13th International Conference on Fracture June 16–21, 2013, Beijing, China -8- Figure 11 Substructure morphology of X80 linepipe material by 0.4% strain amplitude: (a) conventional (Nf =7800), (b) high-strain (Nf =9500) Figure 12 (a) ~ (b) shows the dislocation configuration of X80 conventional linepipe material. With the increment of strain amplitude up to 0.8%, dislocation density in ferrite of X80 conventional linepipe material further decreases, single dislocation line almost disappears and complete dislocation has been formed as figure 12 (a) shown. Dislocation in M/A island begins to be polygonzaition and dislocation cell is going to be formed, in the same time, the amount of dislocation which bypasses second particles and accumulates around increases as shown in figure 12 (b) which results the realignment of dislocation. The formation of dislocation cell and declines of dislocation density causes the cyclic softening rate increase of material under 0.8% strain amplitude. Figure 12 (c) ~ (d) shows the dislocation configuration of X80 high-strain linepipe material. An obvious variation of dislocation configuration happens in X80 high-strain line pipe material with the increment of strain amplitude. Dislocation cell with relative thinner wall has started to be formed in ferrite in some area, which implies that with the occurrence of cellular structure, cyclic softening rate of X80 high-strain line pipe material suffered 0.8% strain amplitude is significantly improved comparing with 0.4% strain amplitude. Furthermore, it also can be observed that the tendency of dislocation cellular structure formation is much more obvious in ferrite surrounded by M/A as shown in figure 12 (c). While, the dislocation configuration in ferrite not surrounded by M/A is dislocation line or tangled dislocation as show in 12 (d). The reason for this phenomenon is that deformation inside material focuses primarily on coordinating deformation among structures. That is to say, deformation firstly happens on ferrite with a lower strength which can induce dislocation annihilation or formation of dislocation cell after dislocation movement and occurrence of ferrite softening [7]. Meanwhile, since deformation of ferrite is restrained by plenty of M/A constituent, once ferrite has been deformed to a certain level, deformation on M/A island begins. Furthermore, since much more ferrite exits in X80 high-strain linepipe material, the resource for dislocation movement is much more than X80 conventional linepipe material. Therefore, deformation in ferrite is further suppressed and the formation of dislocation cell in ferrite is accordingly delayed, which results in the reduction of softening rate of ferrite. Perhaps, that is certain reasons for inducing of much lower softening rate of X80 high-strain line pipe material [8]. (b) (a)
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