WO2023026020A1

WO2023026020A1 - Heat treatable titanium alloys exhibiting high ductility and resistance to impact fracture

Info

Publication number: WO2023026020A1
Application number: PCT/GB2022/052059
Authority: WO
Inventors: Roger Owen THOMAS; David Dye
Original assignee: Roger Thomas Titanium Limited
Priority date: 2021-08-27
Filing date: 2022-08-05
Publication date: 2023-03-02
Also published as: GB202112312D0

Abstract

The present invention is directed towards a titanium alloy consisting of: vanadium in an amount of from 3.5 wt.% to 5.6 wt.%; chromium in an amount of from 1.0 wt.% to 2.5 wt.%; aluminium in an amount of from 0.7 wt.% to 1.7 wt.%; iron in an amount of from 0.2 wt.% to 1.0 wt.%; oxygen in an amount of from 0.10 wt.% to 0.25 wt.%; and optionally, silicon in an amount up to 0.15 wt.%; the balance being titanium and incidental impurities. The present invention further relates to titanium alloy products formed of said titanium alloys, and their use in fan containment casings for aircraft turbofan engines.

Description

HEAT TREATABLE TITANIUM ALLOYS EXHIBITING HIGH DUCTILITY AND RESISTANCE TO IMPACT FRACTURE

FIELD OF THE INVENTION

The present disclosure relates to titanium alloys and titanium alloy products. More specifically, this disclosure relates to titanium alloys products formed into a part or component used in an application in which a key design criterion is the energy absorbed during the deformation of the part, including exposure to impact, explosive blast, and /or other forms of shock loading. These titanium alloys products retain high ductility (as measured by the % Reduction of Area in Room Temperature Tensile Tests) and high energy absorption during fracture at high strain rate (as measured by Charpy V-Notch Impact energy) when heat-treated to a high strength condition. The ductility of these titanium alloys products, relative to those previously used, also facilitates improved manufacture of articles therefrom.

BACKGROUND

Reducing the weight of an aircraft or other motorized vehicle can provide fuel savings. As such, for example, there is a strong drive in the aircraft industry to reduce aircraft weight. Titanium and titanium alloys are attractive materials for achieving weight reduction in aircraft applications because of their higher usable strength-to-weight ratios than those provided by steels and other engineering alloys, and other advantageous mechanical properties.

The most used titanium alloy in aircraft structures and other articles requiring higher strength to weight ratios is Ti 6AI-4V (containing 6% aluminium and 4% vanadium by weight percent). Ti 6AI-4V was patented in 1959 in U.S. Patent 2,884,323. Titanium alloys like Ti 6AI-4V consist of alpha (HCP hexagonal structure) and beta (BCC cubic structure) phases. Ti 6AI-4V in the annealed condition typically demonstrates, at room temperature, a minimum yield strength (0.2% Proof Stress) of 830 MPa and a minimum Reduction of Area of 25%. In general, alloys exhibiting high cold formability can be manufactured more efficiently in terms of energy efficiency, rate of manufacture, and the amount of scrap generated during processing. Generally, it is advantageous to formulate an alloy that can be processed at relatively low temperatures.

Ductility is a property of any given metallic material (i.e., metals and metallic alloys) which can be determined from room temperature tensile tests and is most sensitively expressed in terms of the % Reduction of Area (R of A) of the fractured tensile test piece. The cold formability (also referred to as ‘cold workability’) of a metallic material is based on the material’s room temperature workability and ability to deform without cracking and is closely related to ductility. High strength alpha-beta titanium alloys have low cold formability at or near room temperature. This limits their acceptance of cold rolling, drawing, swaging and other low temperature forming given that the alloys are susceptible to cracking and other breakage when worked at low temperatures. Because of their limited formability at low temperatures, alpha-beta titanium alloys such as Ti-6AI-4V typically are processed by techniques involving extensive hot working in the range 675 to 1200 °C. Titanium alloys that exhibit relatively high ductility generally also exhibit relatively low strength. A consequence of this is that high-strength alloys typically are more difficult to manufacture, requiring extensive working at temperatures above several hundred Celsius to homogeneously deform the HCP and BCC crystal structures.

HCP metals have a significantly reduced number of mathematically possible independent slip systems relative to BCC materials. Several of the independent slip systems in HCP metals and alloys require significantly higher stresses to activate. This process is temperature sensitive, so that at high temperature sufficient slip systems can operate for the alloy to be malleable. At low temperature with insufficient slip systems operating, applied strain can result in cracking.

In combination with the slip systems present in HCP materials, several twinning systems are possible in unalloyed HCP metals. The combination of the slip systems and the twinning systems in elemental titanium enables sufficient independent modes of deformation so that ‘commercially pure’ (‘CP’) titanium can be cold worked at temperatures at room temperature.

The BCC beta phase in titanium alloys has sufficient active slip systems to be more malleable than the HCP alpha phase and promotes cold formability in alloys which are predominantly beta phase. However, such alloys have disadvantages in terms of the cost and density of the elemental additions required to fully stabilise the beta phase, and in terms of the poor elevated temperature creep strength of the beta phase.

Alloying effects in titanium and other HCP metals and alloys tend to increase the difficulty of slip, as well as suppress twinning systems from activation. A result of this inhibition of slip and twinning is that alloys such as Ti 6AI-4V and Ti 6AI-2Sn- 4Zr-2Mo-0.1 Si exhibit relatively high strength due to their high concentration of alpha phase and alloying elements. Aluminium is known to increase the strength of titanium alloys at both room and elevated temperatures, but to adversely affect the malleability and room temperature processing capability of the alloy. The mechanisms for this and the detailed effects have been the subject of many studies including: ‘Solid Solution Strengthening in Ti-AI Alloys’, H.W. Rosenberg & W.D. Nix. Met. Trans. Vol 4, May 1973 - 1333; ‘Deformation Behavior of HCP Ti- AI Alloy Single Crystals’, J.C. Williams; R.G. Baggerly & N.E. Paton. Met and Mat Trans A, Vol 33A, March 2002 - 837; and ‘Mechanical Behaviors of Ti-V-(AI, Sn) Alloys with Alpha Prime Martensite Microstructures’, H. Matsumoto; H. Yoneda; D. Fabregue; E. Maire; A. Chiba; F. Gejima. J. Alloys and Compounds 509 (2011) 2684-2692. Alloys such as Ti 6AI-4V and Ti 6AI-2Sn-4Zr-2Mo-0.1 Si contain sufficient aluminium to form precipitates of low ductility Ti3AI compound up to about SOO'G. This Ti3AI compound improves the elevated temperature mechanical properties of the alloys but makes them more difficult to form and machine.

Well established medium strength alloys with lower aluminium contents have been developed, to facilitate the production and forming of sheets, strips, and tubes. Examples include Ti 3AI-2.5V (ASTM Grade 9); Ti 4AI-1.5Mn (OT4 in Russia); Ti 4.5AI-3V-2Mo-2Fe (SP-700®); and Ti 4AI-2.5V-1.8Fe (ATI425®). These alloys contain nil or, at least, smaller proportions of Ti3AI phase compared with Ti 6AI- 4V. Some of these alloys can be processed through some industrial operations without any preheating.

In order to achieve further improvements in room temperature properties, particularly at high strain rate, the authors of U.S. Patent 10000838, ‘Titanium Alloys Exhibiting Resistance to Impact or Shock Loading’ developed the alloy Ti 3.9V-0.85AI-0.25Fe-0.25Si, commercially known as Timetai® 407 (abbreviated Ti- 407). This titanium alloy does not contain enough aluminium to form Ti3AI precipitates. Ti-407 was intended to be formed into a part or component where a key design criterion is the energy absorbed during deformation of the part when exposed to impact, explosive blast, and/or other forms of shock loading. Timetai® 407 in the annealed condition typically demonstrates, at room temperature, a minimum yield strength (0.2% Proof Stress) of 575 MPa and a minimum % Reduction of Area of 40%.

An important example of this type of application is in an aircraft engine fan containment casing, where the titanium alloy typically takes the form of a ring that surrounds the fan blades and maintains containment of the blades if they become detached from the fan disk or hub. The interaction between blade fragments comprising a range of shapes and masses with a containment system is a complex process, and there is a risk of crack propagation in the event of the containment system being overmatched. Ti-407 offers some advantages compared to Ti 6AI-4V in this application, but an ideal titanium alloy for the containment casing might have strength equal to that of Ti 6AI-4V and ductility equal to or greater than that of Ti-407.

High ductility in conventional room temperature tensile tests (sometimes referred to as ‘quasi-static’ tensile tests) is a necessary but not sufficient requirement for high ductility in high strain rate (~ 1000/sec) deformation of titanium alloys. High strain rate deformation and fracture can be measured definitively using specialized equipment e.g., “Hopkinson’s bar” tensile or compression tests but is more conveniently measured using Charpy Impact tests. Charpy Impact tests measure the energy absorbed when a notched sample is struck quickly by a massive hammer with sufficient force to break it. It was shown in the 1950s (see ‘The Effects of Carbon, Oxygen, and Nitrogen on the Mechanical Properties of Titanium and Titanium Alloys’, H.R. Ogden & R.L. Jaffee. TML Report No. 20, Battelle Memorial Institute, October 1955) that moderate additions of interstitial elements (e.g., O, C, N, H) had limited effect on the tensile ductility of titanium alloys but are detrimental to the Charpy Impact energy. It can be conjectured that the interstitial atoms have strong interactions with the dislocations in titanium alloys, locking them in position or inhibiting cross-slip, and making the material strong but not macroscopically ductile when tested at high strain rate. However, the slow strain rate of quasi static tensile tests, combined with the rapid diffusion of interstitial elements, allows some climb and slip of dislocations resulting in macro ductility in this test. In titanium alloys containing hard precipitates such as carbides, silicides, or intermetallic compounds there can be a combination of obstacles to plastic deformation, and low energy paths to fracture propagation, if brittle precipitates occur in preferred planes. These effects can be particularly deleterious to the Charpy Impact energy of these alloys.

In order to reduce the weight of components such as fan containment casings, and hence improve the efficiency of the machine, the authors of US Patent Application US 2020/0095665 A1 devised a titanium alloy with higher strength than Timetai ®407, and higher ductility and Charpy Impact toughness than Ti 6AI- 4V. That alloy, with preferred composition (wt.%) Ti 7.3% V, 1 .2% Al, 0.8% Fe, 0.05% Si, 0.15% oxygen, (hereafter “the ‘5665 alloy”) has the disadvantages of comparatively high Vanadium content - the most expensive element in the alloy - and comparatively high Iron content. High iron content in Titanium alloys is well known to cause segregation during the solidification of ingots, resulting (see ‘Beta Fleck Formation in Titanium Alloys’, K. Kelkar and A. Mitchell, 14^th World Conference on Titanium, MATEC Web of Conferences, Vol. 321 (2), p.10001 , Jan. 2020) in iron rich features which result in microstructural anomalies known as ‘beta flecks’ in the finished parts. Also, high iron content in an alloy may cause surface fissures during draw down of ingots from cold hearth melting furnaces, or at least limit the drawdown speed that can be used, and hence the productivity of the furnace. It is therefore an object of the present invention to provide a solution to the above- mentioned manufacturing problems and provide a titanium alloy having advantageous mechanical properties including high ductility and high energy absorption during fracture at high strain rate, as well as demonstrating favourable workability, machining and processing capabilities including increased formability at low and high temperatures, so as to provide a mass efficient and low-cost solution.

SUMMARY OF INVENTION

According to a first aspect of the present invention, there is provided a titanium alloy consisting of: vanadium in an amount of from 3.5 wt.% to 5.6 wt.%; chromium in an amount of from 1 .0 wt.% to 2.5 wt.%; aluminium in an amount of from 0.7 wt.% to 1 .7 wt.%; iron in an amount of from 0.2 wt.% to 1 .0 wt.%; oxygen in an amount of from 0.10 wt.% to 0.25 wt.%; and optionally, silicon in an amount up to 0.15 wt.%; the balance being titanium and incidental impurities.

According to a second aspect of the present invention, there is provided a titanium alloy product, the titanium alloy product consisting of: vanadium in an amount of from 3.5 wt.% to 5.6 wt.%; chromium in an amount of from 1 .0 wt.% to 2.5 wt.%; aluminium in an amount of from 0.7 wt.% to 1 .7 wt.%; iron in an amount of from 0.2 wt.% to 1 .0 wt.%; oxygen in an amount of from 0.10 wt.% to 0.25 wt.%; and optionally, silicon in an amount up to 0.15 wt.%; the balance being titanium and incidental impurities; and wherein the titanium alloy product has a V-Notch Charpy Impact energy of at least 45J, preferably at least 50 J. According to a third aspect of the present invention, there is provided a titanium alloy product, the titanium alloy product consisting of: vanadium in an amount of from 3.5 wt.% to 5.6 wt.%; chromium in an amount of from 1 .0 wt.% to 2.5 wt.%; aluminium in an amount of from 0.7 wt.% to 1 .7 wt.%; iron in an amount of from 0.2 wt.% to 1 .0 wt.%; oxygen in an amount of from 0.10 wt.% to 0.25 wt.%; and optionally, silicon in an amount up to 0.15 wt.%; the balance being titanium and incidental impurities; wherein the titanium alloy product has: a Yield Strength (0.2% Proof Stress) of 800 MPa or more; an Ultimate Tensile Stress of 900 MPa or more; a % Reduction of Area of at least 35%; and a V-Notch Charpy Impact energy of at least 30 J.

According to a fourth aspect of the present invention, there is provided a titanium alloy product according to the second or third aspect of the present invention, the titanium alloy product having been produced by a process comprising:

(a) obtaining a titanium alloy according to the first aspect of the present invention; and

(b) applying at least one beta forging and/or rolling thermomechanical treatment, followed by at least one alpha-beta forging and/or rolling thermomechanical treatment to the titanium alloy, and allowing to cool.

According to a fifth aspect of the present invention, there is provided a process of producing a titanium alloy product according to the second or third aspect of the present invention, the process comprising: (a) obtaining a titanium alloy according to the first aspect of the present invention; and

(b) applying at least one beta forging or rolling thermomechanical treatment, followed by at least one alpha-beta forging or rolling thermomechanical treatment, and allowing to cool.

According to a further aspect of the present invention, there is provided an article formed from the titanium alloy product according to the second, third or fourth aspect of the present invention.

According to a further aspect of the present invention, there is provided a use of the titanium alloy product according to the second, third or fourth aspects of the present invention, in fan containment casings for aircraft turbofan engines.

BRIEF DESCRIPTION OF DRAWINGS

The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way.

FIG. 1 is the microstructure of a titanium alloy of the present invention, prepared according to the teachings of the present disclosure.

FIG. 2 is a graph of cooling curves measured for the mechanical tests of Alloy G in Example 3 (see Table 6, Plates 4 to 6).

FIG. 3 shows microstructures exhibited by the samples used in the mechanical tests of Alloy G in Example 3 (see Table 6, Plates 4 to 6).

DETAILED DESCRIPTION

The titanium alloys products (formed from the titanium alloys described herein) of the present invention have been surprisingly and advantageously found to demonstrate advantageous mechanical properties including high ductility and high energy absorption during fracture at high strain rate which provide a performance gain over known titanium alloys products, in particular applications in which the titanium alloy product must contain fractured parts of aircraft engines, or resist penetration by explosive blast or munition fragments.. The titanium alloy products of the present invention demonstrate favourable characteristics in ingot manufacturing, as well as workability, machining and processing capabilities including increased formability at low and high temperatures. This combination of performance and manufacturing capability enables the design of fan containment casings for aircraft turbofan engines in which weight and cost are minimized.

The titanium alloy products (formed from the titanium alloys described herein) of the present invention find use in applications that require the alloy to resist failure under conditions of impact, explosive blast, or other forms of shock loading. The titanium alloy products provide a performance gain in terms of advantageous mechanical properties, and favourable formability and workability as discussed above, as well as cost savings over conventional alloys when used in such harsh applications. Such an increase in strength without a decrease in ductility is an advantageous and surprising result, since in titanium alloy products an increase in strength is generally accompanied by a reduction in ductility (as measured by % Reduction of Area) and Charpy impact energy (measure of energy absorbed during fracture at high strain rate).

In arriving at the titanium alloys and titanium alloy products of the present invention, the inventors have determined optimal constituent elemental components, and specific ranges for the amounts thereof, for the titanium alloys. When identifying the constituent elemental components and their amounts, it is not the case that any combinations of elemental components, in any amount, will be beneficial. As discussed in more detail below, a significant number of metallurgical factors should be considered. For example, when iron or chromium are present in a titanium alloy, this can detrimentally lead to the formation of intermetallic compounds such as Ti2Cr. Furthermore, the introduction of vanadium into titanium alloys should be carefully considered due its role as a isomorphous beta stabiliser, whilst the presence of high levels of iron can result in microstructural anomalies and segregation during the solidification of ingots as discussed above. It is the specific combination of constituent elemental components, and the amounts thereof, that leads to the titanium alloys of the present invention, and thus enables the subsequent formation of the titanium alloy products having the advantageous properties discussed herein.

When vanadium (V), chromium (Cr) and iron (Fe) are added to titanium alloys, they act as beta phase stabilisers. Particularly, V is a substitutional alloying element which is an isomorphous beta stabilizer, meaning that Ti and V are mutually soluble across the full range of binary compositions without forming intermetallic compounds. Typically, when a titanium alloy containing V is heat treated in the alpha-beta phase temperature range (below the beta transus temperature), V has some solubility (~1%) in the alpha phase but partitions preferentially to the beta phase.

Fe and Cr are eutectic forming beta stabilizers. That is, they form intermetallic compounds with Ti, and their binary phase diagrams with Ti have the ‘eutectic’ form, where the melting point is depressed over a composition range related to the formation of those intermetallic compounds. This effect is industrially important, because it results in segregation of the beta eutectoid element during solidification of titanium alloy ingots containing that element. It is also important in the context of the present disclosure because the addition of beta eutectoid elements strengthens the beta phase of Ti alloys, but if the combination of alloy composition and processing conditions leads to the formation of particles of intermetallic compound in the alloy, then the energy required to fracture the Ti alloy sample will be reduced, particularly at high strain rate. Fe is an element with very low solubility in alpha phase Ti and exhibits an extremely high diffusion rate (~ 1 ,000,000 x faster than the diffusion rate of Ti atoms) in Ti alloys, because its comparatively small atomic diameter allows it to occupy interstitial positions in a Ti alloy crystal structure. Accordingly, when the Ti alloys disclosed herein are heat treated in the alpha-beta phase temperature range (below the beta transus temperature), Fe partitions almost entirely to the beta phase. Cr is an element which exhibits behaviour in Ti alloys which might be considered intermediate between that of V and Fe. It does form intermetallic compounds with Ti, but the eutectic depression of the melting point in Ti-Cr alloys is not as severe as in Ti-Fe alloys, and hence chemical segregation of Cr in Ti alloy ingots is not as severe as that of Fe. Also, the diffusion rate of Cr in Ti alloys (~ 1000 x faster that of Ti atoms) is significantly slower than that of Fe.

While V, Cr, and Fe all contribute (either independently or in combination) to the strength of the titanium alloys disclosed herein via solid solution strengthening, V, Cr, and Fe also have the effect of refining the transformation product upon cooling from the beta phase to the alpha phase at a given cooling rate. Selection of the ranges of V, Cr and Fe for the titanium alloys and titanium alloy products of the present invention is made so as to maximise minimum strength whilst achieving desired ductility. The chemical segregation in the manufacture of the alloys and alloy products is also borne in mind, as well as costs. It is reiterated that the selection of the specific components, and amounts thereof, of the titanium alloys and titanium alloy products of the present invention is not straightforward, requiring multiple considerations to be made. The ratio of V to Cr and V to Fe may also be considered to preferentially avoid the formation of intermetallic compounds deleterious to the ductility and Charpy impact energy of the titanium alloys disclosed herein. The ratio of V to Cr may be a minimum of 2:1 . The ratio of V to Fe may be a minimum of 4:1 .

It should also be understood that aluminium (Al) and oxygen (O) are known in the art as alpha phase stabilizers in titanium alloys. Also, Al is a substitutional alloying element, and O is an interstitial alloying element. In the titanium alloys disclosed herein, Al and O are strengthening elements, particularly in strengthening the alpha phase. The minimum amounts of these elements may be derived, in part, from a desired minimum strength of the titanium alloy. Conversely, if the amounts of Al and O present in a titanium alloy are too high, the ductility and Charpy impact energy of the titanium alloy may deteriorate. Selection of the ranges of Al and O for the titanium alloys and titanium alloy products of the present invention is therefore made so as to achieve the desired minimum strength whilst achieving desired ductility and fracture energy under impact loading.

The titanium alloys or titanium alloy products of the present disclosure include a titanium base with additions of aluminium, chromium, iron, oxygen, and vanadium. Silicon may also optionally be present. The balance is made up of incidental impurities.

Specifically, the titanium alloys or titanium alloy products of the present invention comprise, in wt.%, aluminium (Al) in an amount of from 0.7 wt.% to 1.7 wt.%, vanadium (V) in an amount of from 3.5 wt.% to 5.6 wt.%; chromium (Cr) in an amount of from 1 .0 wt.% to 2.5 wt.%; iron (Fe) in an amount of from 0.2 wt.% to 1 .0 wt.%; oxygen (O) in an amount of from 0.10 wt.% to 0.25 wt.%; and optionally silicon (Si) in an amount up to 0.15%, the balance being titanium and incidental impurities.

Therefore, according to a first aspect of the present invention, there is provided a titanium alloy consisting of: vanadium in an amount of from 3.5 wt.% to 5.6 wt.%; chromium in an amount of from 1 .0 wt.% to 2.5 wt.%; aluminium in an amount of from 0.7 wt.% to 1 .7 wt.%; iron in an amount of from 0.2 wt.% to 1 .0 wt.%; oxygen in an amount of from 0.10 wt.% to 0.25 wt.%; and optionally, silicon in an amount up to 0.15 wt.%; the balance being titanium and incidental impurities.

All features of the first aspect of the present invention as described herein, whether essential, preferred or optional, are applicable to all other aspects of the present invention as defined herein, and vice versa.

The amounts refer to the elemental amount of each constituent component.

It will be appreciated that titanium is present in the titanium alloys or titanium alloy products of the present invention in an amount of from 88.4 wt.% to 94.5 wt.%, such as from 89.6 wt.% to 92.8% wt.%, preferably from 90 wt.% to 92 wt.%, such as from 91 wt.% to 92 wt.%, for example 91 .4 wt%.

The term ‘incidental impurities’ is a term commonly used in the art in this field. The term refers to unavoidable impurities present in the titanium alloy or titanium alloy product. These may be carbon (C) in an amount of less than 0.08 wt.%, nitrogen (N) in an amount of less than 0.03 wt.%, yttrium (Y) in an amount of less than 0.001 wt.%, boron (B) in an amount of less than 0.0025 wt.%, or other elements such as Co, Cu, Ga, Hf, Mn, Nb, Ni, S, Sn, P , Ta and Zr, where the maximum amount of any one of the other elements is 0.1 wt.% and the combined maximum amount of the other elements is 0.4 wt.%. It will be appreciated that the incidental impurities may arise from, for example, the use of titanium sponge, recycled titanium alloy scrap or other raw materials optionally used in the formation of the titanium alloys or titanium alloy products of the present invention.

Preferably, Al is present in the titanium alloys or titanium alloy products of the present invention in an amount of from 0.9 wt.% to 1 .5 wt.%.

Preferably, V is present in the titanium alloys or titanium alloy products of the present invention in an amount of from 4.5 wt.% to 5.5 wt%.

Preferably, Cr is present in the titanium alloys or titanium alloy products of the present invention in an amount of from 1 .5 wt.% to 2.1 wt.%.

Preferably, Fe is present in the titanium alloys or titanium alloy products of the present invention in an amount of from 0.2 wt.% to 0.6 wt.%.

Preferably, O is present in the titanium alloys or titanium alloy products of the present invention in an amount of from 0.13 wt.% to 0.23 wt.%.

Si is optionally present in an amount up to 0.08 wt%.

Preferably, the ratio of V to Cr is a minimum of 2:1 , such as 2.5:1 or even 3:1 .

Preferably, the ratio of V to Fe is a minimum of 4:1 , such as 8:1 , or 10:1 , or even 12:1.

Preferably, the titanium alloys or titanium alloy products of the present invention comprise Al in an amount of from 0.9 wt.% to 1 .5 wt.%; V in an amount of from 4.5 wt.% to 5.5 wt.%; Cr in an amount of from 1 .5 wt.% to 2.1 wt.%; Fe in an amount of from 0.2 wt.% to 0.6 wt.%; O in an amount of from 0.13 wt.% to 0.23 wt.%; and optionally Si in an amount of up to 0.08 wt.%, the balance being titanium and incidental impurities.

More preferably, the titanium alloys or titanium alloy products of the present invention comprise Al in an amount of 1 .2 wt.%; V in an amount of 5.0 wt.%; Cr in an amount of 1 .8 wt.%; Fe in an amount of 0.4 wt.%; O in an amount of 0.20 wt.%; and optionally Si in an amount of up to 0.08 wt.%, the balance being titanium and incidental impurities.

The present disclosure primarily relates to titanium alloy products for use in applications in which a key design criterion to the energy absorbed during deformation of the part, including impact, explosive blast, or other forms of shock loading. The titanium alloy products made and used according to the teachings contained herein provides a performance gain, as well as cost savings when used in such harsh applications. When used in an aircraft engine containment casing, the titanium alloy products typically take the form of a ring that surrounds the fan blades and maintains containment of the blade in the event of unintended breakage or release of one or more blades. The incorporation and use of the titanium alloy products in conjunction with other types of applications in which they may be exposed to impact, explosive blast, or other forms of high strain rate loading is contemplated. The titanium alloy products of the present invention possess new combinations of properties that are beneficial when the titanium alloy products are used in engine fan containment casings. The balance of properties provides improvement over conventional titanium alloys commonly used for engine fan containment. The mechanical properties exhibited by the titanium alloy products of the present invention include: a Charpy V-Notch Impact energy of at least 45J, preferably at least 50 J.

Therefore, according to a second aspect of the present invention, there is a provided a titanium alloy product consisting of: vanadium in an amount of from 3.5 wt.% to 5.6 wt.%; chromium in an amount of from 1 .0 wt.% to 2.5 wt.%; aluminium in an amount of from 0.7 wt.% to 1 .7 wt.%; iron in an amount of from 0.2 wt.% to 1 .0 wt.%; oxygen in an amount of from 0.10 wt.% to 0.25 wt.%; and optionally, silicon in an amount up to 0.15 wt.%; the balance being titanium and incidental impurities; and wherein the titanium alloy product has a V-Notch Charpy Impact energy of at least 45J, preferably at least 50 J.

All features of the second aspect of the present invention as described herein, whether essential, preferred, or optional, are applicable, where relevant, to all other aspects of the present invention as defined herein, and vice versa.

As discussed above, and as used herein, the Charpy V-Notch Impact energy represents a measurement of the energy absorbed during fracture of the titanium alloy products of the present invention at high strain rate and is most commonly measured at room temperature. The Charpy V-Notch Impact energy may be measured in the rolled direction of the titanium alloy product. This parameter is related to the superior ductility under high strain conditions of the titanium alloy products of the present invention. Methods of measurement will be well known to a skilled person. Measurements as detailed herein, and in accordance with the present invention, are made in accordance with ASTM E23-18.

Preferably, the titanium alloy product according to the second aspect of the present invention has a V-Notch Charpy Impact energy of at least 60J.

The titanium alloy products according to the second aspect of the present invention may demonstrate additional advantageous mechanical properties.

The titanium alloy products according to the second aspect of the present invention may have a Yield Strength (0.2% Proof Stress) of from 650 to 950 MPa, preferably of from 700 to 900 MPa.

In room temperature tensile tests, titanium alloy products typically do not exhibit a clear ‘yield point’, so the Yield Strength at 0.2% strain (0.2% Proof Stress) is reported. Methods of measurement will be well known to those skilled in the art. Measurements as detailed herein, and in accordance with the present invention, are made according to ASTM E8-16a. Measurements are made at room temperature.

The titanium alloy products according to the second aspect of the present invention may have an Ultimate Tensile Stress of from 700 to 1 ,000 MPa, preferably of from 750 to 950 MPa.

‘Ultimate Tensile Stress’ refers to the maximum tensile stress beyond which the titanium alloy products break when being stretched or pulled. Methods of measurement will be well known to those skilled in the art. Measurements as detailed herein, and in accordance with the present invention, are made according to ASTM E8-16a. Measurements are made at room temperature.

The titanium alloy products according to the second aspect of the present invention may also demonstrate a % Reduction of Area of at least 40%. This is a % Reduction of Area of 40% or more. Preferably, the titanium alloy products according to the second aspect of the present invention may have a % Reduction of Area of at least 45%, such as at least 50%. This is a % Reduction of Area of 45% or more, such as 50% or more. It will be appreciated that the % Reduction of Area cannot exceed 100%.

As discussed above, % Reduction of Area represents the ductility of the titanium alloy under ‘quasi-static’ room temperature tensile tests. ‘% Reduction of Area’ is a comparison between the original cross-sectional area of the titanium alloy and the minimum cross-sectional area of the titanium alloy after fracture failure when subjected to tensile load. Methods of measurement will be well known to those skilled in the art. Measurements as detailed herein, and in accordance with the present invention, are made according to ASTM E8-16a.

The titanium alloy products according to the second aspect of the present invention may also demonstrate a % Elongation of from 18 to 35%. It will be appreciated that % Elongation is another parameter by which the ductility of the titanium alloy products may be represented. ‘% Elongation’ refers to the amount that the gauge length of a tensile test piece will elongate up to fracture. Methods of measurement will be well known to those skilled in the art. Measurements as detailed herein, and in accordance with the present invention, are made according to ASTM E8-16a. Measurements are made at room temperature.

The titanium alloy or titanium alloy products of the present invention may also demonstrate improved hot workability versus a conventional titanium alloy.

The titanium alloy products according to the second aspect of the present invention may demonstrate any combination of the above properties. Preferably, the titanium alloy products according to the second aspect of the present invention have: a V-Notch Charpy Impact energy of at least 45J, preferably at least 50J, or at least 60J; a Yield Strength (0.2% Proof Stress) of from 650 to 950 MPa, preferably of from 700 to 900 MPa; an Ultimate Tensile Stress of from 700 to 1 ,000 MPa, preferably of from 750 to 950 MPa; a % Reduction of Area of at least 40%, preferably of at least 45%, or at least 50%; and optionally, a % Elongation of from 18 to 35 %.

Alternatively, according to a third aspect of the present invention, there is provided a titanium alloy product, the titanium alloy product consisting of: vanadium in an amount of from 3.5 wt.% to 5.6 wt.%; chromium in an amount of from 1 .0 wt.% to 2.5 wt.%; aluminium in an amount of from 0.7 wt.% to 1 .7 wt.%; iron in an amount of from 0.2 wt.% to 1 .0 wt.%; oxygen in an amount of from 0.10 wt.% to 0.25 wt.%; and optionally, silicon in an amount up to 0.15 wt.%; the balance being titanium and incidental impurities; wherein the titanium alloy product has: a Yield Strength (0.2% Proof Stress) of 800 MPa or more; an Ultimate Tensile Stress of 900 MPa or more; a % Reduction of Area of at least 35%; and a V-Notch Charpy Impact energy of at least 30 J.

All features of the third aspect of the present invention as described herein, whether essential, preferred, or optional, are applicable, where relevant, to all other aspects of the present invention as defined herein, and vice versa.

The Yield Strength (0.2% Proof Stress), Ultimate Tensile Stress, % Reduction of Area and V-Notch Charpy Impact energy are measured as detailed above in relation to the second aspect of the present invention.

Preferably, the titanium alloy products according to the third aspect of the present invention have a Yield Strength (0.2% Proof Stress) of from 800 to 1 ,100 MPa, preferably from 850 to 1 ,050 MPa.

Preferably, the titanium alloy products according to the third aspect of the present invention have an Ultimate Tensile Stress of from 900 to 1 ,200 MPa, preferably from 950 to 1 ,150 MPa.

Preferably, the titanium alloy products according to the third aspect of the present invention have a % Reduction of Area of at least 40%, such as at least 45%.

Preferably, the titanium alloy products according to the third aspect of the present invention have a V-Notch Charpy Impact energy of at least 35J.

The titanium alloy products according to the third aspect of the present invention may further have a % Elongation of from 18 to 35%. % Elongation is measured as detailed above in relation to the second aspect of the present invention.

The titanium alloy and titanium alloy products according to the first to third aspects of the present invention are alpha-beta titanium alloys and alpha-beta titanium alloy products, having both alpha and beta phases. The titanium alloys according to the first aspect of the present invention may be produced by any suitable method. Methods of forming the titanium alloy according to the first aspect of the present invention in step (a) will be well known to a skilled person. Such methods include melting the blended raw materials in either a plasma or electron beam cold hearth or skull melting furnace, or a vacuum arc remelting (VAR) furnace. The titanium alloys may be produced, for example, by combining titanium sponge with 15%AI, 85%V master alloy; aluminium shot; electrolytic chromium; pure Fe; and titanium dioxide.

The titanium alloy products according to the second and third aspects of the present invention may be produced by any suitable method.

According to a fifth aspect of the present invention, there is provided a process of producing a titanium alloy product according to the second or third aspect of the present invention, the process comprising:

(b) applying at least one beta forging and/or rolling thermomechanical treatment, followed by at least one alpha-beta forging and/or rolling thermomechanical treatment to the titanium alloy, and allowing to cool. All features of the fourth and fifth aspect of the present invention as described herein, whether essential, preferred or optional, are applicable, where relevant, to all other aspects of the present invention as defined herein, and vice versa.

As noted above, methods of forming the titanium alloy according to the first aspect of the present invention in step (a) will be well known to a skilled person. Such methods include melting the blended raw materials in either a plasma or electron beam cold hearth or skull melting furnace, or a vacuum arc remelting (VAR) furnace. The titanium alloys may be produced, for example, by combining titanium sponge with 15%AI,85%V master alloy; aluminium shot; electrolytic chromium; pure Fe; and titanium dioxide.

For step (b), suitable beta thermomechanical treatments will be well known in the art by the skilled person. This will also be the case for alpha-beta thermomechanical treatments. It will be appreciated that beta thermomechanical treatments are carried out above the beta transus temperature when the titanium alloy is in the beta phase, and alpha-beta thermomechanical treatments are carried out below the beta transus temperature, when the titanium alloy is in the alpha-beta condition. That is, both alpha and beta phases are present.

Thermomechanical treatments such as beta forging and/or rolling, and alpha-beta forging and/or rolling, will be well known in the art. It will be appreciated that beta forging and/or rolling will be carried out above the beta transus temperature when the titanium alloy is in the beta phase (BCC) only. Similarly, alpha-beta forging and/or rolling is will be carried out below the beta transus temperature, when the titanium alloy is in the alpha-beta condition. That is, the alloy exists as a mixture of both alpha (HCP) and beta (BCC) phases - the alpha and beta phases coexist in a thermodynamic equilibrium state.

Step (b) may include more than one beta forging and/or rolling thermomechanical treatment, and more than one alpha-beta forging and/or rolling thermomechanical treatment. The at least one beta forging and/or rolling thermomechanical treatment may be carried out at a temperature above the beta transus temperature, preferably 50 °C or more above the beta transus temperature, more preferably from 50 to 250 °C above the beta transus temperature, and more preferably from 100 to 200 °C above the beta transus temperature.

The at least one beta forging and/or rolling thermomechanical treatment may be carried out at a temperature of greater than 830 °C, such as from 830 °C to 1080 °C, or from 930 °C to 1030 °C.

It will be appreciated that if more than one beta forging and/or rolling thermomechanical treatment is applied to the titanium alloy, the beta forging and/or rolling thermomechanical treatments may be carried out at different temperatures within the stated ranges. Generally, the initial beta forging and/or rolling thermomechanical treatment is carried out at the highest temperature, and any further beta forging and/or rolling thermomechanical treatments are carried out at the same or lower temperatures.

The "beta transus temperature" as used herein is a term well known in the art, and refers to the lowest temperature at which all of the titanium alloy is in the beta form, i.e. transforms from the alpha to beta form. Such a temperature is affected by the elemental composition of the titanium alloy. For the titanium alloys and titanium alloy products of the present invention, this is typically around 825 °C.

The at least one alpha-beta forging and/or rolling thermomechanical treatment may be carried out at a temperature below the beta transus temperature of the titanium alloy, preferably at a temperature of from 10 to 130 °C below the beta transus temperature of the titanium alloy, more preferably at a temperature of from 20 to 110 °C below the beta transus temperature, and most preferably at a temperature from of 25 to 100 °C below the beta transus temperature. This is the alpha-beta phase range of temperature of the titanium alloys of the present invention. The at least one alpha-beta forging and/or rolling thermomechanical treatment may be carried out at a temperature of from 695 to 815 °C, preferably from 715 to 805 °C, and more preferably from 725 to 800 °C. This is the alpha-beta phase range of temperature as described herein.

It will be appreciated that if more than one alpha-beta forging and/or rolling thermomechanical treatments are applied to the titanium alloy, the alpha-beta forging and/or rolling thermomechanical treatments may be carried out at different temperatures within the stated ranges.

The primary alpha phase may be characterized by alpha grains having a size that is less than about 30 micrometres. Preferably, the reduction in cross sectional area of the titanium alloy after the at least one alpha-beta forging and/or rolling thermomechanical treatment may be 40% or more, such as 50% or more. This may preferentially provide a primary alpha phase morphology enabling the desired ductility and impact roughness in the final titanium product.

If alpha-beta rolling is utilised to manufacture plates, the titanium alloy may be ‘cross-rolled’ where it is rolled in both the perpendicular and parallel (to ingot axis) directions. The choice of rolling procedure, and the conditions of its execution, affect both the microstructure of the titanium alloy and the statistical distribution of crystal orientations in the alloy, commonly referred to as the ‘crystal texture’ of the sample. Uniaxial rolling of plates and bars tends to produce elongated microstructures and extreme crystal textures, resulting in anisotropic mechanical properties. Cross rolling of plates tends to produce more refined and spheroidized microstructures and less extreme crystal textures, resulting in improved ductility and less anisotropic mechanical properties.

The process of producing the titanium alloy products according to the second and third aspects of the present invention may further comprise:

(c) applying a heat treatment to the titanium alloy, during which the titanium alloy is heated at a temperature below the beta transus temperature of the titanium alloy, preferably at a temperature of from 10 to 130 °C below the beta transus temperature, or at a temperature of from 20 to 110 °C below the beta transus temperature, and most preferably at a temperature of from 25 to 100 °C below the beta transus temperature;

(d) cooling the titanium alloy; and optionally

(e) applying a further heat treatment to the titanium alloy, during which the titanium alloy is heated at a temperature of from 350 to 700 °C, preferably from 400 to 650 °C, and more preferably from 450 to 600 °C; and

(f) cooling the titanium alloy to produce the titanium alloy product.

In step (c), the titanium alloy may be heated at a temperature of from 500 to 815 °C, such as from 695 to 815 °C, preferably from 715 to 805 °C, and more preferably from 725 to 800 °C.

In step (d), the titanium alloy may be cooled at a rate of less than 25 °C/s, from the temperature at which the titanium alloy is heated for step (c) to 400 °C. This may be achieved through air cooling or vermiculite cooling of the titanium alloy. Oil or water quenching may also be used for thicker sections of the titanium alloy. Such a cooling rate, a slow cooling rate, enables the titanium alloy product according to the second aspect of the present invention to be formed. The titanium alloy is then cooled to room temperature. This is the case whether or not optional steps (e) and (f) are to be applied.

Alternatively, in step (d), the titanium alloy may be cooled at a rate of 25 °C/s or more, from the temperature at which the titanium alloy is heated for step (c) to 400°C. This may be achieved by water or oil quenching on thinner sections to achieve a faster cooling rate. Such a cooling rate, a faster one, enables the titanium alloy product according to the third aspect of the present invention to be formed. The titanium alloy is then cooled to room temperature. This is the case whether or not optional steps (e) and (f) are to be applied.

The titanium alloy may be cooled in step (f) by any suitable cooling mechanism, including air cooling, vermiculite cooling, and water or oil quenching. For this step, the rate of cooling is not particularly important. Typically, the titanium alloy is cooled to room temperature by air cooling.

It will be appreciated that steps (c) to (f), and different selections of the parameters detailed therein, affect the properties of the titanium alloy products to be formed by virtue of the creation of formation and refinement of microstructures within said titanium alloy product. Specifically, the different cooling rates affect the properties of the titanium alloy to be formed, such that the titanium alloy product of either the second aspect or third aspect of the present invention can be formed. This is understood to be by virtue of the formation and refinement of different microstructures within said titanium alloy product. Slow cooling of the titanium alloys typically favours the growth of primary alpha during cooling, and the formation of coarse lathes of secondary alpha phase.

The process of producing the titanium alloy product may comprise each of steps (c) to (f).

It will be appreciated that if steps (e) and (f) are carried out, these steps may be repeated one or more times. It will further be appreciated that machining of the titanium alloy product may occur after step (d) or (f), if step (f) is carried out. Alternatively, machining of the titanium alloy product may occur between repeats of steps (e) and (f) where carried out.

The heat treatments of steps (c) and (e) may be any suitable heat treatments for titanium alloys as are well known in the art. Such heat treatments include stress relieving, annealing, solution heat treating (SHT), ageing, and combinations thereof. All conventional methods of heat treatments for titanium alloys are encompassed by the present invention. Preferably, in step (c), the heat treatment is a solution heat treatment.

If solution heat treatment is utilised, preferably the temperature for solution heat treatment exceeds the temperature of step (b) for the at least one alpha-beta forging and/or rolling thermochemical treatment. The heat treatment of step (c) may be applied to the titanium alloy for from 5 to 500 minutes, such as from 10 to 200 minutes, and preferably from 20 to 100 minutes.

The heat treatment of step (e) may be applied to the titanium alloy for from 30 to 1 ,000 minutes, preferably from 60 to 500 minutes.

A preferable example of the process of producing a titanium alloy product according to the second aspect of the present invention comprises:

(c) applying a solution heat treatment to the titanium alloy, wherein the titanium alloy is heated at a temperature of from 25 to 100 °C below the beta transus temperature of the titanium alloy;

(d) air cooling the titanium alloy at a rate of less than 25 °C/s to 400 °C, followed by air cooling to room temperature;

(e) applying an ageing heat treatment to the titanium alloy, wherein the titanium alloy is heated at a temperature of from 450 to 600 °C; and

(f) cooling the titanium alloy to produce the titanium alloy product.

The resulting titanium alloy product according to the second aspect of the present invention may have a higher proportion of alpha phase than was present at the solution treatment temperature; coarse secondary alpha phase platelets which have grown from the primary alpha on planes of preferred orientation by a process in which solute elements are rejected from the lathes by diffusion; and very fine tertiary alpha formed during step (e).

The resulting titanium alloy product according to the second aspect of the present invention may have a volume fraction of a primary alpha phase that is between about 5% to about 90%. This volume fraction may be dependent upon the heat treatment temperature of step (c), and/or the cooling rate from that temperature in step (d). The higher the solution treatment temperature (closer to the beta transus temperature), the lower the proportion of primary alpha phase, and the slower the cooling rate, the higher the proportion of primary alpha phase.

A suitable process for producing a titanium alloy product according to the second aspect of the present invention may include:

(1 ) combining scrap or recycled alloy materials that contain titanium, aluminium, chromium, iron, oxygen, and vanadium;

(2) mixing the scrap or recycled alloy materials with additional raw materials as necessary to create a blend that comprises the composition of the titanium alloys of the first aspect of the present invention;

(3) melting the blend in either a plasma or electron beam cold hearth furnace, or a skull melting furnace, or a vacuum arc remelt (VAR) furnace;

(4) remelting the ingot in a VAR furnace;

(5) processing the ingot using a combination of beta forging and/or beta rolling, followed by alpha-beta forging and/or alpha-beta rolling;

(6) heat treating at a temperature between 10 °C and 130 °C below the beta transus temperature; and cooling at a rate of less than 25°C/s to 400°C, then cooling to room temperature;

(7) annealing or aging in a heat treatment at a temperature of from 350 °C to 700 °C and cooling to form a titanium alloy product; and

(8) machining to finished dimensions, repeating the annealing or ageing heat treatment as necessary.

The titanium alloy product of Figure 2 was produced according to the methodology set out above. Specifically, it is R2 of Example 1 detailed below.

A suitable process for producing a titanium alloy product according to the third aspect of the present invention may include: (1 ) combining scrap or recycled alloy materials that contain titanium, aluminium, chromium, iron, oxygen and vanadium;

(4) remelting the ingot in a VAR furnace;

(5) processing the ingot using a combination of beta forging and/or rolling, followed by alpha-beta forging and/or rolling to a thin section. The reduction in cross section of the titanium alloy by alpha-beta forging and/or rolling may be 40% or more, preferably 50% or more.

(6) solution heat treating at a temperature between 10 °C and I SO'C below the beta transus temperature, and cooling at a rate of 25 °C/s or faster to 400 °C, then cooling to room temperature;

(7) stress relieving or ageing in a heat treatment at a temperature of from 350 °C to 700 °C and cooling to form a titanium alloy product; and

(8) machining to finished dimensions, repeating the stress relieving or ageing heat treatment as necessary.

A preferable example of the process of producing a titanium alloy product according to the third aspect of the present invention comprises:

(d) air cooling the titanium alloy at a rate of 25 °C/s or more to 400 °C, followed by air cooling to room temperature; (e) applying an ageing heat treatment to the titanium alloy, wherein the titanium alloy is heated at a temperature of from 450 to 600 °C and

(f) cooling the titanium alloy to produce the titanium alloy product.

Using the methodology detailed above for the titanium alloy product of the third aspect of the present invention, the necessary structural refinement to obtain the desired properties of the titanium alloy product may be obtained by solution heat treatment of the titanium alloys followed by quenching to obtain a cooling rate of 25 °C per second or faster down to 400 °C, and then stress relieving or aging in the temperature range of 450 °C to 600 °C.

The resultant microstructure consists of primary alpha phase in a phase proportion representing the equilibrium phase proportion at the solution treatment temperature; fine needles of martensitic secondary alpha, and very fine particles of tertiary alpha formed during stress relieving or aging. The fine needles of secondary alpha phase are responsible for the higher strength and lower ductility of this condition of the titanium alloy. The higher cooling rate prevents secondary alpha lathes from growing by a diffusion process. Further cooling results in a martensitic transformation -a phase change by shear of the crystallographic lattice with negligible diffusion required. Figure 3 shows the finer secondary alpha in the quenched sample, compared with the coarser secondary alpha in the slower cooled samples.

As noted above, the titanium alloys of the present invention may used in in fan containment casings for aircraft turbofan engines.

According to a further aspect of the present invention, there is provided a use of the titanium alloy product according to the second, third or fourth aspects of the present invention, in fan containment casings for aircraft turbofan engines. It will be appreciated that when the titanium alloy product is a fan containment casing, the alpha-beta thermomechanical treatment detailed in relation to the fourth and fifth aspects of the present invention may include ring rolling a forged intermediate product to produce a rolled ring of the required dimensions.

All features of these further aspects of the present invention as described herein, whether essential, preferred or optional, are applicable, where relevant, to all other aspects of the present invention as defined herein, and vice versa.

EXAMPLES

Example 1

Two laboratory scale ‘button’ ingots weighing nominally 200g, designated R1 and R2, were arc melted in an argon atmosphere. Target compositions of the ingots are shown in Table 1 . R1 corresponds to Timetal®407, and R2 is a titanium alloy product of the present invention.

Table 1 : Chemistry and Processing Conditions

The buttons were preheated above the calculated beta transus of the titanium alloys, rolled, cooled to room temperature, and then preheated in the alpha-beta temperature range of the alloys (approximately the calculated beta transus minus 50 °C) and rolled to 13mm square section bars. The bars were then SHT at approximately ‘calculated beta transus minus 25 °C’ for 1 hour, air cooled at a rate of less than 25 °C/s, and then aged at 500 °C for 8 hours and air cooled. Actual temperatures are shown in Table 1 .

Results of Yield Strength (0.2% Proof Stress), Ultimate Tensile Stress, % Elongation, and % Reduction in Area [per ASTM E8-16a]; V-Notch Charpy impact tests [in accordance with ASTM E23-18] ; and Vickers Hardness tests on samples manufactured from the bars are shown in Table 2. V-Notch Charpy Impact energies are recorded in the rolled direction.

Table 2: Mechanical Test Results

It is apparent that the titanium alloy product of the present invention (R2) exhibits a significant strength, and hardness increase over Ti 407, whilst attaining superior Charpy impact energy. This can be considered a surprising result, because the significant content of Cr and Fe in the titanium alloy, and resulting titanium alloy product, might be expected to lead to the formation of intermetallic compound precipitates, and hence cause poor impact energy.

Example 2

Nine further laboratory scale ‘button’ ingots weighing nominally 200g, designated V1 to V9, were arc melted in an argon atmosphere. Compositions of the ingots, and the processing conditions used to roll them to 13mm square bars, are shown in Table 3. These titanium alloy products are six further examples of titanium alloy products according to the present invention.

Table 3: Chemistry and Processing Conditions

The buttons were preheated above the calculated beta transus temperature of the titanium alloys, rolled, cooled to room temperature, and then preheated in the alpha-beta temperature range of the titanium alloys (approximately the calculated beta transus minus 50 °C) and rolled to 13mm square section bars. The bars were then SHT at approximately ‘calculated beta transus temperature minus 25 °C’ for 1 hour, cooled in vermiculite at a cooling rate of less than 25 °C/s, and then aged at 500 °C for 8 hours and air cooled. Actual temperatures are shown in Table 3. Cooling in vermiculite was chosen for this experiment to achieve a slower cooling rate, more representative of the conditions during processing industrial forgings or hot rolled rings. Slow cooling of the alloys from SHT favours the growth of primary alpha during cooling, and the formation of coarse lathes of secondary alpha phase.

Results of Yield Strength (0.2% Proof Stress), Ultimate Tensile Stress, % Elongation, and % Reduction in Area [per ASTM E8-16a] and V-Notch Charpy impact tests [in accordance with ASTM E23-18] on samples manufactured from the bars are shown in Table 4. V-Notch Charpy Impact energies are recorded in the rolled direction.

Table 4: Mechanical Test Results

Table 4 shows that the titanium alloy products according to the present invention exhibit useful combinations of strength, ductility, and impact toughness.

From these results, the preferred constituent elemental components, and the amounts thereof, is derived: Alloy V-6: Ti 5.0% V, 1 ,8%Cr, 1 ,2%AI, 0.4%Fe, 0.20% O, on the basis that the increased Vanadium relative to Alloy V-5 leads to increased strength and improved stability of the beta phase during heat treatment, and adjustment of the ratio of Chromium to Iron reduces the risk of segregation during solidification of ingots.

Example 3

A larger scale laboratory ingot, nominally 30kgs, was manufactured by blending and compacting raw materials, then making a first melt ingot by the VAR method. That ingot was then remelted in an induction skull furnace, and the metal was cast into a permanent mould to make an ingot 115 mm diameter. That ingot was machined to 109mm diameter and cut into segments. Each segment was preheated to 960 °C (a temperature above the beta transus temperature) and forged to 80 x 62mm section and cooled to room temperature. These billets were then reheated to 780 °C (a temperature in the alpha-beta phase temperature range) and rolled to 14 x 90mm sections, typically 1200mm long, applying reheats as necessary. The rolling was all applied parallel to the ingot axis, to form a plank with some anisotropy of properties, and a microstructure which commonly exhibited elongated primary alpha features. The chemical composition of this material, designated ‘Alloy G’, is shown in Table 5.

Table 5: Nominal and Actual chemistry in weight % of ‘Alloy G’

Mechanical testing of Alloy G Plank

A plank of Alloy G was cut to give 6 plates, each nominally 150 x 90 x 14mm. Six plates were heat treated and one plate was kept in the ‘As Rolled’ condition. The heat treatments were:

(Plate 1 B) Stress relieve 525 °C, 2hours, Air Cool.

(Plate 2) Anneal 710°C, 1 hour, Vermiculite Cool - that is Vermiculite insulation was used to retard the cooling relative to Air Cooling. [‘Ann. TIO'C, 1 hr. VC’ in Table 6]. The retarded cooling was selected to mimic the air cooling of industrial scale forgings.

(Plate 3) Solution Heat Treat 780 ^PC, 1 hour, Vermiculite Cool [‘SHT 780 ^PC, 1 hr. VC’ in Table 6].

(Plate 4) Solution Heat Treat 770 °C, 1 hour, Vermiculite Cool, then Age 500 °C for 8 hours, Air Cool [‘SHT 770 °C, 1 hr. VC + Age 500 °C, 8hrs. AC’ in Table 6].

(Plate 5) Solution Heat Treat 780 °C, 1 hour, Air Cool, then Age/Stress Relieve 500 °C for 8 hours, Air Cool [‘SHT 780 °C, 1 hr. AC + Age 500 °C, 8hrs. AC’ in Table 6].

(Plate 6) Solution Heat Treat 780°C, 1 hour, Oil Quench, then Age/Stress Relieve 500°C for 8 hours, Air Cool [‘SHT TSO'C, 1 hr. OQ + Age 500 °C, 8hrs. AC’ in Table 6].

Samples of a commercially produced, cross rolled, Ti 6AI-4V alloy plate were tested for comparison.

Cooling curves measured for the alternative methods of cooling from solution heat treatment (Plates 4 to 6) are shown in FIG. 2. Note that the curves for ‘Air cooled’ and ‘Vermiculite cooled’ samples exhibit deviations around 600 °C which are interpreted as being due to the exothermic transformation from beta phase to alpha phase. Under these conditions the phase transformation takes place by diffusion-controlled growth of primary alpha particles, and of lathes of secondary alpha phase. The cooling curve for the ‘Oil Quench’ process exhibits a minor deviation around 400 °C which is interpreted to be due to the martensitic beta to alpha phase transformation which is favoured at high cooling rates. Microstructures exhibited by samples processed by alternative methods of cooling from SHT (Plates 4 to 6) are shown in FIG. 3.

Samples for mechanical testing were cut from the heat-treated Plates in the Rolling Direction (RD) and Transverse Direction (TD). Results of Yield Strength (0.2% Proof Stress), Ultimate Tensile Stress, % Elongation, and % Reduction in Area [per ASTM E8-16a]; and V-Notch Charpy impact tests [in accordance with ASTM E23-18]; are shown in Table 6.

Table 6: Results of Mechanical Tests on Alloy G

The results in Table 6 demonstrate several attributes of the inventive titanium alloy products: In all tested conditions, the ductility (as measured by R. of A.%) and impact energy absorbed (as measured by the Charpy test) of Alloy G are substantially superior to those of Ti 6-4.

The ductility of Alloy G samples can be further increased, relative to the as-rolled condition, by stress relieving; annealing; or Solution Heat Treating and slow cooling sample plates [Plates 1 B, 2, 3, 4 and 5 compared to Plate 1].

Comparison of the results from Plates 3, 4, and 5 demonstrates that the cooling rate from Solution Heat Treatment has an important effect on the strength of the material. Examination of the microstructures of the Plates (see Figure 3) confirms that the higher strength of Plate 6 is associated with a lower volume fraction of primary alpha phase, and refinement of the secondary alpha phase formed during cooling from the Solution Heat Treatment temperature.

The strength of Alloy G samples in Table 6 shows significant anisotropy, as might be expected in a unidirectionally rolled plank, in contrast to the small anisotropy of the cross rolled Ti 6-4 plate. Note that in the higher strength TD samples, good ductility and Charpy Impact Energy were maintained.

Comparison of the results from Plates 3 and 4 shows that the age hardening response is negligible after Solution Heat Treatment and slow cooling, so the second, lower temperature, heat treatment cycle is more correctly designated ‘stress relieving’ and more economical heat treatment cycles may be devised.

Example 4

A 115mm diameter laboratory ingot, nominally 30kg in mass, was manufactured by the same method as in Example 3 above; blending and compacting raw materials, then making a first melt ingot by the VAR method. That ingot was then remelted using Induction Skull Melting (ISM). That ingot was machined to 110mm diameter and cut into segments. Each segment was pre-heated to 960 °C (a temperature above the beta transus temperature) and forged to 75 x 65mm section, cooled to room temperature, and cut to give six pieces, each nominally 160mm long. These billets were machined to 70 x 60 x 150mm then reheated to 780 °C (a temperature in the alpha-beta temperature phase) for rolling. Initial rolling perpendicular to the ingot axis reduced the thickness from 60mm to 28mm. Then the pieces were reheated and rolled parallel to the ingot axis to a final thickness of 14mm. This ‘cross rolling’ procedure was intended to reduce the anisotropy of mechanical properties exhibited by the plate and improve spheroidization of the primary alpha phase in the microstructure which was expected to increase the ductility of the alloy. The elemental composition of this material, designated ‘Alloy H’, is shown in Table 7.

Table 7: Nominal and Actual chemistry in weight % of ‘Alloy H’

Mechanical testing of Alloy H Plates

One of the six plates of Alloy H was cut to give test blocks for heat treatment and mechanical testing. One block (A) was kept in the ‘As Rolled’ condition. The heat treatments were:

(Block B) Stress relieve 500 °C, 4 hours, Air Cool.

(Block C) Solution Heat Treat 775°C, 1 hour, Vermiculite Cool, then Age 500 °C for 8 hours, Air Cool [‘SHT 775 °C, 1 hr. VC + Age 500 °C, 8hrs. AC’ in Table 8]. Additional test duplicates were taken in this preferred condition.

(Block D) Solution Heat Treat 775°C, 1 hour, Oil Quench, then Age/Stress Relieve 500°C for 8 hours, Air Cool [‘SHT 775°C, 1 hr. OQ + Age 500 °C, 8hrs. AC’ in Table 8]. Samples for mechanical testing were cut from the heat-treated Plates in the Rolling Direction (RD) and Transverse Direction (TD). Results of Yield Strength (0.2% Proof Stress), Ultimate Tensile Stress, % Elongation, and % Reduction in Area [per ASTM E8-16a]; and V-Notch Charpy impact tests [in accordance with ASTM E23-18]; are shown in Table 8.

Table 8: Results Of Mechanical Tests On Alloy H

The results in Table 8 confirm the attributes of the inventive titanium alloy products shown in Example 3. Note the higher strength and lower Charpy impact energy of Block D compared with Block C.

Also:

The Yield Strength (0.2% Proof Stress), Ultimate Tensile Stress, % Elongation, and % Reduction in Area results in Table 8 demonstrate the reduced anisotropy of mechanical properties exhibited after applying the ‘cross-rolling’ procedure. The generally higher level of ductility and Charpy Impact Energy shown in Table 8 compared with Table 6 is due to a combination of the lower oxygen content of the alloy in this case and the improved microstructure generated by the cross rolling procedure.

Claims

38

1 . A titanium alloy consisting of: vanadium in an amount of from 3.5 wt.% to 5.6 wt.%; chromium in an amount of from 1 .0 wt.% to 2.5 wt.%; aluminium in an amount of from 0.7 wt.% to 1 .7 wt.%; iron in an amount of from 0.2 wt.% to 1 .0 wt.%; oxygen in an amount of from 0.10 wt.% to 0.25 wt.%; and optionally, silicon in an amount up to 0.15 wt.%; the balance being titanium and incidental impurities.

2. A titanium alloy product, the titanium alloy product consisting of: vanadium in an amount of from 3.5 wt.% to 5.6 wt.%; chromium in an amount of from 1 .0 wt.% to 2.5 wt.%; aluminium in an amount of from 0.7 wt.% to 1 .7 wt.%; iron in an amount of from 0.2 wt.% to 1 .0 wt.%; oxygen in an amount of from 0.10 wt.% to 0.25 wt.%; and optionally, silicon in an amount up to 0.15 wt.%; the balance being titanium and incidental impurities; and wherein the titanium alloy product has a V-Notch Charpy Impact energy of at least 45J, preferably at least 50 J.

3. A titanium alloy product, the titanium alloy product consisting of: vanadium in an amount of from 3.5 wt.% to 5.6 wt.%; chromium in an amount of from 1 .0 wt.% to 2.5 wt.%; aluminium in an amount of from 0.7 wt.% to 1 .7 wt.%; iron in an amount of from 0.2 wt.% to 1 .0 wt.%; oxygen in an amount of from 0.10 wt.% to 0.25 wt.%; and optionally, silicon in an amount up to 0.15 wt.%; the balance being titanium and incidental impurities; 39 wherein the titanium alloy product has: a Yield Strength (0.2% Proof Stress) of 800 MPa or more; an Ultimate Tensile Stress of 900 MPa or more; a % Reduction of Area of at least 35%; and a V-Notch Charpy Impact energy of at least 30 J.

4. The titanium alloy or titanium alloy product according to any of claims 1 to 3, wherein the vanadium is present in an amount of from 4.5 wt.% to

5.5 wt.%, preferably 5.0 wt.%.

5. The titanium alloy or titanium alloy product according to any of claims 1 to 4, wherein the chromium is present in an amount of from 1 .5 wt.% to 2.1 wt.%, preferably 1 .8 wt. %.

6. The titanium alloy or titanium alloy product according to any of claims 1 to 5, wherein the aluminium is present in an amount of from 0.9 wt.% and

1 .5 wt.%, preferably 1 .2 wt.%.

7. The titanium alloy or titanium alloy product according to any of claims 1 to 6, wherein the iron is present in an amount of from 0.2 wt.% to 0.6 wt.%, preferably 0.4 wt.%.

8. The titanium alloy or titanium alloy product according to any of claims 1 to 7, wherein the oxygen is present in an amount of from 0.13 wt.% to 0.23 wt.%, preferably 0.2 wt.%.

9. The titanium alloy or titanium alloy product according any of claims 1 to 8, wherein the titanium alloy or titanium alloy product consists of: vanadium present in an amount of from 4.5 wt.% to 5.5 wt.%; chromium present in an amount of from 1 .5 wt.% to 2.1 wt.%; aluminium present in an amount of from 0.9 wt.% to 1 .5 wt.%; iron present in an amount of from 0.2 wt.% to 0.6 wt.%; oxygen present in an amount of from 0.13 wt.% to 0.23 wt.%; and 40 optionally, silicon in an amount up to 0.08 wt.%; the remainder being titanium and incidental impurities. The titanium alloy or titanium alloy product according to any of claims 1 to 9, wherein the titanium alloy or titanium alloy product consists of: vanadium in an amount of 5.0 wt.%; chromium in an amount of 1 .8 wt.%; aluminium in an amount of 1 .2 wt.%; iron in an amount of 0.4 wt.%; oxygen in an amount of 0.20 wt.%; and optionally, silicon in an amount up to 0.08 wt.%, the remainder being titanium and incidental impurities. The titanium alloy product according to claim 2, or any of claims 4 to 10 where dependent thereon, wherein the Yield Strength (0.2% Proof Stress) is from 650 to 950 MPa, preferably from 700 to 900 MPa. The titanium alloy product according to claim 2, or any of claims 4 to 11 where dependent thereon, wherein the Ultimate Tensile Stress is from 700 to 1 ,000 MPa, preferably from 750 to 950 MPa. The titanium alloy product according to claim 2, or any of claims 4 to 12 where dependent thereon, wherein the % Reduction of Area is at least 40%, preferably at least 45%, and more preferably at least 50%. The titanium alloy product according to claim 2, or any of claims 4 to 13 where dependent thereon, wherein the titanium alloy product has: a V-Notch Charpy Impact energy of at least 45 J, preferably at least 50J, more preferably at least 60J; a Yield Strength (0.2% Proof Stress) of from 650 to 950 MPa, preferably 700 to 900 MPa; an Ultimate Tensile Stress of from 700 to 1 ,000 MPa, preferably 750 to 950 MPa; and a % Reduction of Area of at least 40%, preferably at least 45%, and more preferably at least 50%.

15. The titanium alloy product according to any of claims 2 to 14, wherein the titanium alloy product is produced by a process comprising:

(a) obtaining a titanium alloy according to claim 1 , or any of claims 4 to 10 where dependent thereon; and

16. A process of producing a titanium alloy product according to any of claims 2 to 14, the process comprising:

17. The titanium alloy product or process according to claim 15 or 16, wherein the process of producing the titanium alloy product further comprises:

(c) applying a heat treatment to the titanium alloy, during which the titanium alloy is heated at a temperature below the beta transus temperature of the titanium alloy, preferably at a temperature of from 10 to 130 °C below the beta transus temperature of the titanium alloy, more preferably at a temperature of from 20 to 110 °C below the beta transus temperature, and most preferably at a temperature from of 25 to 100 °C below the beta transus temperature;

(d) cooling the titanium alloy; and optionally

(e) applying a further heat treatment to the titanium alloy, during which the titanium alloy is heated at a temperature of from 350 to 700 °C, preferably from 400 to 650 °C, and more preferably from 450 to 600 °C-, and

(f) cooling the titanium alloy to produce the titanium alloy product. The titanium alloy product or process according to claim 17, wherein in step (c), the titanium alloy is heated at a temperature of from 695 to 815 °C, preferably from 715 to 805 °C, and more preferably from 725 to 800 °C. The titanium alloy product or process according to claim 17 or 18, wherein in step (d), the titanium alloy is cooled at a rate of less than 25 °C/s from the temperature at which the titanium alloy is heated for step (c) to 400°C, and the titanium alloy product of claim 2, or any of claims 4 to 10 where dependent thereon, is formed. The titanium alloy product or process according to claim 19, wherein cooling is achieved by air cooling or vermiculite cooling, or by quenching thick sections of the titanium alloy in oil or water. The titanium alloy product or process according to claim 17 or 18, wherein in step (d), the titanium alloy is cooled at a rate of 25 °C/s or more from the temperature at which the titanium alloy is heated for step (c) to 400°C, and the titanium alloy product of claim 3, or any of claims 4 to 10 where dependent thereon, is formed. The titanium alloy product or process according to claim 21 , wherein cooling is achieved by water or oil quenching. 43 The titanium alloy product or process according to any of claims 17 to 22, wherein the process of producing the titanium alloy product comprises each of steps (c) to (f). The titanium alloy product or process according to claim 23 when dependent upon claims 17 to 20, wherein the process of producing the titanium alloy product comprises:

(d) air cooling the titanium alloy at a rate of less than 25 °C/s from the temperature at which the titanium alloy is heated for step (c) to 400°C, and then cooling to room temperature;

(e) applying an ageing or stress relief heat treatment to the titanium alloy, wherein the titanium alloy is heated at a temperature of from 450 to 600 °C; and

(f) cooling the titanium alloy to produce the titanium alloy product. The titanium alloy product or process according to any of claims 15 to 24, wherein in step (b), the at least one beta forging and/or rolling thermomechanical treatment is carried out at a temperature above the beta transus temperature, preferably 50 °C or more above the beta transus temperature, more preferably from 50 to 250 °C above the beta transus temperature, and more preferably from 100 to 200 °C above the beta transus temperature. The titanium alloy product or process according to any of claims 15 to 25, wherein in step (b), the at least one alpha-beta forging and/or rolling thermomechanical treatment is carried out at a temperature below the beta transus temperature of the titanium alloy, preferably at a temperature of from 10 to 130 °C below the beta transus temperature of the titanium alloy, 44 more preferably at a temperature of from 20 to 110 °C below the beta transus temperature, and most preferably at a temperature of from 25 to 100 °C below the beta transus temperature.

27. The titanium alloy product or process according to any of claims 15 to 26, wherein in step (b), the at least one alpha-beta forging and/or rolling thermomechanical treatment is carried out at a temperature of from 695 to 815 °C, preferably from 715 to 805 °C, more preferably from 725 to 800 °C.

28. The titanium alloy product or process according to any of claims 15 to 27, wherein in step (b), after the at least one alpha-beta forging and/or rolling thermomechanical treatment, the reduction in cross sectional area of the titanium alloy is 40% or more, preferably 50% or more.

29. An article formed from the titanium alloy product according to any of claims 2 to 28. 30. The article according to claim 29, wherein the article is a fan containment casing for an aircraft turbofan engine.

31 . Use of the titanium alloy product according to any of claims 2 to 28 in fan containment casings, for aircraft turbofan engines.