurs only at high Δ σ y. Synergistic combinations of large Δ σ y and severe NHE, due to helium weakening of grain boundaries, could lead to very large transition temperature shifts in first wall and blanket structures at fusion spectrum dose levels above 50–75 dpa and in SP irradiations at much lower doses.