13th International Conference on Fracture June 16–21, 2013, Beijing, China -9- 4. Discussion Birnbaum and Solfronis [8] calculated the interaction of H interstitials with dislocations and indicated that the motion of dislocations trapping H interstitials becomes easier and planar. Ferreira et al. [9] showed that the cross slip of screw dislocations in pure Al becomes difficult in hydrogen atmosphere. Further, Bond et al. [10] showed that the velocity of dislocations is enhanced and therefore the resistance to cracking is reduced in 7050 and 7075 alloys exposed to hydrogen atmosphere. These theoretical and experimental results strongly support that hydrogen-enhanced local plasticity (HELP) accompanying the reduction of flow stress and the planar slip is induced in Al and its alloys, when they are exposed to environments which enable H atoms to enter the materials. It is also considered that H atoms formed by the reaction of H2O with freshly created surfaces close to crack tips enter the material during the fatigue cracking of 7075 T6 alloy in high humidity. The HELP mechanism works at the crack tips and lowers the flow stress in the plastic zone in front of the crack tips. This deterioration mechanism explains the present results well. However, the difference in the sensitivity to humidity as well as the fracture mode between A and B specimens, which is shown by Figs. 3-7, has not been made clear [13]. As mentioned in sections 3.3 and 3.4, as-received A specimens have finer grains containing dislocations with a high density. In contrast, B specimens solution-treated at 733 K for 10.8 ks exhibit the preferential growth of <111> grains and their dislocation density is lowered. Such grain growth as well as the reduction in dislocation density is manifested in C specimens. These results suggest that the grain size, the structure of texture and the dislocation density influence the HELP mechanism in environmentally assisted fatigue cracking. In A specimens, both of the <111> and <100> grains are relatively hard as is shown by the yield strength in Table 2. In addition, the faction of <100> grains in A specimens is larger compared with that of B specimens. It is hence considered that the propagation of slip between neighboring grains is more difficult in A specimens. On the other hand, the grains in B specimens are softer and mostly governed by <111> grains, and the misorientations between the subgrains are very small (Fig. 11). Therefore it is considered that the texture structure of B specimens enables selected slips to propagate smoothly from grain to grain. This consideration explains well why the shear mode fracture takes place more preferentially in B specimens than in A specimens under the same RB test condition using 50 Hz (Fig. 5). Kariya [11] found by using etch pit technique that the humidity-induced shear mode cracks propagate in <110> direction and the cracks lie on {001} plane in B specimens RB-tested at 50 Hz. This result seems to be consistent with the morphology of observed S-type cracks, since the angle between <111> and <001> is 35.3o. On the other hand, the present EBSD analyses yield such angle of S-type cracks in other geometries as shown in Fig. 14. In a <100> grain, the intersections of two slip planes with the specimen surface make the angle of ±35.3o, when <110> direction of the grain is normal to the surface (Fig. 14(a)). The "V"-shaped crack can be formed only in this geometry. In a <111> grain, a slip direction makes an angle of 35.3o with the specimen surface, when this slip direction is parallel to the surface (Fig. 14(b)). It is suspected from these geometries that the cracks of "V" shape are initiated in such <100> grains emerging at the specimen surface. It should be also noted that the "V"-shaped cracks in <100> grains can induce the slip in neighboring <111> grains with the same angle, when <011> directions of both grains are aligned like Fig. 14(a) and (b). Further the slip in <100> grains can be continuously connected with the slips in <111> grains, when the slips on two slip planes involving the selected direction operate simultaneously in <111> grains. This situation leads to the macroscopic {001} slip plane in <111> grains which is microscopically composed of two slip planes (Fig. 14(c)). As a result, {111} and {001} planes become preferential planes for crack propagation in <100> and <111> grains, respectively. It is also considered that edge dislocations glide on these planes so that the interaction of H atoms with the edge dislocations induces the acceleration of inclined crack propagation. It is suspected that the continuous slip propagation between <100> and <111> grains in A specimens is more difficult than in B specimens
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