In the present work, we use a miniature test procedure to investigate the tensile creep behaviour... more In the present work, we use a miniature test procedure to investigate the tensile creep behaviour of the single crystal superalloy ERBO1. We test precisely oriented [0 0 1], [1 1 0] and [1 1 1] creep specimens and determine the stress and the temperature dependence of characteristic creep rates in limited stress and temperature regimes, where the stress and temperature dependence of characteristic creep rates can be well described by power law and Arrhenius type of relations, with stress exponents n and apparent activation energies Qapp. n-values increase with stress and decrease with temperature. Qapp-values, on the other hand, increase with increasing temperature and decrease with increasing stress. Creep curve shapes gradually evolve from the high temperature low stress to the low temperature high stress (LTHS) regime. This implies that there is a gradual change in elementary deformation and softening mechanisms, which is qualitatively confirmed using transmission electron microscopy. While at high temperatures different loading directions only have a moderate influence on creep, there is a very strong effect of loading direction at low temperatures. The [1 1 0] tests show the fastest deformation rates and the shortest rupture times. In the LTHS creep regime, we confirm the double minimum (DM) type of creep behaviour, which was previously reported but never explained. Further work is required to rationalise DM-creep. The implications of this type of creep behaviour on scatter and on extrapolation of creep data is discussed in the light of previous results published in the literature.
High‐resolution characterization techniques are combined with thermodynamic calculations (CALPHAD... more High‐resolution characterization techniques are combined with thermodynamic calculations (CALPHAD) to rationalize microstructural features of single crystal Ni‐base superalloys. Considering the chemical compositions of dendritic and interdendritic regions one can explain differences in γ′‐volume fractions. Using thermodynamic calculations one can explain, why γ‐nanoparticles are observed in the central regions of large cuboidal γ′‐particles and why tertiary γ′‐nanoparticles form in the γ‐channels. The chemical compositions of the γ‐channels and of the newly formed γ‐particles differ because of the Gibbs–Thomson pressure which acts on the small particles. With increasing size of secondary γ′‐particles, their shape changes from spherical to cuboidal. Some general thermodynamic aspects including the temperature dependencies of the Gibbs free energy G, the enthalpy H, and the entropy S and site occupancies in the ordered L12 (γ′) phase are considered. The importance of cooling rate afte...
ABSTRACT In the present work we study the formation of grooves and ledges (typical size: <... more ABSTRACT In the present work we study the formation of grooves and ledges (typical size: < 100nm) at γ/γ’ interfaces of single crystal Ni-base superalloys (SX). We highlight previous work which documents the presence of such interface irregularities and shows that their number and size increases during high temperature exposure and creep. We use diffraction contrast stereo transmission electron microscopy (TEM) to provide new evidence for the presence of ledges and grooves near dislocations at γ/γ’ interfaces after heat treatment and creep. We present a 2D model of the interfacial region which shows how dislocation stress fields alter local chemical potentials and drive diffusional fluxes which result in the formation of a groove. The results of the numerical study yield realistic groove sizes in relevant time scales. The results obtained in the present study suggest that the formation of grooves and ledges represents an elementary process which needs to be considered when rationalizing the kinetics of rafting, the directional coarsening of the γ᾽ phase.
In the present work, we use a miniature test procedure to investigate the tensile creep behaviour... more In the present work, we use a miniature test procedure to investigate the tensile creep behaviour of the single crystal superalloy ERBO1. We test precisely oriented [0 0 1], [1 1 0] and [1 1 1] creep specimens and determine the stress and the temperature dependence of characteristic creep rates in limited stress and temperature regimes, where the stress and temperature dependence of characteristic creep rates can be well described by power law and Arrhenius type of relations, with stress exponents n and apparent activation energies Qapp. n-values increase with stress and decrease with temperature. Qapp-values, on the other hand, increase with increasing temperature and decrease with increasing stress. Creep curve shapes gradually evolve from the high temperature low stress to the low temperature high stress (LTHS) regime. This implies that there is a gradual change in elementary deformation and softening mechanisms, which is qualitatively confirmed using transmission electron microscopy. While at high temperatures different loading directions only have a moderate influence on creep, there is a very strong effect of loading direction at low temperatures. The [1 1 0] tests show the fastest deformation rates and the shortest rupture times. In the LTHS creep regime, we confirm the double minimum (DM) type of creep behaviour, which was previously reported but never explained. Further work is required to rationalise DM-creep. The implications of this type of creep behaviour on scatter and on extrapolation of creep data is discussed in the light of previous results published in the literature.
High‐resolution characterization techniques are combined with thermodynamic calculations (CALPHAD... more High‐resolution characterization techniques are combined with thermodynamic calculations (CALPHAD) to rationalize microstructural features of single crystal Ni‐base superalloys. Considering the chemical compositions of dendritic and interdendritic regions one can explain differences in γ′‐volume fractions. Using thermodynamic calculations one can explain, why γ‐nanoparticles are observed in the central regions of large cuboidal γ′‐particles and why tertiary γ′‐nanoparticles form in the γ‐channels. The chemical compositions of the γ‐channels and of the newly formed γ‐particles differ because of the Gibbs–Thomson pressure which acts on the small particles. With increasing size of secondary γ′‐particles, their shape changes from spherical to cuboidal. Some general thermodynamic aspects including the temperature dependencies of the Gibbs free energy G, the enthalpy H, and the entropy S and site occupancies in the ordered L12 (γ′) phase are considered. The importance of cooling rate afte...
ABSTRACT In the present work we study the formation of grooves and ledges (typical size: <... more ABSTRACT In the present work we study the formation of grooves and ledges (typical size: < 100nm) at γ/γ’ interfaces of single crystal Ni-base superalloys (SX). We highlight previous work which documents the presence of such interface irregularities and shows that their number and size increases during high temperature exposure and creep. We use diffraction contrast stereo transmission electron microscopy (TEM) to provide new evidence for the presence of ledges and grooves near dislocations at γ/γ’ interfaces after heat treatment and creep. We present a 2D model of the interfacial region which shows how dislocation stress fields alter local chemical potentials and drive diffusional fluxes which result in the formation of a groove. The results of the numerical study yield realistic groove sizes in relevant time scales. The results obtained in the present study suggest that the formation of grooves and ledges represents an elementary process which needs to be considered when rationalizing the kinetics of rafting, the directional coarsening of the γ᾽ phase.
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Papers by Alireza Parsa