Method of forming oxide dispersion strengthened alloys

Abstract

A method of forming an oxide-dispersion strengthened alloy and a method for forming an oxide-alloy powder where the oxide-nanoparticles are evenly distributed throughout the powder. The method is comprised of the steps of forming an oxide-nanoparticles colloid, mixing the oxide-nanoparticles colloid with alloy-microparticles forming an oxide-alloy colloid, drying the oxide-alloy colloid solution to form an oxide-alloy powder, applying pressure to the oxide-alloy powder, and heating the oxide-alloy powder to a sintering temperature. The oxide-nanoparticles are sized to be between 1-10 nanometers in diameter. The ratio of oxide-nanoparticles to alloy-microparticles should be 1-5% by weight. Heating of the oxide-alloy powder can use a spark plasma sintering process.

Description

FIELD OF THE INVENTION

The present invention is related to a method of forming materials with oxide dispersions. More specifically, the present invention relates to a method of forming alloys with oxide dispersions for material strengthening.

BACKGROUND OF THE INVENTION

One physical attribute that provides flexibility in metals is the presence of dislocations within the lattice structure of the metal thus allowing the metal to flex and bend. A bending force causes the dislocation to propagate through the metal. Such a characteristic has several drawbacks. First, the ability of the dislocation to propagate through the metal reduces the ability of the metal to resist bending and shear forces. Further, the ability of a metal to bend becomes more pronounced at higher temperatures. Secondly, repetitive or excessive bending can cause the dislocations to aggregate and cause metal embrittlement. It is known in the field of material science that if an oxide material is added to a metal alloy where the oxide particles have the proper size, quantity, and a substantially uniform dispersion, then these oxide particles can interrupt the propagation of a dislocation through the metal lattice. As a result, the metal is strengthened and will better resist bending forces at high temperatures. This attribute is especially important in applications such as the hot end of a jet turbine. The turbine discs are exposed to the hot exhaust gasses that are weakened by the high operating temperatures. It is preferable to operate turbines at the highest possible temperature to achieve the highest possible operating efficiencies. Thus, any method to produce a stronger turbine disc, which provides for a higher operating temperature, is highly desirable.

For years, people skilled in the relevant art have known of the advantages of oxide dispersion strengthened alloys. However, it is also known that the current state-of-the-art has not developed technology to form an alloy with evenly dispersed oxide particles of a nanometer size required to interrupt the dislocation propagation. Mixing the oxide-nanoparticles into a molten alloy has failed to result in an even dispersion of oxide particles of the proper size and thus has failed to achieve the sought after theoretical strength increases. This is because oxide particles are immiscible with a molten alloy but will clump when added. What is needed is a method of forming an oxide dispersion strengthened alloy with nano-sized oxide particles where the oxide particles are evenly distributed throughout the alloy lattice structure.

SUMMARY OF THE INVENTION

In a first aspect of the invention, a method of forming an oxide-dispersion strengthened alloy is disclosed. In a first step, a quantity of oxide-nanoparticles is mixed in with a liquid to form a colloidal suspension that has the oxide-nanoparticles evenly suspended in the liquid. Preferably, substantially all of the oxide-nanoparticles are suspended and do not clump into larger oxide-nanoparticle groups.

In a next step, the oxide-nanoparticle colloid solution is mixed with a second quantity of alloy-microparticles. This mixture forms an oxide-alloy colloid. The oxide-nanoparticles adhere to the alloy-microparticles. Preferably, most of the oxide-nanoparticles adhere to the alloy-microparticles and not to other oxide-nanoparticles.

In a further step, the oxide-alloy colloid mixture is dried. Drying can occur through any common drying techniques, such as evaporation, vacuum techniques, freezing, centrifugal, and chemical drying. Once the liquid is removed from the oxide-alloy colloid, remaining is an oxide-alloy powder with a substantially uniform distribution of oxide nanoparticles on alloy-microparticles. In another step, the oxide-alloy powder is heated to a temperature that results in sintering of the alloy particles. A number of heating techniques are contemplated, including applying outside heat to the oxide-alloy powder or generating an internal heat by passing an electrical current through the powder.

In one embodiment of the present invention, the oxide-nanoparticles are between 1 and 10 nanometers in diameter. Preferably, the particles are 5 nanometers or smaller in diameter. In another embodiment, the oxide-nanoparticles comprise 1-5% by weight of the resulting oxide-alloy powder. In yet another embodiment, the alloy-microparticles contain nickel. Further, the alloy-microparticles can contain greater than 50% nickel. In another embodiment of the present invention, the oxide-nanoparticles are formed from either alumina (Al₂0₃), silica (Si0₂), or Yttria (Y₂0₃), or any combination thereof. In a further embodiment of the invention, the oxide-alloy powder is subjected to an axial pressure before or during heating or both before and during heating. The pressure is preferably asserted by an axial die, but other techniques are contemplated, including a graphite die. In yet another embodiment of the invention, the oxide-alloy powder is heated by a SPS (spark plasma sintering) process where DC current is pulsed through the oxide-alloy powder. This electric current causes the pressed oxide-alloy powder to heat rapidly, internally, and substantially evenly. The oxide-alloy is heated until it reaches a sintering temperature. In one embodiment of the invention, the SPS process uses a graphite die to apply pressure to the oxide-alloy powder.

In another embodiment of the present invention, before heating and applying pressure to the oxide-alloy powder, the powder is ball-milled. Ball-milling produces a powder that has an enhanced distribution of oxide-nanoparticles within the alloy powder and forms nano-oxide regions within the alloy-microparticles.

In a second aspect of the invention, a method of forming an oxide-alloy powder is disclosed, wherein the oxide particles are nanoparticle sized and are substantially evenly distributed throughout the powder. In a first step, a quantity of oxide-nanoparticles is mixed with a liquid to form a colloidal solution wherein the oxide-nanoparticles are evenly suspended in the solution. Preferably, substantially all of the oxide-nanoparticles are individually suspended in the solution and do not clump into larger oxide-nanoparticle groups.

In a second step, the oxide-nanoparticle colloid solution is mixed with a second quantity of alloy-microparticles. This mixture forms a new oxide-alloy colloid. The oxide-nanoparticles adhere to the alloy-microparticles. Preferably, most of the oxide-nanoparticles adhere to the alloy-microparticles and not to the other oxide-nanoparticles.

In a third step, the oxide-alloy colloid mixture is dried. Drying can occur through any common drying techniques, such as evaporation, vacuum techniques, and chemical drying. Once the liquid is removed from the oxide-alloy colloid, remaining is an oxide-alloy powder. In one embodiment of the present invention, the oxide-nanoparticles are between 1 and 10 nanometers in diameter. Preferably, the oxide-nanoparticles are 5 nanometers or smaller in diameter. In another embodiment, the oxide-nanoparticles comprise 1-5% by weight of the resulting oxide-alloy powder. In another embodiment of the present invention, before heating and applying pressure to the oxide-alloy powder, the powder is ball-milled, which produces an enhanced distribution of oxide-nanoparticles within the alloy.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is better understood by reading the following detailed description of an exemplary embodiment in conjunction with the accompanying drawings.

FIG. 1 illustrates a flow chart of a method of forming a oxide dispersion strengthened alloy in accordance with the principles of the present invention.

FIG. 2 illustrates a flow chart of a method of creating an alloy powder with evenly distributed oxide-nanoparticles in accordance with the principles of the present invention.

FIG. 3A illustrates the lattice structure of an alloy with a dislocation in accordance with the principles of the present invention.

FIG. 3B illustrates the lattice structure of an alloy with a dislocation moved over one lattice plane in accordance with the principles of the present invention.

FIG. 3C illustrates the lattice structure of an alloy with a dislocation blocked by an oxide dispersion region in accordance with the principles of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The following description of the invention is provided as an enabling teaching of the invention which includes the best, currently known embodiment. One skilled in the relevant art will recognize that many changes can be made to the embodiment described, while still obtaining the beneficial results of the present invention. It will also be apparent that some of the desired benefits of the present invention can be obtained by selecting some of the features of the present invention without utilizing other features. Accordingly, those who work in the art will recognize that many modifications and adaptions to the present inventions are possible and can even be desirable in certain circumstances, and are a part of the present invention. Thus, the following description is provided as illustrative of the principles of the present invention and not in limitation thereof. The scope of the present invention is defined by the claims. The terms “nanoparticle,” “nanoparticle powder,” and “nano powder” are generally understood by those of ordinary skill to encompass the same concept as described herein.

FIG. 1 illustrates a flow chart of steps for a method of forming an oxide dispersion strengthened alloy 100, according to the present invention. The method begins at a step 110. At the step 110, a colloid of oxide-nanoparticles is formed. The oxide-colloid is formed by adding nanoparticles, preferably 1-5 nanometers in their greatest dimension, to a liquid that has the properties that the nanoparticles can be suspended, prevents clumping of the nanoparticles, and that the liquid can readily be removed. Preferably, the liquid does not react with the oxide-nanoparticles. The quantity of liquid added to the nanoparticles is preferably sufficient to suspend the oxide-nanoparticles and the quantity of alloy-microparticles to be later added. It will be appreciated that the oxide-nanoparticles can be added to the liquid or the liquid can be added to the oxide-nanoparticles, or a combination of the two.

Alternatively, both the oxide-nano particles and the liquid can be concurrently added to a mixing vessel. The oxide-nanoparticles can be any suitable material that does not react with the alloy-microparticles or liquid. Material such as Alumina (Al₂0₃), Silica (Si0₂), or Yttria (Y₂0₃) that do not react with alloy metal can be used. Preferably, Yttria-nanoparticles are used. The quantity of oxide-nanoparticles used should be measured in proportion to the dry weight of the alloy-microparticles. Preferably, a quantity of oxide-nanoparticles should be selected so that the oxide-nanoparticles comprise 1-5% by weight of the resulting oxide-alloy powder.

In the step 120, the oxide-nanoparticle colloid is mixed with the alloy-microparticle powder. Within the scope contemplated by this invention is either the oxide-nanoparticles colloid being added to the alloy-microparticle powder or the alloy-microparticle powder being added to the oxide-nanoparticle colloid. The resulting mixture should be mixed until the oxide-nanoparticles are evenly distributed with the alloy-microparticles, resulting in an oxide-alloy colloid. It is expected that this is because of Van der Waal forces. The oxide-nanoparticles adhere to the alloy˜microparticles. This adherence of the oxide-nanoparticles to the alloy-microparticles prevents clumping of oxide-nanoparticles and provides a substantially even distribution of oxide-nanoparticles throughout the colloid. Mixing can be accomplished by any standard means including stirring, shaking, tumbling, ultrasound, and pouring.

In the step 130, the oxide-alloy colloid is dried. In this step, substantially all of the liquid is removed such that only the alloy-microparticles with an even distribution of oxide-nanoparticles remain. This step 130 occurs after the mixing of the oxide-nanoparticles and the alloy microparticles. Evaporative drying is preferred, but other techniques are contemplated, such as heat drying, vacuum drying, freeze drying, centrifugal drying, chemical drying, or spray gun drying.

In the method of forming an oxide-alloy metal, there is an optional step for the ball milling of the dried oxide-alloy powder before compression and heating to form the oxide-dispersion strengthened alloy. In the step 132, a decision is made whether to ball mill the dried oxide-alloy powder. If the decision is taken to mill the oxide-alloy powder, then step 135 is performed. If the oxide-alloy powder is not to be milled, then the method continues on to step 140.

In the step 135, the optional processing by ball milling the dried oxide-alloy powder is disclosed. The pounding of the milling-balls is expected to embed the oxide-nanoparticles into the alloy as the microparticles are ground together and broken apart. This ball-milling is expected to further improve the uniformity of distribution of oxide-nanoparticles at the microstructure level within the alloy-microparticles.

In the step optional step 140, pressure is applied to the oxide-alloy powder. Preferably, the pressure is an axial pressure of thousands of pounds per square inch, applied by a mold, preferably a graphite mold. Pressures of 30-50 tons per square inch are typical. The pressure can be applied through a hydraulic press, preferably a punch press. However, other means of applying pressure are contemplated. The pressure can be applied before or during heating. Further, the pressure can be varied during the sintering step 150, preferably increasing the pressure as heat is applied. To handle these pressures, the mold for holding the oxide-alloy powder needs to be able to handle the applied pressures and, if sintering occurs within the same mold, the sintering temperature. The step 140 is optional. The oxide-alloy powder can be sintered without the application of pressure, but preferably pressure is applied to the oxide-alloy powder during sintering.

In the step 150, the oxide-alloy powder is heated to a sintering temperature. The heat can be applied externally or though a SPS (spark plasma sintering) process. The SPS technique pulses high current, preferably though the graphite press molds, and also through the oxide-alloy powder. One advantage of the SPS technique is the uniformity of applying heat. The powder, being electrically heated, heats substantially evenly from inside. The second advantage of the SPS process is the speed of the sintering process. A turbine disc can be formed in minutes as opposed to hours as is typical with a forging process.

FIG. 2 illustrates the steps for the method of forming an alloy microparticle powder with evenly dispersed oxide-nanoparticles. The method begins in step 210 with the forming of an oxide-nanoparticle colloide, which is as described above in step 110 of FIG. 1. The step 220 of mixing the oxide-nanoparticle colloid with alloy-microparticles is as described above in step 120 of FIG. 1. The step 230 of drying the oxide-alloy colloid is as described above in step 130 of FIG. 1. The step 232 of making the decision to optionally ball-mill the oxide-alloy powder is as described above in step 132 of FIG. 1. The optional step 235 of ball-milling the dried oxide-alloy powder is as described above in step 135 for FIG. 1. The process ends at step 240 at either the completion of the ball-milling step 235 or upon a decision in step 232 not to ball-mill the oxide-alloy powder.

FIG. 3A illustrates one mechanism within a lattice structure by which a metal deforms or bends. A metal 300 is typically formed in a crystalline lattice structure with layers of interconnected atoms 310. Some of the lattice layers don't run the entire length of the lattice and terminate between other layers, creating a distortion of the other layers. These layers are referred to as dislocation layers. The dislocation layer 313 is shown terminated between two of the lattice layers, 312 and 313 respectively. The lattice layers 312 and 314 are shown adjacent to the dislocation layer 313 and distorted by the dislocation layer 313. When a shear or bending force 320 is applied to the lattice structure 300, the dislocation layer 313 can propagate as a wave through the metal. One manner by which the displacement wave front shifts through the lattice is for the distortion layer of the lattice to shift and connects to the adjacent layer. FIG. 3B shows the lattice structure after a displacement of one lattice plane. As shown in FIG. 3B, the dislocation layer 313 shifts to the right by one lattice plane width. The distorted plane 314 in FIG. 3A becomes the non-distorted plane 314′ in FIG. 3B. The dislocation layer 313 in FIG. 3A shifts and joins the adjacent layer on the right, becoming a distorted layer 312′ in FIG. 3B. Upper part of the distorted lattice layer 312 become the propagated dislocation layer 312′ in FIG. 3B. The lattice layer 311 in FIG. 3A becomes a distorted layer as shown by 311′ in FIG. 3B.

FIG. 3C illustrates the metal lattice structure of an oxide dispersion enhanced metal. When a shear force 330 is placed on metal 300″, the propagation wave will travel until it encounters the oxide dispersion 340. This oxide dispersion impedes the propagation of the displacement wave. This resistance to displacement strengthens the metal to shear and bending forces.

An exemplary use of the present invention's method for producing an oxide-dispersion strengthened alloy is in the fabrication of a jet turbine disc. First a quantity of oxide-nanoparticle powder is selected for forming the turbine disc, preferably Yttria. The quantity is selected such that the formed disc contains 1-5% Yttria by weight. The Yttria nanoparticles are mixed with a liquid to form a colloid of Yttria nanoparticles. The nanoparticles and liquid are mixed and stirred to prevent the nanoparticles from clumping and forming larger particles. To this Yttria-nanoparticle colloid, an alloy-mircoparticle powder is added. Common alloys used in the fabrication of a turbine disc are nickel super-alloys, such as the Rolls Royce RR1000 or General Electric GE R80. RR1000 is comprised of 50-60% nickel, 14-15% chromium, 14-19% cobalt, 4-5% molybdenum, 3% aluminum, and about 4% titanium. The RR1000 alloy is added as an alloy-microparticle powder to the Yttria-nanoparticle colloid. This produces an Yttria-alloy colloid where Yttria-nanoparticles attach to the surface of the alloy-microparticles. Next, the Yttria-alloy colloid solution is then dried. This forms a powder with a uniform distribution of oxide-nanoparticles on the surface of the microparticles. Next, the Yttria-alloy powder is ball-milled. The optional ball-milling process further improves the uniformity of distribution of oxide-nanoparticles within the oxide-alloy powder. The powder is then placed into a graphite mold and subjected to a high pressure of up to 50 tons per square inch. DC electricity is pulsed through the graphite mold and through the oxide-alloy powder, heating it internally to a sintering temperature. The advantage of electrical heating is that the heating occurs substantially evenly throughout the powder and can be quickly heated. It is expected that the resulting disc is stronger at high operating temperatures than a forged disc formed from an alloy without uniform oxide dislocations. This example is exemplary of the two significant advantages to using the present inventive method to obtain an oxide-dislocation strengthened alloy. First, there is not a cost-effective alternative for creating an alloy with uniformly distributed oxide dispersions having the small geometry needed to stop dislocations from propagating through the alloy. The second advantage is that, in using this method, high strength metal structures can be formed in a cost-effective manner. For example, the forming of a turbine disc is currently accomplished with a forging process, which is both time consuming and expensive. By using an oxide dislocation strengthened powder along with a spark plasma sintering process, a turbine disk can be formed in minutes instead of hours, a small fraction of the time used for prior art methods.

The present invention has been described in terms of specific embodiments incorporating details to facilitate the understanding of principles of construction and operation of the invention. Such reference herein to specific embodiments and details thereof is not intended to limit the scope of the claims appended hereto. It will be readily apparent to one skilled in the art that other various modifications may be made and equivalents may be substituted for elements in the embodiments chosen for illustration without departing from the spirit and scope of the invention as defined by the claims.

Claims

1. A method of forming an oxide-dispersion strengthened alloy comprising the steps: a. mixing a first quantity of oxide-nanoparticles with a liquid forming a colloid of oxide-nanoparticles, wherein the oxide-nanoparticles are 1-5 nanometers in the largest dimension;b. mixing the colloid of oxide-nanoparticles with a second quantity of alloy-microparticles forming an oxide-alloy colloid, wherein the oxide-nanoparticles are substantially uniformly distributed throughout the colloid, and wherein substantially all of the oxide-nanoparticles adhere to the alloy-microparticles and not to the oxide-nanoparticles;c. drying the oxide-alloy colloid, forming an oxide-alloy powder; andd. heating the oxide-alloy powder to a sintering temperature.

2. The method of claim 1, wherein the first quantity of oxide-nanoparticles is between 1% and 5% by weight of the oxide-alloy powder.

3. The method of claim 1, wherein the alloy-microparticles contain nickel.

4. The method of claim 1, wherein the alloy-microparticles contain greater than 50 wt % nickel.

5. The method of claim 2, wherein the first quantity of oxide-nanoparticles is selected from at least one of Alumina, Silica, and Yttria.

6. The method of claim 2, further comprising the step of applying pressure to the oxide-alloy powder.

7. The method of claim 6, wherein the pressure is applied axially with of a force of between 30 and 50 tons per square inch.

8. The method of claim 6, wherein the heating of the alloy powder is performed by a spark plasma process.

9. The method of claim 8, wherein the pressure is applied to the oxide-alloy powder with a graphite die.

10. The method of claim 1, further comprising the step of ball-milling the oxide-alloy powder.

11. A method of forming an oxide-alloy powder comprising the steps: a. mixing a first quantity of oxide-nanoparticles with a liquid forming a colloid of oxide-nanoparticles, wherein the oxide-nanoparticles are 1-5 nanometers in the largest dimension;b. mixing the colloid of oxide-nanoparticles with a second quantity of alloy-microparticles forming an oxide-alloy colloid, wherein the oxide-nanoparticles are substantially uniformly distributed throughout the colloid, and wherein substantially all of the oxide-nanoparticles adhere to the alloy-microparticles and not to oxide-nanoparticles; andc. drying the oxide-alloy colloid, forming an oxide-alloy powder.

12. The method of claim 11, wherein the first quantity of oxide-nanoparticles is between 1% and 5% by weight of the oxide-alloy powder.

13. The method of claim 12, further comprising the step of ball-milling the oxide-alloy powder.

14. The method of claim 12, wherein the first quantity of oxide-nanoparticles is selected from at least one of Alumina, Silica, and Yttria.