Patent Application
20080145691
This application claims priority under 35 U.S.C. §119 to Patent Application number A200613252, filed in the Ukraine on Dec. 14, 2006, the entirety of which is incorporated by reference herein.
The invention relates generally to high strength articles and methods of making the same, and more specifically to articles having a continuous gradient in grain size and methods of making the same.
The continued effort to design and build more powerful and more efficient turbo-machinery, such as gas turbines, steam turbines, and aircraft engines, requires the use of materials having enhanced performance capabilities over a broad range of temperatures. Such performance enhancements require state-of-the-art materials with stability at high temperatures as well as improved mechanical properties such as strength, fatigue resistance, and creep resistance.
Improved material strength is conventionally achieved in a number of ways including grain refinement, solid solution strengthening, composite strengthening, or dispersoid strengthening. Strengthening alloys using grain refinement applies a mechanism referred to as Hall-Petch strengthening. Hall-Petch strengthening relates to dislocation pile-up at grain boundaries and the stress required to propagate dislocations across grain boundaries. According to the Hall-Petch definition of strengthening, the strength of a material is inversely proportional to the square root of the grain size. Grain refinement to nano-scale and sub-micron scale results in a large number of grain boundaries that serve as barriers to dislocation motion, thus increasing the overall strength of the material.
Manufactured articles often have competing requirements and in some applications would benefit from a microstructure engineered to optimize local properties. For example, an airfoil in a gas turbine simultaneously requires good resistance to thermal fatigue (surface cracking associated with thermal cycling during operation) and resistance to creep (the elongation under steady state load at elevated temperatures). A fine grain size can provide high tensile strength and resistance to thermal fatigue. On the other hand, a coarse grain size can provide resistance to creep. Therefore, an airfoil is an example of an article that may benefit from having an engineered structure comprising both fine and coarse grains disposed in strategic regions on the article.
Accordingly, there is a need in the art for improved high strength materials and improved articles having a controlled variation in grain size to enable better overall material performance.
Embodiments of the present invention meet these and other needs. One embodiment is an article comprising an alloy having a minor phase dispersed within a matrix phase and a plurality of substantially equiaxed grains. The article further comprises a continuous gradient in grain size from a first grain size at an outer surface of the article to a second grain size at an inner portion of the article, wherein the first grain size is less than the second grain size.
Another embodiment is a method for forming an article. The method comprises providing a feedstock material comprising a minor phase dispersed within a matrix phase; and extruding the feedstock material through a die having a twist channel configured to apply a torsional strain to the feedstock material as it passes through the die to form an extruded billet, wherein the extruding step is performed using a predetermined combination of temperature, strain, and strain rate such that a. during the extrusion step, the temperature of the feedstock is maintained at below two thirds of the absolute melting temperature of the feedstock material, b. the feedstock is plastically deformed without substantially damaging the die, c. the feedstock material undergoes substantially no recrystallization during the extruding step, and d. the feedstock material undergoes substantially no dynamic recovery during the extruding step.
Another embodiment is a method for forming an article. The method comprises providing a feedstock material comprising a minor phase dispersed within a matrix phase; and extruding the feedstock material through a die having a twist channel configured to apply a torsional strain to the feedstock material as it passes through the die to form an extruded billet, wherein the extruding step is performed using a predetermined combination of temperature, strain, and strain rate such that a. during the extrusion step, the feedstock temperature is in a range from about two thirds of the melting temperature of the feedstock material to a solvus temperature of the minor phase, b. the feedstock is plastically deformed without substantially damaging the die, and c. the feedstock material undergoes at least partial dynamic recrystallization during the extruding step.
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 drawings, wherein:
An article 10 with a microstructure comprising a continuous gradient (G) in grain size from a first grain size at an outer surface 12 of the article 10 to a second grain size at an inner portion 14 of the article 10, is shown in
While certain embodiments of this invention will be discussed in terms of an airfoil, this is not a limitation of the invention. In fact, article 10 may comprise any engineered article that would benefit from a microstructure with a continuous grain size gradient from a first grain size at an outer surface of the article to a second, larger grain size at an inner portion of the article. Turbine assembly components, such as fan blades, fan disks, compressor blades, compressor disks, airfoils, disks, blisks (bladed disks), ducts, frames, casings, hot gas path components, and the like are examples of components encompassed by embodiments of the present invention, as are other components such as shafts and gears.
Article 10 comprises an alloy. The alloy comprises a first phase, such as, for example, a precipitate phase, that is dispersed within a matrix phase. In some embodiments, the first phase is substantially uniformly dispersed within the matrix phase. “Substantially uniformly” as used herein means that the dispersion of first phase is sufficiently uniform such that the mechanical properties of the bulk material would not vary significantly from those of a material having a completely uniform dispersion. Examples of alloys suitable for use in embodiments of the present invention include, but are not limited to, titanium alloys, superalloys, steels, aluminum alloys, copper alloys, magnesium alloys, refractory metal alloys, platinum-group metal alloys, and intermetallic alloys. One particular example of a titanium alloy is known in the art as Ti6-4, which has a nominal composition, in approximate weight percent, of 6% Al, 4% V, balance Ti. Superalloys may include nickel-based superalloys, cobalt-based superalloys, iron-based superalloys, and nickel-iron-based superalloys. One particular example of a nickel-iron based superalloy is known in the art as Alloy 718, which has a nominal composition, in approximate weight percent, of 18% Fe, 18% Cr, 3% Mo, 5% Nb, 1% Ti, 0.5% Al, 0.04% C, and 0.004% B, with the balance being nickel (Ni).
Article 10 comprises a plurality of substantially equiaxed grains. “Substantially equiaxed” in this context means that the grains have axes approximately equal in length, in contrast to grains typically observed in a casting, where the grains tend to have an elongated, columnar shape, especially near the outer surface of the casting.
Article 10 has a continuous gradient G in grain size from a first grain size at an outer surface 12 of the article 10 to a second grain size at an inner portion 14 of the article. As used herein, “continuous gradient” is defined as a continuous proportional change in grain size over a given distance. The first grain size is smaller than the second grain size, so that article has a comparatively fine grain size at its outer surface 12, and the grain size increases as a function of depth beneath the outer surface 12. “Grain size” as used herein refers to the median grain size at a particular depth as measured using standard grain size measurement procedures, such as, for example, ASTM E112. In one embodiment, the first grain size is in the range from about 10 nanometers to about 1 micrometer. In another embodiment, the second grain size is in the range from about 1 micrometer to about 100 micrometers. In particular embodiments, the article 10 comprises a continuous gradient in grain size from a first grain size, in the range from about 10 nanometers to about 1 micrometer, at outer surface 12 to a second grain size, in the range from about 1 micrometer to about 100 micrometers, at an inner portion of article 10. Articles having grain sizes within the ranges described above may offer several advantages over articles with larger grain sizes, including, for example, higher strength and the potential for superplastic formability.
One method 100 for fabricating article 10 with a microstructure comprising a continuous grain size gradient (G) from a first grain size at an outer surface to a second grain size at a center portion is shown in
In some embodiments, the providing step 102 comprises a pre-processing step to provide the feedstock in a form suitable for use in the method 100. Typically, bulk alloy materials, for example, alloys comprising one or more of titanium (Ti), nickel (Ni), aluminum (Al), zirconium (Zr) and the like, benefit from pre-processing to provide a billet of material as feedstock for the extreme deformation in step 104. For example, if the alloy materials are provided as ingots or castings they are typically processed using a thermo-mechanical process such as extrusion, rolling, or forging, to create a billet. Standard thermo-mechanical processes are commonly used in the art for various alloys to convert these alloys from ingots (which in highly alloyed systems often suffer from high degrees of chemical segregation) to billets having more uniform chemistry and/or structure. In another embodiment, the starting feedstock is provided in the form of powders produced by gas atomization or similar processes. The powder feedstock is processed, typically by hot isostatic pressing or extrusion, followed by thermo-mechanical processing to create a billet. In general, the process of creating a billet from starting material such as ingots or feedstocks is called “billetizing” the material.
The feedstock thus provided comprises a first phase dispersed within a matrix phase. In certain embodiments, where the extreme deformation step is performed within a temperature range where recrystallization of the material is likely to occur, this first phase may serve to pin grain boundaries to inhibit growth of the newly formed grains, thereby retaining a refined grain structure. The first phase is often a phase that serves to strengthen the finished article. For example, in nickel-based superalloys, the first phase may comprise the phase known in the art as gamma prime, and in certain alloy systems such as Alloy 718, the first phase may comprise the phase known as gamma double prime or the phase known as delta. In nickel based superalloy compositions, the first phase can be a gamma-prime precipitate phase. In titanium alloys such as Ti-6Al-4V alloys, the first phase may be a grain-boundary alpha phase or a reinforcement phase, such as TiB2 particulates.
Next, the billet is introduced as feedstock into extreme deformation step 104. Typically, the billet is subjected to extreme shear deformation in a twist die in a process called twist extrusion. Twist extrusion involves extrusion of billets through a die 300 having a twist channel 310, as schematically shown in
In order to accommodate such a wide variety of materials, deformation step 104 is typically applied according to certain parameters that vary in accordance with the material being processed. In one embodiment, the temperature of the feedstock in the deformation process is maintained below about two-thirds of the absolute melting temperature (that is, the temperature as measured using an absolute scale, such as the Kelvin scale) of the feedstock material. This embodiment may be referred to herein as the “low-temperature regime.” In another embodiment, the temperature of the feedstock in the deformation process is in a range from about two-thirds of the absolute melting temperature of the feedstock material to about the solvus temperature of the first phase. This embodiment may be referred to herein as the “high temperature regime.”
Regardless of whether the method is being performed in the high temperature regime or the low temperature regime, the temperature at which twist extrusion is performed is selected to be high enough that the flow stress of the material being twist extruded is substantially below that of the material being used as the tooling. The flow stress depends on the material being used, including its composition and metallurgical condition, the temperature of that material, the strain being applied, and the strain-rate at which the billet is being strained. The flow stress ratio, defined by the compressive yield strength of the tool material divided by the flow stress of the billet material, is a good indicator of success. If this flow stress ratio is less than one, then the tooling will likely deform about the billet, or the billet will not move through the die or tool. If the flow stress ratio is greater than one, and typically greater than 3, and often greater than 5, the tool material is likely to survive the twist extrusion process. Table 1 indicates flow stress ratios of a variety of materials at certain twist extrusion conditions.
In the low-temperature regime, the twist extrusion process is performed using a predetermined combination of temperature, strain and strain rate such that a. during the extrusion step, the temperature of the feedstock is maintained at below two thirds of the absolute melting temperature of the feedstock material, b. the feedstock is plastically deformed without substantially damaging the die, c. the feedstock material undergoes substantially no recrystallization during the extruding step, and d. the feedstock material undergoes substantially no dynamic recovery during the extruding step.
Factors a. and b. above represent a balance in the selection of, among other things, the temperature in this regime. The temperature is sufficiently high to avoid damage to the tooling, but sufficiently low to allow for significant deformation to occur without risk of uncontrolled grain growth. In certain embodiments, such as where the feedstock is a superalloy such as Alloy 718, the temperature of the feedstock is less than about 725° C.; this temperature may be less than about 625° C. in particular embodiments, such as where the feedstock is a titanium alloy such as Ti-6-4. Where the temperature is kept in this low temperature regime, the presence of the first phase is typically not required to maintain grain size because there is not sufficient thermal energy to drive significant grain growth. Accordingly, in some embodiments, the first phase is dissolved into the matrix (generally by a solutionizing heat treatment, commonly performed in the art) to form a single-phase material prior to extruding. The dissolution of the first phase may lower the flow stress of the feedstock, thereby allowing the feedstock to pass more easily through the die at these relatively low temperatures.
Strain introduced into the feedstock during twist extrusion is a function of the angle of die twist and the number of passes through the die to which the feedstock is subjected. The strain that is referred to herein represents the strain measured at an outermost surface of the feedstock material, where strain is at a maximum due to the geometry of the die. In some embodiments, the strain is greater than about 0.2, and in particular embodiments, the strain is greater than about 0.4. Generally, high strain correlates to a high driving force for the nucleation of recrystallized grains, and so a comparatively high amount of strain is typically employed where a fine grain size is desired.
The strain rate is generally determined by how quickly the feedstock is forced through the twist die. In the low temperature regime, as described above, no substantial amount of recrystallization or recovery is permitted to occur during the extrusion step. Accordingly, the strain rates can be relatively fast compared to other regimes where dynamic recrystallization or recovery processes are desired. The latter regimes require slower deformation rates to allow for newly formed dislocations to move and form, for example, into desired sub-structures; the former requires no such consideration. Of course, the strain rate should not be so high as to cause undue transient increases in local flow stress sufficient to damage the die. In certain embodiments, the strain rate is in the range from about 0.1 sec−1 to about 0.5 sec−1.
In the high temperature regime, the twist extrusion step is performed using a predetermined combination of temperature, strain, and strain rate such that a. during the extrusion step, the feedstock temperature is in a range from about two thirds of the melting temperature of the feedstock material to a solvus temperature of the first phase, b. the feedstock is plastically deformed without substantially damaging the die, and c. the feedstock material undergoes at least partial dynamic recrystallization during the extruding step.
In this regime, where the temperature is sufficiently high that recrystallization and grain growth during processing may come into play, the presence of the first phase during the extrusion step serves as an inhibitor to undesirably uncontrolled grain growth. In certain embodiments, such as where the feedstock is a titanium alloy such as Ti-6-4, the temperature of the feedstock is at least about 625° C.; this temperature may be at least about 725° C. in particular embodiments, such as where the feedstock is a superalloy such as Alloy 718. The temperature is typically bound on the upper end by the solvus temperature of the first phase, to ensure that sufficient first phase is present to pin the grains of the feedstock during processing. For titanium alloys, for example, this upper temperature limit is typically equal to or less than the beta transus temperature, for example between about 750° C. to about 850° C. For nickel-iron alloy systems such as Alloy 718, this upper temperature limit is typically below about 1000° C.
In some embodiments, the amount of strain introduced into the feedstock material during twist extrusion is the same as described above for the low temperature regime. The rate at which strain is applied in the high temperature regime, however, may differ somewhat from that in the low temperature regime, because in the high temperature regime the strain rate is sufficiently low to allow for at least partial dynamic recrystallization to occur during the extrusion step. As used herein, “partial dynamic recrystallization” means a portion of the microstructure undergoes recrystallization. In certain embodiments, the strain rate is in the range from about 10−4 sec−1 to about 10−2 sec−1.
Regardless of which of the above regimes is used, a post-processing step 106 is applied in some embodiments to provide further property enhancements to the extruded billet or to otherwise form the billet into a desired shape. The post-processing step generally comprises one or more processes used in the metal processing art. Examples of such processes include, but are not limited to, extrusion (such as by conventional extrusion methods), rolling, forging, and heat treating. In some embodiments, one of the primary functions of the post-processing step is to cause recrystallization of grains within the extruded billet. For example, heat treatment or other elevated-temperature processes operated above about two-thirds of the melting temperature (absolute scale) of the material making up the extruded billet is likely to trigger recrystallization where a sufficient driving force (such as retained strain energy from the twist extrusion step) exists within the extruded billet. Generally the post-processing is performed below a solvus temperature for the first phase to avoid uncontrolled grain growth. In certain embodiments, the grain size of the extruded billet material is controlled to provide a balance of desirable properties, as described previously. Fine-grained materials are typically desired for superior strength, for example. In some embodiments, after post-processing, the grains have a median grain size less than about 10 micrometers.
As described previously, the article produced by the above method can have a gradient G in grain size. Where twist extrusion is performed in accordance with embodiments of the present invention, the outer surface of the extruded billet experiences the highest amount of strain, while the centerline of the billet may experience very little strain. As a result, the driving force for recrystallization, and hence the nucleation rate of new grains, is comparatively low at an inner portion of the extruded billet and comparatively high at the outer portion, with a continuous gradient in existing between these points as dictated by the geometry of the die. Where nucleation rates are high, grain size tends to be low when processed according to embodiments of the present invention. The result is that grains are disposed within the extruded billet such that the extruded billet has a microstructure comprising a continuous gradient (G,
The following examples are provided to further illustrate particular embodiments of the invention described above; they should not be construed as limiting the invention in any way.
A feedstock material comprising a nickel-based superalloy or a nickel-iron-based superalloy is provided, and is twist extruded such that the temperature of the feedstock is less than about 725° C. The die used for the extrusion is shaped such that the strain, as measured by the strain at an outermost surface of the feedstock material, is greater than about 0.2. The rate at which the feedstock is forced through the die is controlled so that the strain rate is in the range from about 0.1 sec−1 to about 0.5 sec−1. The extruded billet thus formed may be subjected to a number of repetitions of the twist extrusion step, to increase the total strain accumulated within the material. After twist extrusion, the extruded billet is post-processed to cause recrystallization of grains within the extruded billet.
A feedstock material comprising a titanium alloy is provided, and is twist extruded such that the temperature of the feedstock is less than about 625° C. The die used for the extrusion is shaped such that the strain, as measured by the strain at an outermost surface of the feedstock material, is greater than about 0.2. The rate at which the feedstock is forced through the die is controlled so that the strain rate is in the range from about 0.1 sec−1 to about 0.5 sec−1. The extruded billet thus formed may be subjected to a number of repetitions of the twist extrusion step, to increase the total strain accumulated within the material. After twist extrusion, the extruded billet is post-processed to cause recrystallization of grains within the extruded billet.
A feedstock material comprising a nickel-based superalloy or a nickel-iron-based superalloy is provided, and is twist extruded such that the temperature of the feedstock is in the range from about 725° C. to about 1000° C. The die used for the extrusion is shaped such that the strain, as measured by the strain at an outermost surface of the feedstock material, is greater than about 0.2. The rate at which the feedstock is forced through the die is controlled so that the strain rate is in the range from about 10−4 sec−1 to about 10−2 sec−1. The extruded billet thus formed may be subjected to a number of repetitions of the twist extrusion step, to increase the total strain accumulated within the material. After twist extrusion, the extruded billet is post-processed to cause recrystallization of grains within the extruded billet.
A feedstock material comprising a titanium alloy is provided, and is twist extruded such that the temperature of the feedstock is in the range from about 625° C. to about 1000° C. The die used for the extrusion is shaped such that the strain, as measured by the strain at an outermost surface of the feedstock material, is greater than about 0.2. The rate at which the feedstock is forced through the die is controlled so that the strain rate is in the range from about 10−4 sec−1 to about 10−2 sec−1. The extruded billet thus formed may be subjected to a number of repetitions of the twist extrusion step, to increase the total strain accumulated within the material. After twist extrusion, the extruded billet is post-processed to cause recrystallization of grains within the extruded billet.
Hot rolled Ti-6Al-4V and forged nickel Alloy 718 billets were tested as prototype Ti and Ni-structured alloys, respectively. Twist dies having a rectangular cross-section of 15×25 mm and with 3 different twist angles (30°, 45°, and 60°) were selected for the extrusion process. Prior to the twist extrusion, to achieve a baseline microstructure, the feedstock pieces were heated as follows: Ti-6Al-4V was heat treated at 1000° C. for 1 hour and air cooled; Alloy 718 was heat treated at 1010° C. for 1 hour and air cooled. An auxiliary billet (Zn, Cu or commercially pure Ti) was attached to the front of the feedstock during the twist extrusion process in order to exert a counter-pressure during the deformation. The length and the choice of the auxiliary billet material controlled the level of counter pressure. The use of the counter-pressure billet allows the creation of additional pressure of up to about 1000 MPa. Without the auxiliary billet, the twist deformation, under certain conditions, can result in the formation of microvoids at grain boundary triple points, or even the fracture of the workpiece. The Ti-6Al-4V and Alloy 718 billets were warm extruded by using the twist extrusion process at temperatures in the range between about 500° C. to about 600° C. to avoid dynamic recrystallization. Subsequent heat treatments were then applied to the twist extruded Ti-6Al-4V material to further refine the substructure or grains in the material and place the material in a condition for use in high strength or high temperature structural or rotating components in steam turbines, gas turbines, aircraft engines or other hot gas applications. The twist extruded Ti-6Al-4V material was heat treated and aged according to the schedule in
As discussed above, the continued effort to design and build more powerful and more efficient turbo-machinery, such as gas turbines, steam turbines, and aircraft engines, requires the use of materials having enhanced performance capabilities over a range of temperatures. These components include rotating components, such as fan blades, fan disks, compressor blades, compressor disks, turbine airfoils, blisks, disks, and fixed components, such as ducts, frames, casings, hot gas path components, and airframes. In method 100, bulk metal alloys are processed and subsequently subjected to extreme shear deformation to create a structural material with improved mechanical properties compared to conventional alloys. In addition, method 100 reduces processing inhomogeneities and defects that are commonplace in cast and powder metallurgy components. In fact, the resulting high strength, fine-grained billet material can be used in a variety of applications, over an extremely broad range of temperatures depending on the material system, for example from about 100° C. to about 700° C. for nickel alloys. Applications at the high end of the temperature range can include, as discussed above, high strength structural or rotating components in steam turbines, gas turbines, aircraft engines and other high temperature applications.
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 | Kind |
|---|---|---|---|
| A200613252 | Dec 2006 | UA | national |
