US3203794A - Titanium-high aluminum alloys - Google Patents

Description

United States Patent Office 3,203,794 Patented Aug. 31, 1965 3,203,794 TITANIUM-HIGH ALUMINUM ALLOYS Robert I. .laitee, Worthington, and Horace R. Ogden and Daniel J. Maykuth, Columbus, Ohio, assignors, by

mesne assignments, to Crucible Steel Company of America, Pittsburgh, Pa., a corporation of New Jersey N Drawing. Filed Apr. 1'5, 1957, Ser. No. 652,697

2 Claims. (Cl. 75-'175.5)

This application is a continuation-in-part of our copending applications Serial No. 187,370, filed September 28, 1950 (now abandoned), and Serial No. 401,053, filed December 29, 1953 (now abandoned).

This invention pertains to alloys consisting essentially of titanium and aluminum, the aluminum content of which ranges from about 34% to 46%, and which are characterized at room temperature by high strength, elasticity and elastic modulus in compression, and characterized further in maintaining substantially their room temperature hardness values at elevated temperatures as high as about 1250 C., and in being highly resistant to oxidation and atmospheric contamination at elevated temperatures up to at least 1050 C. The binary Ti-(34- 46%) Al alloys of the invention are further characterized in having a single phase, face-centered, tetragonal microstructure.

The invention also pertains to shaped articles of structural utility made of such alloys, especially those for use at elevated temperatures under atmospheric or other oxidative or contaminating condition, such as jet engine components, for example, as compressor wheels or blades or turbine wheels or buckets, or more generally for applications requiring lightness in weight and retention of strength and good oxidation resistance at elevated temperatures, such as plates, channel or angle members or equivalent structural components, tubes, engine housings or shrouds, etc.

The alloys of the invention may be made by arc melting under contamination-free conditions, for example, in an inert atmosphere such as argon, in a water-cooled copper crucible, or by equivalent techniques, such as skull melting. They may also be produced by powder metallurgy techniques as by admixing the finely comminuted alloying ingredients followed by pressing and sintering.

The shaped structural articles of the invention may be produced by casting the alloy from the molten state. Or where powder metallurgy techniques are employed, by pressing the admixed, powdered alloying ingredients substantially to the final shape desired followed by pressing and sintering.

The metal titanium has at ordinary temperatures, a close-packed hexagonal microstructure commonly called the alpha phase. At a temperature of about 880 C., it is transformed to a body-centered cubic or beta microstructure. The addition of aluminum tends to stabilize the alpha phase. Alloys have been made containing as much as of aluminum and found to consist entirely of equiaxed alpha grains. When the aluminum content is increased to about another phase appears interspersed with the alpha phase in a plate-like structure. This additional phase is not beta titanium but a third crystalline form which, due to its high aluminum content, cannot be called a phase of titanium. For the purpose of this specification, it will be identified as an I(Al-Ti) or gamma phase. Its crystalline form is face-centered tetragonal.

When the aluminum content is increased to about 32% alpha titanium is still present, but at an aluminum content of about 34% the alpha titanium disappears, and the entire microstructure is one of equiaxed 1(Al-Ti) or gamma grains. A microphotograph shows a strong resemblance to many of the brasses. As the aluminum content is further increased, the alloy continues single phase I(Al-Ti) up to an aluminum content of about 46%. At an aluminum content of about 50% the alloy again has a two-phase microstructure.

Up to a temperature of 1050 C., the boundaries of the I(Al-Ti) phase are an aluminum content of about 34% and an aluminum content of about 46%. Alloys in this phase show some extraordinary properties. They have essentially the same structure and hardness whether quenched or slow cooled from temperatures up to 1050 C. Their room temperature hardness is not high, being between about and 255 Vickers, but they maintain substantially their room temperature hardness at temperatures as high as 1250 C. An alloy containing 37% aluminum is typical. When it is enclosed in a heavy sheath of aluminum or stainless steel and the assembly is rolled at temperatures up to 1250 C., the metal of the sheath is reduced to zero thickness without deformation of the alloy contained therein, nothwithstanding there may be, in the case of stainless steel, some alloying with the sheath. They are also highly resistant to oxidation at elevated temperatures. A specimen of an alloy containing 37.5% aluminum, balance substantially pure titanium, when exposed to a temperature of 850 C., for 36 hours, shows a weight gain of only 205 milligram per square decimeter; whereas, under the same conditions, the titanium, from which the alloy was made, gained nearly 3000 milligrams weight per square decimeter. The surface hardness of the alloy remained substantially the same, in fact, showed a slight decrease; Whereas, the surface hardness of the titanium increased from 101 to 415 Vickers, an increase of over 300 Vickers points, and there remained a measurable difference in hardness at a depth of 0.009". When exposed to a temperature of 1050 C., for 24 hours, the alloy gained only 1381 milligrams weight per square decimeter; whereas the titanium, from which the alloy was made, gained 5570 milligrams weight per square decimeter. Again the hardness of the alloy was essentially the same after the test as before; while the surface hardness of the titanium increased from 104 Vickers to 526 Vickers, an increase of over 400 Vickers points, and its hardness at a depth of 0.028" was 343 Vickers, an increase of about 240 Vickers points.

When subjected to a compression test, an allow of commercial titanium with 37% aluminum in the as cast condition showed an elastic modulus of 20,500,000 p.s.i., a proportional limit of 23,000 p.s.i., a 0.1% offset yield strength of 52,000 p.s.i., and a 0.2% offset yield strength of 58,000 psi. Its ultimate strength was not less than 126,000 p.s.i., and it sustained a reduction in length of 7.8% without fracture.

It will be obvious that the high modulus of elasticity together with the low density, the excellent oxidation resistance and other high temperature properties of this alloy render it desirable for many uses particularly in the form of castings or shaped powder metallurgy parts, which in use are exposed to high temperature conditions. Additions of other elements to the binary Ti-(34-46)Al 3 alloy, affect the hardness, and hence the strength of the resulting alloy in the manner shown in Table I below.

TABLE I Hardness of Ti-(3446)Al gamma phase alloys with and without alloying additions of other elements [Iodide Ti base] Composition As cast (balance Ti): Vickers hardness 35Al 206 37Al 238 37Al 231 37.5Al 211 40Al 221 42Al 230 46Al 255 37.5Al-1Ag 211 37Al-0.25B 284 37Al-1B 322 37.5Al-0.25Be 312 37.5Al-1Be 297 37.5Al-0.25C 320 37Al-1Cb 266 37Al-1Cr 282 37.5Al-1Cu 227 37Al-1Fe 244 37.5Al-1In 198 37.5Al-1Mn 226 37.5Al-5Mn 282 35A1-10Mn 347 37Al-1Mo 243 37Al-0.25N 320 37.5Al-lNi 256 37Al-0.25O 320 37Al-1Pb 238 37Al-1Sb 266 37.5A1-1Sn 215 37Al-1Si 239 37Al-1Ta 240 37Al-1Te 247 37Al-1W 231 37Al-lZr 279 The above alloys were produced by cold mold melting and casting in an inert atmosphere. Highest purity materials were employed including, as noted, iodide base titanium. The total impurity content for the binary alloys was estimated not to exceed 0.10% of other elements.

The alloying additions of Table I include the alphaphase promoters such as In, Bi, Pb, Sn, Sb, Ag, C, O, N; beta promoters, such as Mo, V, Cb, Ta, Zr, Mn, Cr, Fe, W, Co, Ni, Cu, Si, Be; and compound forming elements such as B, Ce, As, S, Te and P.

The data of Table I shows that additions of other elements to the binary alloy in general produce an increase in hardness. Carbon, oxygen and nitrogen have a potent hardening action when present even in small amounts. Thus the hardness of the Ti-37.5%Al alloy is increased from about 200 to 320 Vickers by additions of 0.25% of each of C, O and N. The compound formers, such as boron, are also potent hardeners, as shown. As against this, the metallic alpha promoters such as silver, tin, indium, etc., do not greatly increase the hardness when added in amounts up to at least 1%. The hardening action of the beta promoters is in general substantial and varies over a considerable range as shown.

Since minimum hardness and hence maximum ductility is obtained with the binary Ti-(3446)Al alloy, the presence of other elements either as intentional additions or as impurities should be minimized where maximum duc tility is desired. This is particularly true with respect to such potent hardeners as the interstitials and compound formers, which in such case, should not total more than about 0.25% and preferably should not exceed 0.1% in aggregate. The metallic alpha promoters such as silver, tin, antimony, etc., may in general be present up to about 1%, without seriously impairing the ductility of the alloy, and the same in general applies to the beta promoters.

Where ductility is not of controlling importance, the interstitials and compound formers may be present up to a total of about 1%, and the alpha and/or beta promoters up to about 5%. The lower effective limits for the interstitials and compound formers is about 0.05% and for the alpha and/or beta promoters, about 0.1%. Individual upper limits for carbon, oxygen and nitrogen are 1%, 0.5% and 0.3%, respectively, the preferred upper limits being 0.25% C, 0.2% 0 and 0.15% N.

What is claimed is:

1. A titanium-base alloy containing about 34 to 46% of aluminum, and characterized in maintaining substantially its room temperature hardness at temperatures as high as about 1250 C., and in being highly resistant to oxidataion at elevated temperatures up to at least 1050 C.

2. A titanium-base alloy containing about 34 to 46% of aluminum, characterized in having a single-phase, face-centered, tetragonal microstructure, an ultimate strength at room temperature of at least 126,000 p.s.i., and in being highly resistant to oxidation at elevated temperatures and in maintaining substantially its room temperature hardness at elevated temperatures as high as about 1250 C.

References Cited by the Examiner UNITED STATES PATENTS 2,370,289 2/45 Chandler -l75.5 2,880,087 3/59 Jaffee 75-1755 3,008,823 11/61 McAndrew 75175.5

OTHER REFERENCES Tentative Diagram of the Titanium-Aluminum System, from The Constitution of Titanium-Aluminum Alloys, H. R. Ogden, D. I. Maykuth, W. L. Finlay, and R. I. Jaffee; submitted to AIME on April 25, 1961.

Hansen, Constitution of Binary Alloys, McGraw-Hill Book Co., Inc., New York, 1958, pp. 139-142.

DAVID L. RECK, Primary Examiner.

CLYDE C. LE ROY, RAY K. WINDHAM, MARCUS U. LYONS, ROGER L. CAMPBELL, Examiners.

Claims (1)

1. A TITANIUM-BASE ALLOY CONTAINING ABOUT 34 TO 46% OF ALUMINUM, AND CHARACTERIZED IN MAINTAINING SUBSTANTIALLY ITS ROOM TEMPERATURE HARDNESS AT TEMPERATURES AS HIGH AS ABOUT 1250*C., AND IN BEING HIGHLY RESISTANT TO OXIDATAION AT ELEVATED TEMPERATURES UP TO AT LEAST 1050* C.