Patent Application
20170260609
The invention relates generally to a nanostructured ferritic alloy. More particularly the invention relates to a nanostructured ferritic alloy having dual scale dispersions.
Gas turbines operate in extreme environments, exposing the turbine components, especially those in the turbine hot section, to high operating temperatures and stresses. In order for the turbine components to endure these conditions, they are manufactured from a material capable of withstanding these severe conditions. As material limits are reached, one of two approaches is conventionally used in order to maintain the mechanical integrity of hot section components. In one approach, cooling air is used to reduce the part's effective temperature. In a second approach, the component size is increased to reduce the stresses. However, these approaches can reduce the efficiency of the turbine and increase the cost.
In certain applications, super alloys have been used in these demanding applications because they maintain their strength at up to 90% of their melting temperature and have excellent environmental resistance. Nickel-based super alloys, in particular, have been used extensively throughout gas turbine engines, e.g., in turbine blade, nozzle, wheel, spacer, disk, spool, blisk, and shroud applications. In some lower temperature and stress applications, steels may be used for turbine components. However, conventional steels cannot currently be used in high temperature and high stress applications because they do not meet the necessary mechanical property requirements. Designs for improved gas turbine performance require alloys that balance cost with higher temperature capability.
Nickel-based super alloys used in heavy-duty turbine components require specific elaborate processing steps in order to achieve the desired mechanical properties, including three melting operations: vacuum induction melting (VIM), electroslag remelting (ESR), and vacuum arc remelting (VAR). Nanostructured ferritic alloys (NFAs) are an emerging class of alloys that exhibit exceptional high temperature properties, thought to be derived from nanometer-sized oxide clusters that precipitate during hot consolidation following a mechanical alloying step. These oxide clusters are present at high temperatures, providing a strong and stable microstructure during service. Unlike many nickel-based super alloys, that require the cast and wrought (C&W) process to be followed to obtain necessary properties, NFAs are manufactured via a different processing route that requires fewer melting steps.
While NFAs yield enhanced tensile and creep properties compared to conventional steels, additional benefits are sought. In order for any material to be optimally useful in, e.g., large hot section components of heavy duty turbo machinery, it may also desirably exhibit a further increased creep resistance making it useful for gas turbo-machinery applications. Any such alloy will also desirably be capable of being manufactured into the desired article utilizing a less energy intensive and/or time consuming process, than the conventional cast and wrought process.
In one embodiment, an alloy is provided. The alloy includes a matrix phase, and a population of particulate phases dispersed within the matrix. The matrix includes iron and chromium; and the population includes a first subpopulation of particulate phases and a second subpopulation of particulate phases. The first subpopulation of particulate phases include a complex oxide, having a median size less than about 20 nm, and present in the alloy in a concentration from about 0. 1 volume percent to about 5 volume percent. The second subpopulation of particulate phases have a median size in a range from about 30 nm to about 10 microns, and present in the alloy in a concentration from about 1 volume percent to about 15 volume percent.
In one embodiment, an alloy is provided. The alloy includes a matrix phase, and a population of particulate phases dispersed within the matrix. The matrix includes iron and chromium; and the population includes a first subpopulation of particulate phases and a second subpopulation of particulate phases. The first subpopulation of particulate phases include a complex oxide having yttrium and titanium, and having a median size less than about 10 nm, and present in the alloy in a concentration from about 0. 1 volume percent to about 3 volume percent. The second subpopulation of particulate phases include precipitated Laves phase, have a median size in a range from about 50 nm to about 3 microns, and present in the alloy in a concentration from about 1 volume percent to about 6 volume percent.
In one embodiment, a method of forming an alloy is provided. The method includes melting starting materials comprising iron and chromium; atomizing the melt to form an alloy powder; milling the alloy powder in the presence of an oxide until the oxide is dissolved into the alloy powder, thus forming a milled alloy powder; consolidating the milled alloy powder at a first temperature; precipitating a first subpopulation of particulate phases comprising a complex oxide having a median size less than about 20 nm; and establishing a second subpopulation of particulate phases having a median size in a range from about 30 nm to about 10 microns.
In one embodiment, a method of forming an alloy is provided. The method includes the steps of forming a milled alloy powder, consolidating the milled alloy powder at a first temperature, precipitating a first subpopulation of particulate phases including a complex oxide comprising yttrium and titanium, and establishing a second subpopulation of particulate phases by an in-situ precipitation of a Laves phase. Forming a milled alloy powder includes melting starting materials having iron and chromium through a vacuum induction melting process; atomizing the melt to form an alloy powder; and milling the alloy powder in the presence of an oxide until the oxide is dissolved into the alloy powder. The first subpopulation of particulate phases have a median size less than about 20 nm, in a concentration from about 0.1 volume percent to about 3 volume percent of the alloy. Establishing a second subpopulation of particulate phases by an in-situ precipitation of a Laves phase may include hot-working the consolidated, milled alloy powder, and heat-treating the hot-worked, consolidated, milled alloy powder at a second temperature. The Laves phase has a median size in a range from about 30 nm to about 10 microns, in a concentration from about 1 volume percent to about 4 volume percent of the alloy.
In one embodiment, a method of forming an alloy is provided. The method includes the steps of forming a milled alloy powder, adding a particulate phase comprising an oxide, boride, or a combination of an oxide and boride, mixing the added particulate phase, consolidating the milled and mixed alloy powder at a first temperature, precipitating a first subpopulation of particulate phases including a complex oxide comprising yttrium and titanium, and establishing a second subpopulation of particulate phases resulting from the added particulate phases. Forming a milled alloy powder includes melting starting materials having iron and chromium through a vacuum induction melting process; atomizing the melt to form an alloy powder; and milling the alloy powder in the presence of an oxide until the oxide is dissolved into the alloy powder. The first subpopulation of particulate phases have a median size less than about 20 nm, in a concentration from about 0.1 volume percent to about 3 volume percent of the alloy. The second subpopulation of particulate phases have a median size in a range from about 30 nm to about 10 microns, in a concentration from about 1 volume percent to about 4 volume percent of the alloy.
Embodiments of the invention described herein address the noted shortcomings of the state of the art. One or more specific embodiments of the present invention will be described below. In an effort to provide a concise description of these embodiments, all features of an actual implementation may not be described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure.
When introducing elements of various embodiments of the present invention, the articles “a,” “an,” and “the,” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. Moreover, the use of “top,” “bottom,” “above,” “below,” and variations of these terms is made for convenience, but does not require any particular orientation of the components unless otherwise stated.
All ranges disclosed herein are inclusive of the endpoints, and the endpoints are combinable with each other. The terms “first,” “second,” and the like as used herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another.
Approximating language, as used herein throughout the specification and claims, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it may be about related. Accordingly, a value modified by a term such as “about” is not limited to the precise value specified. In some instances, the approximating language may correspond to the precision of an instrument for measuring the value.
In one embodiment, a nanostructured ferritic alloy (NFA) is provided. Typically a nanostructured ferritic alloy includes an iron-containing alloy matrix that is strengthened by nanofeatures disposed in the matrix. The concentration of iron in the alloy matrix may be greater than about 50 wt %. In one embodiment, the iron content in the alloy matrix is greater than about 70 wt %. In one embodiment, the alloy matrix is in the form of the ferritic body-centered cubic (BCC) phase. As used herein, the term “nanofeatures” means particles of matter having a largest dimension less than about 100 nanometers in size. The nanofeatures used herein are typically in-situ formed. in NFA by the dissolution of the initial added oxide and the precipitation of nanometer sized clusters of a modified oxide that can serve to pin the alloy structure, thus providing enhanced mechanical properties.
The nanofeatures of NFA may have any shape, including, for example, spherical, cuboidal, lenticular, and other shapes. The mechanical properties of the nanostructured ferritic alloys may be controlled by controlling, for example, the density (meaning the number density-number of particles per unit volume) of the nanofeatures in the matrix, the composition of the nanofeatures, and the processing used to form the article.
The alloy matrix of the NFA includes iron and chromium. Chromium is important for both phase stability and corrosion resistance, and may thus be included in the NFA in amounts of at least about 5 wt %. Amounts of up to about 30 wt % may be included. In one embodiment, chromium in the alloy matrix is in a range from about 9 wt % to about 14 wt % of the alloy.
In one embodiment, the alloy may have titanium and yttrium. The titanium and yttrium may be present in the metallic or alloy form as a part of the matrix of the alloy, or may be present in the dispersions as a part of the particulate phases of the alloy. In some embodiments, the titanium is present in the alloy in a range from about 0.1 weight percent to about 2 weight percent and yttrium from about 0.1 weight percent to about 3 weight percent of the alloy. In certain embodiments, the alloy matrix includes from about 0.1 weight percent titanium to about 1 weight percent titanium. In addition to its presence in the matrix, titanium and yttrium may play a role in the formation of the oxide nanofeatures, as described hereinbelow.
Vanadium may also be present in the alloy matrix in certain embodiments, where it may serve to strengthen the alloy through the formation of precipitates or by altering phase stability. In some embodiments, the vanadium is present in a range from about 0.1 weight percent to about 2 weight percent, and in particular embodiments the range is from about 0.1 weight percent to about 1 weight percent.
In one embodiment, the alloy includes a population of particulate phases dispersed within the matrix. As used herein, “dispersed within the matrix” would include the dispersion of the particulate phases in the grains and grain boundaries of the matrix. In one embodiment, the particulate phases are substantially dispersed in the grain boundaries of the iron and chromium containing matrix.
The population of particulate phases may include at least two subpopulations. A first subpopulation of particulate phases may be the above-described nanofeatures, providing enhanced tensile and creep properties to the alloy. The nanofeatures of the first subpopulation have a median size less than about 20 nanometers (nm). In a particular embodiment, the particulate phases of the first subpopulation have a median size less than about 10 nm.
The first subpopulation of particulate phases may include a complex oxide. A “complex oxide” as used herein is an oxide phase that includes more than one non-oxygen elements. The complex oxide may be a single oxide phase having more than one non-oxygen elements such as, for example, ABO; or may be a mixture of more than one simple oxide phases (having one non-oxygen element) such as, for example AxByOz. The examples included here are without accounting for any charge balance, hence will include the oxides of elements of different valencies and deviations from stoichiometry.
In one embodiment, an oxide phase may be added to the alloy matrix, and processed to precipitate nanofeatures of the first subpopulation. At least a part of the added oxide phase may be dissolved in the alloy structure and precipitate as the nanofeatures of same oxide or some other oxide phase. In one embodiment, the new oxide in the NFA may include the transition metals present in the starting materials and the metallic element(s) of the initial oxide addition.
In one embodiment, the particulate phases of the first subpopulation include at least two elements from the group of yttrium, titanium, aluminum, zirconium, hafnium, and magnesium. The particulate phases may include a combination of two or more simple oxides; a combination of one or more simple oxide and one or more complex oxides; or a combination of multiple different complex oxides. In a particular embodiment, the particulate phases of the first subpopulation includes a complex oxide with a single phase including more than one non-oxygen elements, such as for example, an yttrium titanium oxide; an yttrium titanium silicon oxide; an aluminum titanium oxide; a magnesium titanium oxide; a zirconium titanium oxide; hafnium titanium oxide; a magnesium zirconium oxide; zirconium hafnium oxide; a yttrium zirconium oxide; a yttrium magnesium oxide; a yttrium zirconium titanium oxide; or a yttrium aluminum titanium oxide.
The nanofeatures of the first subpopulation of particulate phases enhance the tensile and creep properties of the alloy. However, further enhanced mechanical properties of the alloy are desired for certain high temperature and harsh environment applications. The mechanical properties of the alloy may be further increased by an increment in the number density of the first subpopulation of particulates; by an addition of hard second phases of multiple length scales; or by a combination of the higher number density and the second phase addition. However, increasing the number density or adding the second phases may embrittle the alloy thereby decreasing the ductile property of the alloy. It is known in the art that as the volume fraction of the nanofeatures or the hard second phases increases in the alloy, the ductility of the material drops. Therefore, in the case of traditional oxide dispersion strengthened alloys and in NFAs, the volume fraction of any oxide features and any other hard, non-oxide second phase particles are limited to a nominal amount so that the ductility of the material does not drop to insufficient levels.
Traditionally, the total volume fraction of the dispersed particles in an NFA is restricted to less than 2 volume percent of the alloy. A volume fraction of the first subpopulation of particulate phases in the current alloy may vary from about 0.1 volume percent to about 5 volume percent of the alloy. In a particular embodiment, the volume percent of the first subpopulation of particulate phases is in a range from about 1 volume percent to about 5 volume percent of the alloy.
In one embodiment of the current invention, an alloy having a second subpopulation of particulate phases along with the above-described first subpopulation of particulate phases is presented without forgoing the ductility of the alloy. The alloy has enhanced tensile and creep properties as compared to an NFA, while maintaining a desirable level of ductility. The second subpopulation of particulate phases may have a median particulate size in a range from about 30 nm to about 10 microns. In one embodiment, the second subpopulation of particulate phases has a median size in a range from about 50 nm to about 3 microns. A concentration of the second subpopulation of particulate phases in the alloy may vary from about 1 volume percent of the alloy to about 15 volume percent of the alloy. In one embodiment, the concentration of the second subpopulation is in an amount from about 1 volume percent to about 6 volume percent of the alloy. In a particular embodiment, the population of particulate phases (including both first subpopulation and second subpopulation) is in a range from about 2 volume percent to about 6 volume percent of the alloy.
The particulate phases of the second subpopulation may include oxides or non-oxide phases. In one embodiment, the second subpopulation includes an oxide phase, a boride phase, or a combination of an oxide and boride phase. In one embodiment, the concentration of total oxygen in the alloy is in a range from about 0.1 weight percent to about 0.6 weight percent of the alloy.
The oxide or boride phases may be added to the alloy during processing to further strengthen the alloy. One embodiment of the present invention is a method of forming an alloy having a precipitated first subpopulation of particulate phases and an added second subpopulation of particulate phases. The process of fo ling the alloy may start from melting starting materials such as, for example, iron and chromium to form an initial melt. Melting may be by any of the methods known in the art. A vacuum induction melting process may be conveniently used to melt the starting material.
The melted material may be atomized to form an alloy powder that can be milled along with the added oxide material to form a milled alloy powder. The milled alloy powder may be consolidated at a first temperature to precipitate the first subpopulation of particulate phases having a complex oxide with a desired size in a desired volume percent range. The majority, if not substantially all, of the added oxide is dissolved into the alloy matrix during powder attrition and participates in the formation of the aforementioned nanofeatures when the powder is raised in temperature during the consolidation process.
In one embodiment, the second subpopulation of particulate phases is established by adding and mixing an oxide, boride, or a combination of an oxide and boride particulate phase to the milled alloy powder. The composition, size and volume ratio of the second subpopulation of particulate phases may be similar to the added phases.
In an alternate embodiment, an oxide, boride, or a combination of an oxide and boride may be added before the consolidation of the milled alloy powder. The added oxide, boride, or a combination of an oxide and boride may partially react with the matrix. with another added phase, or the first subpopulation of particulate phase, and may remain or form a second subpopulation. Based on the reactivity of the added phase with the other constituents of the alloy-in the making, the composition, size, and volume percent of the second subpopulation of particulate phases may be the same or different from that of the added phases.
Depending on the elements that were added, and thermodynamically favored compositions after certain subsequent process steps to which different NFAs were subjected to, further additional elements may be present in the alloy in order to form oxide or boride particulate phases. The elemental concentration ranges of two exemplary alloy compositions in the ranges of weight % of the alloy are shown in Table 1 when borides or oxides are present in the alloy.
In one embodiment, the alloy includes an in-situ precipitated second subpopulation of particulate phases along with the precipitated first subpopulation of particulate phases. The first subpopulation of particulate phases may be precipitated using the aforementioned process methods. A temperature in a range from about 500° C. to about 1300° C. may be used in the consolidation process to precipitate out the particulate phases of the first subpopulation. Transition metals, such as iron, chromium, titanium, molybdenum, tungsten, manganese, silicon, niobium, aluminum, niobium, or tantalum from the alloy matrix may also participate in the creation of the nanofeatures.
The second subpopulation of particulate phases may be in-situ precipitated in the NFA by varying the components of the starting powder, or the additive phases, or by varying the steps of the process of preparation of NFA. The precipitated second subpopulation of particulate phases may include an oxide, boride, carbide, nitride, carbonitride, or an intermetallic phase. The in-situ precipitation of the particulate phases of the second subpopulation may be formed by varying the process steps of milling, consolidating, by changing the temperature, duration of heating, or the heat-treatment cycles at different stages. For example, a second subpopulation of particulate phases may be formed by a prolonged milling or prolonged heat-treatment of the starting materials and the added powders or by processing the milled alloy powders in a second heat-treatment cycle. In another example, a second subpopulation of particulate phases is formed by hot-working the consolidated, milled alloy powder before subjecting them for a second heat-treatment. Hot-working of the consolidated alloy powder may be carried out by forging, hot extrusion, rolling, or any combination of these methods.
Depending on the desired second subpopulation of particulate phases, the starting materials for the formation of the alloy may include more number or quantity of components than the elements that are reflected as a matrix phase in the alloy. For example, the starting materials may include iron, chromium, titanium, molybdenum, tungsten, manganese, silicon, niobium, aluminum, nickel, tantalum, yttrium, carbon, nitrogen. Some of these elements may be present as a part of the matrix phase, as a part of the first subpopulation of particulate phases, or as a part of the second subpopulation of particulate phases in the alloy. The quantity, composition, and percentage ratios of the elements of the starting material may be different in different phases, and may vary with the variation of process steps executed on the starting materials.
In one embodiment, a part of the added oxide phase to the starting materials may be dissolved in the alloy structure and precipitate as the nanofeatures of same or some other nano-oxide phase. Another part of the added oxide phase may remain as it is in the alloy or may react with some other matrix or dispersion elements and convert in to another oxide with a particulate size in the size range of the second subpopulation of particulate phases.
In one embodiment, one or more non-oxygen elements of the added oxide phase may react with some other starting material or dispersion elements and may precipitate into a different, non-oxide particulate phase in the size range of the second subpopulation of particulate phases.
In one embodiment, the non-oxygen elements of the starting materials may partly form the matrix structure, and may partly precipitate as a part of the second subpopulation of particulate phases.
In one embodiment, the non-oxygen elements of the added materials may partly form the matrix structure, and may partly precipitate as a part of the second subpopulation of particulate phases.
In one embodiment, the non-oxygen elements of the added materials may completely precipitate as a part of the second subpopulation of particulate phases. In one embodiment, a substantial part of the starting materials may precipitate as a part of the second subpopulation of particulate phases.
In one embodiment, depending on the variation in the process steps, the elements to form particulate phases of the second subpopulation may be present as a part of the matrix phase; or as a part of the particulate phase of the first subpopulation at one stage and may precipitate as the second subpopulation on further processing, such as, for example, a second heat-treatment, a heat-treatment at a little higher temperature than for the precipitation of the particulate phases of the first subpopulation, or a longer duration heat-treatment at a particular temperature. In one embodiment, a second heat-treatment at a temperature range of about 550° C. to about 850° C. is provided to the formed NFA structure to precipitate the particulate phases of the second subpopulation.
In one embodiment, a precipitated particulate phase of the second subpopulation is an intermetallic phase. Non-limiting examples of the intermetallic phase may include a Laves phase, a Mu phase, a Z-phase, and a Ni3M structure. A Laves phase may have an AB2 structure and may include magnesium, copper, zinc, nickel, iron, tungsten etc. as the constituting elements. In one particular embodiment, the precipitated Laves phase has a composition having the elements selected from the group consisting of molybdenum, niobium, magnesium, iron, zinc, nickel, copper, and a combination thereof. A Laves phase may be formed by annealing the NFA structure (with first subpopulation of particulate phases) at a temperature below about 850° C. In one embodiment, the NFA is annealed at about 700° C. for precipitating Laves phase as the particulate phase of the second subpopulation at the size ranges and volume fraction as mentioned hereinabove. Table 2 provides a list of non-limiting examples, crystal structure, and additional elements of intermetallic phases that may be formed as a particulate phase of the second subpopulation singularly or as a combination with another phase.
While Table 2 lists some of the phases that were observed, or thermodynamically favored structures by the process steps to which different NFAs were subjected to, depending on the starting materials, added elements, and process steps followed, further additional elements may be present in the precipitated particulate phase of the second subpopulation of the alloy in the ranges of weight % of the alloy, as shown in Table 3.
The particulate phase of the second subpopulation may have a combination of added dispersion phases and in-situ precipitated phases. For example, the particulate phases of the second subpopulation may have a precipitated Laves phase, mu phase, a precipitated carbide, nitride, carbonitride, or an intermetallic phase having a Ni3M structure; along with a non-precipitated oxide or boride phase. The non-precipitated (added) dispersion phases may be added to the NFA structure before the precipitation of the particulate phases of the first subpopulation; before the precipitation of the particulate phases of the second subpopulation and processed to have both the precipitated and added particulate phases of the second subpopulation.
The following example illustrates methods, materials and results, in accordance with a specific embodiment, and as such should not be construed as imposing limitations upon the claims.
A vacuum induction melting furnace was charged with the following composition: Fe-14Cr-0.4Ti-3W-0.5Mn-0.5Si (wt %). Once the alloy was molten and well mixed, it was atomized via argon gas. The powder was sieved to a final cut size of about +325/−100 and sealed in a container. The powder was then transferred to an attrition vessel. In addition to the atomized powder, 0.25 wt % of yttrium oxide and 5 mm diameter steel balls were added to the attrition vessel. The balls were added such that the ball to powder ratio was 10:1 by mass. The powders were then milled for approximately 20 hours or until the yttrium oxide was dissolved in the metal matrix. The powder was separated from the steel balls during unloading of the vessel, while under inert gas. The powder was then loaded into a container (can) for hot isostatic pressing (HIP). The can was then evacuated at room temperature until a leak-back rate of about 15 microns/hour or better was reached. Once evacuated and sealed, the HIP can was subjected to HIP at about 30 ksi for 4 hrs at a temperature of about 1000° C. During this consolidation step, the first dispersion of complex oxides was precipitated.
Following HIP, the material in the HIP can was heated in a furnace with flowing argon to a temperature of about 1000° C. The can was then transferred to an open die forging press and the height was reduced by about 60% at a strain rate of about 0.6 min−1. The forged material was then allowed to cool.
After forging, the transmission electron microscopy (TEM) analysis of the microstructure of the material shows a first dispersion of nm-size complex oxides. The volume fraction of the dispersion, as measured through small angle xray scattering (SAXS) was about 0.8-1 vol %. The size of the particles of the first dispersion, also measured through SAXS, is between about 2-5 nm. A second dispersion of larger particles was not evident here in the post-forging stage.
The forged material was once again heated in a furnace for about 5 hrs at a temperature of about 700° C., to force the precipitation of a second dispersion of particles. TEM analysis of this sample was carried out and a Fe,W based Laves phase was found to be present in the microstructure. The concentration of this second dispersion of particles was measured using image analysis software and was found to be approximately about 2 vol % in this example. The average size of the Laves phase was measured to be between about 100 nm and 500 nm.
While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 13931108 | Jun 2013 | US |
| Child | 15607775 | US |
