13th International Conference on Fracture June 16–21, 2013, Beijing, China -6- Fig. 6. Crack initiations in (a) T-type Fig.7. Relation of crack growth with and (b) S-type modes. the number of cycles in B specimens. 3.3 Microstructure Fig. 8(a) shows the SEM image of the transverse cross-section at the center of B specimen, which is inclined by 70o for EBSD analyses. Fig. 8 (b), (c) and (d) are the maps (IPF maps) of crystallographic orientations of grains in z, x and y directions, respectively, which are obtained from the square region depicted in Fig. 8(a). Grains are distinguishable by those relief and contrast in SEM image. The grain size estimated by using the SE image is 12 μm, which is nearly the same as the grain size evaluated by using large misorientation angle. The IPF image along z axis shows that the transverse surface are dominated by large grains close to <111> direction (hereafter called <111> grains) and the remaining large grains have orientations close to <001> (hereafter called <001> grains). The areal fraction of <111> grains is estimated to be about 80 %. However, the IPF images along x and y axes clearly indicate that these large grains involve subgrains, and the average grain size including the subgrains is estimated to be 3.9 μm by using a misorientation angle of 0.5o for grain boundary detection. The IPF (Fig. 7(e)) along z axis also shows the largest fraction of <111> grains and the second one of <001> grains. The crytallographic orientations are uniformly distributed along <101>-<112> circle in x and y directions, as is shown by Figs. 7(f) and 7(g). Figs. 9 and 10 are the IPF images of A and C specimens, respectively. It is obvious that the grains in C specimens have round corners compared to A specimens, which indicates the grain growth takes place in C specimen by the solution treatment. The grain growth appearing as round grains can be seen in B specimen by comparing Fig. 8(b) with Fig. 9(a). The grain sizes listed in Table 4 show that the as-received A specimen has the smallest grain sizes in both large misorientation and small misorientation including subgrains. In addition the fraction of <111> grains in A specimen is 61 %, while the fraction in the other specimens are nearly 80%. This result indicates that the solution treatments done for B and C specimens induce the preferential growth of <111> grains. Fig. 11(a), (b) and (c) are the pairs of IPF image and IP figure taken from the longitudinal cross-section at the center of B specimen along z, x and y axes, respectively. The longitudinal cross-section is mainly covered with <111> grains which involve the elongated islands of <001> grains (Fig. 11(a)). The extrusion ratio is estimated to be more than 800 % from the shape of island <001> grains. In contrast to the transverse cross-section, subgrains involved in large <111> grains do not have large misorientation between neighboring ones. It is also noted that some of the <001> grains observed in z axis analysis disappear in x and y axes, which indicate that these disappeared grains have continuous relation with surrounding <111> grains. Thus the texture structure is 100 μm 35o 35o (a) 35o 35o 100 μm (b) 0.01 0.1 1 10 0 1 2 3 4 5 6 7 240 MPa 260 MPa 240 MPa 260 MPa Crack length l (mm) Number of cycles N (x105) 25%RH 85%RH
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