For high-temperature components made from nickel-based superalloys such as rotor disks of aero engines, intergranular cracking can be a relevant damage mechanism which is a result of high-temperature fatigue in an oxygen-rich environment. The intergranular stress-assisted diffusion of oxygen is strongly dependent of the grain boundary type and accelerates crack growth along grain boundaries [1]. This damage mechanism is known as dynamic embrittlement (DE) and limits the application range of polycrystalline high-temperature superalloys. DE is highly depending on the material’s microstructure, especially on the grain boundary character that determines diffusion rates and, thus, the local concentration of the embrittling element in the grain boundary. For the computational assessment of DE with microstructure-based models, the mechanical grain boundary properties depending on grain boundary type and oxygen concentration are required. Hence, it is the aim of this work to derive grain boundary properties from atomistic calculations and to transfer the properties to microstructure-based finite-element models resolving the polycrystalline microstructure.

The cohesive properties of grain boundaries are described by traction-separation laws in atomistic calculations from applying a virtual tensile test and making use of the universal binding energy relation (UBER). This is calculated for different grain boundary types in a nickel fcc-crystal using density functional theory (DFT). The traction-separation laws are implemented in a cohesive element [2,3,4] representing the grain boundary in polycrystal finite-element models and describing damage initiation and damage evolution depending on the grain boundary type. Results of finite-element calculations are presented for polycrystals where the properties of the crystals are described by a single crystal cyclic viscoplasticity model. The sensitivities of the modeling approach with respect to the material properties of the traction-separation laws are assessed.

References
[1] U. Krupp; W. Kane; X. Liu, O. Düber; C. Laird; C.J. McMahon Jr Material Science Fracture Mechanics The Effect of Grain-Boundary-Engineering on Oxygen Induced Cracking of IN718, 2003, 349, 213-217.
[2] K.L., Auth; J. Brouzoulis; M. Ekh Journal of the Mechanics and Physics of Solid A fully coupled chemo-mechanical cohesive zone model for oxygen embrittlement of nickel-based superalloys, 2022, 164.
[3] S., del Busto; C. Betegón; E. Martínez-Pañeda Engineering Fracture Mechanics A cohesive zone framework for environmentally assisted fatigue, 2017, 185, 210-226.
[4] L. Benabou; Z. Sun; P.R. Dahoo International Journal of Fatigue A thermo-mechanical cohesive zone model for solder joint lifetime prediction, 2012, 49, 18-30.