On the relation between irradiation induced changes in the master curve reference temperature shift and changes in strain hardened flow stress

Irradiation hardening produces increases in the cleavage transition fracture toughness reference temperature (ΔT0). It is traditional to relate ΔT0 to the corresponding changes in the yield stress, Δσy, as C0 = ΔT0/Δσy. However, it is a strain-hardened flow stress, σfl, in the fracture process zone that controls cleavage, rather than σy. Thus irradiation induced decreases in the strain hardening Δσsh (< 0) must be considered along with Δσy (> 0) in evaluating ΔT0. The Δσsh in reactor pressure vessel (RPV) steels irradiated to low doses at around 300 °C are small, even for large Δσy. However, the Δσsh are much greater for high dose irradiations of tempered martensitic steels (TMS) that are candidates for fusion applications. As a result, for the TMS case, the C0 are less, and in some instances much less, than for RPV steels and irradiation conditions. We address two key questions. First, how does Δσsh influence the C0 = ΔT0/Δσy relation? Second, is it possible to derive a universal relation between ΔT0 and Δσfl averaged over a pertinent range of ε, 〈Δσfl〉, such that a C0′ = ΔT0/〈Δσfl〉 is independent of the individual values of Δσy and Δσsh? The results of this study suggest that 〈Δσfl〉 averaged between 0 and 0.1 provides a similar C0′ for various assumptions about the effect of irradiation on Δσsh. Notably, changes in indentation hardness, ΔH, are also directly related to this same 〈Δσfl〉. Hence, measurements of ΔH should provide a good basis for assessing ΔT0 for a wide range of alloys and irradiation conditions.