This invention relates to Ni—Al—Pt—Hf alloy compositions for high-strength, high temperature and oxidation resistant structural metal alloys.
Aerospace systems, as well as components for gas turbine and rocket engines, routinely require high temperature surface stability during service. Commercially available nickel-based superalloys with controlled microstructures, which rely on the formation of a continuous and adherent thermally grown oxide (TGO) scale of α-Al2O3 for extended resistance to degradation, may be used for high-strength thermal protection components. However, most commercial Ni-based superalloys were developed more for high-temperature strength than for oxidation resistance.
U.S. Pat. No. 7,273,662, incorporated herein by reference, describes alloy compositions and coatings including a Pt-group metal, Al, a reactive element such as Hf, and Ni, which have a predominately γ′-Ni3Al+γ-Ni phase constitution. These alloy compositions are sufficiently low in Al content to be substantially free of β-NiAl, and form metallic coatings with improved reliability and durability. Further, these alloy compositions form highly adherent, slow-growing TGO scales during both isothermal and cyclic oxidation at high temperatures.
To further enhance properties of certain γ′-Ni3Al+γ-Ni alloys such as, for example, strength, toughness and ductility, the present disclosure is based in part on the finding that addition of up to about 20 at % of strengthening elements can be added without substantially altering the γ′-Ni3Al+γ-Ni phase stability. Suitable strengthening elements in this context include, for example, Cr, Si, Co, Mo, Re, Ta, W and the like. The resultant strengthened alloy compositions form highly adherent, slow-growing TGO scales during both isothermal and cyclic oxidation at high temperatures up to at least about 1150-1200° C. The present disclosure is also based on the finding that controlling the Al content of certain γ′-Ni3Al+γ-Ni alloy compositions to below about 16 at % renders them heat treatable.
In one aspect, this disclosure is directed to an alloy including 5 at %≦Al<16 at %, about 0.05 at % to 1 at % of a reactive element selected from the group consisting of Hf, Y, La, Ce, Zr, and combinations thereof, and Ni, wherein the alloy composition has a predominately γ-Ni+γ′-Ni3Al phase constitution.
In another aspect, this disclosure is directed to a bulk alloy including about 13 at % Al to about 15 at % Al, about 0.05 at % to about 0.5 at % Hf, about 2.5 at % to about 5 at % of a Pt-group metal selected from Pt, Ir and combinations thereof, and Ni, wherein the alloy has a predominately γ-Ni+γ′-Ni3Al phase constitution.
In yet another aspect, this disclosure is directed to a method for making an alloy composition including providing a bulk alloy including about 5 at % to about 16 at % Al, about 0.05 at % to about 1.5 at % of a reactive metal selected from the group consisting of Hf, Y, La, Ce, Zr, and combinations thereof, up to about 20 at % of a Pt group metal selected from the group consisting of Pt, Pd, Ir, Rh, Ru, and combinations thereof, and Ni, wherein the alloy has a predominately γ-Ni+γ′-Ni3Al phase constitution; heating the bulk alloy to a temperature sufficient to substantially dissolve the γ′-Ni3Al phase and form a γ-Ni phase; and quenching the bulk alloy at a temperature sufficient to precipitate the γ′-Ni3Al phase within a γ-Ni matrix.
The alloy compositions may be particularly useful as high-temperature components that require both strength and oxidation resistance, such as thermal protection systems used in space re-entry and hypersonic aero systems, as well as for components used in gas turbine and rocket engines. The alloy compositions may be provided in such forms as, for example, bulk alloys, cast shapes, foils, claddings, or overlay-type coatings for metallic parts. The alloy compositions have excellent properties such as high-temperature strength and environmental resistance. Unlike conventional superalloys, the alloys described in this disclosure do not require a separate coating to enhance oxidation resistance at high temperatures.
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.
In the photographs and plots above, all compositions are nominal and set forth in atom percent. Like reference symbols in the various drawings indicate like elements.
In one aspect, the present disclosure is directed to an alloy composition that includes 5 at %≦Al<16 at % Al, about 0.05 at % to 1 at % of a reactive element such as Hf, Y, La, Ce, Zr and combinations thereof, and Ni, and has a phase constitution that is predominately or solely γ-Ni+γ′-Ni3Al. In some embodiments, the alloy further includes up to about 20 at % of a Pt-group metal such as Pt, Pd, Ir, Rh and combinations thereof. In other embodiments, the alloy includes up to about 20 at % of an additional strengthening element such as Cr and/or Si, and may also include refractory element such as such as Mo, Ta, Re, W, Ru, Ti and combinations thereof. As noted above, all at % values specified for all elements in this application are nominal, and may vary by as much as +1-2 at %.
Controlling the amount of Al in the alloy composition has a significant impact on the heat treatability of the composition. To maintain heat treatability, depending on the other elements present in the composition the Al content in the alloy should be maintained in the range of 5 at %≦Al<16 at %, or about 9 at % to about 15 at %, or about 9 at % to about 14 at %, or about 9 at % to about 13 at %, or 13 at %<Al<15 at %.
The addition of reactive elements such as Hf, Y, La, Ce and Zr, and combinations thereof, may tend to stabilize the γ′ phase in the alloy composition. Therefore, if sufficient reactive metal is added to the composition, the resulting phase constitution may be predominately γ′ or solely γ′. The reactive elements Hf, Y, La, Ce and Zr, and combinations thereof, are preferably present in the alloy at about 0.05 at % to about 1 at %. To provide excellent heat treatability, the reactive element is more preferably present at about 0.05 at % to 0.5 at %, and even more preferably at about 0.05 at % to about 0.1 at %.
The γ-Ni+γ′-Ni3Al alloy composition preferably also includes at least one Pt-group metal (PGM) such as, for example, Pt, Pd, Ir, Rh or combinations thereof. Pt and Ir are preferred Pt-group metals, and Pt is particularly preferred. The total concentration of Pt-group metals in the alloy composition is preferably less than about 20 at %, more preferably less than about 10 at %, even more preferably less than about 5 at %, and most preferably about 2.5 at % to about 5 at %. If the Pt-group metals are selected from Pt and Ir, the Pt-group metals are most preferably present in the alloy composition at about 2.5 at % Pt and about 2.5 at % Ir, with a total of about 5 at %.
The γ-Ni+γ′-Ni3Al alloy composition may optionally further include up to about 20 at % of strengthening elements such as Cr and/or Si to enhance certain alloy properties such as, for example, strength and corrosion resistance. The Cr is preferably present in the alloy composition at about 5 at % to about 8 at %. In addition to or in place of the Cr, the alloy composition may optionally include up to about 3 at % Si, more preferably about 0.2 at %.
The γ-Ni+γ′-Ni3Al alloy composition may also optionally include a refractory element or elements for conferring additional alloy strengthening. Refractory elements in this application refer to metals with high melting points such as Mo, Ta, Re, W, Ru, Ti and combinations thereof. The refractory metals may be present in the alloy composition at any concentration as long as the γ+γ′ phase constitution in the composition predominates, but typically are present at up to about 5 at % to about 10 at %, more preferably about 8 at %. It has been found that these refractory elements enhance alloy properties such as creep strength, while properties such as corrosion resistance and high temperature resistance are retained. Preferred refractory elements include W, Ta, Mo and Ti.
The alloy composition may further optionally include up to 1 at % of C, B, N and combinations thereof.
Referring to
As noted above, selection of the Al content in the alloys has a significant impact on whether or not they are heat treatable (i.e., able to be single-phase γ-Ni at some elevated temperature and two-phase γ-Ni+γ′-Ni3Al at lower temperatures). For example, a particularly preferred heat treatable alloy includes about 9 at % Al to about 14 at % Al and about 0.1 at % to about 0.3 at % Hf and the remainder Ni. These alloys may optionally include about 10 at % Cr. Typical examples (at %) include: Ni-13Al-0.1Hf, Ni-13Al-5Cr-0.1Hf, and Ni-13Al-10Cr-0.1Hf.
If a Pt-group metal is present, the heat treatable alloy preferably includes about 9 at % to about 13 at % Al, about 0.1 at % to about 0.3 at % Hf, about 2.5 at % to about 16 at % Pt, and Ni. The Pt-group metal containing alloys may further optionally include about 5 at % to about 10 at % Cr. Examples (at %) include Ni-13Al-5Pt-0.1Hf, Ni-13Al-16Pt-0.1Hf, Ni-13Al-5Pt-8Cr-0.1Hf and Ni-15Al-16.0Pt-5Cr-0.3Hf.
The alloys may be prepared by techniques such as, for example, argon-arc melting pieces of high-purity Ni, Al, Pt-group metals, reactive and/or strengthening metals, as well as optional refractory metals and combinations thereof. The alloys are typically cast using conventional processes and exist in bulk form, which in this application refers to free-standing cast shapes that nominally have substantially the same composition throughout. The cast shapes may be made into a wide variety of structural materials, including foils, sheets, bars, and cladding, and are particularly well suited for structural applications or for protecting an underlying substrate against high temperatures. In this application the term cladding refers to two alloys in contact, with a diffusive bond between them. The alloys may even be applied as a coating on a substrate using, for example, thermal spraying techniques such as plasma-arc spraying and high-velocity oxygen-fuel spraying or physical vapor deposition methods including magnetron sputtering or electron beam-based processes.
When thermally oxidized, the γ-Ni+γ′-Ni3Al bulk alloys described in this application grow a highly adherent α-Al2O3 scale layer during both isothermal and cyclic oxidation at high temperatures up to about 1150-1200° C.
Once a cast shape is formed having an appropriate concentration of Al, one or more reactive metals, and Ni selected to retain a predominately γ-Ni+γ′-Ni3Al phase structure, the cast shape may be thermally treated to obtain a desired microstructure and further enhance the properties of the material for a particular application. A wide variety of thermal treatment processes may be used to tailor the microstructure of the bulk alloy for a particular application.
As noted above, if the concentration of Al is maintained within a selected range, the resulting γ-Ni+γ′-Ni3Al alloy is heat treatable. Suitable thermal treatments include the precipitation heat treatment processes exemplified below, which has at least a solution treatment step, a quenching step and an aging step. However, this application is not limited to such a thermal treatment process, and a wide variety of processes may be used to tailor the microstructure of the bulk alloy for a particular application.
For example, in one precipitation heat treatment process the cast shape with constituent metals selected to have a predominately γ-Ni+γ′-Ni3Al phase structure is first thermally heated to or above a temperature sufficient to substantially dissolve the γ′-Ni3Al phase and form a single γ-Ni phase. This solution treatment step is typically performed by thermally heating the cast shape in pre-heated furnace from room temperature to a temperature of about 1200 to about 1300° C. for about 0.5 to about 6 hours.
The solution treatment step is followed by a first quenching step in which the temperature of the cast shape is quickly returned to room temperature, typically by quenching in water. During the quenching step the γ′-Ni3Al phase precipitates to form a phase assemblage with γ′-Ni3Al precipitates distributed in a γ-Ni matrix.
Following the first quenching step, the cast shape is again thermally treated for a longer period of time at a lower temperature than used in the solution treatment step described above to substantially uniformly distribute the γ′-Ni3Al precipitates, as well as any reactive, strengthening or refractory elements present in the composition, within the γ-Ni matrix. This aging step is typically performed by inserting the cast shape into a pre-heated furnace and heating from room temperature to a temperature of about 800 to about 1000° C. for about 1 to about 24 hours.
The aging step is followed by a second quenching step in which the temperature of the cast shape is quickly returned to room temperature, typically by quenching in water.
During the solutionizing or in the as-quenched stage of the heat treatment, the cast shape may be processed for a particular application, such as, for example, by rolling into a heat-protective foil. Typically, foils with a thickness of about 1 mm or less can provide substantial thermal and corrosion protection for an underlying substrate, and are quite lightweight.
The concentrations of the constituent elements and the precipitation heat treatment conditions may be selected to provide a cast shape with the compositions described above, as well as a desired microstructure for a particular application. Preferred alloys have a microstructure with a phase constitution of about 30 volume % (vol %) to about 70 vol %, or about 30 vol % to about 60 mvol %, of γ′-Ni3Al precipitates distributed substantially uniformly in a γ-Ni matrix.
High purity alloys were Ar-arc melted and drop cast to provide cast shapes with the following alloy compositions (all compositions in the examples below are set forth in at %):
For oxidation testing, the samples were annealed at 1200° C. for 6 hours, followed by thermal treatment at 1150° C. for 48 hours in flowing Ar.
For microstructural characterization, the samples were first placed in a pre-heated furnace and thermally treated at 1300° C. for 1 hour, then rapidly quenched in water to reduce the temperature of the samples to room temperature.
The microstructure of the resulting samples is shown in the photographs of
The samples from Example 1 were then oxidized in air at 1000° C. for 100 hours, and the measured weight change results (due to oxygen uptake) are shown in
Cross-sectional photographs of the
The samples from Example 1 were then oxidized at 1150° C. under thermal cycles, and the results are shown in
The same plot is enlarged in
The oxidation behavior of PGM-modified γ+γ′ alloys with 13 at % Al and 0.1 at % Hf were assessed under both isothermal and cyclic conditions at 1000® C. in air. Cross-sectional SEM images of selected alloys after 100 hour isothermal oxidation are shown in
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 this 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 No. N00014-02-1-0733, awarded by the Office of Naval Research.
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