Crystal cracking of grain-gradient aluminum by a combined CPFEM-CZM method

Gradient nanostructured metals attract extensive attention due to their excellent mechanical properties, while they are prone to microcracks during forming and servicing process and eventually develop into a sudden fracture. Actually, experimental studies show that microcracks occurred in both the grain boundary (GB) and the intragrain, but most of the current reports only focus on the GB cracking, very limited work has studied the intragranular cracking. In this paper, the crystal plasticity finite element method (CPFEM) and cohesive zone model (CZM) are combined to study the cracking mechanism of polycrystalline aluminum (Al) with different grain-gradient structures under tensile load, where molecular dynamics (MD) method is used to determine the cohesive parameters of the intragrain and GB. The results not only show the crack initiation and propagation process of polycrystalline Al with different grain-gradient structures, but also reveal the mechanism of intragrain fracture, GB fracture and crack transgranular. The grain-gradient distribution with optimal comprehensive performance is obtained. Moreover, it is also found that the initial microcracks with different positions, numbers and angles have a great influence on the cracking mechanism and effective properties of the whole material. This study provides a solid theoretical basis for improving the quality and operation life of metal parts.