Patent Application
20080142126
The invention is related to graded metallic structures. More particularly, the invention is related to metallic structures having a gradation in grain size. The invention is also related to a method of making a metallic structure having a gradation in grain size.
Materials having spatial gradients in microstructure or composition are of considerable interest in disciplines as diverse as tribology, geology, optoelectronics, biomechanics, fracture mechanics, and nanotechnology. The potential for improved mechanical properties in graded metallic structures is attractive for many high temperature applications. In particular, metallic structures with graded grain size may achieve a desirable balance of thermal fatigue resistance and creep resistance in a single material. Achieving metallic structures with fine grain size and a systematic gradation in grain size has proven to be a very challenging task. Many deposition techniques such as thermal spraying, electrodeposition, and electrophoretic deposition have been explored as means for preparing ultra-fine grained, graded metallic coatings. These methods generally have not been successful in producing bulk metallic structures having high strength and high temperature stability. There remains a demand for materials having a graded microstructure, especially for materials with a proper balance of thermal fatigue resistance and creep resistance. There is also a demand for methods to produce bulk metallic structures having an engineered gradation in grain size.
The present invention meets these and other needs by providing a metallic structure having a gradation in grain size. The smaller grains at one portion of the material may provide thermal fatigue resistance for example, and bigger grains at another portion may provide creep resistance, for example.
One embodiment of the invention is a metallic structure having a graded microstructure. The metallic structure comprises a graded region comprising a plurality of grains having a gradient in grain size, varying as a function of position, between a first median grain size at an outer region and a second median grain size at an inner region and a plurality of dispersoids dispersed within the microstructure. The first median grain size is different from the second median grain size.
Another embodiment is a method for forming a metallic structure having a graded microstructure. The method comprises: providing a metallic structure comprising at least one reactive species; diffusing at least one reactant at a controlled rate from an outer region of the metallic structure towards an inner region of the metallic structure to form a gradient in reactant activity; reacting the reactant with the reactive species to form a plurality of dispersoids; and heat treating the metallic structure to achieve grain growth so as to form a graded microstructure. The graded microstructure comprises a graded region comprising a plurality of grains having a gradient in grain size. The grain size varies as a function of position between a first median grain size at an outer region and a second median grain size at an inner region. The microstructure further comprises a plurality of dispersoids dispersed within the microstructure. The first median grain size is different from the second median grain size.
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawing, wherein:
In the following description, like reference characters designate like or corresponding parts throughout the several views shown in the figures. It is also understood that terms such as “top,” “bottom,” “outward,” “inward,” “first,” “second,” and the like are words of convenience and are not to be construed as limiting terms. Furthermore, whenever a particular aspect of the invention is said to comprise or consist of at least one of a number of elements of a group and combinations thereof, it is understood that the aspect may comprise or consist of any of the elements of the group, either individually or in combination with any of the other elements of that group.
Referring to the drawings in general, it will be understood that the illustrations are for the purpose of describing one embodiment of the invention and are not intended to limit the invention thereto.
For the purposes of understanding the invention, the term “a graded microstructure” is meant to describe a microstructure wherein median grain size varies as a function of position. “Median grain size”, implies a median grain size in a selected region. In some embodiments, the gradation is substantially continuous, but this does not always have to be the case. For example, the rate-of-change in grain size may itself vary from region to region, increasing slightly in some regions, and decreasing slightly in others. Any and all of these variations in gradations are meant to be encompassed by the term “graded”. The specific grain size profile for a given metallic structure may depend on various factors, e.g., required mechanical and elastic properties, thermal cycling ranges; material composition, actual grain size, and thickness of the metallic structure.
Schematic representation of a metallic structure according to one embodiment of the present invention is shown in
The composition of the metallic structure depends on the end-use application. The metallic structure comprises any suitable metal or a metal alloy. Examples of some suitable metals include, but are not limited to, cobalt, nickel, iron, titanium, various combinations of these, and alloys thereof. In a particular embodiment, the metallic structure comprises a material selected from the group consisting of a cobalt-based super alloy, a nickel-based super alloy, and a titanium-based alloy. In one exemplary embodiment, the metallic structure comprises a nickel-based super alloy. Examples of some suitable alloys include, but are not limited to, alloys designated by Universal Numbering System for Metals and Alloys (UNS) UNS N07718, UNS N13100, UNS N09706; alloys designated by General Electric Company trademarks MX4, RENE104, RENE95, RENE88DT; and UDIMET 720 (a trademark of Special Metals Corporation). In a particular embodiment, the alloy comprises alloy UNS N07718. In another embodiment, the alloy comprises RENE88DT. In yet another embodiment, the alloy comprises MX4.
The resistance to fatigue crack initiation and propagation, in metals and alloys, is known to be influenced by the grain size. Fatigue endurance limit typically increases with decrease in grain size. Studies on grain size variations in metallic materials, in the nano regime, have shown that nanosized grains exhibit enhanced resistance to high cycle fatigue. Accordingly, the grain sizes in the outer and the inner regions are selected based on the elastic properties required in the two regions.
In one embodiment, the first median grain size (in the outer region) has a value in the range from about 100 nanometers to about 1 micrometer. In another embodiment, the first median grain size (in the outer region) has a value in the range from about 100 nanometers to about 500 nanometers. In another embodiment, the first median grain size has a value in the range from about 100 nanometers to about 200 nanometers. In one embodiment, the second median grain size (in the inner region-core region) has a value in the range from about 10 micrometers to about 100 micrometers. In another embodiment, the second median grain size has a value in the range from about 10 micrometers to about 50 micrometers. In one embodiment, the outer region comprises a region from a top surface of the metallic structure to about 5% of the depth of the structure. In another embodiment, the outer region comprises a region from a top surface of the metallic structure to about 10% of the depth of the structure. In another embodiment, the outer region comprises a region from a top surface of the metallic structure to about 15% of the depth of the structure. In another embodiment, the outer region comprises a region from a top surface of the metallic structure to about 40% of the depth of the structure.
Such microstructural designs involving graded transitions, from finer surface grain morphology to relatively coarser interior grain morphology may provide gradual transitions in the properties, from a surface layer resistant to high-cycle fatigue to a core region resistant to fatigue damage and crack growth. Further, a fine-grained surface is expected to yield good thermal fatigue resistance, and coarse grains at the inner region may provide creep resistance.
Typically, the metallic structure comprises a plurality of dispersoids dispersed within the microstructure. The plurality of dispersoids comprise at least one material selected from the group consisting of an oxide, a nitride, a boride, a carbide, a nitride, an intermetallic, a carbo-nitride, and an oxynitride. In one embodiment, the dispersoid comprises an oxide. Examples of suitable oxides include, but are not limited to, alumina, zirconia, yttria, hafnia, thoria, titania, ceria, lanthanum oxide, nickel oxide, and erbium oxide. In an exemplary embodiment, the dispersoid comprises yttria. Dispersoids of suitable size dispersed within the metallic matrix, are expected to pin the grain boundaries and thus provide to desired thermal stability and mechanical strength. Typically, at least about 50% of the plurality of dispersoids is disposed at the grain boundaries of the plurality of grains. In a particular embodiment, at least about 90% of the plurality of dispersoids are disposed at the grain boundaries of the plurality of grains.
The median size of the dispersoids is selected so as to obtain desirable mechanical strength and thermal stability. The dispersoids have a median size in the range from about 10 nanometers to about 1 micrometer. In a particular embodiment, the dispersoids have a median size in the range from about 10 nanometers to about 50 nanometers. If the dispersoids have too large a size, they may be less effective in grain boundary pinning.
The metallic structure is structurally stable up to a high temperature, that is, the metallic structure does not undergo a substantial change in crystal structure, grain growth, or morphology. The temperature up to where the metallic structure is stable depends, in part, on the material composition of the metallic structure. In certain embodiments, the metallic structure is structurally stable at a temperature up to about 600° C., in other embodiments, the metallic structure is structurally stable at a temperature up to about 800° C., in yet other embodiments, the metallic structure is structurally stable at a temperature up to about 1000° C., and in yet other embodiments, the metallic structure is structurally stable at a temperature up to about 1100° C.
The metallic structure is a bulk monolithic structure. As used herein, a “bulk monolithic structure” means a three-dimensional bulk structure constituting a single unit without joint. This is in contrast to a body formed of multiple components, such as a laminated, or a multi-layered structure, or a thin film, or a coated layer deposited on a substrate. Accordingly, in some embodiments, the metallic structure comprises the metal or metal alloy having the composition and the microstructure as discussed in the structure embodiments above. The structure may be in the form of a sheet, a plate, a disc, an annular ring, or a bar, or any other useful form. Of course, those skilled in the art recognize that the metallic structures described herein may be coated with other materials as required for particular applications.
In an exemplary embodiment, the metallic structure is in the form of an annular ring 30 as shown in
The metallic structures of the embodiments with substantially high mechanical strength, structural stability, fatigue and creep resistant properties are suitable for various structural components. They are especially suited for aeroengine components such as discs that require multifunctional properties. For example, the central portion of a disc may require high creep resistance, whereas, the periphery of an aeroengine disc faces high thermal fatigue damage. It is expected that the grain size gradation would ensure that the component optimizes itself well to the differential properties across its thickness. This may result in improved life as well as enhanced high temperature performance of the disc. In one embodiment, the metallic structure comprises a gas turbine component. In an exemplary embodiment, the metallic structure comprises a turbine airfoil. In an exemplary embodiment, the metallic structure comprises an aircraft engine disc. It is expected that a fine-grained microstructure of the metallic structure at the periphery may provide desired thermal fatigue resistance. The coarser grains at the core may provide the desired creep resistance.
Another aspect of the invention is to provide a method for preparing a metallic structure having a graded microstructure. A flow diagram of the method for making a membrane structure is shown in
The step of heat treating to achieve grain growth may be conducted simultaneously with the diffusion step or may be conducted subsequent to the diffusion step. The nature of the gradient in reactant activity may be selected by controlling the partial pressure of the reactant.
A metallic structure comprising at least one reactive species is provided in step 42. The selection of reactive species depends on the thermodynamics of the process. Specifically, the more negative the value is for the free energy of formation (i.e. ΔGo) for a particular dispersoids material, the greater the tendency (i.e. the thermodynamic driving force) to form the dispersoid at a given temperature within the matrix metal or metal alloy. As discussed above, the reactive species comprises a material selected from the group consisting of an oxide former, a carbide former, a nitride former, a carbo-nitride, and an intermetallic. In a particular embodiment, the reactive species comprises a plurality of oxide formers. In specific embodiments, the reactive species comprises at least one selected from the group consisting of aluminum, yttrium, zirconia, hafnium, cerium, erbium, and lanthanum. In a particular embodiment, the reactive species comprises yttrium. Yttrium has a strong tendency to form oxides (substantially low free energy of formation). Further, yttria is effective in pinning the grain boundaries of the metallic structure. Therefore, a yttria dispersed metallic structure is expected to yield a desired graded grain structure, on subjecting to a suitable oxidizing atmosphere and temperature treatment.
The gradient in reactant activity formed in step 44 is expected to create a gradient in precipitation concentration, in step 46. The dispersoids pin the grain boundaries of the metallic structure and hence may control the grain sizes of the metallic material. A gradient in grain size may be obtained by controlling the precipitation formation and the grain growth rate. As discussed in detail above, a metallic structure thus processed comprises a graded microstructure such as a metallic structure 10 as shown in
As discussed in detail in the above embodiments, the metallic structure may comprise any suitable metal or a metal alloy. Examples of some suitable metals are cobalt-based super alloys, nickel-based super alloys, and titanium-based alloys. In an exemplary embodiment, the metallic structure comprises a titanium-based alloy.
Generally, diffusing a reactant comprises exposing the metallic structure to an effective activity of the reactant. In some embodiments, the metallic structure may be exposed to a gaseous phase of the reactant. In other embodiments, the metallic structure may be exposed to a solid phase of the reactant. The concentration of the reactants at any region within the metallic structure, in part, depends on the solubility of the reactant within the metallic structure and its diffusivity. Hence for a given metallic structure and reactant system, the nature of the gradient in reactant activity may be achieved by controlling the partial pressure of the reactant. Any specific microstructure or grain size gradation may be achieved by controlling the reactive species dispersed within the matrix, their volume fraction, partial pressure of the reactant, temperature, and time duration of heat treatment, among other parameters.
In certain embodiments, the metallic structure may be exposed to a gaseous phase of the reactant. Typically, the parent metallic matrix comprising one or more reactive species is produced by casting or any other suitable process. Such a metallic structure may be subjected to a reactant by immersing the sample in a mixture of materials capable of releasing the desired reactant. For example, the sample may be subjected to an oxygen partial pressure by exposing to oxide powders at the desired temperature in a controlled atmosphere. The surrounding oxide at least partially decomposes to yield oxygen that can diffuse into the material to form oxide particles dispersed in the matrix by the process of internal oxidation. The extent of the formation of these oxides with respect to the depth of penetration is a function of the partial pressure, temperature and the surface area exposed. The partial pressure of oxygen may be controlled by adjusting the surrounding oxide mixture, its relative proportions as well as the temperature. An apparatus 50 for carrying out such a process is shown in
The various processing parameters may be evaluated for any specific material system. It is explained, by way of example, where article 52 is a metallic matrix comprising yttrium surrounded by a nickel oxide powder 54:
If the alloy with yttrium is surrounded by NiO, without any additional partial pressure of O2, the existing partial pressure at 1000° C. is computed analyzing the reactions.
NiO→Ni+½O2 [1]
½O2→[O]alloy [2]
2Y+3[O]alloy→Y2O3 [3]
Gibbs energy for the above mentioned reactions are shown below.
ΔG1=234300+85.2T J/mol
ΔG2=85353+18.5T J/mol
ΔG3=−1640382.24+245.31T J/mol
From reactions [1] and [2], using the rate constant calculations, it can be computed that the oxygen concentration at 1000° C. would be 0.0024%. This would also mean a partial pressure of 3.56×10−8 torr. Typically, about 10−5 torr oxygen partial pressure would be needed to form a surface layer. (About 10−5 torr oxygen partial pressure would be needed to form a surface layer of aluminum oxide). The passive layer may hinder the flow of reactants through the surface of the structure. Since the partial pressure is three orders of magnitude smaller, partial pressure may be increased using gaseous oxygen to increase the penetration depth.
From a similar analysis, it can be shown that very small quantity (10−39 torr) of oxygen is needed to react with yttria to form Y2O3. This would mean that Y2O3 would form instantaneously.
To estimate the penetration distance X, the following equation may be used:
X=(2Co/vCs*Dt)½ [4]
where, Co is the concentration of the reactant, v is the stoichiometric value of the reactive species to the reactant, Cs is the concentration of the reactive species, and Dt is the diffusivity of the reactive species. Using the value of oxygen concentration at the surface and the bulk diffusivity of O in Ni as 1.54×10−8 cm2/sec, the penetration distance would be about 40 micrometers. By varying the temperature and the mixture of the oxides to change the partial pressure of oxygen, the penetration distance may be optimized.
Diffusing the reactant at a controlled rate comprises providing the reactant at a controlled partial pressure. The partial pressure required for any reactant-metallic system may be computed, as discussed in detail below. Typically, heat treating the metallic structure to achieve grain growth comprises heating at a temperature greater than about ⅔rd of the melting temperature, as measured on an absolute scale. In some embodiments, the temperature is in a range from about 600° C. to about 1200° C. The exact temperature profile chosen depends on the composition of the metallic structure.
As discussed above, the reactant comprises a material selected from the group consisting of oxygen, boron, carbon, and nitrogen. The dispersoid comprises a material selected from the group consisting of an oxide, a nitride, a boride, and an oxynitride. The first median grain size has a value in the range from about 100 nanometers to about 1 micrometer. The second median grain size has a value in the range from about 10 micrometers to about 50 micrometers.
In an alternative embodiment, the reactant for reacting with the reactive species may be obtained by decomposing a plurality of precursor particles dispersed within the metallic structure. The precursor particles are chemically less stable than the dispersoids. On decomposition, the precursor particles decompose into a product comprising a plurality of secondary reactive species and secondary reactants. The secondary reactants may be further reacted with the reactive species to form dispersoids. Dispersion of precursor particles within the metallic structure gives an additional degree of freedom in altering the reactant activity profile within the metallic structure. This change in the reactant activity profile may be utilized for altering the microstructure of the processed metallic structure.
The metallic structures and methods disclosed herein provide many advantages over conventionally used methods. The method is capable of providing a material having a gradient microstructure. These graded metallic structures may provide multifunctional capabilities in a single component and may also enable high temperature performance of the metallic structure. Metallic structures with a hole (rotating parts) or multiple holes (internal cooling holes) may be processed using the disclosed method to achieve several graded regions.
The embodiments of the present invention are fundamentally different from those conventionally known in the art. There have been reports of graded metallic structured layers. In such cases, the layers are extremely thin (less than a few micrometers). The metallic structures disclosed herein provide bulk structures with the right balance of creep and fatigue properties within the same monolithic structure. The embodiments of the invention provide simpler and versatile methods to obtain bulk structures of graded metallic structures.
The following example serves to illustrate the features and advantages offered by the present invention, and not intended to limit the invention thereto.
The following examples describe the preparation method a graded metallic structures.
Example: Method for fabricating a graded metallic structure by internal oxidation of yttria using nickel oxide.
The part made of Ni based superalloy with nano-structured grains is placed in a bath of nickel oxide powder. The superalloy has the reactant solutes such as yttria that would readily oxidize. This whole set up is placed in an inert gas furnace (if the component is big) or encapsulated in a glass fixture (for small components). Depending on the partial pressure of oxygen required to form the surface oxide layer such as AlO (about 10−5 torr), the excess partial pressure could be supplied by passing O2 gas through the furnace or the fixture. The internal oxidation results in the reactant solute getting oxidized to form the oxide particles in the alloy matrix preferrably at grain boundaries due to enhanced rates of diffusion. The extent of the depth of oxidation is proportional to the temperature and the partial pressure of oxygen. The combination is optimized to get the correct size distribution of the oxide particles up to required depths.
While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. 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. For example, it should be understood that though the above embodiments are discussed with respect to a airfoil disc, the embodiments of the invention may be utilized in any other metallic component, in which the excellent creep and fatigue resistant of these graded metallic structures are essentially beneficial. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.
