Titanium alloy and method of manufacturing a titanium alloy

Abstract

According to an aspect of the disclosure an (α+β)-titanium alloy consists of, in % in weight: Al: 3.0 to 7.0%, (Fe + Cr + Mn): 1.5 to 4.5%, with Cr less than Fe, Mn less than Fe, and Cr less than 1.5%, (Sn + Zr): 1.0 to 5.0%, with Zr less than 0.8%, O: 0.03 to 0.35%, and optionally one or more of (V + Mo + Nb + Ta): less than 3.0%, B: less than 1.0%, C: less than 0.08%, N: less than 0.05%, H: less than 0.015%, the balance Ti and incidental impurities.

Description

TITANIUM ALLOY AND METHOD OF MANUFACTURING A TITANIUM ALLOYTechnical Field

[0001] This disclosure relates to titanium alloys, and in particular to an (a+p ) -titanium alloy with high ballistic performance and / or applicability in medical technology or aerospace applications.Background

[0002] Ti-6A1-4V (Grade 5) is the most widely used(a+p ) -titanium alloy with a market share of more than 50%.(a+p ) -titanium alloys such as Grade 5 are suitable for applications in the high-performance sector due to their balanced mechanical properties and low density.

[0003] Despite the wide range of applications, Grade 5 is not optimized for applications in the defense sector and, in particular, with regard to ballistic protection effect. The process control of manufacturing Ti-6A1-4V is challenging due to undesired oxygen pick-up and a narrow temperature window for forming. There is a limited formability of Ti-6A1-4V due to thermodynamic equilibrium p-phase fractions of 5 - 10 vol . % at room temperature. Moreover, the manufacturing technique is costly because of high temperature annealing required to process the final product.

[0004] Therefore, ongoing research is being conducted to develop a low-cost (a+p ) -titanium alloy that has improved ballistic resistance and / or applicability in other industries such as, e.g. , medical devices or aerospace applications.Summary

[0005] According to an aspect of the disclosure an(a+p ) -titanium alloy consists of, in % in weight: Al: 3.0 to 7.0%, (Fe + Cr + Mn) : 1.5 to 4.5%, with Cr less than Fe, Mn less than Fe, and Cr less than 1.5%, (Sn + Zr) : 1.0 to 5.0%, with Zr less than 0.8%, 0: 0.03 to 0.35%, and optionally one or more of (V + Mo + Nb + Ta) : less than 3.0%, B: less than 1.0%, C: less than 0.08%, N: less than 0.05%, H: less than 0.015%, the balance Ti and incidental impurities.

[0006] According to another aspect of the disclosure a ballistic protection metal sheet is provided which comprises or consists of the (a+p ) -titanium alloy as indicated above.

[0007] According to still another aspect of the disclosure a method of manufacturing an (a+p ) -titanium alloy product having an alloy composition as set out above is provided. The method comprises melting the alloy composition, hot-rolling the (a+p) - titanium alloy and annealing the hot-rolled (a+p) -titanium alloy.Brief description of the drawings

[0008] Figure 1 is a phase diagram displaying the a-phase fraction and the p-phase fraction of conventional Ti-6A1-4V and of Ti-4.5Al-3Fe-l .5Sn-2V according to an example of the disclosure .

[0009] Figure 2 is a diagram illustrating flow curves (stress under compression as a function of degree of forming) obtained by compression test measurement during forming at 855°C ofconventional Ti-6A1-4V and Ti-4.5Al-3Fe-l .5Sn-2V according to an example of the disclosure.

[0010] Figure 3A is a diagram illustrating tensile strength measurement results of conventional Ti-6A1-4V andTi-4.5Al-3Fe-l .5Sn-2V according to an example of the disclosure.

[0011] Figure 3B is a diagram illustrating tensile strength measurement results of conventional Ti-6A1-4V andTi-4.5Al-3Fe-l .5Sn-2V according to an example of the disclosure.

[0012] Figure 4 illustrates exemplary stages of a method of manufacturing an (a+p) -titanium alloy product.

[0013] Figures 5A and 5B are microstructure images of Ti-64 and Ti-4321 alloys taken with an optical microscope, respectively.

[0014] Figure 6 is a schematic perspective view of an exemplary (a+p ) -titanium alloy product.Detailed description

[0015] The titanium base alloys (in the following also referred to as (a+p ) -titanium alloys) disclosed herein exhibit high solid solution strengthening and improved p- s tabili zation . Further, they typically have a relatively low p-transus temperature.

[0016] As a result, improved manufacturing process properties are obtained. For example, energy costs are lowered due to the reduction of process temperature and improved formability. Further, machine wear is reduced due to improved formability.

[0017] Products produced from (a+p ) -titanium alloys disclosed herein feature better performance than products made of conventional Ti-6A1-4V. In particular, as a more p-stabilized titanium base alloy can be obtained, a broader range of mechanical properties may be achieved. For example, a higher maximum tensile strength (about +10 - 15% compared to Ti-6A1-4V) or a higher maximum elongation (about +5% compared to Ti-6A1-4V) can be achieved.

[0018] At the microstructural level, lower susceptibility to strain localization than in conventional Ti-6A1-4V has been observed. This effect comes from the lower anisotropy and higher thermal conductivity of the p-phase as compared to the a-phase. Lower susceptibility to strain localization improves the ballistic protection properties of an (a+p ) -titanium alloy product as disclosed herein.

[0019] Further, due to the typically lower p-transus temperature than in conventional Ti-6A1-4V, diffusion during the manufacturing process is reduced, resulting in less oxygen uptake and less grain growth. The smaller the grains the higher is the tensile strength of the (a+p ) -titanium alloy product.

[0020] The importance and properties of the constituent chemical elements as well as their compositional ranges in the titanium base alloy are described in the following. Throughout this description and the claims, all percentages of the chemical composition are given in percentage in weight (wt.%) unless expressively stated otherwise. The upper, intermediate and lower limits of the individual elements can be freely combined within the compositional ranges set out in the claims.

[0021] Aluminum (Al: 3.0 - 7.0%) is an important a-solid solution strengthening and a-stabilizing element in titanium base alloys. A lower limit is set at 3.0%, otherwise the a-phase may have too low strength. A preferred lower limit is set to 4.0%, because the trade-off of solid-solution strengthening and a- stabilization is optimized in the range above 4.0% (sufficient solid-solution strengthening without losing too much p- stabili zation) . An upper limit is set to 7.0%, because above 7.0% embrittlement occurs and melting metallurgical production becomes more difficult. A preferred upper limit is set to 5.5%, because hardenability is optionally given, while the a-stabilization effect does not yet exceed an undesirable level. Further, a high solid-solution strengthening effect is available below this preferred upper limit.

[0022] More specifically, Al is of specific importance as it is the only known metallic a-stabilizer . The addition of Al therefore increases the stability of the a-phase and thus its phase fraction as well as the strength of this phase, which is increased by solid solution strengthening. In its pure form, titanium is already present in the a-phase, i.e. without additional p-stabilizing elements it acts primarily as a solid solution strengthening agent, whereby the transformation temperature for transformation into p-phase (p-transus) is shifted to higher values (hence a-stabilizer) . Al also reduces the density as an alloying element in Ti alloys, resulting in a weight saving. A maximum Al content of around 6 to 7.0% should not be exceeded due to the tendency to precipitate ordered a- crystals Ti3Al . For example, for the exemplary composition Ti-4321 (see Table 1 further below) with around 80 vol . % a-phase at room temperature, this significantly determines the mechanical properties of the alloy. On the other hand, too low values of Alwould (A) reduce the process temperature too much and (B) cause too little solid solution strengthening in the a-phase.

[0023] Iron (Fe: 1.5 - 4.5%) is a strong p-stabili zer . Fe segregates strongly but can be produced homogeneously up to 3.0% with ISM (Induction Skull Melting) , EBCHM (Electron Beam Cold Hearth Melting) , PBCHM (Plasma Beam Cold Hearth Melting) and / or VAR (Vacuum Arc Remelting) . An upper limit is set to 4.5%, since above this limit, it is difficult to produce a homogeneous material (due to Fe segregation) . A preferred upper limit is set to 3.5% to allow a high degree of p-stabili zation and a high solid-strengthening effect. A preferred lower limit is set to 1.9%, where the degree of p-stabilization and the solidstrengthening effect are still sufficient, so that the alloy concept can be pursued by adjusting the other elements. Below a lower limit of 1.5%, the advantage of Fe as an effective and inexpensive p-stabilizer is largely lost.

[0024] More specifically, Fe is one of the "eutectic" p-stabilizers . It causes a particularly strong p-stabilization and thus increases the p-phase content even in relatively small quantities. It also acts as an effective solid solution solidifier in this phase. Its solubility in the a-phase is extremely low. As mentioned above, excessively high Fe contents in Ti alloys lead to segregation phenomena, which, however, are not observed within the claimed limits. Therefore, a preferred lower limit can even go beyond 1.9% and amount to values of 2.2% or 2.5%.

[0025] Substitution of Fe : Fe may partly be substituted by chromium (Cr) and / or manganese (Mn) , which are also p- stabilizers. (Fe + Cr + Mn) equal to or greater than 1.5% alsoprovides sufficient p-phase stabilization and solid solution strengthening. Therefore, the above range for Fe can optionally be replaced or amended by the further condition: (Fe + Cr + Mn) : 1.5 - 4.5%, with Cr less than Fe, Mn less than Fe, and Cr less than 1.5%.

[0026] Above 1.5%, the segregation of Cr may be too high. Cr contributions less than 1.0% or 0.5% or 0.2% may be preferred, or, as already mentioned, Cr-free titanium base alloy may be provided. Mn has a similar effect as Fe with respect to p-phase stabilization. With a similar high segregation coefficient as Cr it has an even stronger tendency to form detrimental intermetallic phases. It also evaporates easily in VAR and EBCHM / PBCHM, leading to difficulties or even the exclusion of preferred melting routes. Mn contents of less than 0.5% or 0.1% may be preferred, or in particular a Mn-free titanium base alloy may be advantageous .

[0027] Tin (Sn: 1.0 - 5.0%) is an uncritical alloying element and serves as a neutral solid solution strengthener in both phases. Sn is more or less neutral regarding a- or p-phase stabilization with a slight tendency to stabilize a-phase. Sn is known to support the formation of the detrimental a2-phase (TiaAl) . An upper limit is set to 5.0% to limit solid-solution strengthening and a- and a2-stabilization . A preferred upper limit is set to 4.0% to provide an optimized range for neutral solid-solution strengthening. A preferred lower limit is set to 1.5% to limit an optimized range for solid- solution strengthening consolidation. Below a lower limit of 1.0% the effect of solidsolution strengthening is largely lost.

[0028] More specifically, Sn has hardly any influence on the phase proportions, as it is absorbed in the crystal lattices of both phases in approximately the same ratio as a solid solution strengthening agent. It therefore acts as a solid solution strengthening agent for the entire material structure, i.e. in both the a- and p-phases. An appropriate amount of Sn can increase the strength of the alloy without significantly influencing the phase proportions and subsequently the process window. In view of the Ti-Al-Fe-V alloy concept disclosed herein, an excessively high Sn content can lead to embrittlement, which is why this element should only be present in an amount so that sufficient ductility is maintained. In this respect, it is to be noted that the hot forming capacity is reduced if the strength level is too high. Furthermore, with a mass number of 118 g / mol, Sn is the heaviest alloying element used. The density of the alloy is therefore increased in proportion to the Sn addition, which makes it less attractive for applications in the mobility sector (vehicles, aviation, protective elements such as, e.g. , armor plates, etc. ) . This is another reason for a lower upper limit for Sn. Therefore, preferred upper limits for Sn may, e.g. , be 2.5% or 2.0% or even 1.5%, for example

[0029] Substitution of Sn: Sn may partly (up to an amount of 0.8%) be substituted by zirconium (Zr) . Though Sn is slightly a- stabilizing and Zr is slightly p-stabilizing, both elements are nevertheless rather neutral solid solution strengtheners . Therefore, the above range for Sn can optionally be replaced or amended by the further condition: (Sn + Zr) : 1.0 - 5.0%, with Zr less than 0.8%, preferably less than 0.5%. As being a substitution element, Zr can be omitted (Zr = 0%) . Surprisingly, Sn has been found to be a stronger solid solution strengthener than Zr at concentrations from 1.0 - 5.0%, which is the reason Snis preferred over Zr. The probable cause is a similar lattice structure between Zr and Ti, which makes the Ti lattice less distorted. In addition, Sn widens the temperature range during hot forming advantageously compared to Zr.

[0030] Vanadium (V: 0.0 - 3.0%) can be used as an additional p- stabilizer and solid solution s trengthener . V is more expensive than iron but has no tendency to segregate and is not as strong a p-stabilizer than Fe . V is considered cytotoxic and is preferably avoided for materials in medical technology. An upper limit is set to 3.0% due to price and / or toxicity, depending on the respective application. A preferred upper limit may be set to 2.5% for the same reasons (price, toxicity) . A lower limit is 0.0%, as V is not mandatory since Fe (optionally partly substituted by Cr and / or Mn as mentioned above) can provide sufficient stabilization of the p-phase. For example, in particular for medical technology products, the alloy is preferably free of V.

[0031] However, for example in Ti-4321 (see Table 1) , the effect of V is utilized. The addition of V is also associated with p- stabilization, but with a moderate effect compared with Fe . The solid solution strengthening effect is also lower than with Fe, but not in the same proportion as the phase stabilization. Therefore, an optimum ratio between solidification and phase fraction can be set by using both elements (V and Fe) as p- stabilizers. Therefore, at least for some applications, a preferred lower limit of V may, e.g. , be 1.0% or 1.5%.

[0032] Substitution of V: V may partly or fully be substituted by molybdenum (Mo) , niobium (Nb) , and / or tantalum (Ta) , which also act as p-stabili zers . Therefore, the above range for V canoptionally be replaced or amended by the further condition:(V + Mo + Nb + Ta) : 0.0 - 3.0%. Mo is preferably less than V. In particular, Mo may, e.g. , be less than 0.5% or 0.2%. The alloy may also be free of Mo, i.e. Mo = 0%. Although relatively small amounts of Nb and Ta could be acceptable, preferably Nb and Ta are 0%. V is preferred over Mo, Nb and / or Ta because the synergistic effects of solid-solution-strengthening and p- stabilization are best to adjust the desired properties. Larger amounts of Ta and Nb would be necessary for the desired p- s tabili zation, but this would lead to undesirably high solidsolution-strengthening and thus embrittlement. In contrast, smaller amounts of Mo would be required for the desired p- stabilization, but this would lead to insufficient solidsolution-strengthening .

[0033] The effects of individual alloying elements as discussed above are summarized qualitatively in the following listing:with: ++ strongly increased; + increased; ~ slightly increased;0 influence negligible; -decreased; --strongly decreased.

[0034] Oxygen (0: 0.03 - 0.35%) is deliberately added as an interstitial element to improve the strength of the titanium base alloy. Hence, a minimum limit is set to 0.03%. As too much oxygen causes embrittlement at room temperature, an upper limit of 0.35%should not be exceeded. Preferred upper and lower limits are0.25% and 0.03%, respectively. Oxygen acts as an a-stabilizer .

[0035] Boron (B: 0.0 - 1.0%) can be used as an additive for a finer primary grain. This results in a more homogeneous forming and a finer final microstructure. Further, B can reduce oxygen uptake .

[0036] Due to grain refinement and precipitation hardening, B exhibits a strength-increasing effect. In addition, a preforming process during manufacturing can optionally be simplified or skipped when using B as a grain refining additive since the microstructure is already more homogeneous. An upper limit is set to 1.0% since otherwise brittle effects caused by TiB networks could hinder forming. Below a preferred upper limit of 0.5% the formability is good while strength-enhancing and grain refining advantages for, e.g. , improving ballistic protection are still significant. Above a preferred lower limit of 0.08%, grain refining effects are realistic. Even at the lower limit of 0.0%, examples of B-free titanium base alloys according to the disclosure outperformed conventional Ti-6A1-4V in terms of ballistic protection.

[0037] In the following, alloy Ti-4.5Al-3Fe-l .5Sn-2V according to the disclosure is referred to as Ti-4321 and conventional Ti- 6A1-4V is referred to as Ti-64. Table 1 summarizes the chemical compositions of titanium base alloys according to the disclosure (upper and lower limits, preferred upper and lower limits as well as the exemplary composition of Ti-4321) . As mentioned above, Fe, Sn, and V may be partially or completely replaced by the above substitution elements within the ranges indicated. For V, a preferred lower limit may alternatively be 1.0% rather than 0.0%.Table 1: Chemical compositions of titanium base alloy (in weight percent)

[0038] T race element limits for carbon (C) , nitrogen (N) and hydrogen (H) are C: 0.0 - 0.08%, N: 0.0 - 0.05%, and H: 0.0 - 0.015%, respectively. These limits are similar to conventional Ti-64 (Grade 5) and may, e.g. , be in compliance with standards (e.g. ASTM B265, AMS 4911 or others, see Table 2) . As apparent from Table 1, significantly lower limits may be used, e.g. C: less than 0.06% or 0.02%, N: less than 0.03%, H: less than 0.01%, for example.

[0039] Typically no deliberate addition of C, N and H is used.These elements would act as (undesired) a-stabilizers . In particular, the sum of C + N + 0 must not exceed 0.4%.Table 2: Impurity limits according to standards

[0040] Generally, a-stabilizers are elements that raise the p- trans formation temperature and p-stabilizers are elements that lower the p-transformation temperature. The p- trans formation temperature Tp (also known as "p-transus temperature" or, briefly, "p-transus") is the lowest temperature where 100% p-phase exists. Only below the p-transus temperature, the a-phase is thermodynamically stable.

[0041] Figure 1 is a phase diagram showing the a-phase fraction and p-phase fraction of conventional Ti-64 and Ti-4321 as a function of temperature (at thermodynamic equilibrium) . As mentioned above, Ti-4321 is an illustrative example of a titanium base alloy according to the disclosure.

[0042] Tp,i denotes the p-transus temperature of Ti-4321 and Tp,2denotes the p-transus temperature of conventional Ti-64. As apparent from Figure 1, Tp,i < Tp,2-

[0043] ATi is the difference between the temperature at phase fraction equality and T^i, and AT2is the difference between the temperature at phase fraction equality and Tp,2• Phase fraction equality means a fraction of 50% a-phase and a fraction of 50% p- phase. As apparent from Figure 1, AT2> AT2, meaning that the process window in the a-p region is widened for Ti-4321 compared to Ti-64. In other words, when cooling the titanium base alloy down from the p-region, the phase transition into the a-phase starts at lower temperature and proceeds at a smaller rate forTi-4321 as compared to Ti-64.

[0044] Thermodynamic equilibrium phase fractions at room temperature are also significantly different for Ti-4321 andconventional Ti-64. While conventional Ti-64 features a ratio fp / fa ~ 5 / 95 of p-phase fraction fp and a-phase fraction faat room temperature, Ti-4321 has a significantly higher fraction of p-phase alloy at room temperature, namely fp / fc ~ 10 / 90 or more. Depending on the cooling rate after annealing (see Figure 4) , p- phase fractions in Ti-4321 of 15 - 25 vol . % were determined, with 15% for slow, e.g. furnace cooling and 25% in fast, e.g. ambient air cooling. That is, the slower the cooling, the closer the actual phase fractions are to the thermodynamic equilibrium values. This is due to the more effective diffusion with slower cooling .

[0045] For example, slow cooling may be characterized by a cooling rate of 100°C / min or less between 900°C and 700°C. Otherwise, the cooling may be referred to as fast cooling (e.g. , ambient air cooling) .

[0046] Table 3 illustrates the dependency of the p-transus temperature in °C on the chemical compositions "upper limits" and "lower limits" of Table 1.Table 3: Minimum and maximum P-transus temperature in °C for chemical compositions of titanium base alloy (in weight percent)

[0047] The chemical composition "upper limits" has a (maximum) p-transus temperature above l, 050°C while the chemical composition "lower limits" has a (minimum) p-transus temperature below 850°C.

[0048] Figure 2 illustrates flow curves (stress under compression in N / rtim2versus degree of forming <p in 100%, i.e. 1.0 corresponds to <p = 100%) obtained by compression test measurements during forming at 855°C of conventional Ti-64 and Ti-4321 according to an example of the disclosure. The degree of forming <p is the percentage reduction in thickness of a sample plate (or slab, ingot) in relation to the initial thickness (for example, if a sample plate of 100 mm thickness is rolled to 30 mm thickness, this corresponds to a reduction of 70% ( ( 100-30 ) / 100 = 0.7 or 70%) .

[0049] Significantly easier formability is obtained for Ti-4321 than for Ti-64. More specifically, the required forming force is reduced by at least 30% over the illustrated range of degrees of forming at the same temperature. The easier formability of Ti- 4321 is due to optimized phase fractions (i.e. , more p-phase) and their influence on the plastic behavior of the material. Easier formability is beneficial because it allows either forming at lower temperature or using lower forming force at the same temperature. Both measures (lower temperature and / or lower forming force) permit a reduction in manufacturing costs.

[0050] Figures 3A and 3B are diagrams illustrating tensile strength measurement results in longitudinal (L) and transversal (T) direction of product samples of conventional Ti-64 and product samples of Ti-4321 in the similar microstructure condition. The results shown in Figure 3A were obtained from product samples having a thickness of 10 mm, while the results shown in Figure 3B were obtained from product samples of the same composition having a thickness of 41 mm. For the 10 mm comparison (Figure 3A) , the degree of forming was about <p = 90% each (initial thickness of the slab was 98 mm) , for the 40 mmcomparison (Figure 3B) , the degree of forming was about <p = 84% each (initial thickness of the slab was 250 mm) . In Figure 3A, the tensile strength of Ti-4321 was found to exceed the tensile strength of Ti-64 by about 10%. In particular, in Figure 3A, a tensile strength of 1, 090 MPa in longitudinal direction and 1, 155 MPa in transversal direction was measured. In Figure 3B, the tensile strength of Ti-4321 was found to exceed the tensile strength of Ti-64 by more than 20%. In particular, a tensile strength of 1, 137 MPa in longitudinal direction and 1,204 MPa in transversal direction was measured. That is, the effect in terms of tensile strength increases with increasing thickness and / or decreasing degree of forming <p of the product sample plates. The product samples of Ti-4321 were made of the (bor-free) alloy of Example 1 of Table 4 (see below) .

[0051] Further, ballistic VPAM tests (v50, Vi / ioooo) were carried out to investigate the ballistic protection effect (VPAM: Vereinigung der Prufstellen fur angrif f shemmende Materialien und Kons truktionen - Translation: Association of Test Laboratories for Attack Resistant Materials and Constructions) . The v50test at test level 9 yields the projectile velocity at 50% failure (i.e. bullet penetration) probability at 20 mm panel thickness. The vi / ioooo values yields the projectile velocity at 0.01% failure probability, determined from v50test. The tests were carried out and evaluated in accordance with VPAM APR 2006 (APR: Allgemeine PrufRichtlinie ) of 2010 and TEST GUIDELINE "Bullet resistant plate materials" VPAM - PM Edition 3 as of March 15, 2021. It was found that most of the sample armor plates made of Ti-4321 exhibited ballistic protection of vi / ioooo > 805 m / s, while Ti-64 (Grade 5) sample armor plates were measured at a maximum of Vi / ioooo < 784 m / s. Thus, the ballistic protection properties ofTi-4321 exceed those of Ti-64.

[0052] Every damage mechanism in ballistic impact loading causes strain localization. Generally, the more homogeneous the material, the less strain localization occurs.

[0053] The p-phase has fewer anisotropies than the a-phase. Therefore, it is believed that one reason for the better ballistic properties of the titanium base alloys according to the present disclosure is that the proportion of p-phase at room temperature is higher than in Ti-64, resulting in a more homogeneous structure and thus less strain localization. Another reason for the better ballistic properties could be that the p- phase has a better thermal conductivity than the a-phase.Manufacturing process

[0054] As illustrated in Figure 4, the titanium base alloys according to the present disclosure may, e.g. , be produced through ISM (Induction Skull Melting) and one or multiple VAR (Vacuum Arc Remelting) processes. Other possibilities are EBCHM (Electron Beam Cold Hearth Melting) or PBCHM (Plasma Beam Cold Hearth Melting) , each followed by one or multiple VAR processes. Further, it is possible to use multiple VAR processes alone (i.e. without initial ISM or EBCHM or PBCHM) . The titanium base alloy is then cast into ingots under vacuum. The ingots have a fully p- type microstructure.

[0055] The ingots may then be preformed (e.g. forged) to produce slabs. Preforming may be carried out in the p-range (so-called p- forging) . For example, preforming may be carried out at ~ 950°C. If the hot-rolling start temperature is in the p-range, preforming may be carried out at about the hot-rolling start temperature, for example.

[0056] The purpose of preforming is to make the ingot more homogeneous. As mentioned above, the preforming process can be simplified or optionally even skipped by adding boron to the titanium base alloy.

[0057] The titanium base alloy (formed as ingots or slabs) of the present disclosure is then hot-rolled. In some examples, the hot-rolling start temperature may, e.g. , be in the p-range. For example, a hot-rolling start temperature of ~ 950°C may be used.

[0058] In other examples, the hot-rolling start temperature may, e.g. , be in the (a+p) -range, i.e. below p-transus . For example, a hot-rolling start temperature of ~ 860°C or even less may be used .

[0059] In both cases (hot-rolling start temperature above or below p-transus) , hot-rolling is typically terminated in the (a+p) -range. For example, hot-rolling may terminate at ~ 860°C or even less. Typically, a thickness reduction of equal to or greater than 70%, preferably 80% is helpful to adjust the microstructure and mechanical properties of the (a+p ) -structure, for example.

[0060] Subsequently, the hot-rolled titanium base alloy is annealed. In some examples, annealing is carried out in the p-range. For example, annealing may be carried out at ~ 950°C. The duration of annealing (after complete through-heating of the alloy) may, e.g. , be about 45 to 90 minutes, more preferable 45 to 60 minutes. Annealing in the p-range is also referred to as " p- annealing" .

[0061] In other examples, annealing is carried out in the (a+p) -range, i.e. below p-transus . In this case, annealing may be carried out temperatures between, e.g. , ~ 700°C and ~ 900°C.Preferable, annealing may be carried out at temperatures equal to or less than ~ 880°C or ~ 860°C, for example. The duration of annealing (after complete through-heating of the alloy) may, e.g. , be approximately 60 to 90 minutes or even longer. Annealing in the (a+p) -range is also referred to as "solution annealing".

[0062] Possible annealing processes also include stress relief annealing or mill annealing, for example.

[0063] The type of annealing (e.g. , p-annealing or solution annealing) has implications on the microstructure of the (a+p) - titanium alloy product to be produced, p-annealing typically results in a lamellar microstructure, e.g. in a coarse lamellar microstructure 420 or in a fine lamellar microstructure 422. On the other hand, solution annealing results in a bimodal microstructure 440 or in a globular microstructure 442.

[0064] The lamellar microstructure 420, 422 cannot be transformed into a bimodal microstructure 440 or into a globular microstructure 442 by any subsequent heat treatment. For some applications such as ballistic protection, a bimodal microstructure 440 or, in particular, a globular microstructure 442 is preferred. Thus, in some examples, solution annealing may be preferred over p-annealing.

[0065] Aft er annealing, the annealed titanium base alloy is cooled down to room temperature. In some examples ambient air cooling may be used. In particular, ambient air cooling may be used for bimodal microstructures 442. That way, a high strengthof about 1, 170 MPa or more was achieved (e.g. , for a panel thickness of about 20 mm) , for example.

[0066] In other examples, the annealed titanium base alloy may be cooled down to room temperature at a rate slower than air cooling. Slow cooling may be performed in a furnace configured to set a cooling rate smaller than the cooling rate of ambient air. Especially for globular microstructures 442, slower cooling than in ambient air can be used. In this way, a high ductility (e.g. , about 14.3% or more) was achieved.

[0067] Figures 5A and 5B are optical microscope images of the microstructure of Ti-64 and Ti-4321 (Ex. 1) alloys, respectively. The alloys were manufacture by the process of Figure 4. The respective manufacturing temperatures for preforming, hot-rolling und annealing were about 100°C lower for Ti-4321 than for Ti-64. Figures 5A and 5B reveal that about 20 vol . % p-phase is present in Ti-4321, while about 10 vol . % p-phase is present in Ti-64.

[0068] Further, another effect is apparent from the microscope images: Ti-64 (Figure 5A) was rolled to 20 mm thickness and Ti-4321 to 40 mm thickness, with the same initial thickness of the slabs. Ti-4321 already has a similar grain size at a degree of forming <p = 84% as Ti-64 at a degree of forming at <p = 92%.

[0069] In general, it was found that Ti-4321 forms a finer micros tructure / grain size than Ti-64. Lower temperature during hot-rolling and more p-phase mean more forming energy for the microstructural processes during forming. At higher temperatures, diffusion-controlled processes tend to take place, which lead to a rapid reduction in forming energy, whereas at lower temperatures, more stress / inter facial energy is "accumulated", sothat new and finer grains are more likely to form. Figuratively speaking, it becomes more unpleasant for the existing structure when the forming defects accumulate, and it "decides" to remodel the structure. That is, "more p-phase" may mean "the same amount of p-phase as with Ti-64, but at a hot-rolling temperature which is lower by approximately 100°C than the hot-rolling temperature needed for Ti-64 to obtain the "correct" a-phase / p-phase ratio.

[0070] Generally, a lower temperature at all process steps (e.g. , about 100°C less at forging and / or hot rolling and / or annealing for microstructure adjustment) compared to conventional Ti-64 manufacturing can be achieved. This lowers diffusion and may reduce unwanted oxygen uptake (having an embrittlement effect) . At all process steps, lower temperatures lead to a reduction in grain growth, which has a positive effect on strength. Further, as forging and hot rolling forces may be reduced, machines are less worn out and / or life span of machines may increase. Lower temperatures may also provide energy and cost savings at all process steps.

[0071] Referring to Figure 6, titanium base alloys with the above composition and / or manufacturing process can be used, for example, as ballistic protection sheets such as armor plates 600 due to their high ballistic protection effect. The armor plates 600 may preferably be fabricated with a degree of forming <p of about 90% (e.g. , preferably equal to or greater than 80%) .EXAMPLES

[0072] In the following examples, exemplary titanium base alloys according to the present disclosure are compared to reference titanium base alloys not according to the present disclosure. The chemical compositions of the exemplary titanium base alloys(Examples 1 - 9) and reference titanium base alloy (Examples 10 - 13) are presented in Table 4 and Table 5, respectively. All of the compositions were induction skull melted (ISM) , subsequently two times vacuum arc remelted (VAR) and conventionally casted into ingots under vacuum. The ingots were heated up to about 960°C, forged and later hot-rolled into 20 mm plates at a temperature below B-transus. The plates were cut in order to obtain specimens (e.g. , smaller plates) for solution annealing, metallographic characterization, tensile tests, hardness tests, and ballistic impact tests (v50test and vi / ioooo analysis) . The Vickers hardness was measured in accordance with the EN ISO 6507-1 standard.Table 4: Examples - chemical composition (in weight percent) and properties of titanium base alloys according to the present disclosure

[0073] Examples 1, 6 and 7 relate to Ti-4321 with different amounts of B added. It is apparent that the addition of B improves the ballistic properties as well as the Vickers hardness of the titanium base alloy. The p-transus temperature is not significantly affected by addition of B.

[0074] In summary, the claimed alloys and, in particular, Ti- 4321 are characterized by a combination of good processability, attractive specific mechanical properties (based on density) and microstructural characteristics, i.e. with regard to phase proportions at process and application temperature and adjustable microstructure variants, which in turn help to determine the mechanical property profile. It is to be emphasized that due to the coordination of solid solution strengthening and the existing phase proportions, even a low degree of deformation leads to the desired mechanical properties, which distinguishes, e.g. , Ti-4321 from other (a+p) -Ti alloys used. This was investigated and established for roll forming with more than 66% degree of forming <p on pre-forged VAR ingots (or slabs) .

[0075] Similarly, at high thicknesses, i.e. low degrees of forming <p and low associated globularization of the microstructure, higher elongation at break values were also measured in Ti-4321 despite its high strength. This speaks in favour of the strategy of high p-stabilisation (i.e. , more ductile phase) used in the alloy. This is possibly underlined by the microstructure comparison (Figures 5A, 5B) . With Ti-64, a certain, highly remodelled microstructure would be necessary to achieve such high ductility, but not with Ti-4321.

[0076] In the examples of Table 4, Fe, Sn and V were not substituted by any of the substitution elements set out above. However, research considerations suggest that substitutions within the ranges indicated above are likely to (at least partially) preserve the beneficial effects according to the present disclosure.Table 5: Examples - chemical composition (in weight percent) and properties of titanium base alloys not according to the present disclosure

[0077] In Table 5, composition values that fall outside the ranges are in bold. In Example 10, too much Al is present. As a result, the titanium base alloy is difficult to produce, because Al evaporates. Further, this titanium alloy features too high embrittlement because of formation of the brittle a2-phase. Such titanium base alloy is difficult to produce in terms of process technology. In Example 11, too much Fe is present. This results in strong Fe segregation and poor creep resistance due to rapid diffusion. In Example 12, too little Sn has been added. As a result, neutral solidification is missing (i.e. Sn solidifies a- phase and p-phase without stabilizing a certain phases) . In Example 13, too much V is present. Such high V content would cancel out the cost savings and increase cytotoxicity (therefore, omitting V from the alloy may also be preferable, for example) . In Example 14, the content of Sn is too high.

[0078] Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and / or equivalent implementations may be substituted for the specific embodiments shown and described without departing from the scope of the present invention. This application is intended to cover any adaptations or variations of the specific embodiments discussedherein. Therefore, it is intended that this invention be limited only by the claims and the equivalents thereof.

Claims

Claims1. An (a+p ) -titanium alloy consisting of, in % in weight:Al : 3.0 to 7.0%,Fe + Cr + Mn : 1.5 to 4.5%, with Cr less than Fe, Mn less than Fe, and Cr less than 1.5%,Sn + Zr: 1.0 to 5.0%, with Zr less than 0.8%,0: 0.03 to 0.35%, and optionally one or more ofV + Mo + Nb + Ta: less than 3.0%,B: less than 1.0%,C: less than 0.08%,N: less than 0.05%,H: less than 0.015%, the balance Ti and incidental impurities.

2. The (a+p ) -titanium alloy of claim 1, fulfilling the following requirement:Sn : 1.0 to 5.0%.

3. The (a+p ) -titanium alloy of claim 1 or 2, fulfilling the following requirement:Sn: less than 2.5% or 2.0% or 1.5%.

4. The (a+p ) -titanium alloy of any of the preceding claims, fulfilling the following requirement:B: 0.08 to 0.5%.

5. The (a+p ) -titanium alloy of any of the preceding claims, fulfilling the following requirement:Fe : more than 1.5% or 1.9% or 2.2% or 2.5%.

6. The (a+p ) -titanium alloy of any of the preceding claims, fulfilling at least one of the following requirements: Al : 3.5 to 5.5%,Fe + Cr + Mn : 1.9 to 3.5%,Sn + Zr: 1.5 to 4.0%, 0: 0.03 to 0.25%. . The (a+p ) -titanium alloy of any of the preceding claims, fulfilling at least one of the following requirements:Cr: less than Fe, and / or Cr less than 1.0% or 0.5% or 0.2% or Cr = 0%,Mn : less than Fe, and / or Mn less than 0.5% or 0.1% or Mn = 0%.

8. The (a+p ) -titanium alloy of any of the preceding claims, fulfilling the following requirement:Zr: less than 0.5%, or Zr = 0%.

9. The (a+p ) -titanium alloy of any of the preceding claims, fulfilling the following requirement:V: less than 2.5%, or V = 0%.

10. The (a+p ) -titanium alloy of any of the preceding claims, fulfilling the following requirement:V: more than 1.0% or 1.5%.

11. The (a+p ) -titanium alloy of any of the preceding claims, fulfilling at least one of the following requirements:Mo: less than V, and / or Mo less than 0.5% or 0.2% or Mo = 0%.

12. The (a+p ) -titanium alloy of any of the preceding claims, fulfilling at least one of the following requirements: Nb = 0%,Ta = 0%,H: less than 0.01%.

13. A ballistic protection metal sheet comprising the (a+p) - titanium alloy of any of the preceding claims.

14. A method of manufacturing an (a+p ) -titanium alloy product, the method comprising: providing an (a+p ) -titanium alloy composition of, in % in weight : Al : 3.0 to 7.0%, Fe + Cr + Mn : 1.5 to 4.5%, with Cr less than Fe, Mn less than Fe, and Cr less than 1.5%, Sn + Zr: 1.0 to 5.0%, with Zr less than 0.8%, 0: 0.03 to 0.35%, and optionally one or more of V + Mo + Nb + Ta: less than 3.0%, B: less than 1.0%, C: less than 0.08%, N: less than 0.05%, H: less than 0.015%, the balance Ti and incidental impurities; melting the (a+p ) -titanium alloy composition; hot-rolling the (a+p ) -titanium alloy; and annealing the hot-rolled (a+p ) -titanium alloy.

15. The method of claim 14, wherein melting the (a+p ) -titanium alloy composition comprises one or more processes of the group consisting of Induction Skull Melting, Electron Beam Cold Hearth Melting, Plasma Beam Cold Hearth Melting and one or multiple Vacuum Arc Remelting processes.

16. The method of claim 14 or 15, wherein hot-rolling starts at a hot-rolling start temperature which is above p-transus temperature .

17. The method of one or more of claims 14 to 16, wherein annealing is carried out at a temperature above p-transus temperature .

18. The method of one or more of claims 14 to 16, wherein annealing is carried out at a temperature below p-transus temperature, in particular below 860°C.

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