Low thermal stress metal structures

Abstract

A structured three-phase composite which include a metal phase, a ceramic phase, and a gas phase that are arranged to create a composite having low thermal conductivity, having controlled stiffness, and a CTE to reduce thermal stresses in the composite when exposed to cyclic thermal loads. The structured three-phase composite is useful for use in structures such as, but not limited to, heat shields, cryotanks, high speed engine ducts, exhaust-impinged structures, and high speed and reentry aeroshells.

Claims

1. A three or more phase composite which includes a ceramic phase, a non-solid phase, and a metal phase that includes niobium, said non-solid phase and said ceramic phase are segregated into isolated pockets forming a discontinuous phase in said composite, said metal phase is continuous phase in said composite, said composite having a compression modulus that is at least 25% less than a compression modulus of said metal forming said metal phase, said composite also having a thermal conductivity that is at least 40% less than a thermal conductivity of said metal that forms said metal phase, said composite having a density that is at least 20% lower than a density of said metal that forms said metal phase.

2. The three-phase composite as defined in claim 1, wherein a plurality of said ceramic phase is formed of ceramic particles that include a central cavity or plurality of cavities that are filled with a portion of said non-solid phase.

3. The three-phase composite as defined in claim 1, wherein said ceramic phase is formed of one or more materials selected from the group consisting of carbon, SiAlON, Si.sub.3N.sub.4, SiC, SiOC, SiO.sub.2, Al.sub.2O.sub.3, aluminates, zirconates, aluminosilicates, and ZrO.sub.2.

4. The three-phase composite as defined in claim 1, wherein said non-solid phase does not include a gas or includes one or more gasses selected from the group consisting of air, noble gasses, and nitrogen.

5. The three-phase composite as defined in claim 1, wherein said ceramic forms 5-35 vol. % of said composite.

6. The three-phase composite as defined in claim 1, wherein said non-solid phase constitutes 10-40 vol. % of said composite.

7. The three-phase composite as defined in claim 1, where the metal phase also incorporates 1-20 vol. % additional phases including gas porosity or ceramic or intermetallic phases.

8. The three-phase composite as defined in claim 1, wherein at least a portion of a ceramic material of said ceramic phase is coated with a metal material prior to formation of said composite.

9. The three-phase composite as defined in claim 1, further including a coating or surface modification on an outer surface of said composite to improve corrosion/oxidation protection of said composite, said coating includes one or more materials selected from the group of iridium, platinum, rhenium, rhodium, silicides, MCrAl, MCrAlY, aluminum, aluminum alloy, and chromium alloys.

10. The three-phase composite as defined in claim 1, further including a coating or surface modification on an outer surface of said composite to increase surface temperature limits of said composite by about 50-250 C.

11. The three-phase composite as defined in claim 10, wherein said coating is applied by a process that creates a bond between said coating and said outer surface of said composite that is at least 5000 psig strength.

12. The three-phase composite as defined in claim 1, wherein metal phase further includes one or more metals selected from the group consisting of magnesium, aluminum, vanadium, titanium, calcium, manganese, zirconium, lithium, nickel, iron, molybdenum, tantalum, hafnium, and tungsten.

13. The three-phase composite as defined in claim 1, wherein said composite includes a corrosion-resistant coating.

14. The three-phase composite as defined in claim 1, wherein an outer surface of said composite includes insulation.

15. The three-phase composite as defined in claim 14, wherein said insulation includes one or more materials selected from the group consisting of rigid polyurethane and isocyanate foams, insulation blankets, and aerogel-containing insulation blankets, molded refractories.

16. The three-phase composite as defined in claim 1, wherein said composite has a ductility at room temperature (25 C.) that is greater than about 2% strain to failure.

17. The three-phase composite as defined in claim 1, wherein said composite has a ductility at cryogenic or elevated temperatures greater than about 2% strain to failure.

18. The three-phase composite as defined in claim 1, wherein said composite is designed for use above 700 C. surface temperature.

19. The three-phase composite as defined in claim 1, wherein said composite is fabricated using powder metallurgy or casting processes.

20. A composite that includes a ceramic phase, a non-solid phase, and a metal phase; said non-solid phase and said ceramic phase are segregated into isolated pockets forming a discontinuous phase in said composite; said metal phase is continuous phase in said composite; said ceramic phase is formed of one or more materials selected from the group consisting of carbon, SiAlON, Si.sub.3N.sub.4, SiC, SiOC, SiO.sub.2, Al.sub.2O.sub.3, aluminates, zirconates, aluminosilicates, and ZrO.sub.2; said metal phase includes niobium, aluminum; said non-solid phase formed of a) a vacuum in one or more cavities in said composite, and wherein said one or cavities is located in said metal phase and/or said ceramic phase, and/or b) a low thermal conductivity gas, and wherein said low thermal conductivity gas includes one or more gases selected from the group of air, one or more noble gasses, and nitrogen; said ceramic phase constitutes 5-35 vol. % of said composite; said non-solid phase constitutes 10-40 vol. % of said composite; a plurality of said ceramic phase is formed of ceramic particles that include one or more cavities that are filled with a portion of said non-solid phase; said metallic phase forms about 1-20 vol. % of the composite; said composite having a compression modulus that is at least 25% less than a compression modulus of said metal forming said metal phase, said composite also having a thermal conductivity that is at least 40% less than a thermal conductivity of said metal that forms said metal phase, said composite having a density that is at least 20% lower than a density of said metal that forms said metal phase.

21. The composite as defined in claim 20, wherein at least a portion of a ceramic material of said ceramic phase is coated with a metal material prior to formation of said composite.

22. The composite as defined in claim 20, further including a coating or surface modification on an outer surface of said composite to improve corrosion/oxidation protection of said composite, said coating includes one or more materials selected from the group of iridium, platinum, rhenium, rhodium, silicides, MCrAl, MCrAlY, aluminum, aluminum alloy, and chromium alloys.

23. The composite as defined in claim 20, wherein said composite includes a corrosion-resistant coating.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 illustrates a prior art NASA Metallic TPS encapsulated insulation concept.

(2) FIG. 2 illustrates a prior art BF Goodrich/Rohr, Inc. metallic TPS System.

(3) FIG. 3 illustrates low density, thermally-insulating syntactic metal composite isogrid panels in accordance with the present invention.

(4) FIG. 4 illustrates an isogrid flat panel structure in accordance with the present invention.

(5) FIG. 5 illustrates an isogrid concept in accordance with the present invention incorporating a three-phase isogrid and high efficiency non-structural insulation.

(6) FIG. 6 illustrates MR&D-designed structural TPS using syntactic panels in a panel design.

(7) FIG. 7 illustrates a typical three-phase syntactic composite structure along with one method to manufacture such a composite in accordance with the present invention.

(8) FIGS. 8-11 are tables which illustrate properties of typical three-phase composite systems in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

(9) The invention pertains to the design and materials of construction of structures in which thermal or other strain-induced loads (such as constrained thermal growth and thermal gradient loads) contribute a large portion of the stresses. In particular, the present invention pertains to a structured three-phase composite which includes a metal, a ceramic, and a gas phase that are arranged to create a low thermal conductivity, have controlled stiffness, and a CTE to reduce thermal stresses in the structure when exposed to cyclic thermal loads. The structured three-phase composite is useful for use in structures such as, but not limited to, heat shields, cryotanks, high speed engine ducts, exhaust-impinged structures, and high speed and reentry aeroshells.

(10) The structured three-phase composite has reduced stiffness, reduced thermal conductivity and a CTE for use in thermally-loaded structures. The structured three-phase composite configuration, enabled by a new engineered metal-ceramic-gas three-phase composite, combines the hot and cold structure techniques into an insulating structure.

(11) The present invention advances current hot structure designs and incorporates a multifunctional (thermally-insulating) structured three-phase composite, replacing the honeycomb and thermal insulation with a metal skin-stringer or isogrid-type construction. The structured three-phase composite serves as a load-bearing structural element, and can be stiffened with ribs and/or supports. The structured three-phase composite can be constructed as panels that are attached to a frame, or as an isogrid or skin-stringer or other J-stiffened panel construction or I-stiffened panel construction. These types of constructions can be used to reduce or eliminate the large costs, reliability concerns, and fabrication difficulties (e.g., complex curved panels, etc.) of the honeycomb, while still retaining a large stiffness, thermally-insulating, single-component three-phase composite. The structured three-phase composite has very low thermal conductivity (2.4 g/cc Ti 6242 structured three-phase composite has thermal conductivity equivalent to modern ceramic thermal barrier coatings) and low thermal leakage. The use of the structured three-phase composite of the present invention can result in an advantage for use in multifunctional materials. Because the materials are metallic and have low flaw sensitivity (particularly at elevated temperatures), they can be threaded, bolted, riveted, and assembled using techniques used for the last 100 years of aviation structures. FIG. 3 illustrates a non-limiting metallic isogrid design 30 in accordance with the present invention. The isogrid design is a formed of a low density, thermally-insulating syntactic metal composite that can be used to replace honeycomb panels for low cost metallic TPS and hot structure systems. The panel can include stiffening ribs 32.

(12) In accordance with the present invention, a three-phase or syntactic metal isogrid or skin-stringer structure can be used to produce an integrated insulating structural system. This structure can also integrate the low density, high strength, but lower stiffness three-phase stiffeners/supports and facesheets with low density, extremely low thermal conductivity non-structural rigid or non-rigid insulation, such as aerogels or insulation blankets. Table 1 illustrates the compression properties of various three-phase composite systems in accordance with the present invention.

(13) An isogrid panel is a plate (or face sheet) with triangular integral stiffening ribs (often called stringers). The triangular pattern was found to be very efficient because it takes advantage of the fact that triangular trusses are very efficient structures. The term isogrid is used because the structure acts like an isotropic material. Isogrids are used in many aerospace systems where weight lightening is required with continued structural strength. Isogrid structures can be made into flat panels 40 (See FIG. 4) as well as in curved or cylindrical shapes not limiting the proposed structure to only flat area TPS. An isogrid structure can be formed that optimizes the benefits of the syntactic composite (SComP) materials allowing for the highest possible loadings with the lowest possible weight and then integrate insulation in the void space between the stiffening ribs. By controlling stiffness (modulus and width), spacing, and geometry of the grid elements while controlling CTE and thermal conductivity, the structure can be optimized for performance in its intended temperature range. Very low density insulation 52 can be added to the isogrid spaces of the isogrid panel 50 and underneath to prevent heatsoak, as illustrated in FIG. 5.

(14) System Proposed and Modeled for High Temperature TPS

(15) The structured three-phase composite can potentially be used for structural insulators for rocket engines, TPS systems, thermal energy storage systems, and cryotanks to reduce parasitic weight in a multilayer (ML1) high- or low-temperature system. The structured three-phase composite can act as a load bearing member, as well as a high temperature insulation primary structure. The closed-cell/impermeable nature of the structured three-phase composite can also mitigate the offgassing/condensation issues of ceramics and honeycomb structures, and the structural nature allows for ease of fabrication and integration (they can be formed, drilled, threaded, brazed, or even welded). Integrating thermal and structural functions (as well as potential impact damage mitigation) while reducing thermomechanical stresses by more than 50% can greatly reduce parasitic insulation and thermal protection system mass impacts, while enabling greatly simplified, lower cost, and more robust TPS designs to be realized.

(16) FIG. 6 illustrates a basic integral TPS or tank panel subelement design 60, which includes a structured three-phase composite facesheet 62, standoff 64 and a structural support 66, in a 2020 panel design.

(17) Three-Phase Composite Materials

(18) The structured three-phase composite can be formed of a discontinuous gas phase, a discontinuous ceramic phase or phases (of which some of the phase surrounds the gas phase), and a continuous metallic phase. The most common form of the structured three-phase composite is referred to as a syntactic composite, in which the gas phase is spherical and normally added to the metal during processing as ceramic hollow microspheres. FIG. 7 illustrates a typical three-phase syntactic composite structure along with one non-limiting method to manufacture such composite. In general, the structured three-phase composite can be fabricated by powder metallurgy techniques or via melt casting techniques. As illustrated in FIG. 7, a hollow ceramic microballoon 70 is provided that includes a cavity 72 which is filled with a gas such as air. The outer surface of the microballoon is coated with a metal material 80. The type of coating process used, the composition of the metal coating, and the thickness of the metal coating will depend on the final application for the formed three-phase composite. The coated microballoon is then consolidated together to formed the composite 90. An additional metal 100 can be added during the consolidation process.

(19) Due to the high density difference between the microballoons and the metal phase, and the reactivity of materials at high temperatures, higher temperature materials (including titanium, niobium alloys (C103, CB752), molybdenum (TZM), tungsten, tantalum, nickel alloys, and superalloys such as IN909, IN718, MA956, IN625) can be fabricated using powder metallurgy techniques. Lower melting and lower density materials, such as magnesium and aluminum can be manufactured using casting techniques such as thixomolding (semi-solid forming), squeeze casting, pressure or pressureless infiltration casting, or even stir casting. Powder metallurgy techniques include spark plasma sintering, other field-assisted sintering, pressureless sintering, hot pressing, hot isostaic pressing, injection molding, and other powder metallurgy techniques.

(20) To provide strengthening during pressing to prevent crushing of the more fragile microballoons, control over the powder and the balloon-forming materials is generally required. Several techniques have been demonstrated, including adding a metal coating by such techniques as powder coating (wurster or other encapsulation technique), with or without presintering, CVD or other vapor solid coating, electroplating, molten salt plating, and other techniques. One non-limiting technique is powder encapsulation through a mixing process, such as wurster or other fluid bed coating, or ribbon or high shear blending of a ceramic or preceramic polymer balloon, metal powders, a clean burning (thermal removal) or easily removed binder such as PEG, wax, polysacharrides, and other low ash content or easily vaporized binders, and a solvent. In one non-limiting embodiment, heated balloons are added to a bed of solid binder plus metal powder, which is adhered to the surface. Wax, PVB (polyvinyl butyral), and other low melting binders can be used in that process. A binder can be applied to the microballoon surface and heated to its melting point; the binder-coated balloon is then added to a bed of metal powder. In general, the metal powder size is less than 1/10.sup.th of the microballoon size, and typically less than 1/50.sup.th of the microballoon size.

(21) Using the powder or metal-coated ceramic microballoons, additional metal is added by blending. Normally, the mixing is used to fill between the ceramic balloons to maximum green packing density. Multiple size balloons can be used for similar purposes to fill intersticies, generally with particle cuts .sup.th-.sup.th of the prior size cut.

(22) For melt-infiltrated materials, a wetting coating can be used for infiltration techniques and for other casting (thixocasting). This wetting coating can be a ceramic such as TiN or tungsten, but is most commonly a eutectic-forming or active metal. The wetting coating can be added as a powder or coated onto the surface. The metals can include the matrix metal, zinc, nickel, copper, titanium, silicon, palladium, among others. Most often, active metals (titanium, silicon, zirconium, palladium) are used to enhance wetting.

(23) To control separation of the gas and ceramic phases, it is desirable to ensure that the ceramic phases are not contiguous (not touching). In most cases, a contiguity below 0.2, and generally below 0.1 or 0.05 is desired. The desired contiguity can be controlled by wetting, powder mixing, and/or by adding a powder coating to separate the microballoons during the infiltration processes. The desired contiguity can also be achieved or enhanced by post-fabrication deformation processing, such as by constrained rolling or forging.

(24) In one non-limiting embodiment, the ceramic microballons are coated with a metal or metal alloy powder and then melt infiltrated with the matrix material. The powder-coated balloons can be presintered or added to a container and constrained while infiltration takes place; however, this is not required.

(25) The microballoons can be made from a low thermal conductivity ceramic and generally include amorphous or glassy ceramics including carbon, Si.sub.3N.sub.4, SiC, SiOC, SiO.sub.2, Al.sub.2O.sub.3, or ZrO.sub.2. Lower densities are desirable, but should be strengthened or properly distributed since they are also lower in strength. Where density is less of an issue, such as ground-based thermal energy storage systems, cenospheres or other low cost glass or ceramic spheres can be used.

(26) For titanium, niobium, aluminum, magnesium, and superalloy three-phase composites that are useful for cryotanks and hypersonic structures, glassy carbon or preceramic-derived ceramic microspheres can be used. These types of microspheres generally have a bulk density of less than 0.2 g/cc, and typically less than 0.05 g/cc, with microspheres sizes ranging from 20-200 microns, and typically less than 100 microns. For heavy ceramic spheres, high melting glass such as borosilicate or sol-gel-derived silica, or cenespheres aluminosilicate microspheres can be used. These types of material are more rigid and can be processed with less attention to precoating or premixing before infiltration or pressing.

(27) Properties of typical three-phase composite systems in accordance with the present invention are illustrated in Tables 1-4.

(28) For improved corrosion resistance (mainly oxidation) and to increase temperature limits, the structured three-phase composite thermal structure can be coated. For hypersonic and other high temperature structures, a thermal barrier coating such as, but not limited to, mullite, BSAS or other aluminosilicates, or zirconia (generally yittria-stabilized zirconia) can be applied. These are generally applied between 10-100 mils (0.01-0.1) using plasma spray processes; solution spray or slurry coating can also be used.

(29) Oxidization coatings can be used and typically depend on the materials system. For cryotanks, a magnesium structured three-phase composite can be used, but generally requires coatings for H.sub.2 and LO.sub.2 resistance. Aluminum coating or surface layers, formed during fabrication (such as by roll-bonding aluminum sheet), or applied by cold spray, can be used. For titanium systems, MCrAlY or NiCuCrAl alloy coatings applied by thermal spray, PVD, or slurry coating are very effective. For niobium alloys, silicide slurry coatings containing chrome and silicon, such as R512E from HiTempco are very effective. Titanium-chromium-silicon, and vanadium-chromium-silicon can be used, but molybdenum-, tungsten-, and iron-containing silicide systems can also be effective. For superalloy and nickel-based alloy systems, MCrAlY coatings and diffusion coatings, including those applied by pack aluminizing, can be used. For molten salt storage systems, three-phase composites made from nickel-based alloys, such as Haynes420, exhibit excellent resistance to molten salts. Additional aluminizing, or using a lower cost iron alloy with a graded, coated, or layered high nickel alloy surface can be used for operation above 650 C.

EXAMPLES

(30) Example 1: A high speed vehicle airframe panel was designed using a coated syntactic CB752 niobium structure. The structure included a CB752 niobium alloy three-phase composite that is fabricated by premixing 50 vol. % 60 micron D50 glassy carbon microballoons with 25 vol. % 1-10 micron powder mixture of 87.5 wt. % NbH.sub.2, 10 wt. % tungsten, and 2.5 wt. % zirconium (as ZrH.sub.2) using a spray-coating technique using 2% of 3000 MW PEG in acetone to adhere them to the microballons and then subsequently blending in an additional 25 vol. % of 325 mesh prealloyed CB752 (niobium-10 wt. %-2.5 wt. % zirconium) powder. The material was isostatically formed at 4500 psig compaction force using rubber molds into a 1 thick, curved panels of 2016 dimensions. After vacuum sintering at 1525 C. for 4 hours, the material was machined into an isogrid panel. The panel was coated with a commercial RT512E slurry coating from Hitempco Inc. and the top surface was further coated with a zirconia (ZrO.sub.2) based thermal barrier coating using a typical atmospheric plasma spray application process. The panels were mechanically attached (e.g., bolted, etc.) to titanium bulkheads to form a biconic surface for a hypersonic vehicle airframe. Surface temperature capabilities of 2500-2800 F. for 10-1000 hour life ratings were achieved for the hypersonic vehicle airframe.

(31) Example 2: A three-phase composite panel was fabricated using powder metallurgy processing from IN909 alloy (35-40 wt. % nickel, 12-14 wt. % cobalt, 4.3-5.2 wt. % niobium, 1.3-1.8 wt. % titanium, 0.25-0.5 wt. % silicon, 0.15 wt. % max aluminum, 0.06 wt. % max carbon, balance iron) powders using the sequence described in Example 1, but sintered at 1280 C., and a zirconia thermal barrier coating was applied to the outer mold line surface. The panel was suitable for long term use at 1600 F.

(32) Example 3: Isogrid panels of Haynes420 were fabricated using a powder metallurgy processing as described in Example 1, except the materials were sintered by hot isostatic pressing at 3000 psig at 1080 C. 33 curved panels were molded to near net shape, with 0.75 thick facesheets and 2.5 long (thickaxial direction)0.75 wide stringers spaced at 8 intervals. The panels were mechanically assembled onto a carbon steel frame using steel standoffs and the intersticies filled with high efficiency insulation (kaowool) to a thickness of 6. An additional 12 of rigid isocyanate insulation was applied to the outer section and covered with an aluminum weather shield. At the panel intersticies, the gaps were welded closed with 420 welding rods (0.28 wt. % carbon, 0.42 wt. % manganese, 0.37 wt. % Si, 0.15 wt. % molybdenum, 0.03 wt. % S, 13.13 wt. % chromium, balance iron) using TIG welding techniques. These panel were used in a tank construction and were useful for thermal energy storage to as high as 800 C. using mixed chloride salts.

(33) Example 4: Magnesium three-phase composite cryotanks were fabricated by melt infiltrating a calcium-modified AZ61 alloy containing 2 wt. % calcium and 1 wt. % cesium into a 45 vol. % SiOC microballoon preform space using prealloyed AZ61 (92 wt. % manganese, 5.80-7.2 wt. % aluminum, 0.4-1.5 wt. % zinc, 0.15 wt. % manganese, 0.1 wt. % silicon, 0.05 wt. % copper, 0.005 wt. % nickel, 0.005 wt. % iron)+titanium powders applied (blended) with the balloons using squeeze casting. The 2 thick billets were roll-reduced using constrained rolling at 450 C. to thickness, then creep-formed to form a curved structure, and then machined into isogrid panels with 0.125 thick surface with stringers arranged hexagonally at 6 spacing. The inner surface was cold sprayed with commercially pure aluminum to a thickness of 0.01. The isogrid panels were assembled into a magnesium alloy frame. The panel interfaces were arc welded and then overcoated with aluminum using a portable cold spray system. The finished tank was then further overcoated with 0.3 of rigid polyisocyanate insulation to provide additional insulation. A 52% weight savings over an aluminum cryotank, and a 12% weight savings over a composite cryotank was achieved.

(34) It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficiently attained, and since certain changes may be made in the constructions set forth without departing from the spirit and scope of the invention, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense. The invention has been described with reference to preferred and alternate embodiments. Modifications and alterations will become apparent to those skilled in the art upon reading and understanding the detailed discussion of the invention provided herein. This invention is intended to include all such modifications and alterations insofar as they come within the scope of the present invention. It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described and all statements of the scope of the invention, which, as a matter of language, might be said to fall there between. The invention has been described with reference to the preferred embodiments. These and other modifications of the preferred embodiments as well as other embodiments of the invention will be obvious from the disclosure herein, whereby the foregoing descriptive matter is to be interpreted merely as illustrative of the invention and not as a limitation. It is intended to include all such modifications and alterations insofar as they come within the scope of the appended claims.