High Temperature Oxidation Behavior of Gamma-nickel+gamma'-nickel Aluminum Alloys and Coatings Modified with Platinum and Reactive Elements

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ProQuest, 2007 - 188 pages
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Materials for high-pressure turbine blades must be able to operate in the high-temperature gases (above 1000C) emerging from the combustion chamber. Accordingly, the development of nickel-based superalloys has been constantly motivated by the need to have improved engine efficiency, reliability and service lifetime under the harsh conditions imposed by the turbine environment. However, the melting point of nickel (1455C) provides a natural ceiling for the temperature capability of nickel-based superalloys. Thus, surface-engineered turbine components with modified diffusion coatings and overlay coatings are used. Theses coatings are capable of forming a compact and adherent oxide scale, which greatly impedes the further transport of reactants between the high-temperature gases and the underlying metal and thus reducing attack by the atmosphere. Typically, these coatings contain beta-NiAl as a principal constituent phase in order to have sufficient aluminum content to form an Al2O3 scale at elevated temperatures. The drawbacks to the currently-used beta-based coatings, such as phase instabilities, associated stresses induced by such phase instabilities, and extensive coating/substrate interdiffusion, are major motivations in this study to seek next-generation coatings.
  

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Contents

INTRODUCTION
1
Ellingham diagram showing the standard free energy as a function of temperature for selected oxides 40
8
Logarithmic and inverse logarithmic oxidation 43
10
Schematic representation of scale formation according with Wagners model 41
12
A schematic representation of oxidation modes of alloy AB of variable composition where B is the less noble metal
18
A schematic showing the early stages of simultaneous formation of immiscible oxides AO and BO 54
20
Dependence of the mechanism of oxidation for NiAl alloys on temperature and alloy composition according to Pettit 59
23
A schematic showing the L12 crystal structure of γNi3Al
24
Cross section of an asdeposited nickel aluminide coatings on CMSX4
33
Crosssectional SEM image showing the surface rumpling of Ptmodified βNiAl coating after 1000 cycles at 1150C
35
SEM image showing the formation of secondary reaction zone
36
EXPERIMENTAL PROCEDURES
37
Schematic showing the automated furnace used for the earlystage oxidation tests
39
RESULTS
43
A section of the equilibrium NiAlPt ternary phase diagram at 1150C with the alloy compositions superimposed
44
The earlystage anisothermal oxidation kinetics of the γNi alloys affected by Pt additions
46

Isothermal phase equilibria on NiPtAl alloys at 1150C 38
25
The weight change of Ni22Al30Pt and Ni22Al30Pt0 8Hf in at alloys at 1150C after 500 cycles
27
Stability relationships of oxides arranged in periodic table 69
29
Cyclic oxidation results for the Ren N5 superalloy with different treatments at 1149C in air 74
32
SEM images showing the surface morphology of scales formed on Ni 12 5AlxPt x0 10 and 20 alloys
47
GDOES composition profiles from the surface of oxidized Ni12 5Al after 90 180 and 1800 seconds
49
DISCUSSION
131
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