Creep, microstructure evolution and crack growth in precipitation hardened microstructures
The project aims on investigations of the mechanical anisotropy and microstructural evolution during high temperature / low stress creep of a single crystalline Ni-based superalloy. Tests are performed in different crystallographic directions and interrupted after specific strains and times. Scanning electron microscopy (SEM) is used to quantify the microstructural evolution during creep.
A technique was developed to investigate interfacial dislocations by means of SEM. The development of the interfacial dislocation networks was studied for  and  loading directions. Mechanical behavior is correlated to microstructural changes to gain a deeper understanding of the deformation processes.
Fracture and failure of single crystalline Ni-based superalloys are studied by means of single etch notch tension samples and CT-like specimens. The alloy shows a high resistance for the formation of cracks. Hence crack growth seems to be very fast, and there seems to be no influence of the crystallographic directions.
Coarsening of precipitates of binary Ni-Al with and without external stresses has been simulated by the phase field method (see Figure). For the first simulation the material was treated elastically. Rafting in this case occurs by diffusion solely. The direction of rafting dependent on the materials parameter calculated by the phase field method is in good agreement with the theoretical predictions. In order to include creep in simulations the functionality of plastic deformation is being developed.
Rafting of Ni-Superalloy LEK 94: axial stress is applied in the horizontal direction. Long chanels of precipitates and matrix develop perpendicular to the loading direction.
Scanning Transmission Microscopy (STEM) The white lines are dislocations that populate the matrix of the material. The particles have fewer dislocations.
γ/γ’-interface of a single crystalline Ni-based superalloys showing bright lines which represent dislocation traces (SEM micrograph).
Concentration field of a γ+γ'/γ+γ' diffusion couple annealed at 1273 K.The left couple is a γ+γ' alloy with 60 vol.% γ' phase, while the right one is a γ+γ' alloy with 40 vol.% γ' phase
Creep curves of the single crystalline Ni-based superalloy LEK94 along different crystallographic directions.
Corresponding SEM micrographs of dislocation traces (bright lines) in crept samples. Creep tests were conducted at 1020°C and 160 MPa along left: , middle:  10° off to  and right:  10° of .