Effects of particle agglomeration in the γ′-free zone on the damage evolution of single-crystal Ni-based superalloys

Non-metallic inclusions are known to be a significant cause of microstructural deterioration in the single-crystal Ni-based superalloy DD6, resulting in the initiation and propagation of damage. Previous experimental results have revealed that AlN particles are dispersed individually in several regions within the γ′ free zone, situated beneath the oxide layer. However, areas where these particles agglomerate, whether they contain solely AlN particles or clustered with Al2O3 particles, are the most damaging and attractive sites for surface cracks arising in the oxide layers. Interfacial debonding between the particle and matrix, particle fracture, and matrix cracking caused damage initiation in the agglomerate region. To study the damage behavior of the agglomerate area belonging to the γ′-free region and how those particles influence the mechanical drivers of damage nucleation, we developed a new framework based on the Extended Finite Element Method (XFEM) and crystal plasticity (CP) theory. We employed a creep-damage model in a crystal plasticity (CP) framework combined with the XFEM approach to predict the micro-cracks in the agglomerate area (γ′-free zone). We also used the cohesive zone method (CZM) to simulate the interfacial debonding between the γ′ free region and each of the AlN and Al2O3 particles, and cohesive behavior-based XFEM is used to model the particle fracture. The fracture properties of the interface layer between the γ′ free zone and particles, as well as the agglomerated particles, were determined., followed by an assessment of the effects of particle agglomerate on damage mechanisms, including interfacial debonding, particle fracture, and matrix cracking in the agglomeration region. Interfacial partial decohesion of agglomerated AlN particles was prevalent with interparticle microcracking, making the agglomeration area between the AlN particles attractive for surface cracks. In contrast, the damage initiation delay with relatively low rates caused by particle fracture was the damage behavior of the agglomerated Al2O3 model. The consistency between numerical and experimental results confirmed the ability of this framework to capture microstructure-sensitive microcracks.