Ni-base superalloys

The use of Ni-base superalloys as turbine blades in an actual end-use atmosphere produces deterioration of material properties. This deterioration can result from erosion or corrosion. Erosion results from hard particles impinging on the turbine blade and removing material from the blade surface. The particles may enter through the turbine inlet or can be loosened scale deposits from within the combustor. 

Fig. 2.42. Energy-filtered TEM (three-window method), imaging ofthe Cr distribution in the Ni-base superalloy SCI 6.

Nickel (Ni)/Copper (Cu)/Zinc (Zn) Nickel exhibits a mixture of ferrous and nonferrous metal properties, and Ni-based alloys are characterized by corrosion resistance. Therefore, Ni has been widely used in stainless steel (about 65% of the Ni consumed in the Western World) and superalloys/nonferrous alloys (12%). Turbine blades, discs and other critical parts of jet engines and land-based combustion turbines are fabricated from superalloys and Ni-based superalloys. The remaining 23% of consumption is applied in alloy steels, rechargeable batteries, catalysts and other chemicals, coinage, foundry products, and plating (USGS, 2006). 

Gas Atomization 50-300 Standard deviation 1.9-2.5 <10-50 at high gas pressures with close-coupled atomizer Solder materials. Precious metals, Cu, Fe, Al, Mg, Co, Ti, Zn, Al-6Cr-3Fe-2Zr alloy. Low-alloy steels. High speed steels. Stainless steels, Specialty alloys, Ni-base superalloys, Alumina, Intermetallics io3-.o5 1-70 Spherical smooth particles. Cleanliness, Rapidly-solidified structures, Acceptable production rates High cost, Low 1 volume, Low energy efficiency (EE), Gas-filled porosity in particles H... 

Main uses of the metal. It is especially used as an alloying element in the steel industry and in the preparation of non-ferrous alloys (V-Ti alloys, Ni-based superalloys, etc.). 

Solution databases now exist for a niunber of the major metallic alloy systems such as steels, Ni- based superalloys and other alloy systems, and highly accurate calculation have been made which even a few years ago would have been considered impossible. The number of substance databases are increasing and the numbers of substances they include is reaching well into the thousands. Substance and solution databases are increasingly being combined to predict complex reactions such as in gas evolution in cast-irons and for oxidation reactions, and it is already possible to consider calculations of extreme complexity such as the reactions which may occur in the burning of coal in a industrial power generator or the distribution of elements in meteorites. 

Unfortunately, the dilute solution model is limited in its applicability to concentrated solutions. This causes problems for alloys such as Ni-based superalloys, high alloy steels, etc., and systems where elements partition strongly to the liquid and where solidification processes involve a high level of segregation. It is also not possible to combine dilute solution databases which have been assessed for different solvents. The solution to this problem is to use models which are applicable over the whole concentration range, some of which are described below. 

Earlier work on systems such as Ni-Al-Cr reported in Sanchez et al. (1984b) used FP methods to obtain information on phases for which there was no experimental information. In the case of Ni-base alloys, the results correctly reproduced the main qualitative features of the 7 — 7 equilibrium but cannot be considered accurate enough to be used for quantitative alloy development. A closely related example is the work of (Enomoto and Harada 1991) who made CVM predictions for order/disorder (7 — 7 ) transformation in Ni-based superalloys utilising Lennard-Jones pair potentials. 

Validation of the database. This is the final part in producing an assessed database and must be undertaken systematically. There are certain critical features such as melting points which are well documented for complex industrial alloys. In steels, volume fractions of austenite and ferrite in duplex stainless steels are also well documented, as are 7 solvus temperatures (7 ) in Ni-based superalloys. These must be well matched and preferably some form of statistics for the accuracy of calculated results should be given. 

Figure 10J9 Comparison between calculated and experimental critical temperatures for Ni-based superalloys (from Saunders 1996c).

Figure 10.45 Comparison between calculated and observed compositions of 7 and 7 in Ni-based superalloys (a) Al, (b) Co, (c) Cr, (d) Mo and (e) W. (at %) (from Saunders I99te)...

Figure 10.49 Calculated mole % phase vs temperature plot for a CMSX-4 Ni-based superalloy.

Figure 10.52 (a) Calculated mole % phase vs temperature plots for an IN625 Ni-based superalloy, (b) Expanded region of Fig. 10.50(a). 

Simulations for Ni-based superalloys have been carried out by Saunders (1995, 1996b), Chen et al. (1994) and Boettinger et al. (1995). Saunders (1995) used a straightforward Scheil simulation to predict chemical partitioning and the formation of interdendritic phases in a modified single crystal U720 alloy containing... 

In terms of cyclic plasticity, Shenoy et al. [139, 140], Wang et al. [137], and McDowell [141] performed hierarchical multiscale modeling of Ni-based superalloys employing internal state variable theory. Fan et al. [142] performed a hierarchical multiscale modeling strategy for three length scales. 

M.M. Shenoy et al Estimating fatigue sensitivity to polycrystalline Ni-base superalloy microstructures using a computational approach. Fatigue Fract Eng. Mat. Struct 30, 889-904 (2007)... 

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