13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- mode was intergranular (Fig. 3c). This local intergranular fracture mode was observed in both conditions. These zones correspond to the SZ evidenced in Fig. 1a. They contained very often MnS inclusions (Fig. 3b) and were surrounded by cleavage facets as observed in Fig. 3b. This observation is thus similar to previous examinations on a segregated PWR material (see Fig. 1 in [2]). The ratio of intergranular/transgranular cleavage fracture was similar in both conditions. Figure 3. SEM observations: a) and b) As received material; c) Aged (450°C – 5000 h) condition. 4. Intergranular segregation – Results and theoretical considerations Auger spectrometry measurements showed that the phosphorus concentration (atomic %) at grain boundaries increased significantly between the as-received condition ( ) 12.5 6% gb pC = ± and the aged condition( ) 23 9% gb pC = ± . The grain boundary concentration in carbon tended to decrease with aging ( 37 14% gb cC = ± in the as-received condition compared to 28 10% gb cC = ± in the aged condition). Full details about these measurements can be found elsewhere [10]. Intergranular P phosphorus segregation was shown to occur during cooling after the stress-relieving heat treatment. The relatively low cooling rate (~ 30°C/h below 615°C) thus produces temper-embrittlement effect in the segregated zones enriched in phosphorus. The theory of isothermal intergranular segregation has been developed by McLean [6] in his pioneering work. This theory was extended by Guttmann [7] to account for the interaction of an impurity element (here P) with other elements, in particular carbon atoms. A comprehensive review of these theories can found in the recent book of Lejcek [11]. The grain boundary concentration at equilibrium, Cgb,eq in a system containing N elements in solid solution CB can be written as: , , exp 1 1 R gb eq B seg i i N gb eq N B j o j j o j C C G C C T = = ⎡ Δ ⎤ = ⎢ − ⎥ −Σ −Σ ⎣ ⎦ (1) The segregation energy seg iGΔ is expressed as: seg o E o o E i i i i i G G G H T S G Δ =Δ +Δ =Δ −Δ+Δ (2) a) b) c)
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