This section represents the overall content of the thesis. Fig. 1.19 shows the chapters and their relationship in the present study. Each chapter of this thesis is separately summarized in this section to convince the reader.
Chapter two examines the creep behavior of three different generations of Ni-based single crystal superalloys with aluminide coating on different surface orientations.
Various superalloys used in this study are aimed to study the alloying element effect and its significance for the enhancement of creep behavior of aluminized Ni-based single crystal superalloys. Ni-based single crystal superalloy with the higher content of rhenium has greatly improved the creep rupture life of Ni-based single crystal superalloy.
Chapter three focuses on the effect of temperature on anisotropic creep response of aluminized Ni-based single crystal superalloy. The aluminized specimens with different
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surface orientations are tested by creep at two different temperatures namely intermediate and high temperatures. The typical creep curves for each condition is unlike. The intermediate temperature shows an extensive primary creep region which has important role for the prolongation of the creep rupture life of Ni-based single crystal superalloy and it is contrary to the high temperature creep. The deformation mechanisms in both conditions are also described.
Chapter four is about the effect of specimen thickness on anisotropic creep behavior of aluminized Ni-based single crystal superalloy. The specimens with two different cross-sections are evaluated by creep in tension. In fact, the thin specimens show the more creep anisotropy than the thick specimens. The most interesting from this study, the creep rupture test is performed with two different stress orientations, that is, [001]
and [011] orientations for further investigation using thin specimen. The plastic anisotropy of each orientation of the specimens is elaborated in this chapter.
Chapter five evaluates the high temperature oxidation characteristics of Ni-based single crystal superalloy without and with aluminide coating. The uncoated and coated specimens with two different surface orientations are thermally oxidized at 1100 °C for a specific period of 500 h. The two different surface orientations are intended to investigate the anisotropy property on oxidation behavior in Ni-based single crystal superalloy. It is found that the anisotropy phenomenon also occurs on oxidation behavior.
Chapter six is the conclusion which discusses the overall results obtained from this thesis and the possible future works in this area.
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Fig. 1.1. A gas turbine engine SGT-600 made by Siemens [1].
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Fig. 1.2. The evolution of Ni-base superalloy over 60 years period [8].
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Fig. 1.3. The microstructures of the cast turbine blades made by these three techniques:
(a) conventional casting, (b) directionally solidified and (c) single crystal [11].
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Fig. 1.4. Two-phase γ/γ' microstructure in fully heat-treated AM3 first generation single crystal superalloy [10].
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Fig. 1.5. Atomic size factors of elements for solid solution formation with nickel [19].
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Table 1.1. Chemical compositions of first generation single crystal Ni-based superalloys (mass%)
Alloy Cr Co Mo W Al Ti Ta Nb Hf
PWA-1480 10 5 - 4 5 1.5 12 - -
AM1 7.8 6.5 2 5.7 5.2 1.1 7.9 - -
AM3 8 5 2.25 5 6 2 3.5 - -
CMSX-2 8 4.6 0.6 8 5.6 1 6 - -
CMSX-3 8 4.6 0.6 8 5.6 1 6 - 0.1
CMSX-6 9.8 5 3 - 4.8 4.7 2 - 0.1
Rene N4 9 8 2 6 3.7 4.2 4 0.5 -
Table 1.2. Chemical compositions of second generation single crystal Ni-based superalloys (mass%)
Alloy Cr Co Mo Re W Al Ti Ta Nb Hf
CMSX-4 6.5 9 0.6 3 6 5.6 1 6.5 - 0.1
PWA-1484 5 10 2 3 6 5.6 - 8.7 - 0.1
Rene N5 7 8 2 3 5 6.2 - 7 - 0.2
SC180 5 10 2 3 5 5.2 1 8.5 - 0.1
MC2 8 5 2 - 8 5 1.5 6 - -
SMP14 4.8 8.1 1 3.9 7.6 5.4 - 7.2 1.4 -
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Table 1.3. Chemical compositions of third generation single crystal Ni-based superalloys (mass%)
Alloy Cr Co Mo Re W Al Ti Ta Nb Hf
CMSX-10 2 3 0.4 6 5 5.7 0.2 8 0.1 0.03
TMS-75 3 12 2 5 6 6 - 6 - 0.1
Rene N6 4.2 12.5 1.4 5.4 6 5.75 - 7.2 - 0.15
TMS-113 2.89 11.93 1.99 5.96 5.96 6.56 - 5.96 - 0.1
Table 1.4. Chemical composition of fourth generation single crystal Ni-based superalloys (mass%)
Alloy Cr Co Mo Re W Al Ta Hf Ru
TMS-138 3 6 3 5 6 6 6 0.1 2
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Fig. 1.6. Variation of the creep rupture lives of the SC superalloy TMS-75 and TMS-82+, as a function of the amount of γ' volume fraction. The creep rupture life is largest when the γ' fraction is about 70% [23].
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Fig. 1.7. A schematic representation of the typical constant load creep behavior associated with the stages of deformation [26].
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Fig. 1.8. Typical creep response of Ni-base single crystal superalloys at three different temperature regimes: (a) low temperature/high stress, (b) intermediate temperature/intermediate stress, and (c) high temperature/low stress creep conditions.
Strain (%)
Time (h) (c)
Strain (%)
Time (h) (b)
Strain (%)
Time (h) (a)
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Fig. 1.9. A schematic illustration of this metal-scale-gas system [35].
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Fig. 1.10. Oxidation growth curves for linear and parabolic rate laws [37].
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Table 1.5. Characteristics of protective and non-protective oxide layers
Protective (stable) oxide layer Non-protective (unstable) oxide layer Formation of a porous and cracked
oxide film on the metal surface.
Formation of a non-porous film on the metal surface.
Because of porosity, O2 penetrates to the metal surface and reacts with the metal to form more oxide.
O2 can only react with the metal ions through diffusion.
Finally, the entire metal will be consumed.
Growth rate decreases as the oxide scale thickness increases.
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Table 1.6. Rupture life data of MAR-M247 single crystal tested at 774 °C and 774 MPa [41]
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Fig. 1.11. Creep rupture lives shown as a function of orientation for MAR-M247single crystals tested at 774 °C and 724 MPa. The number associated with each orientation is the life of the crystal in hours; the corresponding letter in parentheses is the specimen identification [41].
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Fig. 1.12. Orientation dependence of the stress rupture life (in hours) at 760 °C and 750 MPa of CMSX-2 single crystals, as a function of γ' precipitate size : (a) 0.23 μm, (b) 0.3 μm, and (c) 0.45 μm [42].
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Fig. 1.13. Effect of orientation and γ' size on the high temperature stress rupture life of single crystals: (a) CMSX-2, at1050°C/120 MPa ; (b) alloy 454, at 980°C/200 MPa [42].
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Table 1.7. Comparison of pack cementation and other processes Name of
process Advantages Disadvantages
Pack cementation
1. Good repeatability of manufacturing process.
2. Low equipment cost.
3. Simplicity of process.
1. Limited coating thickness achieved in this process.
2. Deposition rate cannot be controlled.
3. It is not readily applicable to coating of such internal surfaces
4. Frequent pack-particles entrapment in the outer layer of the coating occurs.
Out of pack
1. Improve the coating
uniformity in the internal channels and the blade cooling holes.
2. Ensure better control over the process run.
3. Increase the coating purity without confinement of the powder particles in it.
4. Have no contact between the powder mixtures and the specimens.
1. Deposition rate cannot be controlled.
2. The coating thickness is not uniform between external and internal coatings.
Chemical vapor deposition
1. The capability of producing highly dense coating.
2. Can produce uniform films with good reproducibility and adhesion at reasonably high deposition rates.
3. Deposition rate can be adjusted and controlled readily.
1. Chemical and safety hazards caused by the use of toxic, corrosive,
flammable and/or explosive precursor gases.
2. High production cost.
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Fig. 1.14. The schematic illustration of pack cementation process.
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Fig. 1.15. The schematic illustration of out-of-pack/vapor phase aluminizing process [52].
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Fig. 1.16. The schematic diagram of CVD low activity co-deposition apparatus for aluminum and one or more reactive elements [54].
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Fig. 1.17. Features of TCP phases in aluminized Ni-base single crystal superalloys after 300 h exposure at 1100 °C: (a) TMS-138A, (b) TMS-198 and (c) TMS-199 [57].
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Table 1.8. Chemical compositions of TMS-138A, TMS-198 and TMS-199 [57]
Alloys
Elements (mass%)
Co Cr Mo W Al Ta Hf Re Ru Ni
TMS138A 5.8 3.2 2.8 5.6 5.7 5.6 0.1 5.8 3.6 Bal.
TMS198 5.8 4.8 2.4 5.2 5.8 5.6 0.1 5.6 3.6 Bal.
TMS199 5.8 6.4 2.2 4.6 5.7 5.6 0.1 5.6 3.6 Bal.
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Fig. 1.18. Microstructure of Pt-modified aluminide coating on MC-NG [58].
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Fig. 1.19. Chapter and their relationship in the present study
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