Patent Application
20060039820
This invention relates to a nickel-base superalloy such as that used in aircraft gas turbine engines, and in particular to a single-crystal nickel-base superalloy such as that used in gas turbine blades and vanes.
In an aircraft gas turbine (jet) engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is combusted, and the resulting hot combustion gases are passed through a turbine mounted on the same shaft. The flow of hot combustion gas turns the turbine by contacting an airfoil portion of the turbine blade, which turns the shaft and provides power to the compressor. The hot exhaust gases flow from the back of the engine, driving it and the aircraft forward. There may additionally be a bypass fan that forces air around the center core of the engine, driven by a shaft extending from the turbine section.
The higher the temperature of the hot combustion gas, the greater the efficiency of the engine. There is therefore an incentive to operate the materials of the engine at ever-higher temperatures and loadings. A variety of techniques and structures have been used to achieve higher temperatures, including, for example, improved alloy compositions, oriented and single crystal turbine blades, thermal barrier coatings that provide environmental protection and act as insulation applied to the turbine blades, and physical cooling techniques. Nickel-based superalloys are widely used as the materials of construction of gas turbine blades and vanes.
Advanced nickel-base superalloys include substantial amounts of refractory alloying elements, such as rhenium, tungsten, tantalum, and molybdenum, that inhibit atomic diffusion of other elements and consequently improve the high-temperature mechanical properties such as creep. The presence of the large amounts of refractory alloying elements may also lead to various types of microstructural instabilities in the articles made from the superalloys. One such microstructural instability is the formation of topologically close-packed (TCP) phases after prolonged exposure of the superalloy to temperatures above about 1800° F. TCP phases are brittle and also cause partitioning of solid-solution-strengthening elements to the TCP phases, resulting in a loss of high-temperature strength.
There is a need for an approach that allows the refractory elements to be present to perform their function of reducing diffusion of other elements and improving elevated temperature mechanical properties, while reducing or ideally avoiding instabilities such as TCP phase formation, and while retaining or improving high-temperature mechanical properties. The present invention fulfills this need, and further provides related advantages.
The present invention provides a nickel-base superalloy with high levels of refractory metals but reduced tendency toward microstructural instability. High-temperature mechanical properties are also improved. Castability and heat treatability are not sacrificed in the nickel-base superalloy.
A composition of matter consists essentially of, in weight percent, from about 0.4 to about 6.5 percent ruthenium, from about 3 to about 8 percent rhenium, from about 5.8 to about 10.7 percent tantalum, from about 4.25 to about 17.0 percent cobalt, from 0.1 to about 2.0 percent hafnium, from about 0.02 to about 0.4 percent carbon, from about 0.001 to about 0.005 percent boron, from 0 to about 0.02 percent yttrium, from about 1 to about 4 percent molybdenum, from about 1.25 to about 10 percent chromium, from about 0.5 to about 2.0 percent niobium, from about 0.05 to about 0.5 percent zirconium, from about 5.0 to about 6.6 percent aluminum, from 0 to about 2.0 percent titanium, from about 3.0 to about 7.5 percent tungsten, and from about 0.1 to about 6 percent of an element selected from the group consisting of platinum, iridium, rhodium, and palladium, and combinations thereof. The balance of the composition of matter is nickel and incidental impurities.
A preferred composition of matter within this broad range has a nominal composition, in weight percent, of about 3 percent ruthenium, about 5.5 percent rhenium, about 8.25 percent tantalum, about 16.5 percent cobalt, from 0.5 to about 2.0 percent hafnium, about 0.03 percent carbon, about 0.004 percent boron, about 0.01 percent yttrium, about 2.0 percent molybdenum, about 2 percent chromium, from about 1 to about 2 percent niobium, from about 0.1 to about 0.5 percent zirconium, about 5.5 percent aluminum, from 0 to about 2.0 percent titanium, about 6 percent tungsten and the balance nickel and incidental impurities. It is preferred that this composition has from about 0.5 to about 2.0 percent of the element selected from the group consisting of platinum, iridium, rhodium, and palladium, and combinations thereof.
An article comprises a substantially single crystal piece having a composition as set forth above and elsewhere herein. The single crystal piece is desirably substantially free of TCP phase. The article may be a component of a gas turbine engine such as a gas turbine blade or a gas turbine vane. In some applications, there may be a protective coating overlying the substantially single-crystal piece, such as an aluminide protective coating and optionally a ceramic thermal barrier coating.
The present approach achieves mechanical properties in the superalloy that equal or exceed those of conventional superalloys, while minimizing, delaying, or, ideally, completely avoiding the formation of microstructural instabilities such as TCP phases. The presence of an elevated level of iridium, rhodium, palladium, and/or platinum improves mechanical properties. The hafnium and zirconium contribute to improved strength and environmental resistance, both in the case of an uncoated article as well as a coated article with an aluminide protective coating. Good castability and heat treatability are retained.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.
The article must be substantially a single crystal (i.e., single grain). It may not be a polycrystal, either a random polycrystal or an oriented polycrystal such as produced by directional solidification. In the polycrystalline alloys, it has been conventional to add higher levels of elements that are known to strengthen grain boundaries, such as carbon, boron, hafnium, and zirconium. Zirconium and hafnium are chemically reactive, modify the morphologies of precipitate phases, and may adversely affect the heat treatment of the alloys. Because these elements are not needed to strength high-angle grain boundaries, which are not present in substantially single-crystal articles, it has therefore been the general industry practice to omit them from single-crystal articles except in very minor amounts to strengthen the low-angle grain boundaries that may be present. The present alloy departs from that approach and adds substantial amounts of hafnium and zirconium even to a substantially single-crystal alloy.
The turbine blade 20 is mounted to a turbine disk (not shown) by a dovetail 24 which extends downwardly from the airfoil 22 and engages a slot on the turbine disk. A platform 26 extends longitudinally outwardly from the area where the airfoil 22 is joined to the dovetail 24. In some articles, a number of optional cooling channels extend through the interior of the airfoil 22, ending in openings 28 in the surface of the airfoil 22. A flow of cooling air is directed through the cooling channels, to reduce the temperature of the airfoil 22.
The substantially single-crystal article is preferably manufactured by the directional solidification of a melt of the alloy composition as discussed next. Directional solidification techniques to produce substantially single-crystal pieces of other materials and compositions are known in the art, and the same techniques may be used here.
The article is formed of a nickel-base superalloy. As used herein, “nickel-base” means that the composition has more nickel present than any other element. The nickel-base superalloys are typically of a composition that is strengthened by the precipitation of gamma-prime phase or a related phase.
The article and nickel-base superalloy preferably have a composition consisting essentially of, in weight percent, from 0.4 to 6.5 percent ruthenium, from 3 to 8 percent rhenium, from 5.8 to 10.7 percent tantalum, from 4.25 to 17.0 percent cobalt, from 0.1 to 2.0 percent hafnium, from 0.02 to 0.4 percent carbon, from 0.001 to 0.005 percent boron, from 0 to 0.02 percent yttrium, from 1 to 4 percent molybdenum, from 1.25 to 10 percent chromium, from 0.5 to 2.0 percent niobium, from 0.05 to 0.5 percent zirconium, from 5.0 to 6.6 percent aluminum, from 0 to 2.0 percent titanium, from 3.0 to 7.5 percent tungsten, and from 0.1 to 6 percent of an element selected from the group consisting of platinum, iridium, rhodium, and palladium, and combinations thereof. The balance of the composition is nickel and incidental impurities.
More preferably, within this broad range the article has specific elemental limitations, in weight percent, of 3 percent ruthenium, 5.5 percent rhenium, 8.25 percent tantalum, 16.5 percent cobalt, from 0.5 to 2.0 percent hafnium, 0.03 percent carbon, 0.004 percent boron, 0.01 percent yttrium, 2.0 percent molybdenum, 2 percent chromium, from 1 to 2 percent niobium, from 0.1 to 0.5 percent zirconium, 5.5 percent aluminum, from 0 to 2.0 percent titanium, 6 percent tungsten and the balance nickel and incidental impurities. It is preferred that this composition has from 0.5 to 2.0 percent of the element selected from the group consisting of platinum, iridium, rhodium, and palladium, and combinations thereof.
The amount of ruthenium may not be below about 0.4 percent by weight because smaller amounts of ruthenium would not effectively stabilize the microstructure against detrimental TCP formation. The amount of ruthenium may not be above about 8 percent by weight because the stability benefits of larger amounts of ruthenium do not outweigh the cost and density increases to the alloy.
The amount of rhenium may not be below about 3 percent by weight because smaller amounts would not provide sufficient creep rupture strength through solid solution strengthening. The amount of rhenium may not be above about 8 percent by weight because larger amounts lead to microstructural instability, increased cost, and increased density of the material.
The amount of tantalum may not be below about 5.8 percent by weight because smaller amounts would not provide sufficient creep rupture strength through precipitation strengthening. The amount of tantalum may not be above about 10.7 percent by weight because the strength benefits above this level do not outweigh the cost and density increases to the alloy.
The amount of cobalt may not be below about 4.25 percent by weight because smaller amounts would not effectively act to stabilize the microstructure against detrimental TCP formation. The amount of cobalt may not be above about 17.0 percent by weight because larger amounts are detrimental to high-temperature strength by reducing the temperature of the gamma prime solvus.
The amount of hafnium may not be below about 0.1 percent by weight because lower amounts would not provide sufficient low angle grain boundary strengthening, precipitation strengthening, and environmental resistance. The amount of hafnium may not be above about 2.0 percent by weight because larger amounts cause difficulty in properly heat treating the alloy to obtain the optimum strength.
The amount of carbon may not be below about 0.02 percent by weight because smaller amounts would not effectively serve to clean the alloy melt of detrimental inclusions. The amount of carbon may not be above about 0.4 percent by weight because larger amounts cause excessive carbide formation that may reduce fatigue properties.
The amount of boron may not be below about 0.001 percent by weight because smaller amounts would not provide sufficient low-angle grain boundary strength. The amount of boron may not be above about 0.006 percent by weight because larger amounts cause excessive incipient melting.
The amount of yttrium may not be above 0.02 percent by weight because larger amounts cause excessive incipient melting.
The amount of molybdenum may not be below about 1 percent by weight because smaller amounts do not provide sufficient creep rupture strength through solid solution strengthening. The amount of molybdenum may not be above 4 percent by weight because larger amounts degrade environmental resistance.
The amount of chromium may not be below about 1.25 percent by weight because smaller amounts would not provide sufficient environmental resistance. The amount of chromium may not be above about 10 percent by weight because larger amounts cause microstructural instability.
The amount of niobium may not be below about 0.5 percent by weight because smaller amounts would not provide sufficient creep rupture strength through precipitation strengthening. The amount of niobium may not be above about 2 percent by weight because larger amounts degrade environmental resistance
The amount of zirconium may not be below about 0.05 percent by weight because smaller amounts do not provide sufficient low-angle grain-boundary strengthening, precipitation hardening, and environmental resistance. The amount of zirconium may not be above about 0.5 percent by weight because larger amounts cause excessive incipient melting.
The amount of aluminum may not be below about 5.0 percent by weight because smaller amounts do not provide sufficient environmental resistance. The amount of aluminum may not be above 6.6 percent by weight because larger amounts cause microstructural instability.
The amount of titanium may not be above about 2.0 percent by weight because larger amounts degrade environmental resistance.
The amount of tungsten may not be below about 3.0 percent by weight because smaller amounts do not provide sufficient creep rupture strength through solid solution strengthening. The amount of tungsten may not be above about 7.5 percent by weight because larger amounts cause microstructural instability.
The amount of platinum, iridium, rhodium, or palladium, or combinations thereof, may not be less than about 0.1 percent by weight because smaller amounts do not provide sufficient creep strength. The amount of platinum, iridium, rhodium, or palladium, or combinations thereof, may not be greater than about 6 percent by weight because the strength benefits of larger amounts do not outweigh the cost and density increase to the alloy.
The single-crystal piece 40 may be used with bare surfaces. However, at the elevated service temperatures in a combustion gas environment, the bare surfaces are rapidly oxidized with significant metal loss. Optionally but preferably, a protective coating 42 is applied overlying the surfaces 44 of the substantially single-crystal piece 40. The protective coating 42 reduces oxidation and hot-gas damage to the surfaces 44.
Although a particular embodiment of the invention has been described in detail for purposes of illustration, various modifications and enhancements may be made without departing from the spirit and scope of the invention. Accordingly, the invention is not to be limited except as by the appended claims.
