Microstructure-based intergranular fatigue crack nucleation model: Dislocation transmission versus grain boundary cracking

Intergranular fatigue crack nucleation has been reported to occur commonly at high angle grain boundaries (HAGBs) but seldom at low angle GBs (LAGBs) during persistent slip band (PSB)-GB interactions. However, the mechanistic understanding of the role of GB misorientation angles in affecting the GB fatigue cracking remains limited due to the lack of quantitative descriptions. Here a theoretical framework based on the competition between dislocation transmission across GB and GB cracking is established for investigating the GB fatigue crack nucleation. We show that, HAGBs usually provide higher resistances to dislocation transmissions compared with LAGBs, causing a more significant dislocation pile-up and stress concentration, which facilitates the GB crack nucleation. But the quantitative analysis indicates that even at HAGBs with poor plastic compatibility, the critical stress of GB cracking is much larger than the critical transmission stress. This demonstrates the crucial role of GB fatigue damage accumulation, which is associated with PSB extrusion growth, in promoting GB crack nucleation. In this study, the critical stress of dislocation transmission is derived in terms of the energy balance, and the nucleation criterion of GB crack is formulated using the fracture theory, both of which display the dependence on GB misorientation angles and grain sizes. It is found that the GB fatigue cracks preferentially nucleate at HAGBs with poor plastic compatibility and large grain sizes. The competition between dislocation transmission and GB cracking at varying fatigue cycles are also discussed. This theoretical framework offers quantitative analyses for the GB crack nucleation during PSB–GB interactions, clarifies the role of microstructures in affecting the critical cycles of GB cracking, and might offer important guidelines for designing fatigue-resistant metallic materials.