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).