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Dive into the research topics where Alexander Epishin is active.

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Featured researches published by Alexander Epishin.


Philosophical Magazine | 2004

Mechanisms of high-temperature creep of nickel-based superalloys under low applied stresses

Alexander Epishin; Thomas Link

[001] single-crystal specimens of the superalloys CMSX-4 and CMSX-10 were tested for creep at 1100°C under tensile stresses between 105 and 135 MPa, where they show pronounced steady creep. The deformed superalloys were analysed by density measurements, scanning electron microscopy and transmission electron microscopy which supplied information about porosity growth, evolution of the γ–γ′ microstructure, dislocation mobility and reactions during creep deformation. It is shown that, under the testing conditions used, steady creep strain mostly results from transverse glide–climb of (a/2) ⟨011⟩ interfacial dislocations. A by-product of the interfacial glide–climb are vacancies which diffuse along the interfaces to growing pores or to a ⟨100⟩ edge dislocations climbing in the γ′ phase. Climb of a ⟨100⟩ dislocations in the γ′ phase is a recovery mechanism which reduces the constraining of the γ phase by the γ′ phase, thus enabling further glide of (a/2) ⟨011⟩ dislocations in the matrix. Moreover the γ′ dislocations act as vacancy sinks facilitating interfacial glide–climb. The creep rate increases when the γ–γ′ microstructure becomes topologically inverted; connection of the γ′ rafts results in extensive transverse climb and an increase of the number of a⟨100⟩ dislocation segments in the γ′ phase.


Acta Materialia | 2001

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

Alexander Epishin; Thomas Link; U. Brückner; Pedro Dolabella Portella

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.


Acta Materialia | 2000

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

Thomas Link; Alexander Epishin; U. Brückner; Pedro Dolabella Portella

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.


Acta Materialia | 2000

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

Alexander Epishin; Thomas Link; Pedro Dolabella Portella; U. Brückner

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.


International Journal of Materials Research | 2013

Mechanism of porosity growth during homogenisation in single crystal nickel-based superalloys

Alexander Epishin; Thomas Link; Igor L. Svetlov; Gert Nolze; Romeo Saliwan Neumann; Henning Lucas

Several mechanisms for porosity growth in single crystal nickel-based superalloys during homogenisation heat treatment have been proposed in the literature. They were carefully checked using different experimental methods, namely quantitative light microscopy, scanning electron microscopy, transmission electron microscopy, X-ray diffraction and density measurements. It is shown that the main mechanism is the Kirkendall–Frenkel effect, i.e. generation of voids due to uncompensated efflux of Al atoms from dissolving γ/γ′-eutectic areas. The Al diffusion is supported by the afflux of vacancies from surrounding γ-matrix which results in porosity growth. This conclusion is confirmed by the estimation of the vacancy afflux towards the dissolving eutectic.


Materials Science Forum | 2005

Dendritic Stresses in Nickel-Base Superalloys

U. Brückner; Alexander Epishin; Thomas Link; Bernard Fedelich; Pedro Dolabella Portella

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).


International Journal of Materials Research | 2010

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

Alexander Epishin; U. Brückner; Thomas Link; Bernard Fedelich

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.


International Journal of Materials Research | 2011

Synchrotron measurement of the 3D shape of X-ray reflections from the γ/γ′-microstructure of nickel-base superalloys

Alexander Epishin; Thomas Link; Alexander Ulbricht; Ivo Zizak; Mamta Bansal

Abstract The 3D shape of X-ray reflections from the γ/γ′-microstructure of a nickel-base superalloy was investigated using synchrotron X-ray radiation and a position sensitive area detector. The measurements were performed on the 4th generation single-crystal nickel-base superalloy TMS138. The results show that X-ray reflections from non-cubic crystallographic planes have a complex 3D shape which changes during rafting. The 3D intensity distributions contain information about the spacing of the planes and their orientation as well. Whereas h00 reflections show the usual splitting into a γ′ and one γ-subreflection, the hh0 and hhh reflections show two and three γ-peaks respectively, resulting from the different types of {100} matrix channels. Therefore, these 3D diffraction measurements supply additional information about the spatial distribution of microstrains.


Materials Testing-Materials and Components Technology and Application | 2009

Low Cycle Fatigue of the Single-Crystal Nickel-Base Superalloy CMSX-4 — Anistropy and Effect of Creep Damage*

Hellmuth Klingelhöffer; Alexander Epishin; Thomas Link

Abstract Low cycle fatigue of the single-crystal nickel-base superalloy CMSX-4 was investigated in the temperature range 700—950°C under strain controlled cyclic loading. The points of interest were the crystallographic anisotropy of low cycle fatigue behaviour and the effect of creep damage on low cycle fatigue life. For the testing conditions used the low cycle fatigue life of CMSX-4 is strongly orientation dependent as well as very sensitive to the formation of rafted γ/γ’-microstructure.


Materials Science and Engineering A-structural Materials Properties Microstructure and Processing | 2005

〈1 0 0〉 Dislocations in nickel-base superalloys: Formation and role in creep deformation

Thomas Link; Alexander Epishin; M. Klaus; U. Brückner; A. Reznicek

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Thomas Link

Technical University of Berlin

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U. Brückner

Bundesanstalt für Materialforschung und -prüfung

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Pedro Dolabella Portella

Bundesanstalt für Materialforschung und -prüfung

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Bernard Fedelich

Bundesanstalt für Materialforschung und -prüfung

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Gert Nolze

Bundesanstalt für Materialforschung und -prüfung

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Hellmuth Klingelhöffer

Bundesanstalt für Materialforschung und -prüfung

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Ivo Zizak

Helmholtz-Zentrum Berlin

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