RU2690257C1 - Titanium-based alloy - Google Patents

Abstract

FIELD: metallurgy.SUBSTANCE: invention relates to metallurgy, namely to titanium alloys intended for use as high-strength structural high-tech material for production of power structures of shipbuilding, aircraft and space equipment, power plants operating at temperatures of up to 350 °C. Titanium-based alloy contains, wt. %: aluminum 5.3–6.0; vanadium 7.3–7.9; molybdenum 4.8–5.3; chromium 1.3–1.8, iron 0.4–0.7; zirconium 0.5–0.8; oxygen 0.10–0.18, carbon 0.01–0.02, silicon 0.005–0.02, nitrogen 0.005–0.02, hydrogen 0.003–0.015, other impurities not more than 0.30; titanium – the rest.EFFECT: alloy is characterized by high strength, plasticity and corrosion resistance in water NaCl solution.1 cl, 7 tbl, 2 ex

Description

The invention relates to the field of non-ferrous metallurgy, in particular to the creation of high-strength structural high-tech pseudo-P titanium alloys with high strength, plastic characteristics and increased corrosion-mechanical strength. The alloy can be made deformed materials of a wide range (large forgings, stamping, rolled and forged plates, as well as semi-finished products of small section, such as rods, sheet metal), which can be used for power structures of shipbuilding, aviation and space technology, power plants , long working at temperatures up to 350 ° C.

Traditionally, titanium alloys are used in those applications that require a combination of specific strength and performance in corrosive environments. The use of high-strength titanium alloys allows reducing weight and size parameters of structures and increasing the life of products.

Abroad for aerospace engineering produce six grades of β-alloys:

- Ti - 10V - 2Fe - 3Al, AMS 4983/14, TIMET, 1971;

- 5Al-5V - 5Mo - 3Cr - 0.5Fe (Ti-5553) VSMPO, 1997 and its European

variant - Ti - 5Al - 5Mo - 3Cr - 1Zr;

- Ti - 15V - 3Al - 3Sn - 3Cr, AMS 4914, TIMET, 1978;

- Beta 21S, AMS 4897, TIMET, 1989;

- Beta C, AMS 4957, RTI, 1969;

- Alloy C, P & W, 1990. [J.D. Cotton ets. Titanium Alloys for Airframe Applications. - JOM, June 2015.]

The alloy Ti - 10V - 2Fe - 3Al applied in the form of large-sized forgings with a tensile strength σ _in ≥1100 MPa for power elements Boeing 757 chassis 777 and other projects.

The disadvantages of the alloy is the tendency to micro-segregation of iron in the smelting of ingots, low hardenability when cooled in air (in sections of no more than 130 mm), low characteristics of fracture toughness _{Q Q} ≥33 MPa ^/ m ^1/2

Known alloy Ti-5553, used in the details of the chassis of the Boeing 787. The modification of the alloy Ti-55531 is used for the chassis of the Airbus A380.

Known high strength titanium alloy VT22 (Ti - 5A - 5V - 5Mo -1 Cr - 1Fe), which is the most widely used in power structures of aviation technology in Russia [A.L. Yakovlev, N.A. Nochovna, S.V. Putyrsky, V.A. Krokhin. Prospects for the use of high-strength titanium alloy VT22 and its modifications // Titan, 2018. №2 (60). 42-47.]

For the manufacture of high-loaded power parts and airframe structures, we use stampings weighing up to 300 kg from high strength titanium alloy VT22 with the following properties: σ _in ≥1030 MPa; σ ₀ , ₂ ≥960 MPa; K _IC ≥58 MPa⋅m ^1/2

The alloy was designed and certified in 1965 by the FSUE “VIAM”. The structure and properties of the alloy substantially depend on its composition within OST 90013 and GOST 19807-91. The chemical composition of the alloy VT22 was repeatedly specified. The final form of the chemical composition took in 1979.

The chemical composition of alloy VT22 according to GOST 19807-91, wt. %: aluminum 4.4-5.7, molybdenum 4.0-5.5, vanadium 4.0-5.5, iron 0.5-1.5, chromium 0.5-1.5, impurities (not more than): carbon 0.10, silicon 0.15, zirconium 0.30, oxygen 0.18, nitrogen 0.05, hydrogen 0.015, the sum of other impurities 0.30.

For products MS-21, SSJ-100 and other FSUE "VIAM" developed alloy VT22M. In the VT22M alloy, compared with the VT22 alloy, the intervals of the content of alloying elements are narrowed, the maximum permissible content of impurity elements — carbon, oxygen and nitrogen — is reduced. The VT22M alloy has a higher strength (σ _0.2 to 1080 MPa) of increased crack resistance (K _IC up to 69 MPa ^1/2 ) as compared with the commercial alloy VT22.

Known alloy VST-5553 (patent RU 2122040), developed jointly by PJSC "VSMPO-AVISMA Corporation" and the company "Boeing" and being the development of alloy VT22. The principal differences of the VST-5553 alloy are an increased chromium content up to 3.6% and a low iron content up to 0.2-0.8%.

The disadvantage of the above alloys is the insufficient level of strength of massive large-sized parts with a thickness of more than 150-200 mm hardened in air.

There are similar alloys patented by PJSC "VSMPO-AVISMA Corporation" of the following compositions, shown in Table 1.

Iron and chromium are effective hardeners in titanium alloys. However, according to state diagrams, the solubility of iron and chromium in α-titanium does not exceed 0.5% at temperatures of 595 ° C and 670 ° C, respectively, and at 20 ° C it becomes negligible. When the content of chromium and iron is higher than the indicated values, the probability of precipitating intermetallic compounds TiCr ₂ and TiFe is high, as a result, a significant loss of plasticity.

The disadvantages of the above listed alloys with a high content of chromium and (or) iron are an increased likelihood of reducing the corrosion-mechanical strength of the metal of massive large-sized products with a thickness of more than 120-200 mm, subjected to heat treatment with air cooling.

Known domestic high-strength titanium alloys based on β-phase:

VT32 (2.0-4.0 Al, 7.0-9.0 Mo, 7.0-9.0 V, 0.5-2.0 Fe, 0.5-2.0 Cr) and alloy VT35 (2.0-4.0 Al, 14-16 V, 2.0-4.0 Cr, 2.0-4.0 Sn, 0.5-2.0 Mo, 0.5-2.0 Zr , 0.01-0.04 Nb) [Aviation materials: Reference book in 12 volumes. -7th ed. Recycling and add. Under the general ed. E.N. Kablova. -M .: VIAM, 2010. - 96 p.], Having a high level of mechanical properties (σ _in ≈850-950 MPa in the hardened state and σ _in ≥1150 MPa after hardening heat treatment). VT32 and VT35 did not find application in the aviation industry, due to a number of technical and economic factors, which include special requirements for the technological process of producing high-quality industrial ingots, the high cost of charge set and the high content of scarce molybdenum in the alloy of grade VT32. [E.N. Kablov, N.A. Nochovnaya, Yu.A. Gribkov, A.A. Shiryaev. Development of high-strength titanium pseudo-β-alloy and technologies for producing semi-finished products from it. Questions of materials science, 2016, №3 (87). Pp. 23-31]

Known alloy based on titanium (patent RU 2 569 285 C1, publ. 20.11.2015), containing, by weight. %: aluminum 1.5-3.5, molybdenum 1.0-3.0, vanadium 8.0-12.0, chromium 2.5-5.0, iron 0.3-1.8, zirconium 0, 4-2.0, tin 0.4-2.0, yttrium and / or gadolinium 0.01-0.16. The use of tin and rare earth metals (REM) in the composition of the alloy significantly complicates the technology of smelting ingots, increases the specific density of the alloy, reduces the elastic characteristics (modulus of normal elasticity).

The disadvantage of the alloy is a high content of chromium and iron, which reduces the corrosion-mechanical strength. The molybdenum equivalent is defined by the authors in the range from 15.5 to 20 units, which is not optimal for large-sized products with large sections. Examples of implementation are given for cold-rolled sheets with a thickness of 2 mm.

Known alloy based on titanium (RU 2 606 677 C1, publ. 10.01.2017), containing wt. %: aluminum - 1.8-3.5, molybdenum - 1.0-3.0, vanadium - 8.0-12.0, chromium - 2.5-4.6, iron - 0.3-1, 6, zirconium - 0.4-2.0, ruthenium - 0.01-0.16, titanium - the main. The alloy may additionally contain yttrium and / or gadolinium - 0.01-0.16. The alloy is characterized by high characteristics of tensile strength at 20 ° C in the hardened and thermally hardened state while maintaining the limit of technological plasticity of the alloy at a satisfactory level and corrosion resistance against crevice and pitting corrosion.

The content of molybdenum and vanadium specified in the patent provides high technological effectiveness of the alloy and the possibility of obtaining by means of hardening heat treatment moderately high strength characteristics (σ _0.2 = 1220-1300 MPa) in 2 mm thick sheets.

The disadvantage proposed in the patents RU 2 569 285 C1 and RU 2 606 677 C1 alloy compositions is Mo _eq = 11.0 - 23.8 at the lower and upper doping levels, respectively, which is not optimal for ensuring the hardenability of large-sized products with section thicknesses 150 - 200 mm. Examples of implementation are given only for cold-rolled sheets 2 mm thick.

Known alloy based on titanium (RU 2 610 657 C1, publ. 02/14/2017), characterized in that it additionally contains sulfur, in the following ratio of components, wt. %: aluminum 1.0-6.0, molybdenum 5.0-10.0, vanadium 5.0-10.0, iron 0.3-3.5, chromium 0.3-3.5, tin 0, 1-2.0, zirconium 0.1-2.0, sulfur 0.0001-0.30, oxygen 0.01-0.20, nitrogen 0.005-0.050, hydrogen 0.003-0.020, carbon 0.005-0.100, silicon 0 , 01-0.10, titanium - the rest.

The micro-alloying of the gray alloy provides an increase in processability, which consists in improving the workability (cutting, turning, etc.), reduces the wear of the cutting tool and increases its service life. The increase in sulfur content improves the machinability due to the release of intermetallic compounds.

The results of studies of the effect of intermetallic sulfur emissions (with contents of more than 0.015 wt.%) On the corrosion and corrosion-mechanical strength of the alloy are not given. There is also no data confirming the possibility of using the proposed alloy for the manufacture of large parts of plates and forgings with thicknesses of sections over 100-200 mm. Exemplary embodiments are made for thin sheets obtained from a day duck.

Known alloy based on titanium (RU 2 614 356 C1 publ. 03.24.2017), containing, by weight. %: aluminum 1.5-4.5, vanadium 13.5-19.0, chromium 2.0-5.0, tin 2.0-4.0, molybdenum 0.5-2.5, zirconium 0, 5-2.5, niobium 0.01-0.40, yttrium 0.005-0.150.

The proposed combination of α- and β-stabilizers makes it possible to carry out a hardening stepwise heat treatment in vacuum and argon vacuum furnaces, to improve the characteristics of plasticity and thermal stability in a thermally hardened state.

Exemplary embodiments are given for cold rolled thin-sheet semi-finished products.

The closest analogue, taken as a prototype, is an alloy based on titanium (patent RU 2 496 901, publ. 10/27/2013), which is an improvement of alloy VST-5553 and containing, by weight. %: aluminum 5.3-5.7, vanadium 4.8-5.2, iron 0.7-0.9, molybdenum 4.6-5.3, chromium 2.0-2.5, oxygen 0, 12-0.16, the rest is titanium and impurities, if necessary, one or more additional elements selected from N, C, Nb, Sn, Zr, Ni, Co, Cu and Si in an amount of less than 0.1% each, and the total the content of additional elements is less than 0.5 wt. %

Under static tension, this alloy reaches a yield strength (TYS) of 170 ksi (1172 MPa), a tensile strength (UTS) of 180 ksi (1233 MPa), an elongation of 10%, a relative narrowing (RA) of 25% and an elastic modulus of 16.0 Msi ( 110.3HPa). The alloy provides a fatigue life of at least 200,000 cycles in the fatigue test of a smooth specimen with axial load in accordance with ASTM E606 with variable deformation + 0.6% and -0.6% until a fatigue crack appears.

The disadvantages of the prototype alloy are narrow spans of elements that are difficult to implement in industrial products from large ingots and the need for hot deformation of large forgings and plates made of alloy with heating temperatures in the α + β-region.

Also the disadvantages of the prototype alloy is the tendency to segregation in the smelting of large-sized ingots of an alloy with a high content of iron and chromium, which can lead to instability of the level of mechanical properties and corrosion-mechanical strength of the material.

The technical objective of the proposed invention is the creation of high-strength titanium alloy with high strength, plastic characteristics and increased corrosion-mechanical strength in a thermally hardened state with satisfactory processability at the stages of smelting, hot working by pressure and heat treatment and intended for the manufacture of large semi-finished products (forgings, plates, plate bent blanks of plates).

The technical result of the proposed invention is to increase the corrosion-mechanical strength of products in the temperature range from zero to 80 ° C in hardened and thermally hardened states while maintaining the strength and plastic characteristics of the alloy at a satisfactory level.

To achieve the technical result, a titanium-based alloy containing aluminum, vanadium, molybdenum, chromium, iron, zirconium, oxygen, and the rest titanium and impurities in the following ratio, wt. %: aluminum 5.3-6.0, vanadium 7.3-7.9, molybdenum 4.8-5.3, chromium 1.3-1.8, iron 0.4-0.7, zirconium 0, 5-0.8, oxygen 0.10-0.18, carbon 0.01-0.02, silicon 0.005-0.02, nitrogen 0.005-0.02, hydrogen 0.003-0.015, other impurities not more than 0.30 .

The technical result is also achieved in the product, which can be made of any proposed alloy based on titanium.

The proposed alloy belongs to the class of pseudo-β-titanium alloy with Mo _eq > 13.0. The alloy is complexly doped with isomorphic (V, Mo, Nb) and eutectoid (Cr, Fe) β-stabilizers, α-stabilizers (Al, O, C) and neutral reinforcers (Zr, Sn).

In the proposed alloy, the range of aluminum content is expanded to 5.3-6.0% (in the prototype alloy 5.3-5.7%), which provides a greater increase in the strength of the α-phase.

Doping with vanadium, molybdenum and chromium simultaneously leads to an increase in plastic characteristics and yield strength.

The addition of molybdenum, in addition to increasing the strength of the alloy, also increases its thermal stability, increasing the thermal hardening capacity of the alloy. Molybdenum also improves corrosion resistance in seawater.

The content of iron and chromium in the alloy is reduced relative to the prototype alloy to 0.4-0.7% and to 1.3-1.8%, respectively, in order to reduce the segregation of alloying elements and impurities in the ingot and inside the grain, which contributes to the thermal stability of the alloy , as well as improving the corrosion and mechanical strength.

To ensure Mo _eq > 13.0, the vanadium content is increased relative to the prototype alloy to 7.3-7.9%. Mo _eq = 13.2-15.8 when the content of β-stabilizing elements at the lower and upper levels, respectively. In a separate implementation, Mo _eq about 14.5, Al _eq about 8.4. Vanadium increases not only strength, but also plasticity, which is associated with its effect on the parameters of the a-titanium lattice. Most of the alloying elements in titanium increase the ratio of the axes of c / a and bring it closer to the theoretical value of 1.633, which makes it difficult to slip along prismatic and pyramidal planes. Vanadium, on the contrary, somewhat reduces the ratio of the axes of c / a and thereby increases the ability of the α-phase to plastic deformation.

Doping the alloy with zirconium to 0.5-0.8% increases the uniformity of decomposition of the metastable β-phase during aging, reduces the negative effect of segregation of alloying elements on the structure of high-alloyed β-alloys, contributing to a more uniform participation of elements in plastic deformation under loading. Zirconium increases thermal stability, corrosion resistance of Ti-Mo alloys, increases hardenability, suppresses the formation of ω-phase at low temperatures of aging and reduces oxidation.

Oxygen stabilizes the α-phase, dissolving well in α-titanium, significantly strengthens titanium. Every 0.1% O (by mass) increases the strength properties of titanium by 130 MPa, which is associated with a strong distortion of the lattice of α-titanium due to the incorporation of oxygen atoms into octahedral voids. In the proposed alloy expanded the range of oxygen content to 0.10-0.18% relative to the alloy of the prototype, which provides a greater increase in strength.

In the area of low concentrations of carbon increases the limits of strength and fluidity of titanium; at carbon concentrations greater than 0.2%, solid carbides are formed, which reduce the toughness and impede machining. In this regard, the carbon content in the proposed alloy is limited to an interval of 0.01-0.02%.

In structural titanium alloys, where high toughness is required, silicon is considered a harmful impurity, since, being present even in small quantities (hundredths of a percent), it sharply reduces this characteristic, therefore the silicon content is limited to the interval of 0.005-0.02%.

Nitrogen is a harmful impurity in titanium alloys, significantly reducing plasticity, and therefore its content in the proposed alloy is regulated in the range of 0.005-0.02%.

Hydrogen forms an impurity type solution and also belongs to the category of harmful impurities, as it causes hydrogen embrittlement of titanium alloys. In the proposed alloy, the hydrogen content is limited to the interval of 0.003-0.015%.

Examples of implementation

Example 1. The proposed alloy (in accordance with table 2) in the form of ingots with a mass of about 30 kg was smelted by the method of double vacuum-arc remelting. The ingots were subjected to comprehensive forging from heating above and below the temperature of complete polymorphic transformation (T _PP ) to obtain a forged billet ∅ 100 × L mm. After machining, the rods were rolled to ∅ 60 mm from a heating temperature (T _pp +80) ° C, some of which were rolled to rods ∅ 25 mm with a heating temperature (T _pp - 20) ° C.

Table 2 shows the chemical composition of the ingots produced.

Rods ∅ 60 and 25 mm were subjected to a two-stage heat treatment: Stage I — solution processing — heating to a temperature (T _PP - 30) ° C, holding at it, cooling with a furnace to a temperature of 770, 740 and 730 ° C, respectively, for the alloy No. 1, No. 2 and No. 3, shutter speed with it and the subsequent cooling in air; Stage II - aging with air cooling.

Further, the following characteristics of the obtained semi-finished products (rods) were determined:

- yield strength, tensile strength, elongation and contraction, Young's modulus was determined by conducting tensile tests of samples at room temperature according to GOST 1497;

- low-cycle durability at a neural stretching of samples with an annular notch (diameter 8 mm, diameter notch 5 mm with R = 0.10 mm, stress concentration factor Kα = 3.5) with a maximum nominal cycle stress σ _max equal to 0.7 of yield point σ _0.2 in air and in 3% aqueous solution of NaCl according to GOST 25.502;

- stress intensity factor K _IC in a plane stress state according to the three-point bending pattern on samples with a cross section of 35 × 50 mm in air and in a 3% aqueous solution of NaCl according to GOST 25.506-85 and in accordance with the IMMAN methods 32-329-05 MI and RD 5.IEІSH.3649-2013.

Table 3 shows the standard mechanical properties of the proposed alloy and the alloy of the prototype.

The test results of samples for low-cycle durability in air and in synthetic sea water are presented in table 4.

Table 5 shows the results of tests for fracture toughness.

As can be seen from Tables 3-5, in the proposed alloy No. 3 compared to prototype alloys No. 1 and No. 2, the durability in a corrosive environment of specimens with a sharp notch is at least 52% higher, the stress intensity factor K _q in a corrosive environment is 2- 13%, the remaining characteristics, including the yield strength are kept at approximately the same level.

Example 2. The proposed alloy tested in a production environment in the manufacture of large ingots with a diameter of 850 mm and a mass of 5500 kg, produced by the method of double vacuum-arc remelting.

Table 6 shows the chemical composition of the melted ingots from the alloy of the prototype and the proposed alloy.

Of the ingots made of deformed semi-finished products, namely plate.

Standard mechanical properties of the proposed alloy and the alloy of the prototype, obtained on the metal of rolled plates, are shown in Table 7.

The conformity of standard mechanical properties of deformed semi-finished products of the experimental alloy and the prototype alloy was confirmed.

The proposed alloy can be used as a structural material for the manufacture of large parts and structures operating in corrosive environments such as a 3% aqueous solution of NaCl. The alloy possesses high reliability in comparison with the known alloys due to the increased characteristics of crack resistance in a corrosive environment and can be used for products of a long service life (for example, in shipbuilding).

Claims (2)

High-strength pseudo-beta titanium alloy containing aluminum, vanadium, molybdenum, chromium, iron, zirconium, oxygen, impurities and titanium else, in the following ratio, wt.%: aluminum 5.3-6.0 vanadium 7.3-7.9 molybdenum 4.8-5.3 chromium  1.3-1.8 iron 0.4-0.7 zirconium 0.5-0.8 oxygen 0.10-0.18 carbon 0.01-0.02 silicon 0,005-0,02 nitrogen 0,005-0,02 hydrogen 0,003-0,015 other impurities no more than 0.30 titanium rest