Fraunhofer Institute for Mechanics of Materials IWMHigh-temperature components such as turbine discs in aircraft engines and gas turbines can benefit from enhanced reliability through prevention of brittle intergranular cracking. In this project, researchers developed a cross-scale modeling approach that combines atomistic simulations (density functional theory DFT and molecular dynamics MD), finite element modeling and diffusion calculations to predict oxygen-driven grain boundary embrittlement in the nickel-based superalloy Alloy 718. By linking microstructural information with grain boundary cohesion properties, the project created a novel tool for the targeted optimization of materials and processes. Industry partners benefit from more precise service life predictions and a deeper understanding of damage mechanisms.
Cross-scale, microstructure-dependent evaluation of intergranular cracking during high-temperature fatigue with dwell times in polycrystalline superalloys
Completed research project
Project description
High-temperature components in aircraft engines, stationary gas turbines and, increasingly, in additive manufacturing experience extreme thermomechanical stresses. The polycrystalline nickel-based superalloy Alloy 718 is one of the most widely used materials for such applications; however, so-called dynamic embrittlement (DE) - time-dependent brittle intergranular cracking caused by oxygen diffusion along grain boundaries - limits its operating temperature to approximately 700 degrees Celsius. At the same time, there is enormous market demand to increase operating temperatures and service life in order to improve resource and energy efficiency.
Existing service life models can be improved by incorporating the dependence of dynamic embrittlement on the individual grain boundary structure (orientation, character, chemical composition). In this project, researchers have developed a cross-scale modeling approach that couples atomistic simulations (density functional theory (DFT) and molecular dynamics) for determining grain boundary cohesion properties with microstructure-based finite element models (crystal plasticity, cohesive zone elements) and finite-difference calculations for stress-assisted oxygen diffusion. This multiscale approach makes it possible to quantitatively predict the influence of grain boundary characteristics - including tilt and rotation angles - on intergranular crack propagation.
The project combines experimental validation (bicrystal bending tests, dwell-time fatigue tests, three-dimensional electron backscatter diffraction (EBSD) and atom probe tomography (APT)) with numerical modeling, thereby laying the foundation for microstructure-based service life assessment, which will ultimately also enable targeted materials optimization through thermomechanical process control (grain boundary engineering). This development offers opportunities for industry to establish more well-founded safety margins, optimize maintenance intervals and specifically tailor materials development to higher operating temperatures.
Fraunhofer IWM Work Packages:
The Fraunhofer IWM has developed structural models for representative tilting and rotating grain boundaries in face-centered cubic (fcc) nickel and determined their cohesive properties using density functional theory (DFT). To this end, virtual tensile tests were conducted and traction-separation laws were derived using the Universal Binding Energy Relation (UBER). The influence of alloying elements (Fe, Cr, Co, etc.) as well as oxygen as an embrittling species on grain boundary strength as a function of structure and chemical composition was systematically investigated.
The DFT results were used to validate and calibrate interatomic potentials, which were subsequently employed in molecular dynamics (MD) simulations. MD simulations extend beyond the scope of DFT: they enable the investigation of fracture behavior at finite temperatures, for arbitrary grain boundary orientations and for realistic alloy compositions. In this way, angle-dependent parameters for tensile and shear fracture are determined and the influence of temperature and oxygen content is systematically characterized.
Based on this project work, Fraunhofer IWM can offer industrial companies the following new R&D services:
- Atomistically Informed Grain Boundary Analysis
Quantitative determination of the cohesive properties (strength, separation work) of grain boundaries in nickel-based superalloys and other high-temperature materials using density functional theory (DFT) and molecular dynamics (MD) as a basis for informed materials selection and optimization.
- Evaluation of the embrittlement resistance associated with oxygen and other species
Simulation of the influence of surrounding atoms (O, S, H) on interfacial strength as a function of grain boundary type and temperature.
- Parameterization of cohesive zone models from atomistic data
Provision of calibrated traction-separation laws for grain boundary types that can be directly applied in finite-element life-cycle calculations.
- Custom interatomic potentials
Development and validation of potentials for alloy systems (e.g., Ni-Cr-Fe-Nb-O) that enable molecular dynamics (MD) simulations at realistic temperatures and compositions.
- Consulting on Grain Boundary Engineering
Based on the obtained structure-property relationships, recommendations can be provided for thermomechanical process control to specifically increase the proportion of tough grain boundaries.
