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.