13th International Conference on Fracture June 16–21, 2013, Beijing, China -6- transcrystalline mechanism that means that tr dσ < int dσ and tr CS < int CS , where tr CS and int CS are the critical stresses for propagation of transcrystalline and intercrystalline microcracks. The difference of tr dσ and int dσ is less for 2Cr-Ni-Mo-V steel for which some fraction of intercrystalline fracture is observed. For irradiated steels (Fig. 2b) tr dσ and int dσ decrease. For carbides located on grain boundary, segregation of impurities and arising of internal stresses caused by dislocation loops occur more intensively. Intercrystalline phosphorus segregation caused by irradiation decreases also int CS as sharp microcrack nucleation and propagation on phosphorus monolayer weakening atomic bonds, require less energy. Nevertheless, for irradiated 2.5Cr-Mo-V steel the conditions tr dσ < int dσ and ≈ tr CS int CS are more possible, so that transcrystalline brittle fracture is more typical. For irradiated 2Cr-Ni-Mo-V steel the situation is possible when tr dσ ≈ int dσ (this steel is more sensitive to the P segregation) and ≈ tr CS int CS as so that mixed trans- and intercrystalline brittle fracture is observed. After post-irradiation annealing at Tann=475 oC (Fig. 2c) phosphorus segregations dissociate in a grain only, and do not dissociate on grain boundary [16, 17]. Radiation-induced dislocation loops and precipitates may be dissociated practically completely both in a grain and on grain boundary. Therefore the tr dσ value increases up to the value for the unirradiated condition but the int dσ and int CS values remain less than for the unirradiated condition. As a result, for 2.5Cr-Mo-V steel a fraction of intercrystalline fracture may increase as compared with irradiated specimens as the values of tr dσ and int dσ become close, although this annealing results in full recovery of the mechanical properties (Ttr and σY). For 2Cr-Ni-Mo-V steel the situation is possible when intergranular carbides become “weakest link” and intercrystalline fracture predominates, and although σY may recover fully, Ttr recovers not fully so that ( ) ( ) ( ) irr tr ann tr unirr tr T T T < < . The reason is clear: for this case the brittle fracture resistance is controlled by int dσ that does not recover fully. Annealing at Tann≈600 oC (Fig. 2d) results in full recovery of int dσ as dissociation of grain boundary P segregations occurs at this temperature [16] and the situation become close to the unirradiated condition (Fig. 2a). Full recovery of the mechanical properties and the fracture modes is observed. σd unirradiated irradiated annealed, 475 0C annealed, 600 0C 2.5Cr-Mo-V steel σd unirradiated irradiated annealed, 475 0C annealed, 600 0C 2Cr-Ni-Mo-V steel a) b) c) d) Figure 2. Variation of tr dσ (shaded bar) and int dσ (open bar) for RPV steels in various conditions: unirradiated (a), irradiated (b), after post-irradiation annealing (c) and (d) [14]. Let us analyze the above experimental findings from viewpoint of criterion (1). It is clear that criterion (1) may explain an appearance of intercrystalline fracture for irradiated steel but does not
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