Fig. 9 Fractured fatigue sample and cross section of rolled SZ (4 mm): (a) Fractured fatigue sample, (b) Cross section, (c)-(e) Crack
The microstructure of the region near the crack initiation region (in Fig. 10 (a)) was observed using EBSD. The crack after electrode polishing is shown in Fig. 10 (b). The observed region was indicated in Fig. 10 (b). The orientation + grain boundary image of the indicated region is shown in Fig. 10 (c). The band contrast + grain boundary image of the micro zone in Fig. 10 (c) is shown in Fig. 10 (d). According to Fig. 10 (c) and (d), the thin fine grain layer with a thickness of ~10 μm between two layers was observed. The grain boundary angle distribution of the fine grain region and coarse grain region was analyzed as shown in Fig. 10 (e) and (f). It was observed that the fine grain layer had a fraction of high angle grain boundary (HAGB) of 65.1 %. However, the fraction of HAGB of coarse grain layer was only 47.4 %. From the optical images in Fig. 9, the fine grain layer was just the dark line between the lamellar structure. Combining the debonding of lamellar structure and fine grain layer in Fig. 8 (a), Fig. 9 (d), and (e), it was concluded that the debonding of the lamellar structure was based on the crack of fine grain layer.