A TITANIUM-BASED INTERMETALLIC ALLOY

Abstract

A titanium-based intermetallic alloy includes, in atomic percent, 16% to 26% Al, 18% to 28% Nb, 0% to 3% of a metal M selected from Mo, W, Hf, and V, 0.1% to 2% of Si, 0% to 2% of Ta, 1% to 4% of Zr, with the condition Fe+Ni≦400 ppm, the balance being Ti, the alloy also presenting an Al/Nb ratio in atomic percent lying in the range 1.05 to 1.15.

Claims

1. (canceled)

2. A titanium-based intermetallic alloy comprising, in atomic percent, 16% to 26% Al, 18% to 28% Nb, 0% to 3% of a metal M selected from Mo, W, Hf, and V, 0.1% to 2% of Si, 0% to 2% of Ta, 1% to 4% of Zr, with the condition Fe+Ni 400 ppm, the balance being Ti, the alloy also presenting an Al/Nb ratio in atomic percent lying in the range 1.05 to 1.15.

3. An alloy according to claim 2, comprising 20% to 22% Nb, in atomic percent.

4. An alloy according to claim 2, comprising 23% to 24% Al, in atomic percent.

5. An alloy according to claim 2,comprising 0.1% to 0.8% Si, in atomic percent.

6. An alloy according to claim 2 claim 1, comprising 0.8% to 3% of M, in atomic percent.

7. An alloy according to claim 2, comprising 1% to 3% Zr, in atomic percent.

8. An intermetallic alloy according to claim 2, wherein: the content of Al lies in the range 20% to 25%, in atomic percent; the content of Nb lies in the range 20% to 22%, in atomic percent; the content of M lies in the range 0.8% to 3%, in atomic percent; and the content of Zr lies in the range 1% to 3%, in atomic percent.

9. An alloy according to claim 2, wherein the condition M+Si+Zr+Ta≧0.4% is also true.

10. A turbomachine including a part including an alloy according to claim 2.

11. An engine including a turbomachine according to claim 10.

12. An aircraft including an engine according to claim 11.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Other characteristics and advantages of the invention appear from the following description given with reference to the accompanying drawings, in which: [0042] FIG. 1 shows the variation in creep resistance of various alloys at 650° C. under a stress of 310 MPa; [0043] FIG. 2 shows the influence of the Al/Nb ratio on the resistance to oxidation when hot; and [0044] FIGS. 3A to 3D show results obtained in terms of mechanical properties for a preferred alloy of the invention.

EXAMPLES

Example 1

Fabricating an Alloy of the Invention

[0045] Starting from raw materials constituted by titanium sponges and granules of parent alloys, a mixture was prepared to obtain the chemical composition S12 set out in Table 1 above. The powder mixture was then homogenized and then compressed in order to constitute a compact constituting an electrode. The electrode was then remelted in a vacuum by creating an electric arc between the electrode, which is consumed, and the bottom of a water-cooled crucible (a technique known as vacuum arc remelting (VAR)). The resulting ingot was then reduced into a bar by deformation at high speed (by pestle forging or by extrusion) in order to reduce grain size. The last step was isothermal forging of slugs cut off from the bar at a temperature immediately below the β transus temperature with deformation at low speed (a few 10.sup.−3).

[0046] Such an alloy of S12 composition, which contains 1.3% zirconium, presents very good resistance to oxidation when hot. Specifically, this alloy does not present spalling after being exposed to air at 700° C. for 1500 hours, with an oxide layer made of alumina and zirconia being formed that is fine and very adherent, and thus protective. Alloys not containing zirconium can present less good resistance to oxidation when hot.

Example 2

Improving the Resistance to Creep When Hot by Using a Limited Content of Fe+Ni

[0047] The resistances to creep of three alloy compositions P1, P2, and P3 set out in Table 2 has been compared.

TABLE-US-00002 TABLE 2 Composition at % Ti Al Nb Mo Fe Ni Alloy P1 55.2 23.9 20.3 0.40 0.09 0.01 Alloy P2 53.9 25.3 20.3 0.40 0.07 0.01 Alloy P3 55.5 23.8 20.3 0.40 0.01 0.02

[0048] Those alloys include Fe and Ni trace elements which are present in the form of impurities, and which result naturally from the fabrication method. The elements Fe and Ni are impurities coming from the stainless steel container used for preparing titanium powders. It is thus preferable to use a titanium powder of great purity taken from the center of the volume defined by the container, where the pollution coming from the walls is negligible in order to be sure of obtaining the condition Fe+Ni≦400 ppm. As shown in FIG. 1, an improvement in resistance to creep at 650° C. under stress of 310 MPa is observed when the contents of trace elements are reduced so as to satisfy the relationship Fe+Ni≦400 ppm. Specifically, as shown in FIG. 1, creep reached 1% after 250 hours with an alloy of the invention (P3), whereas this value of creep was reached after only 40 hours with a prior art alloy (P1).

Example 3

Improving the Resistance to Corrosion While Hot by Using Al/Nb at an Atomic Percent Ratio Lying in the Range 1 to 1.3

[0049] The resistance to corrosion when hot of various alloys has been compared. The results are given in FIG. 2. The compositions of alloys S3, S5, S9, and S11 are given above in Table 1.

[0050] During this testing, the change in weight as a result of the surface of the alloy spalling was measured. This test shows the resistance to oxidation of the alloys at 800° C. It can be seen that a loss of weight associated with metal being consumed by oxidation is observed for the alloys S3, S5, and S9 which do not present an Al/Nb ratio lying in the range 1 to 1.3. In contrast, this loss of weight does not occur with the alloy S11, which presents an Al/Nb ratio in the range 1 to 1.3.

Example 4

Comparing the Performance of the Alloy Fabricated in Example 1 With Other Types of Alloy

[0051] The results of tests grouped together in FIGS. 3A and 3D show that the composition S12 presents good results both in traction and in creep. More particularly: [0052] FIG. 3A shows, for various alloys how the elastic limit (R.sub.0.2) varies as a function of temperature; [0053] FIG. 3B shows, for various alloys, how elongation of rupture (ductility) varies as a function of temperature; [0054] FIG. 3C compares creep (time for creep to reach 1%) of various alloys at temperatures of 600° C. and of 650C°; and [0055] FIG. 3D compares times for creep rupture of various alloys at temperatures of 600° C. and 650° C.

[0056] The term “comprising a” should be understood as “comprising at least one”.

[0057] The term “lying in the range . . . to . . . ” should be understood as including the bounds.