Corrosion Resistant Alloy Compositions with Enhanced Castability and Mechanical Properties

Abstract

Disclosed are novel nickel-base alloy compositions that may be cast as a single crystal or directionally solidified alloy consisting essentially of, by weight: 8-12% Cr, 10-14% Co, 0.3-0.9% Mo, 3-7% W, 2-8% Ta, 2.0-5.5% Al, 1.5-5.0% Ti, up to 2% Nb, less than 0.1% B, less than 0.1% Zr, 0.05-0.15% C, less than 0.5% Hf, 2-4% Re, 0.05-0.2% Si, up to 0.015% S, up to 0.1% La, up to 0.1% Y, up to 0.1% Ce, up to 0.1% Nd, up to 0.1% Dy, up to 0.1% Pr, up to 0.1% Gd, balance is Ni, and wherein (La+Y+Ce+Nd+Dy+Pr+Gd) is 0.001-0.1%. The compositions for the nickel-base superalloy have a balance between oxidation resistance, corrosion resistance, castability, and mechanical properties, such as creep resistance and thermo-mechanical fatigue resistance.

Description

[0001]This application claims benefit of the 10 Aug. 2007 filing date of U.S. Provisional Patent Application No. 60 / 955,092.FIELD OF THE INVENTION[0002]This invention relates in general to the field of nickel base superalloys possessing improved oxidation resistance, corrosion resistance, castability, and mechanical properties such as creep resistance and thermo-mechanical fatigue resistance. The present invention relates to alloys that may be cast as a single crystal or directionally solidified.BACKGROUND OF THE INVENTION[0003]Nickel base superalloys are alloys composed primarily of nickel with the addition of several other elements selected for their ability to survive an overall high temperature, high stress, and highly oxidative environment. Typically, this environment is that of a gas turbine engine.[0004]The greatest difficulty encountered with the gas turbine environment is that the goals of creep resistance, thermo-mechanical fatigue resistance, and corrosion resistance are ...

AI Technical Summary

Benefits of technology

[0012]The novel alloy compositions disclosed herein offer an improved set of properties that are particularly useful for application in Integrated Gasification Combined Cycle (IGCC) syngas fired gas turbines where relatively low quality fuels and high operating temperatures result in a highly corrosive environment.

[0013]Alloys of the present invention include Chromium (Cr) from 8-12 weight percent or about 10 weight percent to provide the compositions with a desired level of high temperature corrosion resistance.

[0014]Alloys of the present invention may include Aluminum (Al) at a concentration that is lower than is generally observed in highly oxidation resistant alloys, for example as low as 2 weight percent or in the range of 2.0-5.5 weight percent. Rare earth elements, Yftrium (Y), Silicon (Si), and Hafnium (Hf) are included to compensate for the reduction in Aluminum (Al) and to provide increased oxidation resistance.

[0015]The alloy compositions disclosed herein have intentional additions of rare earth elements Lanthanum (La), Yttrium (Y), Gadolinium (Gd), Praseodymium (Pr), Dysprosium (Dy), Neodymium (Nd), and Erbium (Er) in combined amounts of up to 0.1 weight percent. These rare earth additions provide improved oxidation resistance of the inventive alloys and enhance the compatibility of the alloy compositions with various coatings. The rare earth additions also aid in increasing the life of any overlying protective ceramic coating. The increase in coating life through the addition of rare earth elements is attributed to their ability to form sulfides and oxi-sulfides that reduce the residual Sulfur (S) content and prevent the diffusion of sulfur atoms to the alumina scale that is formed at the boundary between the coating and the substrate alloy.

[0016]Silicon (Si) is intentionally added to the present alloys to support the formation of a protective silicon dioxide surface oxide layer. The silicon dioxide provides enhanced oxidation resistance as the film is less susceptible to cracking compared with other protective oxide films. Excessive additions of silicon are detrimental to the performance of the alloy; consequently, an addition of less than 0.15 weight percent or in the range of 0.05-0.2 weight percent is preferred.

[0017]The intentional addition of Hafnium (Hf) at levels similar to those of Silicon or less than 0.5 weight percent serves to further enhance the oxidation resistance.

Problems solved by technology

The greatest difficulty encountered with the gas turbine environment is that the goals of creep resistance, thermo-mechanical fatigue resistance, and corrosion resistance are at odds with each other.

Alloys with greater creep resistance typically sacrifice hot corrosion resistance and thermo-mechanical fatigue resistance.

Alloys with greater hot corrosion resistance come at the expense of poor creep resistance and thermo-mechanical fatigue resistance.

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