3.5.2 Debonding mechanism of lamellar structure
As the discussed in section 3.5.1, the debonding of lamellar structure relied on the cracking of fine grain layer between two layers. The thin fine grain layer was considered as the boundary between two layers. In polycrystalline materials, dislocation tended to piled up on the boundary as the hindering effect of grain boundary[30, 31] or phase boundary[32] on dislocation. In this investigation, the boundary in lamellar structure also contributed to the dislocation pile-up.
The schematic of dislocation slip in lamellar structure is shown in Fig. 14 (a). For fine grain layer, it had a higher hindering effect on dislocation slip as its more grain boundary and higher grain boundary. Therefore, the dislocation slipping in lamellar structure was hindered in fine grain layer. However, the dislocation slipping in coarse grain layer had a higher speed as the less grain boundary. Therefore, the dislocation with a higher slipping speed was slowed when interacted with fine grain layer as shown in Fig. 14 (a). It contributed to the dislocation pile-up and hardening of fine grain layer. During the high-frequency loading process of fatigue test, the brittle fracture tended to occurred in the hardened fine grain layer as shown in Fig. 14 (b).