Mechanisms governing the distortion of alumina-forming alloys upon cyclic oxidation
-
N. R. Rebollo
,
C. G. Levi
Abstract
The lateral extension of alumina scales as they grow at high temperature can induce substantial deformation of the substrate, especially under thermal cycling. The phenomenon is relevant to the understanding of failure mechanisms in thermal barrier coatings, and is explored here using model FeCrAlY substrates, bare as well as coated with electron beam physically vapor deposited (EB PVD) yttria-stabilized zirconia. Model surface imperfections consisting of shallow grooves experience significant distortions that evolve over time. The effect is most pronounced under cyclic oxidation, absent the thermal barrier coating (TBC). Distortions around the grooves are considerably reduced when the TBC is present.
A procedure for simulating thermally grown oxide (TGO) displacements has been used to predict the groove distortions. Those absent the TBC are shown to closely match those found experimentally for certain choices of TGO growth stress. It is also shown that the presence of an intact TBC entirely suppresses the distortions. Overlaying the simulated and measured TGO profiles absent the TBC reveals that, for consistency, the TGO growth stress should be lower on convex segments of the TGO than on flat segments, by about a factor 2.
Funding statement: Support for this investigation was provided by the Advanced Gas Turbine Research Program of the Department of Energy (Contract No. 01-01-SR093), and by the National Science Foundation (Grant DMR-0099695). Additional support for N. R. R. through a CONACyT scholarship is gratefully acknowledged. The authors would like to thank Dr. S. G. Terry, D. Stave and J. Saleta for technical assistance with the experiments, and Dr. A. Karlsson for useful discussions.
Appendix Approximate model of groove distortion
To gain some appreciation for the window governing groove distortion, the corner region is modeled as a solid cylinder, radius of curvature Rs, with a convex TGO, thickness h. The base is envisaged as a cylindrical channel with radius of curvature R2, and a concave TGO. The stresses in the corner region can be estimated by using elasticity, with misfit caused by either thermal contraction or TGO growth. Equating these stresses to the yield condition then allows derivation of some useful scaling results.
Using the finite element method, under generalized plane strain conditions, the elastic magnitudes of the equivalent stresses in the TGO and the alloy have been determined (Fig. A1). Note that the stress in the alloy is uniform, whereas that in the TGO varies from the interface to the surface. For present purposes, the stress near the interface is relevant.
The equivalent stresses in the TGO present on the surface of an elastic cylinder: (a) stresses caused by a thermal expansion misfit, (b) stresses induced by lateral growth in the TGO.
Upon cooling, the increases in the equivalent stress in the TGO and in the alloy are related by (h/Rs ≤ 0.8):
with
Note that the equivalent stress at the TGO surface is:
During TGO growth, the stresses differ because all of the strain is imposed parallel to the interface. In particular, the equivalent stress in the TGO near the interface is appreciably larger, relative to the stress in the alloy. The stresses are given by (h/R ≤ 0.5):
with
The requirement for groove distortion is that the bond coat attain yield at some stage during the thermal cycle. For cyclic conditions, bond coat yield is most likely to be realized during cooling. The most stringent condition arises when the TGO is at yield in compression at the temperature maximum. Then, the alloy near the corner is assured to yield on cooling when the ratio of bond coat to TGO yield strength satisfies:
Rearranging to correlate with the simulations indicates that the TGO strength must exceed:
For representative material properties (Table 1) and TGO thickness, the right-hand side of Eq. (A4) is negative for low yield strength alloys. Accordingly, distortions are expected even at low
Summary of material properties: E = Young’s modulus; v = Poisson’s ratio; σ = yield strength.
| Material | E (GPa) | ν | σ (MPa) |
|---|---|---|---|
| TGO | 400 | 0.2 | 200–1000 |
| Bond coat | 200 | 0.3 | 10–100 |
The existence of a difference between groove distortion upon isothermal and cyclic conditions requires that the TGO attain yield during growth. For the relatively soft alloys of present relevance, which yield during growth (as revealed in the simulations), the condition that the TGO reach yield becomes:
This condition places strict bounds on conditions that enable a difference between isothermal and cyclic distortions. The simulations conducted for Rs/h = 3.3 reveal this effect (Fig. 8b) and exhibit a difference when
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© 2003 Carl Hanser Verlag, München
Artikel in diesem Heft
- Frontmatter
- Editorial
- Editorial
- Articles/Aufsätze
- The role of oxidation-induced cavities on the failure of the thermally grown oxide on binary β-NiAl alloys
- Phase stability of Y + Gd co-doped zirconia
- Mechanisms governing the distortion of alumina-forming alloys upon cyclic oxidation
- High-temperature oxidation of FeCrAl alloys: the effect of Mg incorporation into the alumina scale
- Nonlinear dielectric properties at oxide grain boundaries
- TEM observations of singular grain boundaries and their roughening transition in TiO2-excess BaTiO3
- Processing of dense MgO substrates for high-temperature superconductors
- Microwave-induced crystallization of polysilazane-derived silicon carbonitride
- Schottky barrier formation in liquid-phase-sintered silicon carbide
- SrTiO3: a model electroceramic
- Optical properties and electronic structure of oxidized and reduced single-crystal strontium titanate
- Spreading of liquid Ag and Ag–Mo alloys on molybdenum substrates
- Nanoalloying in mixed AgmAun nanowires
- Never ending saga of a simple boundary
- Comparison of interfacial chemistry at Cu/α-alumina and Cu/γ-alumina interfaces
- Microstructure of Cu2O/Si interfaces, made by epitaxial electrodeposition
- Metal/oxide interfaces and their reaction with hydrogen
- Amorphous films at metal/ceramic interfaces
- Some thoughts on source monochromation and the implications for electron energy loss spectroscopy
- Determination of the contrast transfer function by analysing diffractograms of thin amorphous foils
- Progress in the preparation of cross-sectional TEM specimens by ion-beam thinning
- Quantification of interfacial segregation by analytical electron microscopy
- Quantification of elemental segregation to lath and grain boundaries in low-alloy steel by STEM X-ray mapping combined with the ζ-factor method
- Microstructure of Al/Ti metallization layers
- Connectivity of CSL grain boundaries and the role of deviations from exact coincidence
- Effect of laser shock processing on the microstructure and mechanical properties of pure Cu
- Growth and microstructure of iron nitride layers and pore formation in ε-Fe3N
- Phase diagram of the Al–Cu–Fe quasicrystal-forming alloy system
- Notifications/Mitteilungen
- Personal/Personelles
- Gesellschaftsnachricht
- International Conferences
Artikel in diesem Heft
- Frontmatter
- Editorial
- Editorial
- Articles/Aufsätze
- The role of oxidation-induced cavities on the failure of the thermally grown oxide on binary β-NiAl alloys
- Phase stability of Y + Gd co-doped zirconia
- Mechanisms governing the distortion of alumina-forming alloys upon cyclic oxidation
- High-temperature oxidation of FeCrAl alloys: the effect of Mg incorporation into the alumina scale
- Nonlinear dielectric properties at oxide grain boundaries
- TEM observations of singular grain boundaries and their roughening transition in TiO2-excess BaTiO3
- Processing of dense MgO substrates for high-temperature superconductors
- Microwave-induced crystallization of polysilazane-derived silicon carbonitride
- Schottky barrier formation in liquid-phase-sintered silicon carbide
- SrTiO3: a model electroceramic
- Optical properties and electronic structure of oxidized and reduced single-crystal strontium titanate
- Spreading of liquid Ag and Ag–Mo alloys on molybdenum substrates
- Nanoalloying in mixed AgmAun nanowires
- Never ending saga of a simple boundary
- Comparison of interfacial chemistry at Cu/α-alumina and Cu/γ-alumina interfaces
- Microstructure of Cu2O/Si interfaces, made by epitaxial electrodeposition
- Metal/oxide interfaces and their reaction with hydrogen
- Amorphous films at metal/ceramic interfaces
- Some thoughts on source monochromation and the implications for electron energy loss spectroscopy
- Determination of the contrast transfer function by analysing diffractograms of thin amorphous foils
- Progress in the preparation of cross-sectional TEM specimens by ion-beam thinning
- Quantification of interfacial segregation by analytical electron microscopy
- Quantification of elemental segregation to lath and grain boundaries in low-alloy steel by STEM X-ray mapping combined with the ζ-factor method
- Microstructure of Al/Ti metallization layers
- Connectivity of CSL grain boundaries and the role of deviations from exact coincidence
- Effect of laser shock processing on the microstructure and mechanical properties of pure Cu
- Growth and microstructure of iron nitride layers and pore formation in ε-Fe3N
- Phase diagram of the Al–Cu–Fe quasicrystal-forming alloy system
- Notifications/Mitteilungen
- Personal/Personelles
- Gesellschaftsnachricht
- International Conferences
