High-strength titanium-based alloy and article manufactured from same

Abstract

The invention relates to non-ferrous metallurgy, in particular to the development of titanium base alloys used for the manufacture of a wide range of high strength semi-finished products, mainly die-forgings and forgings, sheets, plates, billets, bars, including large ones, as well as semi-finished products for the manufacture of parts operated at elevated temperatures, which can be used in aerospace, power, chemical industries, machine building, and other industrial sectors. A high strength titanium base alloy containing aluminum, molybdenum, niobium, iron, silicon, oxygen, according to the invention additionally contains tungsten, carbon, nitrogen, hydrogen with the following ratio of components, wt. %:Aluminum5.0-7.0Molybdenum 6.0-12.0Niobium2.0-6.5Tungsten2.5-4.5Silicon0.1-0.3Iron 0.2 max.Oxygen0.17 max.Carbon0.05 max.Nitrogen0.01 max.Hydrogen0.01 max.Titanium and inevitable impurities—the balance.

Description

[0001] The invention relates to non-ferrous metallurgy, in particular to the development of titanium base alloys used for the manufacture of a wide range of high strength semi-finished products, mainly die-forgings and forgings, sheets, plates, billets, bars, including large ones, as well as semi-finished products for the manufacture of parts operated at elevated temperatures which can be used in aerospace, power, chemical industries, machine building, and other industrial sectors.

[0002] Titanium base alloys are one of the prime structural materials. Their application is governed by the inherent properties of titanium and its alloys: high strength, corrosion resistance in many corrosive environments, non-magnetization, good high temperature strength at operating temperatures as high as 500-600° C.

[0003] The unique application of titanium alloys in recent decades is characterized by their increasing use in various structures which leads to both production ramp-up and expansion of the mix of semi-finished products and finished parts.

[0004] High strength β-titanium alloys are the main materials for making heavily loaded structural elements due to the combination of high strength, fracture toughness, hardenability, and other properties. The primary property of these alloys is specific strength which justifies their use despite higher cost as compared to other structural materials. Thus, at strength of 1500 MPa, replacement of a steel element with an element made of a high strength β-titanium alloy leads to weight saving of about 20-25%.

[0005] Therefore, due to increasingly stringent technical requirements, critical applications require specialists to further improve the performance of components by developing new compositions of titanium alloys which are capable of meeting strict requirements for physical and mechanical properties which is quite a challenging task.

[0006] There is a known titanium base alloy with the tensile strength of more than 1300 MPa, consisting of, in weight percentages, 6.6 to 7.2 molybdenum, 3 to 5.1 aluminum, 2.6 to 4.1 chromium, 2.7 to 3.1 niobium, 0.17 to 0.25 oxygen, 0 to 0.5 iron, and the balance—titanium and impurities (Patent No. CN110016588A, published on 16 Jul. 2019, IPC C22C14 / 00).

[0007] The known alloy contains up to 0.5 wt. % of iron, the presence of which in the alloy is undesirable because of the significant decrease in creep resistance at elevated temperatures and excessive segregation during melting.

[0008] The closest to the described invention is an alloy consisting of, in weight percentages, 2.0 to 4.0 aluminum, 5.5 to 8.0 molybdenum, 0.4 to 0.8 iron, 8.5 to 10.5 vanadium, 0.4 to 0.8 chromium, 2.1 to 3.0 niobium, 0.11 to 0.25 silicon, and the balance—titanium (Patent RF2192493, published on 10 Nov. 2002, IPC C22C 14 / 00)-prototype.

[0009] The defective feature of the prototype is low oxidation resistance and creep resistance at elevated temperatures attributed to high concentration of vanadium in the alloy.

[0010] The problem to be solved by this invention is the development of a versatile high strength titanium alloy for the manufacture of a wide range of products, including die-forgings and forgings, billets, sheets, plates, and bars, including large ones, suitable for use at elevated temperatures.

[0011] The technical result of the invention is a titanium alloy having a complex of high mechanical and performance properties, including high strength with satisfactory ductility, as well as an enhanced creep resistance and oxidation resistance.

[0012] This technical result is ensured by the fact that according to the invention a high strength titanium base alloy containing aluminum, molybdenum, niobium, iron, silicon, oxygen, additionally contains tungsten, carbon, nitrogen, hydrogen with the following ratio of components, wt. %:Aluminum5.0-7.0Molybdenum 6.0-12.0Niobium2.0-6.5Tungsten2.5-4.5Silicon0.1-0.3Iron 0.2 max.Oxygen0.17 max.Carbon0.05 max.Nitrogen0.01 max.Hydrogen0.01 max.Titanium and inevitable impurities—the balance,

[0014] The value of molybdenum equivalent [Mo]eq, determined from the formula:

[0015] [Mo]eq=[Mo]+[V] / 1.5+[Cr]×1.25+[Fe]×2.5 wt. % is 8 minimum. The ratio of Mo (wt. %) to Nb (wt. %) is 2 to 4. The ratio of Al (wt. %) to W (wt. %) is 1.4 to 2.2.

[0016] In addition, an article made of this alloy is proposed.

[0017] The alloy contains alloying elements from various groups of stabilizers having a complex effect of strengthening the α and β phases in the alloy structure: α stabilizers: aluminum, oxygen, carbon, nitrogen; β stabilizers: molybdenum, niobium, tungsten, silicon, iron.Group of α Stabilizers (Al, O, C, N)

[0018] Aluminum is the main a stabilizer, it increases the beta transus temperature. The aluminum content in the alloy is taken to be 5.0 to 7.0 wt. %. To ensure the optimal strengthening effect, the preferable concentration of aluminum is in the range of 5.0 wt. % minimum and 7.0 wt. % maximum, since higher concentrations promote formation of Ti3Al phase in the α phase which affects the performance properties of the alloy, in particular it deteriorates ductility and fracture toughness.

[0019] The content of oxygen, nitrogen, and carbon within the specified limits, together with an increase in strength, increases the temperature of the allotropic transformation of titanium and maintains high level of strength and ductility. Higher concentrations of oxygen, carbon, and nitrogen reduce the processing ductility and impact strength of the alloy.Group of β Stabilizers (Mo, Nb, W, Si).

[0020] Alloying with β stabilizers reduces the beta transus temperature. The authors defined that simultaneous introduction of molybdenum, niobium, and tungsten in the proposed concentration of the alloy components contributes to the alloy strengthening and improvement of high temperature strength behaviour.

[0021] Introduction of molybdenum and niobium is dictated by their ability of unlimited dissolution in the β phase of titanium. Molybdenum content over 6.0 wt. % increases the solid solution strengthening of the alloy; molybdenum content over 12.0 wt. % increases the density of the alloy which results in the increased weight of the products. Niobium content in the range of 2.0 to 6.5 wt. % allows to improve the oxidation resistance and to ensure its uniform content in titanium phases.

[0022] Tungsten concentration in the alloy is limited to the range of 2.5 to 4.5, which, together with other β stabilizers, has a stabilizing effect of fixing the metastable phase during air cooling after forging / rolling using strengthening heat treatment at the final stage of alloy manufacturing to achieve the highest possible strength characteristics. Tungsten, having low diffusion coefficient and high elastic coefficient, forms a solid solution with the β phase of titanium which enhances high temperature strength of the alloy.

[0023] Silicon concentration in the alloy is set in the range of 0.1 to 0.3 wt. %. Silicon in the alloy composition present in the titanium solid solution increases strength without reducing ductility and increases creep resistance. Silicon concentration over 0.3 wt. % leads to the formation of large particles of titanium silicides which deteriorate the alloy ductility and also affect the resistance to fatigue failure.

[0024] In the claimed alloy the molybdenum equivalent [Mo]eq is controlled and determined from the following formula:[Mo]eq=[Mo]+[V] / 1.5+[Cr]×1.25+[Fe]×2.5 wt. %,

[0025] To ensure stability of the β solid solution of titanium during the alloy air cooling after deformation and after solid solution heat treatment the value of [Mo]eq is 8 minimum. The ratio of Mo (wt. %) to Nb (wt. %) in the claimed alloy is in the range of 2 to 4, and the ratio of Al (wt. %) to W (wt. %) is in the range of 1.4 to 2.2 which allows to maintain high creep resistance and good oxidation resistance of the alloy at elevated temperatures.

[0026] The mixture of elements in the alloy composition in the claimed ratio which are individually characterized by a favourable effect on the performance of titanium as well as controlled ratios of alloying elements result in an additive effect in terms of achieving high levels of creep resistance and oxidation resistance versus the known alloys.

[0027] The industrial applicability of the invention is confirmed by an example of its specific implementation.

[0028] To study the properties of the proposed alloy, ingots of 3 chemical compositions were melted via vacuum arc remelting practice. The chemical compositions of the alloys of this invention and the beta transus temperature (BTT) of the alloys determined by microstructural method are given in Table. 1.

[0029] The alloy ingots were hot worked via forging followed by rolling to produce 13 mm thick plates which were subsequently subjected to strengthening heat treatment.TABLE 1AlloyElements, wt. %BTT,Mo / Al / compositionTiAlMoNbWSiFeOCNH° C.NbW[Mo]eq.1base5.839.472.823.720.220.100.130.006<0.0030.00238903.361.579.72base6.27.83.840.250.100.110.006<0.0030.00199152.051.558.13base6.510.32.63.40.210.100.100.005<0.0030.00228953.961.9110.6

[0030] The study of mechanical properties included tensile tests at room temperature and creep resistance tests with the determination of the tensile strength, elongation, and creep rupture strength at 400° C. The values of mechanical properties of samples in comparison with the prototype alloy are given in Table 2. As can be seen, mechanical properties of the proposed alloy exceed mechanical properties of the prototype alloy: the room temperature tensile strength is 50-60 MPa higher, the creep rupture strength at 400° C. is 50 MPa higher.

[0031] To study the possibility of using the proposed alloy at elevated temperatures its creep resistance was evaluated. For this purpose the samples were isothermally annealed in static laboratory air at 700° C. with holding time of 100 hours. After that the oxidation resistance was studied by calculating the weight gain of the samples expressed in mg / cm2. The results of studies of the oxidation resistance of the proposed alloy in comparison with the known commercial high temperature alloys are shown in FIG. 2. The proposed alloy has a 30% less weight gain as a result of oxidation as compared to the commercial high temperature alloys Ti6242S and Ti6246 which testifies to its advantage.TABLE 2Creep ruptureAlloyTensile strengthElongationstrengthcompositionσB, MPaδ, %σ100400, MPaComposition 1 of1350111010the inventionComposition 2 of1350121010the inventionComposition 3 of1360101010the inventionPrototype alloy130011960

[0032] Therefore, the claimed alloy, as compared to the known alloys, has higher strength and creep resistance, improved oxidation resistance as compared to the commercial high temperature titanium alloys and can be used for the manufacture of a wide range of semi-finished products including those for production of parts operated at elevated temperatures which can be used for various industrial applications.

Claims

1. A high strength titanium base alloy containing aluminum, molybdenum, niobium, iron, silicon, wherein the alloy additionally contains tungsten, oxygen, carbon, nitrogen, hydrogen with the following ratio of components, wt. %:Aluminum5.0-7.0Molybdenum 6.0-12.0Niobium2.0-6.5Tungsten2.5-4.5Silicon0.1-0.3Iron 0.2 max.Oxygen0.17 max.Carbon0.05 max.Nitrogen0.01 max.Hydrogen0.01 max.Titanium and inevitable impurities—the balance.

2. A high strength alloy according to claim 1, wherein the value of molybdenum equivalent [Mo]eq determined from the formula:[Mo]eq=[Mo]+[V] / 1.5+[Cr]×1.25+[Fe]×2.5 wt. %is 8 minimum.

3. A high strength alloy according to claim 1, wherein the ratio of Mo (wt. %) to Nb (wt. %) is 2 to 4.

4. A high strength alloy according to claim 1, wherein the ratio of Al (wt. %) to W (wt. %) is 1.4 to 2.2.

5. An article made of a high strength titanium base alloy, wherein the article comprises the high strength alloy according to claim 1.

6. The article according to claim 5, wherein the article comprises a sheet, a plate, a billet, or a bar.

7. The high strength alloy according to claim 1, wherein the tensile strength of the high strength alloy at 400° C. is at least 1350 MPa.

8. The high strength alloy according to claim 1, wherein the creep rupture strength at 400° C. is at least 1010 MPa.

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