Nickel-base superalloy, unidirectional-solidification process therefor, and castings formed therefrom

Abstract

An alloy composition and method by which the incidence of freckling can be reduced in castings produced with properties similar to the René N5 nickel-base superalloy. The casting has a unidirectional crystal structure and a composition consisting of, by weight, 6% to 8% chromium, 6% to 9% cobalt, 0% to 2% molybdenum, 4% to 6% tungsten, 6.4% to 6.9% tantalum, 0% to 2% titanium, 5% to 7% aluminum, 2.7% to 3.0% rhenium, 0.3% to 0.7% haffiium, 0.04% to 0.08% carbon, 0.002% to 0.006% boron, 0% to 0.075% yttrium, 0.002% to 0.004% zirconium, the balance being nickel and incidental impurities.

Description

BACKGROUND OF THE INVENTION[0001]The present invention generally relates to nickel-base superalloy compositions and processes for producing directionally-solidified castings therefrom. More particularly, the invention is directed to a nickel-base superalloy that is castable as single-crystal articles suitable for use as components of gas turbines and other high temperature applications.[0002]Components of gas turbines, such as blades (buckets), vanes (nozzles) and combustor components, are typically formed of nickel, cobalt or iron-base superalloys characterized by desirable mechanical properties at turbine operating temperatures. Because the efficiency of a gas turbine is dependent on its operating temperatures, there is an ongoing effort to develop components, and particularly turbine buckets, nozzles, and combustor components, that are capable of withstanding higher temperatures. As the material requirements for gas turbine components have increased, various processing methods an...

AI Technical Summary

Benefits of technology

[0008]According to a preferred aspect of the invention, the unidirectional crystal structure of the casting is substantially free of freckle defects. As used herein, the characterization of a casting being substantially free of freckles is meant to indicate a total combined length of all freckles of not more than 0.5 inch (about 12 mm). A significant advantage of this invention is that, though the composition of the casting contains the same alloying constituents as N5 and the alloying levels ofthese constituents are within or overlap the broadest ranges disclosed for N5, it has been determined that the employment of particular levels and relative proportions ofthese constituents are able to produce unexpected superior results as compared to the N5 alloy processed under identical conditions. In particular, the reduction in the incidence of freckling is greater than was expected for composition modifications alone, and the result is the absence of freckling that would otherwise be an expected property for N5 under the same processing conditions.

Problems solved by technology

Such defects can reduce fatigue life and act as grain initiators during the solidification process that cause unacceptable off-axial grains.

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