creep-resistant titanium alloys

Description

[0001] This patent application is a divisional application of European Patent Application No. 19867058.0 which claims a titanium alloy and method of making the same.FIELD OF THE TECHNOLOGY

[0002] The present disclosure relates to creep resistant titanium alloys.DESCRIPTION OF THE BACKGROUND OF THE TECHNOLOGY

[0003] Titanium alloys typically exhibit a high strength-to-weight ratio, are corrosion resistant, and are resistant to creep at moderately high temperatures. For example, Ti-5Al-4Mo-4Cr-2Sn-2Zr alloy (also denoted "Ti-17 alloy," having a composition specified in UNS R58650) is a commercial alloy that is widely used for jet engine applications requiring a combination of high strength, fatigue resistance, and toughness at operating temperatures up to 800°F. Other examples of titanium alloys used for high temperature applications include Ti-6Al-2Sn-4Zr-2Mo alloy (having a composition specified in UNS R54620) and Ti-3Al-8V-6Cr-4Mo-4Zr alloy (also denoted "Beta-C", having a composition specified in UNS R58640). However, there are limits to creep resistance at elevated temperatures in these alloys. Accordingly, there has developed a need for titanium alloys having improved creep resistance at elevated temperatures.SUMMARY

[0004] The titanium alloy of the invention is given in claim 1. Preferred embodiments are given in claims 2 to 13. It's method of manufacture is given in claim 14.BRIEF DESCRIPTION OF THE DRAWINGS

[0005] The alloy of the invention may be illustrated by the Figures. FIG. 1 is a graph plotting creep strain over time for certain non-limiting embodiments of titanium alloys according to the present disclosure in comparison to certain conventional titanium alloys. FIG. 2 includes a micrograph of a non-limiting embodiment of a titanium alloy according to the present disclosure, and a graph showing results of an energy dispersive X-ray (XRD) scan of the alloy prior to sustained load exposure; FIG. 3 includes a micrograph of the titanium alloy of FIG. 2, and a graph showing results of an XRD scan of the alloy and the partitioning of Zr / Si / Ge to an intermetallic precipitate after the alloy was heated at 482°C (900°F) for 125 hours under a sustained load of 358.5MPa (52 ksi); and FIG. 4 shows elemental maps for the titanium alloy of FIG. 3.

[0006] The reader will appreciate the foregoing details, as well as others, upon considering the following detailed description of certain non-limiting embodiments according to the present disclosure.DETAILED DESCRIPTION OF CERTAIN NON-LIMITING EMBODIMENTS

[0007] Articles and parts in high temperature environments may suffer from creep. As used herein, "high temperature" refers to temperatures in excess of about 93°C (200°F). Creep is time-dependent strain occurring under stress. Creep occurring at a diminishing strain rate is referred to as primary creep; creep occurring at a minimum and almost constant strain rate is referred to as secondary (steady-state) creep; and creep occurring at an accelerating strain rate is referred to as tertiary creep. Creep strength is the stress that will cause a given creep strain in a creep test at a given time in a specified constant environment.

[0008] The creep resistance behavior of titanium and titanium alloys at high temperature and under a sustained load depends primarily on microstructural features. Titanium has two allotropic forms: a beta ("β")-phase, which has a body centered cubic ("bcc") crystal structure; and an alpha ("α")-phase, which has a hexagonal close packed ("hcp") crystal structure. In general, β titanium alloys exhibit poor elevated-temperature creep strength. The poor elevated-temperature creep strength is a result of the significant concentration of β phase these alloys exhibit at elevated temperatures such as, for example, 482°C (900°F). β phase does not resist creep well due to its body centered cubic structure, which provides for a large number of deformation mechanisms. As a result of these shortcomings, the use of β titanium alloys has been limited.

[0009] One group of titanium alloys widely used in a variety of applications is the α / β titanium alloy. In α / β titanium alloys, the distribution and size of the primary α particles can directly impact creep resistance. According to various published accounts of research on α / β titanium alloys containing silicon, the precipitation of silicides at the grain boundaries can further improve creep resistance, but to the detriment of room temperature tensile ductility. The reduction in room temperature tensile ductility that occurs with silicon addition limits the concentration of silicon that can be added, typically, to 0.3% (by weight).

[0010] The present relates to titanium alloys as given in the claims.

[0011] Aluminum may be included in the alloys according to the present disclosure to increase alpha content and provide increased strength. In certain non-limiting embodiments according to the present disclosure, aluminum may be present in weight concentrations, based on total alloy weight, of 5.5-6.5%, or in certain embodiments, 5.9-6.0%.

[0012] Tin may be included in the alloys according to the present disclosure to increase alpha content and provide increased strength. It is present based on total alloy weight, of 1.5-2.5%, or in certain embodiments, 1.7-2.1%.

[0013] Molybdenum may be included in the alloys according to the present disclosure to increase beta content and provide increased strength. It is present in weight concentrations, based on total alloy weight, of 1.3-2.3%, or in certain embodiments, 1.7-2.1%.

[0014] Zirconium may be included in the alloys according to the present disclosure to increase alpha content, provide increased strength and provide increased creep resistance by forming an intermetallic precipitate. It is present in weight concentrations of 0.1-10% Preferably, zirconium may be present in weight concentrations, based on total alloy weight, of 3.4-4.4%, or in certain embodiments, 3.5-4.3%.

[0015] Silicon may be included in the alloys according to the present disclosure to provide increased creep resistance by forming an intermetallic precipitate. In certain non-limiting embodiments according to the present disclosure, silicon may be present in weight concentrations, based on total alloy weight, of 0.01-0.30%. In certain non-limiting embodiments, silicon may be present in weight concentrations, based on total alloy weight, of 0.03-0.11%, or in certain embodiments, 0.06-0.11%.

[0016] Germanium may be included in embodiments of titanium alloys according to the present disclosure to improve secondary creep rate behavior at elevated temperatures. In certain non-limiting embodiments, germanium may be present in weight concentrations, based on total alloy weight, of 0.1-2.0%, or in certain embodiments, 0.1-0.4%. Without intending to be bound to any theory, it is believed that the germanium content of the alloys in conjunction with a suitable heat treatment may promote precipitation of a zirconium-silicon-germanium intermetallic precipitate. The germanium additions can be by, for example, pure metal or a master alloy of germanium and one or more other suitable metallic elements. Si-Ge and Al-Ge may be suitable examples of master alloys. Certain master alloys may be in powder, pellets, wire, crushed chips, or sheet form. The titanium alloys described herein are not limited in this regard. After final melting to achieve a substantially homogeneous mixture of titanium and alloying elements, the cast ingot can be thermo-mechanically worked through one or more steps of forging, rolling, extruding, drawing, swaging, upsetting, and annealing to achieve the desired microstructure. It is to be understood that the alloys of the present disclosure may be thermo-mechanically worked and / or treated by other suitable methods.

[0017] The method of making the titanium alloy is given in claim 14. Upon completion of the solution treatment, the titanium alloy is cooled to ambient temperature at a rate depending on a cross-sectional thickness of the titanium alloy.

[0018] The solution treated titanium alloy is subsequently aged at an aging temperature, also referred to herein as an "age hardening temperature", that is in the α+β two-phase field below the β transus temperature of the titanium alloy. In a non-limiting embodiment, the aging temperature is in a temperature range from 552°C (1025°F) to 607°C (1125°F). The aging time is 8 hours. General techniques used in STA processing of titanium alloys are known to practitioners of ordinary skill in the art and, therefore, are not further discussed herein.

[0019] While it is recognized that the mechanical properties of titanium alloys are generally influenced by the size of the specimen being tested, in certain non-limiting embodiments of the titanium alloy according to the present disclosure, the titanium alloy exhibits a steady-state (also known as secondary or "stage II") creep rate less than 8×10 -4< (24 hrs) -1< at a temperature of at least 890°F (477°C) under a load of 52 ksi, (358 MPa). Also, for example, certain non-limiting embodiments of titanium alloys according to the present disclosure may exhibit a steady-state (secondary or stage II) creep rate less than 8×10 -4< (24 hrs) -1< at a temperature of 900°F (482°C) under a load of 52 ksi (358 MPa). ksi. In certain non-limiting embodiments according to the present disclosure, the titanium alloy exhibits an ultimate tensile strength of at least 130 ksi (890 MPa) at 900°F (482°C). In other non-limiting embodiments, a titanium alloy according to the present disclosure exhibits a time to 0.1% creep strain of no less than 20 hours at 900°F (482°C) under a load of 52 ksi (358 MPa).

[0020] The examples that follow are intended to further describe non-limiting embodiments according to the present disclosure, without restricting the scope of the present invention. Persons having ordinary skill in the art will appreciate that variations of the following examples are possible within the scope of the invention, which is defined solely by the claims.EXAMPLE 1

[0021] Table 1 lists elemental compositions of certain non-limiting embodiments of titanium alloys according to the present disclosure ("Experimental Titanium Alloy No. 1," "Experimental Titanium Alloy No. 2," and "Experimental Titanium Alloy No. 3"), along with a comparative titanium alloy that does not include an intentional addition of germanium ("Comparative Titanium Alloy"). Table 1AlloyAl (wt%)Sn (wt%)Zr (wt%)Mo (wt%)Si (wt%)O (wt%)Ge (wt%)C (wt%)N (wt%)Comparative Titanium Alloy, UNS R58650 (B5P41) 5.91.84.11.90.070.160.00.0130.001Experimental Titanium Alloy No. 1 (B5P42) 5.91.94.01.80.060.120.10.0030.001Experimental Titanium Alloy No. 2 (B5P43) 5.91.93.91.90.070.130.20.0030.001Experimental Titanium Alloy No. 3 (B4M35) 6.02.03.71.80.110.130.40.0080.001

[0022] Plasma arc melt (PAM) heats of the Comparative Titanium Alloy, Experimental Titanium Alloy No. 1, Experimental Titanium Alloy No. 2, and Experimental Titanium Alloy No. 3 listed in Table 1 were produced using plasma arc furnaces to produce 9 inch (23 cms) diameter electrodes, each weighing approximately 400-800 lbs (181-363 Kg). The electrodes were remelted in a vacuum arc remelt (VAR) furnace to produce 10 inch (25.4 cms) diameter ingots. Each ingot was converted to a 3 inch (7.6 cms) diameter billet using a hot working press. After a β forging step to 7 inch (17.8 cms) diameter, an α+β prestrain forging step to 5 inch (12.7 cms) diameter, and a β finish forging step to 3 inch (7.6 cms) diameter, the ends of each billet were cropped to remove suck-in and end-cracks, and the billets were cut into multiple pieces. The top of each billet and the bottom of the bottom-most billet at 7 inch (17.8 cms) diameter were sampled for chemistry and β transus. Based on the intermediate billet chemistry results, 2 inch (5.1 cms) long samples were cut from the billets and "pancake"-forged on the press. The pancake specimens were heat treated to a solution treated and aged condition as follows: solution treating the titanium alloy at 1780°F to 1800°F (971-982°C) for 4 hours; cooling the titanium alloy to ambient temperature at a rate depending on a cross-sectional thickness of the titanium alloy; aging the titanium alloy at 1025°F to 1125°F (552-607°C) for 8 hours; and air cooling the titanium alloy.

[0023] Test blanks for room and high temperature tensile tests, creep tests, fracture toughness, and microstructure analysis were cut from the STA processed pancake specimens. A final chemistry analysis was performed on the fracture toughness coupon after testing to ensure accurate correlation between chemistry and mechanical properties. Certain mechanical properties of the experimental titanium alloys listed in Table 1 were measured and compared to that of the comparative titanium alloy listed in Table 1. The results are listed in Table 2. The tensile tests were conducted according to the American Society for Testing and Materials (ASTM) standard E8 / E8M-09 ("Standard Test Methods for Tension Testing of Metallic Materials", ASTM International, 2009). As shown by the results listed in Table 2, the experimental titanium alloy samples exhibited ultimate tensile strength and yield strength at room temperature comparable to the comparative titanium alloy, which did not include an intentional addition of germanium. Table 2AlloyHeat TreatmentRoom Temperature (72°F)Elevated Temperature (900°F)UTS (ksi)YS (ksi)%el%RAUTS (ksi)YS (ksi)%el%RAComparative Titanium Alloy, UNS R58650 (B5P41) 117816313451251091763Experimental Titanium Alloy No. 1 (B5P42) 117515713391301031864Experimental Titanium Alloy No. 2 (B5P43) 11781571439130951759Experimental Titanium Alloy No. 3 (B4M35) 21771586121331061341Heat Treatments: 1 - Solution treating at 1780°F (971°C) for 4 hours, water quenching, aging at 1100°F (593°C) for 8 hours, and air cooling 2 - Solution treating at 1800°F (982°C) for 4 hours, water quenching, aging at 1100°F (593°C) for 8 hours, and air cooling

[0024] Creep-rupture tests according to ASTM E139 were conducted on the alloys listed in Table 1. The results are presented in FIG. 1. The experimental titanium alloys of the present disclosure exhibited very favorable secondary creep rates relative to the comparative titanium alloy. Referring to FIGS. 2-4, precipitation of a zirconium-silicon-germanium intermetallic phase was detected in Experimental Titanium Alloy No. 2 after creep exposure to a sustained load and elevated temperature in excess of the time for primary (or stage I) creep. As shown by FIG. 1, the experimental titanium alloy samples of the present disclosure exhibited steady-state creep after approximately 30 hours at 900°F (482°C) under a load of 52 ksi (358 MPa). The Comparative Titanium Alloy exhibited a time to 0.1% creep strain of 19.4 hours at 900°F (482°C) under a load of 52 ksi (358 MPa). Experimental Titanium Alloy No. 1, Experimental Titanium Alloy No. 2, and Experimental Titanium Alloy No. 3 all exhibited a significantly greater time to 0.1% creep strain at 900°F (482°C) under a load of 52 ksi (358 MPa): 32.6 hours, 55.3 hours, and 93.3 hours, respectively.

[0025] Samples examined prior to the creep exposure (but after the heat treatments) did not reveal the presence of intermetallic precipitates. Referring to FIG. 2, an elemental scan by energy dispersive x-rays (EDS) of Experimental Titanium Alloy No. 2 prior to creep exposure showed a substantially uniform distribution of germanium in the α / β microstructure without the intermetallic particles. In FIGS. 3-4, partitioning of zirconium, silicon, and germanium to intermetallic particles is visible after the creep exposure. The intermetallic particles generally exhibit depletion of aluminum relative to the surrounding alpha particle. The precipitation of the intermetallic particles after the creep exposure was particularly unexpected and surprising. Without intending to be bound to any theory, it is believed that the intermetallic particles may improve secondary creep for the alloys without substantially impacting high temperature yield strength.

[0026] The potential uses of alloys according to the present disclosure are numerous. As described and evidenced above, the titanium alloys described herein are advantageously used in a variety of applications in which creep resistance at elevated temperatures is important. Articles of manufacture for which the titanium alloys according to the present disclosure would be particularly advantageous include certain aerospace and aeronautical applications including, for example, jet engine turbine discs and turbofan blades. Those having ordinary skill in the art will be capable of fabricating the foregoing equipment, parts, and other articles of manufacture from alloys according to the present disclosure without the need to provide further description herein. The foregoing examples of possible applications for alloys according to the present disclosure are offered by way of example only, and are not exhaustive of all applications in which the present alloy product forms may be applied. Those having ordinary skill, upon reading the present disclosure, may readily identify additional applications for the alloys as described herein.

Claims

1. A titanium alloy comprising, in weight percentages based on total alloy weight: 5.5 to 6.5 aluminum; 1.5 to 2.5 tin; 1.3 to 2.3 molybdenum; 0.1 to 10.0 zirconium; 0.01 to 0.30 silicon; 0.1 to 2.0 germanium; and optionally: 0 to 0.30 oxygen; 0 to 0.30 iron; 0 to 0.05 nitrogen; 0 to 0.05 carbon; 0 to 0.015 hydrogen; and 0 to 0.1 each of niobium, tungsten, hafnium, nickel, gallium, antimony, vanadium, tantalum, manganese, cobalt, and copper; balance titanium, and impurities, wherein the titanium alloy comprises an intermetallic precipitate comprising zirconium, silicon, and germanium.

2. The titanium alloy of claim 1 comprising, in weight percentages based on total alloy weight: 1.7 to 2.1 tin; 1.7 to 2.1 molybdenum; 3.4 to 4.4 zirconium; 0.03 to 0.11 silicon; 0.1 to 0.4 germanium.

3. The titanium alloy of claim 1 comprising, in weight percentages based on total alloy weight: 5.9 to 6.0 aluminum; 1.9 to 2.0 tin; 1.8 to 1.9 molybdenum; 3.5 to 4.3 zirconium; 0.06 to 0.11 silicon; 0.1 to 0.4 germanium.

4. The titanium alloy of claim 1, wherein an aluminum content in the alloy is, in weight percentages based on total alloy weight, 5.9 to 6.0.

5. The titanium alloy of claim 1, wherein a tin content in the alloy is, in weight percentages based on total alloy weight, 1.7 to 2.1.

6. The titanium alloy of claim 1, wherein a tin content in the alloy is, in weight percentages based on total alloy weight, 1.9 to 2.0.

7. The titanium alloy of claim 1, wherein a molybdenum content in the alloy is, in weight percentages based on total alloy weight, 1.7 to 2.1.

8. The titanium alloy of claim 1, wherein a molybdenum content in the alloy is, in weight percentages based on total alloy weight, 1.8 to 1.9.

9. The titanium alloy of claim 1, wherein a zirconium content in the alloy is, in weight percentages based on total alloy weight, 3.4 to 4.4.

10. The titanium alloy of claim 1, wherein a zirconium content in the alloy is, in weight percentages based on total alloy weight, 3.5 to 4.3.

11. The titanium alloy of claim 1, wherein a silicon content in the alloy is, in weight percentages based on total alloy weight, 0.03 to 0.11.

12. The titanium alloy of claim 1, wherein a silicon content in the alloy is, in weight percentages based on total alloy weight, 0.06 to 0.11.

13. The titanium alloy of claim 1, wherein a germanium content in the alloy is, in weight percentages based on total alloy weight, 0.1 to 0.4.

14. A method of making a titanium alloy, the method comprising: solution treating a titanium alloy at 971°C (1780°F) to 982°C (1800°F) for 4 hours; cooling the titanium alloy to ambient temperature at a rate depending on a cross-sectional thickness of the titanium alloy; aging the titanium alloy at 552°C (1025°F) to 607°C (1125°F) for 8 hours; and air cooling the titanium alloy, wherein the titanium alloy has the composition recited in any one of the preceding claims.