Pedro Dolabella Portella

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.

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.

Biaxial path-dependence of low-cycle fatigue behaviour and microstructure of alloy 800 H at room temperature

Low-cycle fatigue experiments under combined axial-torsional loading have been carried out on alloy 800 H tubular specimens at room temperature. In comparison with proportional loading, an extra cyclic hardening effect produced by nonproportional loading was observed. The microstructure study highlights the fact that the dislocation arrangement under proportional loading is significantly different from that under nonproportional loading. Mechanical twinning was found in specimens cycled under axial loading and nonproportional loading. It is suggested that mechanical twinning depends not only on shear stress but also on normal stress on the plane of maximum shearing. The extra hardening can be interpreted in terms of the deformation microstructure. Fatigue cracking was initiated generally at the specimen surfaces along the plane of maximum shearing, but under nonproportional loading cracks were found also in the bulk of the specimens. Transcrystalline crack propagation was observed in the specimens after proportional and nonproportional LCF tests.

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).

Investigation of microstructure and properties of a chromium-rich austenitic material with high nitrogen content

Alloy 33 is a recently developed Cr-Fe-Ni alloy with nominally 33 wt.% Cr, 32 wt.% Fe, 31 wt.% Ni which exhibits a fully austenitie structure due to the nitrogen content of 0.4 wt.%. In this work we describe the microstructure of Alloy 33 samples aged at 700 °C and 900 °C for 100 h as characterized by electron microscopy (SEM, EPMA, TEM) and several microanalytical methods (EDS, WDS, EELS). Aging at 900 °C leads solely to the precipitation of σ-phase at the grain boundaries. The microstructural evolution at 700 °C is much more complex, the main features being the precipitation of intermetallic phases at the grain boundaries and the discontinuous precipitation of Cr-rich phases at migrating boundaries starting at grain boundaries. Previous results concerning the stable corrosion resistance of this new alloy in boiling azeotropic nitric acid as well as the changes in the mechanical properties after long term aging are discussed in view of the observed microstructures. Finally, a first assessment of the microstructure of welded joints is given.

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.