Fig.1 The dependence of yield stresses on the irradiation fluence estimated from (a)hardness and (b)positron lifetime measurements.
It is well known that the irradiation embrittlement of reactor pressure vessel steel is due to the irradiation hardening by irradiation induced defect clusters such as interstitial dislocation loops, microvoids and irradiation induced copper precipitation. However, it is still obscure which is a main contributer to the hardening. Microvoids have been regarded as a potential contributes to the hardening, although they are too small to be identified by Transmission Electron Microscope (TEM). In this work, the correlation between irradiation hardening and microstructure of Fe-Cu model alloys will be investigated in order to make clear the mechanism of irradiation embrittlement of model alloys for reactor pressure vessel steel.
The materials used were pure-Fe, Fe-0.35C, Fe-0.3Cu, and Fe-0.35C-0.3Cu alloys which were made by arc melting method. After the annealing for normalization, the button ingots were cold rolled to 0.15mm thickness, then punched out into 3mmf disks. The specimens were annealed at 880oC for 4 hours, then quenched into iced water (Q). A part of the specimens were subsequently tempered at 660oC for 22.5 hours. Neutron irradiations were performed in so-called multi-section multi-division controlled irradiation capsule in JMTR (constant temperature; 190oC, constant flux). Irradiation doses were 1.9 x 1017, 4.2 x 1018, 1.4 x 1019, 3.2 x 1019n/cm2. After the irradiation micro-Vickers hardness tests, positron lifetime measurements and TEM observations were carried out at room temperature.
Irradiation-induced hardening of quenched pure-Fe and Fe-Cu alloy increase with increasing neutron fluence. The hardening of the quenched and tempered Fe-Cu alloy also increases with the fluence, while the hardness of pure-Fe is nearly constant. The quenched carbon added alloys show very high hardness mainly caused by martensitic transformation. The hardening of Fe-C-Cu alloy (QT) and Fe-Cu alloy (QT) show almost the same dependence on the fluence, suggesting that there is no significant effect of carbide precipitation on the irradiation hardening.
As for the results of 2-component analysis of positron lifetime measurement, the lifetime of long life component (t2), which reflects the size of microvoids, increases with increasing the fluence in pure Fe and Fe-Cu alloy. However, the intensity (I2), which reflects the density of microvoids, decreases with increasing the fluence. In Fe-Cu and Fe-C-Cu alloy, no clear dependence was observed for both the lifetime (t2) and the intensity (I2).
It has been considered that there are three dominant factors controlling irradiation hardening of Fe-Cu model alloys; 1) interstitial type dislocation loops, 2) microvoids and 3) copper precipitation. However, the obtained experimental results indicate that there is a reverse correlation in the fluence dependence between irradiation hardening and positron data. It is concluded that microvoids are not main contributer to the irradiation hardening in Fe-Cu model alloys.
Fig.1 The dependence of yield stresses on the irradiation fluence estimated from (a)hardness and (b)positron lifetime measurements.