Titanium-based alloys with improved mechanical properties

Abstract

Titanium-based alloys exhibiting improved mechanical properties. The present invention relates to a titanium-based alloy in which one or more alloying elements are present, the alloy satisfying the following conditions: - 10 ≤ MOeq ≤ 14.5, - 2.77 < Bo < 2.80, and - 2.34 eV < Md < 2.38 eV, where Moeq denotes the mass content of beta-stabilizing elements in the alloy in molybdenum equivalent, where ei denotes the number of valence electrons of alloying element i, ai denotes the atomic radius of alloying element i, and xi denotes the mole fraction of alloying element i in the alloy, the sum being performed over all the alloying elements present in the alloy, Bo denotes the average bond order of the covalent bonds between titanium and the alloying elements, and Md denotes the average energy level in eV of the d orbitals corresponding to the covalent bonds between titanium and the addition elements.

Description

2 7 9 2 1 1 Background of the invention The invention relates to new titanium alloys having improved mechanical properties. Commercial titanium alloys can exhibit low or even zero work hardening at room temperature as well as relatively low ductility (10 to 15% on average). This type of behavior is linked to the hardening modes of known titanium alloys which make it possible to obtain good properties in terms of mechanical resistance but, on the other hand, can limit the deformation capacities of the material and, consequently, lead to low ductilities. The known titanium alloys therefore do not constitute optimal materials for producing parts that must potentially undergo large deformations while retaining good level static properties, such as, for example, casings that must ensure the retention of objects in the event of ingestion or bursting of parts. Some of the known titanium alloys can therefore be excluded from this type of application in favor of substantially heavier steels. There is therefore a need to have new titanium alloys exhibiting work hardening as well as high ductility. OBJECT AND SUMMARY OF THE INVENTION To this end, the invention proposes, according to a first aspect, a titanium-based alloy in which one or more addition elements are present, the alloy satisfying the following conditions: - 4.10 sts 4.16, - 10 s Moég s 14.5, - 2.77 Bo 2.80, and - 2.34 eV Md 2.38 eV, WHERE Moég denotes the mass content of betagen elements in the alloy in molybdenum equivalent , 302 7 9 2 1 2 5- = where ei denotes the number of valence electrons of element a ai of addition i, ai denotes the atomic radius of addition element i and xi denotes the mole fraction of the addition element i in the alloy, the sum being made over all the addition elements present in the alloy, Bo designates the average bond index of the covalent bonds between the titanium and the elements of addition and Md denotes the average energy level in eV of the d orbitals corresponding to the covalent bonds between the titanium and the addition elements. By “titanium-based alloy”, it should be understood that titanium 10 constitutes the base metal of the alloy, that is to say that the alloy comprises titanium in a mass content greater than or equal to 50% , for example greater than or equal to 60%, for example greater than or equal to 70%, for example greater than or equal to 80%. The quantity Moég is given by the following equation: Moég = 15 moizi where zi denotes the mass fraction in the alloy of the addition element i and Mo corresponds to the ratio (betagenic character of the addition element i) / (betagenic character of Mo), the sum being made over all the addition elements present in the alloy. Thus, the sum relates both to the betagen addition elements but also to the 20 alphagenic addition elements possibly present in the alloy, the latter having a negative coefficient Mo. For each of the elements of addition, the magnitudes Me and e1 / a1 are tabulated. Table 1 below gives the values ​​of these quantities for some examples of addition elements. 25 Al Cr Mo, 1 -0.33 + 1.60 + 0.67 3 4 6 5 Table 1 Bo quantifies the average cohesion strength of the covalent bonds between titanium and the addition elements. More precisely, the quantity Bo is calculated as follows: Bo =EBoixi where xi 302 7 9 2 1 3 denotes the molar fraction of the addition element i in the alloy, the sum relating to all the elements addition. Bol values ​​are tabulated and are given for various addition elements in Table 2 below. Md designates the average energy level of the 5 d orbitals corresponding to the covalent bonds resulting from the interaction between the titanium and the addition elements. More specifically, the quantity Md is calculated as follows: Md=Md i xi where xi designates the molar fraction of the addition element i in the alloy, the sum relating to all the addition elements. Mdi values ​​are tabulated and are given for various addition elements in Table 2 below. Al Sn Cr V 0; 2.43 2.28 2.78 2.81 billion; 2.20 2.10 1.48 1.87 Table 2 The parameters Mocl, Bo and Md are known from the literature. In particular, various publications detail the calculation of the Bo and Md parameters. In this respect, mention may be made, for example, of the publication Abdel-Hady et al. "General approach to phase stability and elastic properties of .6-type Ti-alloys using electronic parameters", Scripta Materialia 55 (2006) 477-20 480, the publication Marteleur et al. "On the design of new / 3 -metastable titanium alloys with improved work hardening rate thanks to simultaneous TRIP and 71A / IP effects", Scripta Materialia 66 (2012) 749-752 and the publication Sun et al. “Investigation of early stage deformation mechanisms in a metastable / 3-titanium alloy showing combined twinning-25 induced plasticity and transformation-induced plasticity effects”, Acta Materialia 61(2013) 6406-6417. Unless otherwise stated, in the chemical formulas of alloys used below, the number located in front of a chemical element is the mass content in % of this element in the alloy. For example, the Ti-8.5Cr-1.5A1 alloy 30 is a titanium-based alloy comprising Cr in a mass content equal to 8.5% and Al in a mass content equal to 302 7 9 2 1 4 at 1.5%. The alloys according to the invention advantageously exhibit high work hardening, a high breaking load as well as good ductility.The choice of the ranges of parameters explained above makes it possible to harden the alloy and to activate deformation modes making it possible to obtain high ductility by involving mechanisms of twinning and transformation of phase p into phase a . In the alloys according to the invention, a combination of an effect of plasticity induced by twinning (“TWIP” effect: “Twinning Induced Plasticity”) and of an effect of plasticity induced by phase transformation (“TRIP” effect): “Transformation Induced Plasticity”) is advantageously activated. The invention results in the choice of particular alloys defined by means of the parameters described above making it possible both to activate a martensitic transformation mechanism as well as twinning and sliding mechanisms. Thanks to the activation of these phenomena, the alloys according to the invention can in particular exhibit ductilities of the order of 40% while retaining high elasticity limits (beyond 500 MPa). Such performances are a technological breakthrough compared to the performances of known titanium alloys. In an exemplary embodiment, the alloy may comprise at least one addition element chosen from the following list: Cr, Al, Sn and V. In an exemplary embodiment, the alloy may comprise Cr 25 and Al as addition items. In an exemplary embodiment, the alloy may comprise Cr and Sn as addition elements. In an exemplary embodiment, the alloy may comprise V and Al as addition elements. The alloy can be a binary alloy or a ternary alloy. Preferably, the alloy can constitute a ternary Ti-Cr-Al or Ti-CrSn alloy. The alloy may also constitute a ternary Ti-V-Al alloy. The alloy may also be a quaternary alloy, such as for example the Ti-10V-4Cr-1Al alloy. . In an exemplary embodiment, the alloy may comprise Cr and Al as addition elements and the mass content of Cr in the alloy may be between 6% and 9% and the mass content of Al in the alloy can be between 1% and 3%. In particular, the alloy can have the following chemical formula: Ti-xCr-yAl where x is between 6 and 9 and y is between 1 and 3. In an exemplary embodiment, the alloy may comprise Cr and Sn as addition elements and the mass content of Cr in the alloy may be between 6% and 9% and the mass content of Sn in the alloy can be between 1% and 5° / 0. In particular, the alloy can have the following chemical formula: Ti-x'Cr-zSn where x' is between 6 and 9 and z is between 1 and 5. The alloy according to the invention can, in particular, have one of the following chemical formulas: - Ti-8.5Cr-1.5A1, Ti-8.5Cr-1.5Sn, Ti-7.5Cr-1A1, - Ti-7.5Cr-2A1, Ti-9 .5Cr-2A1, Ti-7Cr-2Sn, or Ti-13V-2.5A1. The present invention also relates to a turbomachine comprising a part comprising an alloy as defined above. Said part can be formed from an alloy as defined above. Preferably, the part is a turbomachine casing. Brief description of the drawings Other characteristics and advantages of the invention will emerge from the following description of particular embodiments of the invention, given by way of non-limiting examples, with reference to the appended drawings, in which: - the figures 1 and 2 represent electronic diagrams showing the positioning of examples of alloys according to the invention, - figure 3 shows the "TRIP" effect in which there is a phenomenon of transformation of a phase 13 into a phase a" in an alloy according to the invention Ti-8.5Cr-1.5A1, 302 7 9 2 1 6 - Figures 4A and 4B are photographs showing the phenomenon of twinning in an alloy according to the invention Ti-8.5Cr- 1.5Sn, and - figures 5 and 6 represent results of tensile tests of alloys according to the invention Detailed description of embodiments Figures 1 and 2 are electronic diagrams on which titanium alloys have been been positioned.These electronic diagrams 10 indicate the deformation mechanisms implemented when the alloy is subjected to stress. Bo is represented on the ordinate of the electronic diagrams of FIGS. 1 and 2. As mentioned above, Bo quantifies the average cohesion strength of the covalent bonds between the titanium and the addition elements. Md is represented on the abscissa of the electronic diagrams of FIGS. 1 and 2. As mentioned above, Md denotes the mean energy level of the d orbitals corresponding to the covalent bonds resulting from the interaction between the titanium and the addition elements. 20 The electronic diagrams provided in FIGS. 1 and 2 indicate various regions corresponding to the different deformation mechanisms implemented: sliding (“slip”), twinning (“twin”) and martensitic transformation (“SIM Transformation”: “Stress Induced Martensitic Transformation”. "). Some examples of alloys according to the invention are, as illustrated, positioned on the electronic diagrams of FIGS. 1 and 2 in the zone corresponding to the activation of twinning phenomena. One can for example have: 2.77 Bo 2.79 and 2.34 eV Md 5_ 2.38 eV for the alloys according to the invention. FIG. 3 is a photograph showing the obtaining, in an alloy according to the invention, of a phase a" from a phase I (activation of the mechanism for transforming a phase 13 into a phase a" during of the application of a constraint). The activation of such a phase transformation advantageously participates in obtaining high ductility. FIGS. 4A and 4B show, for their part, the activation of a twinning phenomenon obtained in an alloy according to the invention which also participates in obtaining high ductility. Figure 5 shows tensile test results obtained for a Ti-8.5Cr-1.5A1 alloy. For this alloy, we have: e / a = 4.129 and Moeq = 12.1. This alloy has a high ductility of the order of 40%, a breaking load of 1150 MPa and retains a high elastic limit. Similar results are obtained for the Ti-8.5Cr-1.5Sn alloy for which we have: Moeq=13.6 and e / a=4.16 (see FIG. 6). The tensile tests carried out were carried out at ambient temperature at a strain rate of 10 −3 s −1 on test specimens 50 mm long, 0.5 mm thick and 5 mm wide. Example A Ti-8.5Cr-1.5A1 alloy ingot was fabricated by compacting the sponge elements of titanium, grain chromium and powdered aluminum and then using the arc melting technique. In the compacted mixture, the following mass contents were respected: Ti at 90% by mass, Cr at 8.5% by mass and Al at 1.5% by mass. This ingot was then deformed to obtain a sheet 0.5 mm thick. This sheet was heat treated at 900°C in the beta region followed by rapid cooling. Flat tensile specimens were cut from this sheet and were used in the tensile test described above in connection with Figure 5. The expression "comprising / containing a" must be understood as "comprising / containing at least one". The expression "between ... and ..." or "ranging from ... to ..." must be understood as including the limits.

Claims

DEMANDS 1. Turbomachine casing comprising a titanium-based alloy in which one or more alloying elements are present, the alloy satisfying the following conditions: - 4.10 <-< 4.16, a - 10 < Moeq < 14.5, - 2.77 < Bo < 2.80, and - 2.34 eV < Md < 2.38 eV, where Moeq denotes the mass content of beta-reactive elements in the alloy in molybdenum equivalent, Xs where ™ denotes the number of valence electrons of element i and denotes the mole fraction of element i in the alloy, the sum being performed over all the elements present in the alloy, Bo denotes the average bond index of the covalent bonds between titanium and the alloying elements and Md denotes the average energy level in eV of the d orbitals corresponding to the covalent bonds between titanium and the alloying elements, the alloy constituting: - a ternary Ti-Cr-Al alloy in which the mass content of Cr in the alloy is between 6% and 9% and the mass content of Al in the alloy is between 1% and 3%, or - a ternary Ti-Cr-Sn alloy in which the mass content of Cr in the alloy is between 6% and 9% and the mass content of Sn in the alloy is between 1% and 5%.

2. Turbomachine comprising a titanium-based alloy in which one or more alloying elements are present, the alloy satisfying the following conditions: P - 4.10 <-< 4.16, a - 10 < MOeq < 14.5, 2.77 < Bo < 2.80, and - 2.34 eV < Md < 2.38 eV, where Moeq denotes the mass content of beta-reactive elements in the winged tissue in molybdenum equivalent, - - ) —X; or ~ denotes the number of valence electrons of element i and xt denotes the mole fraction of element i in the alloy, the sum being carried out over all the elements present in the alloy, Bo denotes the average bond order of the covalent bonds between titanium and the alloying elements and Md denotes the average energy level in eV of the d orbitals corresponding to the covalent bonds between titanium and the alloying elements, the alloy constituting: - a ternary Ti-Cr-Al alloy in which the mass content of Cr in the alloy is between 6% and 9% and the mass content of Al in the alloy is between 1% and 3%, or - a ternary Ti-Cr-Sn alloy in which the mass content of Cr in the alloy is between 6% and 9% and the mass content of Sn in the alloy is between 1% and 5%.

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