Fig. 10 (a) Cross section etched using Keller solution, (b) Cross section electrode etched using HClO₄(10 vol. %)+ethanol solution, (c) orientation + grain boundary image, (d) Band contrast + grain boundary image, (e) Grain boundary distribution of fine grain layer, (f) Grain boundary distribution of coarse grain layer

The fractured SZ sample (3 mm) under a stress of 315 MPa is shown in Fig. 11 (a). The cross section of the fractured sample is shown in Fig. 11 (b) and (c). The crack initiation region was indicated in Fig. 11 (b) and (c). The macro morphology of fracture surface is shown in Fig. 11 (d). Three regions, including region 1, region 2, and region 3, were distinguished. The microstructure of region 1 is shown in Fig. 11 (e) and (f). The fracture step was observed in Fig. 11 (f). The fracture step on the fractured surface relied on the debonding of lamellar structure as shown in Fig. 9 (e) and Fig. 11 (e1). Therefore, the fatigue crack initiated in region 1. For region 2, the remarkable fatigue striation was observed as shown in Fig. 11 (g). Therefore, the fatigue crack propagated in region 2. The microstructure of region 3 is shown in Fig. 11 (h) and (i). The brittle fracture character was observed. Therefore, the formation of region 3 relied on the complete fracture during fatigue test.