The present disclosure relates to Ni-based alloy and coating compositions having a γ′-Ni3Al matrix phase and possessing resistance to high-temperature oxidation and hot-corrosion.
Many high-temperature mechanical systems, such as, for example, gas-turbine engines, produce complex, multi-oxidant gaseous environments that can aggressively degrade the surface of structural components. The resulting multi-oxidant process environments can involve both gaseous and deposit-induced attack. For metallic alloys and coatings, it is often the formation and maintenance of a thermally grown oxide (TGO) scale that is required for surface protection. Alternatively, a stable and durable environmental barrier coating is needed. However, even then it is desirable to have an underlying surface that is capable of forming a reasonably protective TGO scale. The components of high-temperature mechanical systems are often made of a nickel-based superalloy that is based on the γ-Ni+γ′-Ni3Al phase constitution. Ideally, the high-temperature oxidation and corrosion resistance of the Ni-based superalloy is provided by a TGO scale of Al2O3.
U.S. Pat. No. 7,273,662 describes alloy and coating compositions including a Pt-group metal, Ni, Al, and a reactive element such as Hf, wherein the concentration of Al is limited such that the alloy includes substantially no β-NiAl phase. The alloy has a predominately γ+γ′ phase constitution, where γ refers to the solid-solution Ni phase and γ′ refers to the solid-solution Ni3Al phase. As further described in U.S. Published Application No. US2006/0210825, this alloy or coating composition may optionally include at least one of Cr and Si to further enhance its hot corrosion resistance, while maintaining excellent oxidation resistance. An advantage of these Pt-modified γ+γ′ alloys is their compatibility with superalloy substrates in terms of phase constitution, which in turn can provide minimal coating/substrate inter-diffusion and minimal differences in thermal expansion behavior.
Pt-group metals are currently very expensive constituents, which can render the alloys and coating compositions described in U.S. Pat. No. 7,273,662 and U.S. Published Application No. US 2006/0210825 impractical for use in certain applications. The present disclosure relates to γ+γ′ alloy and coating compositions that are free of Pt-group metals. When used as a standalone coating or as a bond coating in a thermal barrier coating (TBC) system on a substrate such as a gas turbine component, these Pt-group metal free compositions can protect the substrate during extended periods of high temperature use, and provide protection comparable to or better than conventional aluminide coatings. The γ+γ′ phase constitution of these compositions is chemically and mechanically compatible with the superalloy substrates commonly used in gas turbine components, and the presently disclosed compositions can be much more cost effective than the Pt-group metal containing materials described in U.S. Pat. No. 7,273,662 and U.S. Published Application No. US 2006/0210825. The alloy and coating compositions are particularly useful as a bond coat layer applied on a superalloy substrate used in a high-temperature resistant mechanical component, or as a non-heat-treatable bulk alloy used in a high temperature application. The alloy and coating compositions described herein exhibit oxidation resistance due to the formation of an Al-rich oxide scale.
In one aspect, the present disclosure is directed to an alloy including about 16 at % to about 23 at % Al; about 3 at % to about 10 at % Cr; up to about 5 at % Si; up to about 0.3 at % of at least two reactive elements selected from Y, Hf, Zr, La, and Ce; and Ni. The alloy has a volume fraction of γ′-Ni3Al phase greater than about 75%, which separates it from conventional Ni-based superalloys.
In another aspect, the present disclosure is directed to a thermal barrier coated article including a superalloy substrate and a bond coat on the substrate. The bond coat includes about 16 at % to about 23 at % Al; about 3 at % to about 10 at % Cr; up to about 5 at % Si; up to about 0.1 at % Y and up to about 0.2 at % of at least one other reactive element selected from Hf and Zr; and Ni; and wherein the bond coat has a volume fraction of γ′-Ni3Al phase greater than about 75%.
In yet another aspect, the present disclosure is directed to a thermal barrier coated article including a Ni-based superalloy substrate and a bond coat on the substrate. The bond coat includes about 16 at % to about 23 at % Al; about 3 at % to about 10 at % Cr; up to about 5 at % Si; up to about 0.1 at % Y and up to about 0.2 at % of at least one other reactive element selected from Hf and Zr; and Ni. The bond coat has a volume fraction of γ′-Ni3Al phase greater than about 75%. The article further includes an adherent layer of oxide on the bond coat and a ceramic coating on the adherent layer of oxide.
In yet another aspect, the present disclosure is directed to an alloy including about 18 at % to about 21 at % Al; about 5 at % to about 8 at % Cr; about 1 at % to about 2 at % Si; about 0.1 at % Y; about 0.2 at % of at least one of Hf and Zr; and Ni. The alloy has a volume fraction of γ′-Ni3Al phase greater than about 75%.
In another aspect, the present disclosure is directed to a method for making a heat resistant substrate. The method includes applying on the substrate a metallic coating including about 16 at % to about 23 at % Al; about 3 at % to about 10 at % Cr; up to about 5 at % Si; up to about 0.1 at % Y and up to about 0.2 at % of at least one other reactive element selected from Hf and Zr; and Ni. The metallic coating has a volume fraction of γ′-Ni3Al phase greater than about 75%.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.
Like reference symbols in the various drawings indicate like elements. All elemental contents in the figures of this application are in at %.
In one aspect, this disclosure is directed to Ni-based alloy or coating compositions including Al and Cr in amounts selected such that the matrix is γ′-Ni3Al phase. In the present application this γ′ matrix assemblage means that γ′-Ni3Al phase is present in a volume fraction of at least 75%. The alloy and coating compositions further include Si and at least two reactive elements selected from Y, Hf, Zr, La and Ce.
The alloy and coating compositions have a volume fraction of γ′-Ni3Al phase of at least 75%, and include substantially no β-NiAl phase, preferably no β-NiAl phase. In some embodiments, the alloy and coating compositions have γ′-Ni3Al phase present in a volume fraction of at least about 80%, and in other embodiments the γ′-Ni3Al phase is present at a volume fraction of at least about 85%, at least about 90%, or at least about 95%. The volume fraction of γ′-Ni3Al phase can be measured by standard quantitative metallographic techniques. For example, metallographically prepared cross-sections may be viewed under an optical microscope and, assuming that the samples are isotropic in structure, the area fraction of a particular phase constituent may be assumed to be equal to the volume fraction of that phase constituent. Concentration profiles can also be obtained from samples by either energy (EDS) or wavelength (WDS) dispersive spectrometry, with the former utilizing a secondary electron microscope (SEM) and the latter an electron probe micro-analyzer (EPMA).
To provide a γ′ volume fraction in the alloys and coating compositions of greater than about 75%, Al is preferably present at a level greater than about 16 at %. In some embodiments, Al is present in the alloys and coatings at about 16 at % to about 23 at %, and in some embodiments Al is present at about 18 at % to about 21 at %. The atomic percentage (at %) values specified for all elements in this application are nominal, and may vary by as much as ±1-2 at %.
The alloy and coating compositions further include Cr to promote primary formation of a continuous alumina (Al2O3) scale, but the Cr content should be limited to avoid formation of the β-NiAl phase. Based on these constraints, in some embodiments Cr is present at about 3 at % to about 10 at %, and in some embodiments Cr is present at about 5 at % to about 8 at %.
The alloy and coating compositions are co-doped with at least two reactive elements such as Hf, Y, La, Ce and Zr. The co-dopants should be present at relatively low concentrations to avoid significant oxidation of the reactive elements, which can be detrimental to cyclic oxidation resistance. In some embodiments the at least two reactive elements are present in the alloy and coating compositions at up to about 0.3 at %. In some embodiments, the reactive elements include up to about 0.1 at % Y and up to about 0.2 at % of at least one other reactive element such as Hf, La, Ce and Zr. In other embodiments, the reactive elements include up to about 0.1 at % Y and up to about 0.2 at % of at least one of Hf and Zr. In other embodiments, the reactive element include about 0.03 at % to about 0.07 at % Y and about 0.03 at % to about 0.12 at % of at least one of Hf and Zr. The reactive element included with Y can be either Hf or Zr or a combination thereof, and Hf is preferred.
The oxidation properties of the alloy and coating compositions can be further improved by addition of up to about 5 at % Si. In some embodiments, Si is present in the alloy and coating compositions at about 1 at % to about 2 at %.
In addition, other typical superalloy constituents such as, for example, Co, Mo, Ta, and Re, and combinations thereof, may optionally be added to or present in the alloy and coating compositions to the extent that at least 75% volume fraction of γ′ phase constitution is present. In some embodiments, up to about 3 at % manganese (Mn) can be added to the alloy and coating compositions to improve corrosion resistance in lower temperature (less than about 1050° C.) applications, depending on the oxidizing environment.
Referring to
The alloys may be prepared by conventional techniques such as, for example, argon-arc melting pieces of high-purity Ni, Al, Cr and optional reactive and/or superalloy metals and combinations thereof.
The compositions described herein may be applied on a substrate as high temperature resistant coatings (as stand-alone metallic coatings or as a bond coating in a thermal barrier coating (TBC) system), and may also be used as non-heat treatable bulk alloys. Any conventional Ni- or Co-based superalloy may be used as the substrate, including, for example, those available from Martin-Marietta Corp., Bethesda, Md., under the trade designation MAR-M 002; those available from Cannon-Muskegon Corp., Muskegon, Minn., under the trade designation CMSX-4, CMSX-10, and the like.
The coating compositions may be applied to the substrate using any known process, including for example, plasma spraying, chemical vapor deposition (CVD), physical vapor deposition (PVD) and sputtering to create a coating and form a temperature-resistant article. Typically this deposition step is performed in an evacuated chamber.
The thickness of the coating may vary widely depending on the intended application, but typically will be about 5 μm to about 100 μm, preferably about 5 μm to about 50 μm, and most preferably about 10 μm to about 50 μm.
If the coating is a bond coat layer in a TBC system, a layer of ceramic typically consisting of partially stabilized zirconia may then be applied using conventional PVD processes on the bond coat layer to form a ceramic topcoat. Suitable ceramic topcoats are available from, for example, Chromalloy Gas Turbine Corp., Delaware, USA. The deposition of the ceramic topcoat layer conventionally takes place in an atmosphere including oxygen and inert gases such as argon. The presence of oxygen during the ceramic deposition process makes it inevitable that a thin oxide scale layer is formed on the surface of the bond coat. The thermally grown oxide is typically an adherent layer of alumina, α-Al2O3. The bond coat layer, the TGO layer and the ceramic topcoat layer form a thermal barrier coating system on the superalloy substrate.
The alloy compositions described herein, when utilized as a coating layer, are both chemically and mechanically compatible with typical Ni- and Co-based superalloys. Protective coatings formulated from these compositions will have coefficients of thermal expansion (CTE) that are more compatible with the CTEs of Ni-based superalloys than those of β-NiAl-containing coatings. The former, when used as a bond coating, would provide enhanced thermal barrier coating stability during the repeated and severe thermal cycles experienced by mechanical components in high-temperature mechanical systems.
When thermally oxidized, the compositions described herein grow an α-Al2O3-rich scale layer at a rate comparable to or slower than the TGO scale layers formed on conventional aluminides, such as Pt-modified β-NiAl, and this provides excellent oxidation resistance.
The compositions described herein may be applied as a coating to any metallic part to provide resistance to severe thermal conditions and salt-induced hot corrosion. Suitable metallic substrate parts include Ni- and Co-based superalloy components for gas turbines, particularly those used in aeronautical and marine engine applications.
In addition, the alloys described herein may be used in bulk alloy form such as, for example, foils, sheets, and the like, to take advantage of the high-temperature oxidation and hot corrosion resistant properties that the alloys provide.
The alloy and coating compositions described herein may be used in an as-fabricated “bare” state or with a “pre-formed” thermally grown oxide layer on the surface. With regard to the latter, the alloy or coating can be exposed to an oxidizing atmosphere at an elevated temperature so as to cause a reaction leading to the formation of an oxide scale layer. This scale layer will be rich in Al2O3.
The alloy and coating compositions will now be described with reference to the following non-limiting examples.
The equilibrated samples were first analyzed using X-ray diffraction (XRD) for phase identification and then prepared for metallographic analyses by cold mounting them in an epoxy resin followed by polishing to a 0.5 μm finish. Microstructure observations were initially carried out on etched samples using an optical microscope. Concentration profiles were obtained from un-etched (i.e., re-polished) samples by either energy (EDS) or wavelength (WDS) dispersive spectrometry, with the former utilizing a secondary electron microscope (SEM) and the latter an electron probe micro-analyzer (EPMA). Differential thermal analysis (DTA) was also conducted on selected samples to determine thermal stability of different phases.
A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.
The U.S. Government has a paid-up license in the presently claimed invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided by the terms of Contract Number N00014-02-1-0733, awarded by the Office of Naval Research.