High temperature creep resistant aluminum superalloys

Abstract

This invention relates to a series of castable aluminum alloys with excellent creep and aging resistance, high electrical conductivity and thermal conductivity at elevated temperatures. The cast article comprises 0.4 to 2% by weight iron, 0 to 4% by weight nickel, 0.1 to 0.6 or about 0.1 to 0.8% by weight zirconium, optional 0.1 to 0.6% by weight vanadium, optional 0.1 to 2% by weight titanium, at least one inoculant such as 0.07-0.15% by weight tin, or 0.07-0.15% by weight indium, or 0.07-0.15% by weight antimony, or 0.02-0.2% by weight silicon, and aluminum as the remainder. The aluminum alloys contain a simultaneous dispersion of Al.sub.6Fe, Al.sub.3X (X=Fe, Ni) and/or Al.sub.9FeNi intermetallic in the eutectic regions and a dispersion of nano-precipitates of Al.sub.3Zr.sub.xV.sub.yTi.sub.1-x-y (0x1, 0y1 and 0x+y1) having L1.sub.2 crystal structure in the aluminum matrix in between the eutectic regions. The processing condition for producing cast article of the present invention is disclosed in detail.

Claims

1. An aluminum alloy comprising: about 0.4 to 2% by weight iron; 0.25 to 0.8% by weight zirconium; about 0.07 to 0.15% by weight tin, indium or antimony; about 0.02 to 0.2% by weight silicon; and aluminum as the remainder.

2. The aluminum alloy of claim 1, further comprising up to about 4% by weight nickel.

3. The aluminum alloy of claim 1, further comprising about 0.5 to 3.5% by weight nickel.

4. The aluminum alloy of claim 1, comprising about 0.25 to 0.55% by weight zirconium.

5. The aluminum alloy of claim 1, comprising about 0.27 to 0.42% by weight zirconium.

6. The aluminum alloy of claim 3, comprising about 0.5 to 1.5% by weight iron.

7. The aluminum alloy of claim 1, comprising about 0.82 to 1.22% by weight iron.

8. The aluminum alloy of claim 7, comprising about 0.25 to 0.55% by weight zirconium.

9. The aluminum alloy of claim 8, further comprising about 1.65 to 2.35% by weight nickel.

10. The aluminum alloy of claim 4, further comprising up to about 4% nickel by weight.

11. The aluminum alloy of claim 5, further comprising up to about 4% nickel by weight.

12. The aluminum alloy of claim 1, further comprising about 0.1 to 0.6% by weight vanadium.

13. The aluminum alloy of claim 1, further comprising about 0.1 to 2% by weight titanium.

14. The aluminum alloy of claim 1, further comprising: about 0.1 to 0.6% by weight vanadium and about 0.1 to 2% by weight titanium.

15. The aluminum alloy of claim 1, further comprising a dispersion of intermetallic Al.sub.6Fe, Al.sub.3X (X=Fe, Ni) and Al.sub.9FeNi in intermetallic phases having an average diameter of about 200-600 nm.

16. The aluminum alloy of claim 1, further comprising a dispersion of Al.sub.3Zr.sub.xV.sub.yTi.sub.1-x-y (0x1, 0y1, and 0x+y1) nano-precipitates, having L1.sub.2 crystal structure and an average diameter in the range of about 6-40 nm.

17. The aluminum alloy of claim 1, further comprising nickel at a concentration of up to about 4% by weight and a dispersion of intermetallic Al.sub.6Fe and/or Al.sub.3Fe in intermetallic phases having an average diameter of about 200-600 nm.

18. The aluminum alloy of claim 1, further comprising nickel at a concentration of up to about 4% by weight a dispersion of intermetallic Al.sub.6Fe, Al.sub.3X (X=Fe, Ni) and Al.sub.9FeNi in intermetallic phases having an average diameter of about 200-600 nm, and a dispersion of Al.sub.3Zr.sub.xV.sub.yTi.sub.1-x-y (0x1, 0y1, and 0x+y1) nano-precipitates having L1.sub.2 crystal structure and an average diameter in the range of about 6-40 nm.

19. The aluminum alloy of claim 18, further comprising filler materials or reinforcement materials selected from the group of materials consisting of silicon carbide (SiC), aluminum oxide (Al.sub.2O.sub.3), boron carbide (B.sub.4C), boron nitride (BN), titanium carbide (TiC), yttrium oxide (Y.sub.2O.sub.3), graphite, diamond particles, and their mixtures, the volume fraction of the filler materials comprising up to about 25% by volume of the metal matrix.

20. The aluminum alloy of claim 1, further comprising a dispersion of intermetallic Al.sub.6Fe and/or Al.sub.3Fe in intermetallic phases having an average diameter of about 200-600 nm and a dispersion of Al.sub.3Zr.sub.xV.sub.yTi.sub.1-x-y (0x1, 0y1, and 0x+y1) nano-precipitates having L1.sub.2 crystal structure and an average diameter in the range of about 6-40 nm.

21. The aluminum alloy of claim 20, further comprising filler materials or reinforcement materials selected from the group of materials consisting of silicon carbide (SiC), aluminum oxide (Al.sub.2O.sub.3), boron carbide (B.sub.4C), boron nitride (BN), titanium carbide (TiC), yttrium oxide (Y.sub.2O.sub.3), graphite, diamond particles, and their mixtures, the volume fraction of the filler materials comprising up to about 25% by volume.

22. The aluminum alloy of claim 1 lacking scandium.

23. The aluminum alloy of claim 1, having a compression yield of at least about 55 MPa at a testing temperature of about 375 C.

24. The aluminum alloy of claim 1, having a compression strength of at least about 60 MPa at a testing temperature of about 375 C.

25. The aluminum alloy of claim 1, having a thermal conductivity at about 20 C. that is at least about 185 W/m.Math.K.

26. The aluminum alloy of claim 1, which retains its room temperature strength after at least 7 months exposure to a temperature of about 375 C.

27. A cast aluminum article comprising the aluminum alloy of claim 1.

28. The cast aluminum article of claim 27, further comprising a dispersion of intermetallic Al.sub.6Fe, Al.sub.3X (X=Fe, Ni) and Al.sub.9FeNi in intermetallic phases having an average size range of about 200-600 nm and a dispersion of Al.sub.3Zr.sub.xV.sub.yTi.sub.1-x-y (0x1, 0y1, and 0x+y1) nano-precipitates having L1.sub.2 crystal structure and an average size range of about 6-40 nm.

29. A method for manufacturing an aluminum alloy comprising the steps of casting an aluminum alloy mixture at about 750 to 950 C., the aluminum alloy mixture comprising: about 0.4 to 2% by weight iron; about 0.1 to 0.8% by weight zirconium; about 0.07 to 0.15% by weight tin, indium or antimony; about 0.02 to 0.2% by weight silicon; an inoculant selected from the group consisting of Sn, In, Sb and their mixtures; and aluminum as the remainder; quenching the cast alloy after solidification; and heat aging the cast article at a temperature in the range of about 300 to 450 C. for about 2 to 72 hours.

30. The method claim 29, wherein the heat aging is at about 400 to 450 C. for about 24 to 72 hours.

31. The method of claim 29, wherein the alloy mixture lacks scandium.

Description

DESCRIPTION OF THE DRAWINGS

(1) FIGS. 1. (A) and (B) are images from the scanning electron microscopic technique showing the interdendritic Al.sub.6Fe, Al.sub.3X (X=Fe, Ni) and Al.sub.9FeNi dispersoids in the invented aluminum alloy.

(2) FIG. 2. is a graphical illustration of the excellent creep resistance of an example of the invented aluminum alloys compared with the prior art, scandium-containing aluminum alloys. The testing temperature for all alloys present in the graph is 400 C. (752 F.).

(3) FIG. 3. shows compression stress strain curves of an example of the invented aluminum alloys at different testing temperatures of 20 C. (RT), 100 C., 200 C., and 375 C. The alloy is at T5 condition prior to testing.

(4) FIG. 4. is a graphical illustration of the excellent aging resistance of an example of the invented aluminum alloys at 375 C. operating temperature for 7 months.

(5) FIG. 5. is a graphical illustration of the effect of two-step aging versus one-step aging for an example of the invented aluminum alloys. For the one-step aging process, the cast article is aged at temperature in the range of 425-475 C. for 4 to 48 hours to achieve optimal properties. For the two-step aging process, in the first step, the cast article is aged at temperature in the range of 330-375 C. for 2 to 8 hours followed by the second step aging at 425-475 C. for 1 to 24 hours.

DETAILED DESCRIPTION OF THE INVENTION

(6) A series of heat resistant castable aluminum alloys with excellent creep and aging resistance and high electrical and thermal conductivity are disclosed. The outstanding creep resistance for these alloys results from: i) the presence of Al.sub.3Zr.sub.xV.sub.yTi.sub.1-x-y (0x1, 0y1 and 0x+y1) nano-precipitates with L1.sub.2 crystal structure and the average diameter in the range of about 6-20 or about 6-40 nm, and ii) the reinforcement phase in the form of Al.sub.6Fe, Al.sub.3X (X=Ni, Fe) and Al.sub.9FeNi intermetallic precipitates with an average diameter in the range of about 200-600 nm. The average diameter of the reinforcement phase and nano-precipitates are measured via scanning electron microscopy (SEM) and atom probe tomography (APT) techniques, respectively. For SEM measurements, at least three images from three different regions are analyzed by means of common image analysis software to obtain the diameter of the captured intermetallic phase, and the average diameter is recorded for all captured intermetallics. For APT analysis, the obtained data is analyzed utilizing IVAS software to reconstruct the studied volume three-dimensionally that include nano-precipitates. The average diameter of the captured nano-precipitates is measured by the same software. The presence of Al.sub.3X (X=Fe, Ni) precipitates in the interdendritic regions and Al.sub.3Zr.sub.xV.sub.yTi.sub.1-x-y (0x1, 0y1 and 0x+y1) nano-precipitates in the dendritic regions creates a strong pinning force against dislocation motions at ambient and elevated temperatures. Since both hardening phases are thermally stable, the mechanical properties are maintained at elevated temperatures as high as 400 C. (752 F.).

(7) The alloys disclosed herein are advantageous since they can be produced via low cost traditional casting method. In addition, utilizing low alloying element concentrations to produce these alloys allows superior electrical and thermal conductivity (at least 45% of annealed copper).

(8) Another advantage is that these alloys are produced without using scandium as an alloying element, which results in a very low alloy cost.

(9) The high number density of Al.sub.3Zr.sub.xV.sub.yTi.sub.1-x-y (0x1, 0y1 and 0x+y1) nano-precipitates, with the average diameter in the range of about 6-20 or about 6-40 nm, is produced by introducing an inoculant element into the alloy. The inoculant elements used in the invented alloys include Sn, In, and Sb. Thus the presence of the inoculant elements is important to obtain good strength and creep resistance at ambient and elevated temperatures.

(10) The disclosed aluminum alloys provide light weight, low cost, highly electrically and thermally conductive, heat resistant, and creep resistant properties that are thermally stable in the temperature range of about 25-400 C. (about 77-752 F.).

(11) These properties are achieved by aluminum alloys produced through traditional casting that contain: 0.4 to 2% by weight iron, 0 to 4% by weight nickel, 0.1 to 0.6 or 0.1 to 0.8% by weight zirconium, Optionally 0.1 to 0.6% by weight vanadium, Optionally 0.1 to 2% by weight titanium, 0.07-0.15% by weight tin or 0.07-0.15% by weight indium, or 0.07-0.15% by weight antimony, 0.02-0.2% by weight silicon.

(12) The present invention comprises a series of aluminum alloys with combination of outstanding creep resistance and high electrical and thermal conductivity at ambient and elevated temperatures. The invented alloys are marked by an ability to perform in cast form, which is suitable for elevated temperatures. The high creep resistance of the invented alloys results from two main strengthening mechanisms: the intermetallic dispersion hardening and nano-precipitation hardening.

(13) The intermetallic dispersion hardening relies on the formation of dispersed intermetallic phase in the interdendritic regions during solidification. In the present invention, about 0.4-2% by weight iron and about 0-4% by weight nickel is utilized to form a fine dispersion of Al.sub.6Fe, Al.sub.3X (X=Fe, Ni) and Al.sub.9FeNi intermetallic phases in the interdendritic regions. The volume fraction, shape, and diameter of Al.sub.3X (X=Fe, Ni) and Al.sub.9FeNi intermetallic phases depend on the concentration of Fe and Ni, the Fe/Ni concentration ratio, and the specific heat treatment conditions. FIGS. 1(A) and (B) show a distribution of such intermetallic phases in the aluminum alloy produced according to the present invention.

(14) The nano-precipitation hardening relies on the formation of nano-precipitates in the aluminum matrix through specific heat treatment conditions. In the present invention, about 0.1 to 0.6 or about 0.1 to 0.8% by weight zirconium, about 0.1 to 0.6% by weight vanadium, and about 0.1-2% by weight titanium are used to create a high number density of coherent L1.sub.2 Al.sub.3Zr.sub.xV.sub.yTi.sub.1-x-y (0x1, 0y1 and 0x+y1) nano-precipitates in the aluminum alloy. Since the atomic radius of vanadium (0.132 nm) is smaller than zirconium (0.159 nm) and titanium (0.176 nm), the lattice mismatch of the Al.sub.3Zr.sub.xV.sub.yTi.sub.1-x-y (0x1, 0y1 and 0x+y1) type precipitates can be reduced to closely match the lattice parameter of aluminum matrix. As a result, the coarsening resistance of the precipitates can be further increased by controlled addition of vanadium and titanium to zirconium-containing alloy. The volume fraction, diameter, and lattice mismatch of Al.sub.3Zr.sub.xV.sub.yTi.sub.1-x-y (0x1, 0y1 and 0x+y1) nano-precipitates depend on the concentration of Zr, V, and Ti, the corresponding Zr:V:Ti concentration ratio, and the specific heat treatment conditions.

(15) The high creep resistance of the invented alloys relies on the presence of the heat resistant Al.sub.6Fe, Al.sub.3X (X=Fe, Ni), and Al.sub.9FeNi intermetallic phases and the heat resistant Al.sub.3Zr.sub.xV.sub.yTi.sub.1-x-y (0x1, 0y1 and 0x+y1) nano-precipitates. While the former contributes to strengthening of the interdendritic regions, the latter strengthens the dendritic regions. The strengtheners create barriers to dislocation glide and dislocation climb (i.e. the main creep mechanism) at elevated temperatures.

(16) The specific concentration of alloying elements and heat treatment conditions are necessary to create the desired microstructure with desired diameter and volume fraction of both nickel-iron intermetallic phases and Al.sub.3Zr.sub.xV.sub.yTi.sub.1-x-y (0x1, 0y1 and 0x+y1) nano-precipitates. FIG. 2 is a graph showing a comparison of an alloy according to the present invention with three alloys of prior art. The testing temperature for all alloys present in the graph is 400 C. (752 F.). The graph shows the outstanding creep resistance of the cast alloy produced according to this invention compared to the scandium-containing alloys. It is noted that the threshold stress of the cast articles produced according to this invention, is at least 60% higher than that of the scandium-containing alloys, tested at 400 C. (752 F.). The threshold stress is the one below which no observable creep occurs in the alloy.

(17) Generally, the invented alloys after optimal processing contain about 0.3-0.8% by volume fraction Al.sub.3Zr.sub.xV.sub.yTi.sub.1-x-y (0x1, 0y1 and 0x+y1), and about 0-20% by volume fraction Al.sub.3X (X=Fe, Ni) and Al.sub.9FeNi.

(18) In order to activate the strengtheners and achieve outstanding mechanical properties, the cast articles must have specific chemical compositions and heat treatments. These conditions are designed to maximize the strengthening effects through optimized formation of Al.sub.3Zr.sub.xV.sub.yTi.sub.1-x-y (0x1, 0y1 and 0x+y1), Al.sub.3X (X=Fe, Ni), and Al.sub.9FeNi phases.

(19) The high-temperature strength is achieved in the present invention when using a T5 heat treatment consisting of aging at 400-450 C. (752-842 F.) for 24 to 72 hours. The unique composition and the corresponding heat treatment allow nearly full precipitation of Al.sub.3Zr.sub.xV.sub.yTi.sub.1-x-y (0x1, 0y1 and 0x+y1) nano-precipitates with high number density and average diameter in the range of 6-20 or 6-40 nm. Additionally, the fragmentation of the nickel-iron intermetallic phases during the heat treatment allows further improvement in ductility while maintaining the strength. FIG. 3 shows compression stress strain curves of an example of the invented aluminum alloys at different testing temperatures of 20 C. (RT), 100 C., 200 C., and 375 C. The alloy is at T5 condition prior to testing. Additionally, table 1 summarizes the values obtained from stress strain curves for the example alloy. At testing temperature of 375 C., as observed, compression yield and compression strength of 55 and 67 MPa are obtained for the example alloy, respectively, using the designed heat treatment conditions.

(20) TABLE-US-00001 TABLE. 1 Thermal Compressive conductivity Specs. 0.2% .sub.Y (MPa) Strength (MPa) (W/m .Math. K) Testing RT 200 C. 300 C. 375 C. RT 200 C. 300 C. 375 C. RT 375 C. temp. Alloy 167 125 102 55 259 184 113 63 185 .sup.1 values in compressive mode

(21) The aging-resistance of the present invention stems from the presence of heat resistant Al.sub.6Fe, Al.sub.3X (X=Fe, Ni) and Al.sub.9FeNi intermetallic phases, and Al.sub.3Zr.sub.xV.sub.yTi.sub.1-x-y (0x1, 0y1 and 0x+y1) nano-precipitates. The aging resistance is described as the retained room temperature strength after exposure to high temperature for several months. FIG. 4 shows an excellent aging resistance of the example alloy of this invention at 375 C. (707 F.). No drop in strength was observed for this alloy after 7 months exposure to 375 C. (707 F.).

(22) The alloys produced according to the present invention are processed using traditional casting. The casting temperature is in the range of 750-950 C. (1382-1742 F.). The cast alloys are quenched in a quenching medium after solidification to maximize the content of solute atoms in the solid solution. This allows cooling rate that exceeds 10 C./s (50 F./s). The most preferred quenching medium is water at temperature between 25 to 40 C. (77 to 104 F.). After quenching, the cast article is aged at temperature in the range of 300 to 475 C. or 400 to 475 C. (572 to 887 F. or 752 to 842 F.) for 2 to 72 or 24 to 72 hours to achieve optimal properties.

(23) The alloy of present invention may be heat treated in one or two-step aging processes. The two-step aging is performed on cast alloys to maximize room-temperature strength. While the first step aging at lower aging temperature creates a high number density of nuclei due to the higher chemical driving force, the second step aging at higher temperature accelerates the kinetics of precipitate growth to achieve optimal strength. For the one-step aging process, the cast article can be aged at temperature in the range of 350-475 C. for 2 to 72 to achieve optimal properties. For the two-step aging process, in the first step, the cast article can be aged at temperature in the range of 330-375 C. for 2 to 24 hours followed by the second step aging at 425-475 C. for 1 to 24 hours. The effect of two-step aging versus one-step aging is presented in FIG. 5 for an invented example alloy. A noticeable increase in microhardness values is observed for the alloy aged by the two-step aging process.

(24) The alloy of the present invention can be used in either a bulk alloy form or as an alloy matrix for producing metal matrix composite (MMC). The metal matrix composite comprises the aluminum alloy of the present invention as the alloy matrix and filler materials in the form of particles, whiskers, chopped fibers, and continuous fibers. The MMC comprising the aluminum alloy of present invention and the filler materials can be produced by common techniques such as mechanically mixing and stirring the filler materials into the molten metal (that is compo-casting or stir-casting) or using ultrasonic waves to mix and distribute the filler materials into the molten metal.

(25) The filler materials should not be confused with Al.sub.6Fe, Al.sub.3X (X=Fe, Ni) and Al.sub.9FeNi intermetallic phases or Al.sub.3Zr.sub.xV.sub.yTi.sub.1-x-y and (0x1, 0y1 and 0x+y1) nano-precipitates. The average diameter of the filler materials or the reinforcement material is typically in the 1-25 microns range. The filler materials or reinforcement materials include silicon carbide (SiC), aluminum oxide (Al.sub.2O.sub.3), boron carbide (B.sub.4C), boron nitride (BN), titanium carbide (TiC), yttrium oxide (Y.sub.2O.sub.3), graphite, diamond particles, and any combination of aforementioned particles. The volume fraction of the filler materials can vary up to 25% by volume.

(26) The present invention has been described in detailed embodiments thereof. It is understood by those skilled in the art that modifications and variations in this detail may be made without departing from the spirit and scope of the claimed invention.