13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- interfaces [16] and result in decreasing the interface strength (i.e. in decreasing σd). This means that the nucleation of cleavage microcracks becomes easier compared with unirradiated steel. The mechanisms of the effect of radiation defects on cleavage microcrack nucleation were proposed in [13, 14]. It should be noted that for RPV steels in unirradiated condition the cleavage microcrack initiators are mainly globular carbides [19] and additional initiators do not arise under irradiation as practically all radiation defects are very small to nucleate a sharp cleavage microcrack. (For example, sizes of dislocation loops are near 5÷20 nm and the nucleus microcrack size estimated from the Griffith's condition is 100÷400 nm.) The proposed mechanisms explain how radiation defects affect cleavage microcrack nucleation on carbides. The proposed mechanisms allow the determination of the dependence of critical parameter σd on neutron fluence [13, 14]. These mechanisms are divided into two groups. The first group includes the mechanisms [13, 14] connected with decreasing the strength of carbide-matrix interface. One mechanism of decreasing σd is the impurity (P) segregation on ferrite-carbide interfaces caused by the P diffusion accelerated by irradiation. Another mechanism for decreasing σd is the arising of internal stresses caused by irradiation-induced dislocation loops and precipitates on carbide-matrix interface. These internal stresses result in rupture of the interface at stress σnuc being less than σnuc for unirradiated material. The second group includes the mechanism connected with easier formation of dislocation pile-ups near initial microcrack initiators (for example, globular carbides) due to increasing the concentration of radiation defects [13, 14]. This process may be described by increasing the probability of dislocation pile-up formation in material with high concentration of barriers for dislocations. The detailed consideration is given in [13, 14] where the probability of pile-up formation is described with the Weibull function and the Orovan stress used as a characteristic parameter for concentration of various barriers. 3.3. On the intergranular fracture mode Brittle fracture of unirradiated RPV steels occurs, as a rule, by transcrystalline cleavage and microcleavage. Neutron irradiation and post-irradiation annealing may result in appearance of intercrystalline fracture mode. It is interesting that a fraction of intercrystalline fracture is not always correlated with mechanical properties. For irradiated steels the mechanical parameters (DBTT and σY) speak about significant embrittlement, however a fraction of intercrystalline fracture is usually small (near 20% of fracture surface). After annealing, significant recovery of mechanical properties is observed, at the same time, a fraction of intercrystalline fracture may increase. These findings may be explained with criterion (2) and the proposed mechanisms of the effect of radiation defects on the critical stress σd [13, 14]. In deterministic statement, value of σd is determined by minimum value of the critical stresses for nucleation of transcrystalline tr dσ and intercrystalline int dσ microcracks. The fracture mode may be trans- or intercrystalline that depends on which value is less if the critical stress SC is the same for both types of microcracks. In probabilistic statement, mixed fracture is expected if the difference of tr dσ and int dσ is not large. The interpretation for variation of the parameters tr dσ and int dσ is schematically shown in Figure 2, for RPV steels in various conditions (unirradiated, irradiated and after post-irradiation annealing). Two steels are considered: 2.5Cr-Mo-V and 2Cr-Ni-Mo-V (steels for WWER-440 and WWER-1000 RPV respectively). For unirradiated steels (Fig. 2a) brittle fracture occurs mainly by the
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