HIGH MODULUS ALUMINUM ALLOYS

Abstract: High modulus aluminum-base alloys comprise mechanically alloyed aluminum-base compositions contain 10-25% titanium part of which may be replaced by vanadium or zirconium. Within described limits the alloys can contain elements other than oxygen and carbon ordinarily derived from the process control agent used in mechanical alloying.

Description: The present invention is concerned with aluminum-base alloys and, more particularly, with aluminum-base alloys having high room and elevated temperature strength, a modulus of elasticity at room temperature of at least 90 GPa and good ductility. In aircraft and in other structures, there is often a need for a light metal, i.e. one having a density less than about 3 g/cm³, which is both strong (in terms of tensile and yield strength) and stiff. It is known that light metal (aluminum) composites with silicon carbide can have moduli measuring in excess of about 90 GPa and measuring as high as even 140 GPa. While these aluminum-silicon carbide or boron carbide composites are useful, they are not particularly strong at high temperatures and, at the higher moduli, are relatively brittle. It is the object of the invention to provide aluminum-base alloys having a combination of high moduli of elasticity and strengths and more particularly to provide aluminum-base alloys which have reasonable tensile elongations coupled with high room and elevated temperature strengths and high moduli. According to the invention a mechanically alloyed aluminum-base alloy having a modulus of elasticity at room temperature of at least 90 GPa contains at least one transition element selected from the group consisting of titanium, vanadium, zirconium, niobium, tantalum, yttrium, tungsten, cerium, erbium, chromium, iron, cobalt, nickel and copper, with the provisos that the content of titanium is from 10 to 25% by wt., said other said transition elements, if present, may partly or wholly replace titanium on an atom-for-atom basis, in amounts such that the contents of vanadium, zirconium, nickel, copper, cerium and erbium do not exceed 5% by wt. each, and the contents of niobium, tantalum, yttrium, tungsten, chromium, iron and cobalt do not exceed 10% by wt. each; said alloy optionally also containing one or more of lithium in an amount up to 3% by wt., silicon and beryllium in an amount up to 5% by wt. each and boron in an amount up to 1%, with the proviso that the total amount of all the above-mentioned transition and other elements, other than aluminum and titanium, does not exceed 10%; said alloy also containing from 1 to 4% by wt. carbon and from 0.1 to 2% oxygen, the balance, apart from impurities, being aluminum, with or without up to 2% by wt. of stable added oxidic material in excess of the oxide indicated by the specified oxygen content of 0.1 to 2% by wt. The replacement of titanium by other transition elements on an atom-for-atom basis up to the limits specified above means, as a practical matter, for example that vanadium can replace titanium on an equal weight basis up to 5% by wt. vanadium and zirconium can replace up to 2.5% by wt. titanium on the basis of two parts by wt. of zirconium for one part by wt. of titanium. The amount of titanium, as such or replaced atom-for-atom by other transition elements, is preferably from 10 to 20%, more narrowly 10 to 16% and still more narrowly 10 to 14% by wt. Those skilled in the art will appreciate that inclusion of elements other than titanium and elements substituted for titanium will generally tend to increase the hardness of the alloy while lowering ductility. Accordingly, it is advantageous to limit incorporation of such elements by reference to the defined range of titanium and elements substituted for titanium such that at the high end of the range, above 15% titanium, say from 15-20% by weight titanium, such elements in the alloy are minimized, e.g. up to a total of 2% by weight and below 15% by weight of titanium the permissible amount of such elements, if any, gradually increases to the total maximas set forth hereinbefore. A like situation exists with regard to deliberately added oxidic materials such as alumina, yttria or yttrium-containing oxide such as yttrium-aluminum-garnet and the like and carbon. In total the optional oxidic materials can be present in a total amount up to 2% with the maximum being present only when titanium contents are low and auxiliary elements are either in low concentration or absent. Similarly except when the defined range is less than about 15%, carbon should be maintained at a maximum of 2%. As stated, the alloys of the present invention consisting of aluminum and the aforestated elements and compounds in the aforestated ranges are made by mechanically alloying elemental or intermetallic ingredients (e.g. Al₃Ti) as previously described in U.S. Patent Nos. 3,740,210, 4,600,556, 4,624,705, 4,643,780, 4,668,470, 4,627,959, 4,668,282, 4,668,470 and 4,557,893. In mechanically alloying ingredients to form the alloys of the present invention a processing aid such as stearic acid or mixtures of stearic acid and graphite is used. The result of milling particulate aluminum and titanium with or without additional elements along with stearic acid is the formation of amounts of oxide and carbide essentially stoichiometrically equivalent to the amount of carbon and oxygen in the process control agent. In the alloys of the invention these oxides and carbides are primarily Al₂O₃ and aluminum carbide with or without modification by titanium. Relatively little titanium carbide is present in the alloy. After mechanical alloying is complete, that is powder ingredients are thoroughly intermingled by repeated fracturing and refracturing of composite particles and have achieved or substantially achieved saturation hardness, the milled particles, sieved to exclude fines, are placed in a container, degassed under reduced pressure, for example, at 500°C for 2 to 12 hours, compacted in vacuum under applied pressure and are then extruded. As practical ranges the extrusion ratio can be from about 5 to 1 to about 50 to 1 and the extrusion temperature from about 250°C to about 600°C. Compositions, in weight percent, of some high modulus aluminum-base alloys of the present invention are set forth in Table 1. These exemplified alloys conform to the range of about 10-16% titanium, about 1.3-2% carbon, about 0.5-1.2% oxygen, up to about 2.5% vanadium, balance essentially aluminum. After preparing the alloys set forth in Table 1 as described hereinbefore, the alloys were examined as to microstructure. Basically the microstructure shows a large volume fraction of Al₃Ti intermetallic phase present as ultra-fine (usually less than 0.2 micrometer in size) grains very uniformly distributed through a fine grain aluminous matrix. Carbon is essentially present as a very finely divided Al₄C₃ or a titanium-doped modification thereof and oxygen is present as grain boundary aluminum oxide. Room and elevated temperature mechanical characteristics of alloys Nos. 2-5 are set forth in Table 2. Table 2 shows that the alloys of the present invention are strong at high temperatures compared to the general run of aluminum alloys made by conventional melting and casting technology. Moduli of elasticity at room temperature, determined by the method of S. Spinner et al, "A Method of Determining Mechanical Resonance Frequencies and for Calculating Elastic Modulus from the Frequencies", ASTM Proc. No. 61, pages 1221-1232, 1961, for alloys of the present invention are set forth in Table 3. Table 3 shows the high, room temperature moduli of elasticity exhibited by alloys of the present invention and also shows with respect to alloy 1 that the modulus of elasticity is not degraded by exposure to high temperature. An additional test of mechanical characteristics shows for alloy 2 that at 427°C the 0.2% yield strength is 121 MPa, the ultimate tensile strength is 132 MPa and the elongation is 5.4%. Laboratory work with mechanically alloyed aluminum alloys has recently shown that mechanical characteristics of this nature at temperatures about 427°C make the alloy amenable to hot working production processes such as rolling and forging thereby significantly increasing the utility of hard, aluminum alloys containing a solid insoluble intermetallic phase. A method of hot working such alloys is the subject-matter of EP-A- 0 340 789 (reference HLP-00044) filed simultaneously herewith. While specific embodiments of the invention are illustrated and described herein, those skilled in the art will understand that the invention is not limited to these embodiments.

Claims: A mechanically alloyed aluminum-base alloy having a modulus of elasticity at room temperature of at least 90 GPa and containing at least one transition element selected from the group consisting of titanium, vanadium, zirconium, niobium, tantalum, yttrium, tungsten, cerium, erbium, chromium, iron, cobalt, nickel and copper, with the provisos that the content of titanium is from 10 to 25% by wt., said other said transition elements may partly or wholly replace titanium on an atom-for-atom basis, in amounts such that the contents of vanadium, zirconium, nickel, copper, cerium and erbium do not exceed 5% by wt. each, and the contents of niobium, tantalum, yttrium, tungsten, chromium, iron and cobalt do not exceed 10% by wt. each; said alloy optionally also containing one or more of lithium in an amount up to 3% by wt., silicon and beryllium in an amount up to 5% by wt. each and boron in an amount up to 1%, with the proviso that the total amount of all the above-mentioned transition and other elements, other than aluminum and titanium, does not exceed 10%; said alloy also containing from 1 to 4% by wt. carbon and from 0.1 to 2% oxygen, the balance, apart from impurities, being aluminum, with or without up to 2% by wt. of stable added oxidic material in excess of the oxide indicated by the specified oxygen content of 0.1 to 2% by wt. An alloy according to claim 1 that contains titanium and also contains a dispersion of titanium aluminide. An alloy according to claim 1 wherein said transition element is one or more of titanium, vanadium, zirconium and niobium. An alloy according to claim 1 that contains 10 to 16% titanium, 1.3 to 2% carbon, 0.5 to 1.2% oxygen and up to 2.5% vanadium, the balance, apart from impurities, being aluminum, with or without up to 2% by wt. of stable added oxidic material in excess of the oxide indicated by the specified oxygen content of 0.1 to 2% by wt. An alloy according to any preceding claim wherein the stable added oxidic material is alumina or yttrium-containing oxide. A method of producing an alloy according to any preceding claim which comprises mechanically alloying a charge that contains process control agents such as to introduce the from 0.1 to 2% oxygen therefrom and optionally an addition of up to 2% by wt. of stable oxidic material. A method according to claim 6 wherein said added stable oxidic material is alumina or yttrium-containing oxide.

Inventors: BENN, RAYMOND CHRISTOPHER, MATTSON, WALTER ERNEST, MIRCHANDANI, PRAKASH KISHINCHAND
Applicants: INCO ALLOYS INTERNATIONAL, INC.
Application year: 1989
Main classification: B22F9
IPC codes: C22C32, C22C1, C22C21