Monocrystalline alloys with controlled partitioning

Abstract

Nickel-based superalloys, for fabrication of monocrystalline turbine components to be used in industrial and aircraft turbine engines, having the following composition (in wt %): 5.6-8.1% Al, 4.1-14.1% Ru, 6.1-9.9% Ta, 3.6-7.5% Re, and the remaining balance Ni. The partitioning of alloying elements can be controlled to achieve a wide range of precipitate shapes and exceptional resistance to degradation under high temperature exposure conditions.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS [0001] This application claims the benefit of U.S. Provisional Application No. 60 / 537,481, filed on Jan. 16, 2004. The disclosure of the above application is incorporated herein by reference.FIELD OF THE INVENTION [0002] The present invention relates to alloys and, more particularly, relates to nickel-based superalloys for the manufacture of monocrystalline structures. BACKGROUND AND SUMMARY OF THE INVENTION [0003] The present invention generally relates to advanced materials for high temperature components in industrial power and aircraft turbines and, specifically, monocrystalline superalloy blades and vanes. To maximize the efficiency of these turbine systems, the operating temperatures of blades and vanes must be maximized to prevent damage and premature failure. By way of background, it should be recognized that premature damage accumulation may occur along grain boundaries when such components are operated near their melting point. Accor...

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Benefits of technology

[0003] The present invention generally relates to advanced materials for high temperature components in industrial power and aircraft turbines and, specifically, monocrystalline superalloy blades and vanes. To maximize the efficiency of these turbine systems, the operating temperatures of blades and vanes must be maximized to prevent damage and premature failure. By way of background, it should be recognized that premature damage accumulation may occur along grain boundaries when such components are operated near their melting point. Accordingly, Bridgman-type processes may be utilized to eliminate boundaries and, thus, permit use of superalloys in monocrystalline form. At high temperatures, monocrystalline blades and vanes undergo degradation due to creep, phase instabilities, or oxidation and, consequently, must be periodically replaced. It is desirable in many cases to minimize these characteristics in order to maximize the useful life and operating properties of turbines.

[0004] The addition of refractory alloying elements, such as rhenium (Re) and tungsten (W), are desirable for improving the maximum temperature capability of these monocrystalline alloys. As a result, the addition of refractory alloying elements serves to strengthen the monocrystal and, thus, delay the onset of creep damage. However, conversely, high levels of refractory alloying elements may lead to phase instabilities. One form of phase instability is the formation of brittle topologically close packed phases (TCPs). These phases form during long-term, elevated-temperature exposures and tend to degrade mechanical properties. To avoid precipitation of detrimental TCP phases during service, low levels of chromium (Cr) are recommended. Low levels of Cr, however, may result in poor oxidation and corrosion resistance. That being said, it has recently been shown that the addition of small amounts of ruthenium (Ru) decreases the propensity for the precipitation of detrimental TCP phases. Another consequence of refractory alloying additions is their tendency to cause a breakdown of single crystal solidification. It is essential to design alloys within composition ranges where it is possible to produce them as monocrystals to avoid the disadvantages of the prior art.

[0006] The present invention goes well beyond the prior art in the use of higher levels of ruthenium (Ru) (up to about 14.1 wt %) to control precipitate morphology and rafting behavior, suppress precipitation of TCP phases, and improve creep properties. This is possible through controlled partitioning, where differing amounts of Ru affect the partitioning of elements in the alloy, particularly the Re and W, to the gamma and gamma prime phases. The exceptional aspect of the present invention is that alloys with positive, zero, or negative misfit, no TCP phases and high levels of Re can be designed. This is significant because rafting can be completely suppressed or rafts parallel to or normal to the applied tension applied stress A-A can form with zero, positive, or negative misfit, respectively.

[0007] Furthermore, it has been demonstrated that with higher levels of Ru, higher ratios of Cr / Re can be achieved, simultaneously improving oxidation and creep behavior. Cr is important in controlling partitioning. Since the three major mechanisms of high temperature degradation (TCP phase formation, creep damage, and oxidation) are improved, the alloys of the present invention are capable of increasing the useful life and temperature capability of critical turbine components.

Problems solved by technology

However, conversely, high levels of refractory alloying elements may lead to phase instabilities.

One form of phase instability is the formation of brittle topologically close packed phases (TCPs).

These phases form during long-term, elevated-temperature exposures and tend to degrade mechanical properties.

Low levels of Cr, however, may result in poor oxidation and corrosion resistance.

Another consequence of refractory alloying additions is their tendency to cause a breakdown of single crystal solidification.

Phase instability may further occur in monocrystalline alloys when the directional coarsening or “rafting” of the Ni3Al-γ′ precipitates under the action of an externally applied stress.

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