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- Creep (5)
- Superalloys (5)
- CMSX-4 (2)
- Dendritic structure (2)
- Misfit (2)
- Nickel alloys (2)
- Nickel-base superalloys (2)
- Rafting (2)
- Residual stresses (2)
- Segregation (2)
The influence of small rhenium additions on the lattice spacing of a nickel solid solution was investigated using a nickelrhenium single crystal with an axial macrosegregation of rhenium. The effect of rhenium on the lattice spacing of the nickel solid solution was found to be similar to that of tungsten.
A new experimental technique (repeated load annealing of flat wedge shaped specimens) was proposed for characterization of microstructural degradation under creep conditions. This technique was applied to investigate the microstructural degradation of the nickel-base superalloy CMSX-4 in a wide range of temperatures and stress levels. The results obtained allowed to describe analytically the kinetics of rafting, which is important to predict the reduction of fatigue lifetime and yield stress.
α(1 0 0) Dislocations are observed in nickel-base superalloys after creep deformation at high temperatures and low stresses. They are formed in the γ/γ´ interfaces by a three step mechanism. First, primary dislocations with Burgers vector α/2(1 0 1) and 60° character are left behind in the interfaces when dislocation loops glide through the γ channels. The 60° dislocations move into edge orientation and react forming secondary dislocations α/2(1 1 0). In the third step, new primary dislocations are knit into the already existing meshes of primary and secondary dislocations, which results in hexagonal misfit dislocation networks consisting of secondary dislocations α/2(1 1 0) and tertiary dislocations α(1 0 0), both of edge type. This self-organisation process is explained by dislocation theory and crystallography.
Creep deformation accelerates when the α(1 0 0) interfacial dislocations enter the γ´ phase. During their climb towards the opposite interface, they attain a characteristic rectangular shape. This shape and the importance of the α(1 0 0) super dislocations for creep deformation are discussed.
Changes in the structure of single-crystal (SC) superalloy SRR99 during creep at 980 °C and 200 MPa were investigated. Geometrical parameters of the y/y'-microstructure, y/y'-misfit, stresses in the phases and local misorientation of the structure were determined at defined points of the dendritic structure by use of X-ray diffraction (XRD), wavelength dispersive X-ray spectroscopy (WDXS), scanning (SEM) and transmission (TEM) electron microscopy. Correlation of the microstructural period and local misorientation of the structure with creep strain were found.