U. Brückner

Kinetics of the topological inversion of the γ/γ'-microstructure during creep of a nickel-based superalloy

In undeformed superalloys, the disordered γ-solid solution of nickel is hardened by coherently embedded small cuboids of the ordered γ-phase (Ni3Al). During high-temperature creep the γ-phase coalesces, coarsens and finally surrounds the γ-phase, i.e., it becomes topologically the matrix. The kinetics of this so-called topological inversion during creep of the superalloy SRR99 at 980°C and 200 MPa has been investigated quantitatively by analysis of scanning electron microscope images. The topological state of the γ/γ-microstructure was characterized by the parameter R: the ratio of area densities of transverse terminations of γ- and γ-lamellae. The topological inversion is explained by the formation of junctions connecting neigh- bouring γ-rafts and separating the γ-phase. One reason is the generation of dislocations in the γ-channels during primary creep. Another reason is the dissolution of γ-edges in the γ-phase, which is more diffusionally penetrative. The released γ-forming atoms move along the interface towards dislocation concentrations, resulting in the formation of junctions between the γ-rafts.  2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

Increase of misfit during creep of superalloys and its correlation with deformation

In superalloys the loss of coherency during creep results in the increase of misfit of the {gamma}/{gamma}{prime}-interface. The kinetics of this process were measured locally by TEM (Moire fringes) and X-ray diffraction. Two materials were creep tested (SRR99 and CMSX-4) in two temperature ranges (stable {gamma}{prime}-morphology and rafting), and the morphology changes were quantified. A microstructural model allows calculation of the equilibrium misfit and the increase of plastic strain on the basis of these data. At high temperatures and low stresses the model describes quantitatively creep kinetics up to 30 h. Here the processes controlling primary creep are propagation of dislocation loops along matrix channels and thickening of the matrix channels oriented perpendicular to the load direction.

The influence of the dendritic structure on the γ/γ′-lattice misfit in the single-crystal nickel-base superalloy CMSX-4

Abstract The γ / γ ′ - lattice misfit was measured locally by X-ray diffraction in the rhenium-containing single-crystal nickel-base superalloy CMSX-4. Different sections of a single crystal casting and positions in the dendritic structure have been analyzed. A remarkable difference of the misfit caused by segregation of refractory elements was found between the dendrite center and periphery. Problems of measurement and characterization of the γ / γ ′-lattice misfit in superalloys of new generations with a high refractory element level are discussed.

Evolution of the γ/γ′ microstructure during high-temperature creep of a nickel-base superalloy

Abstract The evolution of the γ/γ′ microstructure of the superalloy SRR99 during creep at 980°C and 200 MPa has been characterised by Fourier analysis of scanning electron microscope images. Different kinetics of this process were found in the primary and secondary dendrite arms. Changes of the structure period and interface tilt are correlated with the accumulated creep strain. Possible mechanisms for the correlation of microstructural evolution and high-temperature creep deformation are discussed.

Influence of small rhenium additions on the lattice spacing of nickel solid solution

Abstract The influence of small rhenium additions on the lattice spacing of a nickel solid solution was investigated using a nickel–rhenium 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.

Moiré fringes for misfit measurements at incoherent γ/γ′-interfaces of nickel-base superalloys

In standard heat treated superalloys the cuboidal {gamma}{prime}-precipitates are coherently embedded in the {gamma}-matrix. The relative difference in lattice-spacings, the misfit, is compensated elastically: lattice-planes perpendicular to the coherent {gamma}/{gamma}{prime}-interface show zero misfit. During high temperature creep or long-term annealing coherency is lost: applied and misfit stresses press dislocation loops into the matrix channels between the {gamma}{prime}-cubes, leading to the formation of dislocation networks in the interfaces. Misfit measurements on such incoherent interfaces are interesting, because in long-term annealed specimens they give information about the unconstrained misfit, in deformed specimen about the internal stresses during deformation. In 1997, it was shown that the misfit in undeformed superalloys is not homogeneously distributed. Its absolute value is the highest in the primary dendrite arm (PDA) and the lowest in the interdendritic region (IR). This has to be considered when analyzing the misfit in deformed material. With the present work the authors introduce misfit analysis by Moire-fringes in incoherent interfaces as a simple method with high spatial resolution. It is applied to measure locally misfit changes during high temperature creep. The results are compared with those obtained by X-ray diffraction, transmission electron microscopy (TEM) analysis of misfit dislocation networks and theoretical considerations.

Dendritic Stresses in Nickel-Base Superalloys

Nickel-base superalloys are used as blade material for gas turbines. They are solidified by dendritic growth, which results in the segregation of the alloying elements, i.e. in differences in the chemical composition between the dendrite arms (DAs) and the interdendritic regions (IRs). Because the segregation of the slowly diffusing refractory elements rhenium, tungsten, tantalum and molybdenum can not be fully removed within an acceptable homogenization time, superalloys are used with a significant residual segregation. This chemical inhomogeneity influences the structural stability and consequently the mechanical behavior of superalloys. One effect of the segregation are residual stresses within the dendritic cell which arise due to the different thermal contraction of DAs and IRs during cooling. The occurrence of these stresses in heat treated CMSX-4 has been proved by independent methods: scanning electron microscopy (SEM) of the microstructure, dilatometric analysis, finite element (FE) modeling and X-ray diffraction (XRD).

X-ray reflections from the γ/γ′-microstructure of nickel-base superalloys: effect of the plane tilting

Abstract The shape of X-ray reflections from the cuboidal /′-microstructure was investigated. The measurements were performed on the 4th generation single-crystal nickel-base superalloy TMS138. It is shown that reflections from non-cubic crystallographic planes split not only in d-scale due to the different spacing of the - and ′-lattices but also in the pole figure due to the tilting of -lattice planes. This tilting results from the elastic distortion of the -lattice caused by the /′-misfit. The results obtained are discussed under the methodical aspect of misfit measurement.

Rupture Life Time Prediction and Deformation Mechanisms during Creep of Single-Crystal Nickel-Base Superalloys

Mechanisms of creep deformation of single-crystal superalloys at low and high temperature are considered. Rupture life time of a single-crystal superalloy during creep is described in a wide temperature-stress range as a function of temperature and stress taking into account a change of creep mechanism with rising temperature. This phenomenological description is confirmed by creep tests. Anisotropy of creep behavior of single-crystal superalloys is discussed with respect to a choice of the optimal crystallographic direction for solidification of the turbine blades.