Density-optimized molybdenum alloy

Abstract

To provide an alloy system based on Mo-Si-B that has a lower density than that of known Mo-Si-B alloy systems and accordingly can be utilized advantageously as a structural material for rotating or flying applications, in particular in aviation technology and aerospace technology, e.g., as a turbine material.SOLUTION: A molybdenum alloy has 5 to 25 atom% of silicon, 0.5 to 25 atom% of boron, and 3 to 50 atom% of vanadium, with the remainder consisting of molybdenum. The molybdenum alloy has a molybdenum-vanadium mixed crystal matrix and at least one silicide phase distributed therein. The density of the molybdenum alloy is less than 8 g / cm3.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This invention relates to a molybdenum-silicon-boron (Mo-Si-B) based alloy with optimized density and high temperature resistance, a method for producing the same, and its use as a structural material.

[0002] The aforementioned ternary Mo-Si-B alloy system not only has an extremely high melting temperature (exceeding 2000°C), enabling use at temperatures well above 1000°C, but also exhibits excellent oxidation resistance, outstanding creep resistance, and sufficient ductile-brittle transition temperature and fracture toughness.

[0003] Based on these properties, the ternary Mo-Si-B alloy system is particularly suitable as a structural material for manufacturing structural components that operate at extremely high temperatures, such as turbine blades and disks in gas turbines, and for structural components subjected to high stress in aerospace technology, as well as for tools in forming technology.

[0004] A particular advantage for the aforementioned high-temperature applications is the extremely good oxidation resistance of this alloy system when the silicide content is greater than 50%. Therefore, protective measures to prevent oxidation, such as the use of protective gases or the application of protective layers, can be omitted in the case of extremely fine-grained materials with a particle size of less than 10 μm and a homogeneous phase distribution, manufactured by powder metallurgy or other methods.

[0005] Pure molybdenum, as a refractory metal, has a melting point of 2623°C and is, in principle, suitable for high-temperature applications. However, the problem lies in its already low oxidation resistance at temperatures above 600°C.

[0006] A significant increase in oxidation resistance has been achieved by alloying molybdenum with silicon and boron, and by forming silicides bound to it. This type of oxidation-resistant ternary Mo-Si-B alloy is described, for example, in European Patent No. 0804627 (EP 0 804 627 B1). This ternary alloy system forms a boron-silicate layer at temperatures above 540°C, which prevents further penetration of oxygen into the solid or structural member.

[0007] West German Patent Application Publication No. 2534379 (DE 25 34 379 A1) relates in particular to Mo-Si-B alloys, which may also contain vanadium. However, these are excellent in terms of high thermal stability, that is, they are amorphous alloys that are stable even at high temperatures and do not begin to crystallize.

[0008] West German Patent Application Publication No. 1155609 (DE 11 55 609 A) similarly describes a Mo alloy containing at least one metallic boride selected from chromium boride, titanium boride, and zirconium boride as an essential component, and which may also contain Si, B, and V. None of the numerous examples explicitly listed contain V in addition to Mo. The objectives here are solely to increase oxidation resistance and strength, but not to improve toughness as desired by the present invention.

[0009] International Publication No. 2005 / 028692 (WO 2005 / 028692 A2) describes a Mo-Si-B alloy having Mo silicide and Mo-B silicide as essential components. It may selectively contain further elements that form mixed crystals with Mo, in which case vanadium is particularly mentioned, and Mo mixed crystals may exist. However, here, the aforementioned one or more further elements are present exclusively in the mixed crystals but not in the silicides.

[0010] According to U.S. Patent Application Publication No. 2016 / 0060734 (US 2016 / 0060734 A1), the density of a ternary Mo-Si-B alloy can be reduced by the partial substitution of the heavy metal Mo with the lighter metal Ti. However, it is noted that the partial substitution of Mo with Ti impairs its oxidation resistance. To compensate for this, additional elements, such as iron and / or yttrium, must be added.

[0011] Given the outstanding property profile described above, this ternary Mo-Si-B alloy system may have been a very promising candidate as a structural material at high temperatures, such as a turbine material, for rotational or aerial applications.

[0012] The drawback of this type of application, but also in other applications, is that it is typically 8.5-9.5 g / cm³. 3 It has a high density. For example, the alloy Mo-9Si-8B has a density of 9.5 g / cm³. 3 It has a density of .

[0013] Therefore, the object of the present invention was to provide a Mo-Si-B based alloy system having a lower density than known Mo-Si-B alloy systems, and thus advantageously usable as a structural material, for example, as a turbine material, for rotational or flight applications, particularly in aerospace and space technology. Furthermore, the alloy system should maintain the advantages of the ternary alloy system Mo-Si-B, particularly with respect to its oxidation resistance.

[0014] This problem is solved by an alloy system having 5-25 atomic percent silicon (Si), 0.5-25 atomic percent boron (B), 3-50 atomic percent vanadium (V), and the remainder molybdenum, wherein the molybdenum alloy has a molybdenum-vanadium mixed crystal matrix and at least one silicide phase distributed therein, and the density of the molybdenum alloy is 8 g / cm³. 3 It is less than.

[0015] According to a preferred embodiment, the molybdenum alloy has a vanadium content of 10 to 50 atomic percent and at least one silicide phase selected from (Mo,V)3Si, (Mo,V)5SiB2 and (Mo,V)5Si3.

[0016] Preferably, the Mo content is greater than 10 atomic percent, and more preferably at least 20 atomic percent. Particularly preferred is a Mo content of at least 40 atomic percent. The preferred content ranges are 8-15 atomic percent for Si, 7-20 atomic percent for B, and 10-40 atomic percent for V.

[0017] Preferably, the alloy system according to the present invention has a silicide phase ratio of at least 30%, and particularly at least 50%.

[0018] Vanadium has a melting point of 1910°C, and even less than 2000°C, and belongs to the so-called expanded refractory metals, however, 10.28 g / cm³ 3 6.11 g / cm³ at 293.15 K compared to molybdenum containing [unclear] 3 It has a significantly lower density. A further advantage of vanadium is that it has a similar atomic radius (134 pm) and the same crystal structure as molybdenum (145 pm), namely a body-centered cubic structure. This results in good miscibility and exchangeability between both elements in its crystal lattice, and consequently, good alloying properties between the two elements. Furthermore, since vanadium has high ductility, its addition does not worsen the toughness of the ternary Mo-Si-B alloy.

[0019] The alloy according to the present invention, with the addition of vanadium, in particular, has a concentration of 8 g / cm³ at 293.15 K. 3 It has a density of less than 1.

[0020] It was found that the aforementioned alloyed vanadium dissolves into the respective Mo mixed crystal phase and silicide phase, but does not alter the structural characteristics of the known phases in the Mo-Si-B alloy.

[0021] The ternary Mo-Si-B system has a Mo mixed crystal matrix that itself has good toughness. In this case, in the Mo phase, boron is inserted at interstitial positions, and silicon is inserted at ordered lattice positions.

[0022] Additionally, already during its pre-alloying, a silicide phase can be formed, for example, during an extremely long high-energy alloying process or during powder atomization. A silicide phase forms at the latest during consolidation and / or heat treatment of the powder. These phases, in particular Mo3Si (A15) and Mo5SiB2 (T2), do impart high strength to the system, but reduce its toughness on account of their brittleness. As the concentrations of silicon and boron increase, the proportion of the silicide phase increases, and when the silicide phase exceeds a critical proportion (about 50% when produced by mechanical alloying processes), it can form the matrix phase in the structure. As a result, in addition to a reduction in toughness, a shift of the brittle-ductile transition temperature towards higher temperatures is also expected. Therefore, in order to avoid these drawbacks, the objective is to produce an alloy having a Mo mixed crystal phase as the matrix phase.

[0023] The addition of V not only does not deteriorate the toughness of Mo-Si-B alloys, but also stabilizes the Mo mixed crystal phase, slightly increases the proportion of mixed crystals, and improves the toughness of the entire system.

[0024] Furthermore, the substitution of V atoms in the Mo mixed crystal lattice results in a further improvement in strength.

[0025] As a result, it can be confirmed that the addition of vanadium to the ternary Mo-Si-B alloy system not only reduces the density, but also simultaneously improves the strength without changing the toughness. Moreover, the alloy system according to the present invention has a structure in which, as a result of the addition of V, even when the silicide phase proportion is more than 50%, the silicide phase is distributed and present in the Mo mixed crystal matrix.

[0026] According to a preferred embodiment, titanium (Ti) can be added to the Mo-Si-B-V based alloy in an amount of 0.5 to 30 atomic%. It has been confirmed that addition of 0.5 to 10 atomic% results in stabilization of the mixed crystal (Mo,V)₃Si-(Mo,V)₅SiB₂ structure, and addition of 10 to 30 atomic% promotes production of the four-phase alloy mixed crystal (Mo,V)₃Si-(Mo,V)₅SiB₂-(Mo,V)₅Si₃. (Mo,V)₅Si₃ is the T1 phase. Furthermore, 4.51 g / cm 3 addition of Ti, which has a density of only that value, contributes to a further reduction in the density of the alloy.

[0027] If necessary, the base alloy according to the present invention may contain one or more additional alloying elements selected from the group consisting of Al, Fe, Zr, Mg, Li, Cr, Mn, Co, Ni, Cu, Zn, Ge, Ga, Y, Nb, Cd, Ca and La, each in a content of 0.01 atomic% to 15 atomic%, preferably up to 10 atomic%, and / or one or more alloying elements selected from the group consisting of Hf, Pb, Bi, Ru, Rh, Pd, Ag, Au, Ta, W, Re, Os, Ir and Pt, each in a content of 0.01 atomic% to preferably at most 5 atomic%. The latter group has a density greater than 9 g / cm 3 and consists of heavy elements with higher density. These elements should be added in as small an amount as possible to avoid an increase in the density of the alloy.

[0028] The aforementioned additional alloying elements can also be added in the form of their oxides, nitrides and / or carbides and composite phases (e.g., oxynitrides) at a concentration of up to 15% by volume of the alloy.

[0029] Constrained by manufacturing technology, the alloy according to the present invention may further contain interstitially soluble elements such as oxygen, nitrogen and hydrogen. These are unavoidable impurities that cannot always be completely excluded from the process. However, these impurities are only present in the ppm range, typically less than 100 ppm.

[0030] The alloys according to the present invention are non-eutectic alloys, but also alloys near the eutectic and eutectic alloys. Non-eutectic alloys are alloys that do not correspond to the stoichiometric composition of a eutectic. Conversely, alloys near the eutectic are alloys that are located near the eutectic in terms of their composition.

[0031] The production of non-eutectic alloys according to the present invention is advantageously carried out by powder metallurgy methods. In this process, a powder mixture consisting of the corresponding alloy components is treated by mechanical alloying, in which elemental powders and pre-alloyed powders can be used. For this mechanical alloying, various high-energy mills, such as attritors, gravity ball mills, vibratory mills, and planetary ball mills, can be used. In this process, the metal powder is vigorously mechanically treated and homogenized to the atomic level. This pre-alloying can also be selectively carried out by an atomization process under protective gas conditions.

[0032] Next, the mechanically alloyed powder can be compacted by FAST (Field Assisted Sintering Technology). A suitable FAST process is, for example, carried out at 1600°C under vacuum, with a pressure of 50 MPa and a holding time of 15 minutes, during which heating and cooling are performed at 100 K / min. Alternatively, the powder can also be compacted by cold isostatic pressing, for example at 1600°C, and hot isostatic pressing (HIP) at 1500°C and 200 MPa. However, the FAST process is preferable because the sintering process time is considerably shorter compared to hot pressing.

[0033] Furthermore, homogeneous material properties can be achieved even in the case of larger structural members. In addition, using FAST, higher strength and hardness, expressed here as microhardness, can be obtained, because grain growth during the process is inhibited, clearly based on a shorter process time. Fine grains in the structure result in better strength compared to coarser grains.

[0034] In contrast to powder metallurgy processes, the density-optimized alloys according to the present invention can be produced by addition processes, such as selective laser melting (SLM) or laser metal deposition (LMD). These processes are carried out based on mechanically alloyed or atomized, and consequently, pre-alloyed powders, which have a lower melting point than pure ternary Mo-Si-B alloys based on alloying of V (and optionally Ti or other alloying elements), and consequently are more easily processed by this type of method.

[0035] The advantage of this additive manufacturing method is that structural members that closely resemble the final structure can be obtained in a cost-, time-, and material-efficient manner. This type of additive manufacturing method is publicly known and is described, for example, in International Publication No. 2016 / 188696 (WO 2016 / 188696 A1).

[0036] In particular, alloys near the eutectic and eutectic alloys can be processed by addition, because it is possible to produce a particularly fine-grained structure with good mechanical strength. Such alloys fall within the compositional range of Mo-(7..19)Si-(6...10)B-(5...15)V or Mo-(7..19)Si-(6...10)B-(5...15)V-(5...18)Ti. Furthermore, these alloys are also suitable for other molten metallurgical methods, particularly for directional solidification in the known Bridgman process.

[0037] The alloy system according to the present invention is characterized in more detail below based on examples and drawings. [Brief explanation of the drawing]

[0038] [Figure 1] The X-ray diffraction pattern of the alloy sample MK6-FAST (Mo-40V-9Si-8B) is shown; [Figure 2] The microstructure of the alloy sample MK6-FAST after compaction by the FAST method is shown in Figure 1, which is presented as a binarized image; and [Figure 3] The results of the microhardness test, taking into account the standard deviation of the alloy samples from the examples, are shown.

[0039] A) Sample preparation 1. mechanical alloying Four alloys containing 10, 20, 30, and 40 atomic percent vanadium were prepared. The atomic content of silicon (9 atomic percent) and boron (8 atomic percent) remained constant across all alloy systems. 30 g was prepared from each alloy system. To do this, each alloy component was weighed out under an argon protective gas atmosphere and then transferred to a grinding container under the protective gas atmosphere. The resulting powder mixture was ground in a planetary ball mill (model PM 4000) of Retsch GmbH using the following parameters: [Table 1] The resulting alloy was given the following name: [Table 2]

[0040] 2. heat treatment The alloy obtained in step 1 was heat-treated. Each sample was transferred to a ceramic dish and annealed under argon protection gas for the entire duration of the heat treatment. To achieve this, approximately 10g of each alloy present in the initial state was transferred and heat-treated at 1300°C for 5 hours in a Type Losic tubular furnace from HTM Retz GmbH.

[0041] The obtained sample was given the following name: MK3-WB, MK4-WB, MK5-WB and MK6-WB

[0042] 3. Preparation of alloy samples using FAST Sample MK6-WB was compacted by FAST. To do this, the sample was heated and cooled at 100K / min under vacuum at a pressure of 50 MPa and held at 1100°C for 10 minutes and 1600°C for 15 minutes. The obtained sample was given the name MK6-FAST.

[0043] B) Structural survey 1. X-ray diffraction method (XRD) The structures of the pulverized samples MK3-WB, MK4-WB, MK5-WB, MK6-WB, and MK6-FAST were investigated by X-ray diffraction analysis using the PANalytical X'pert pro X-ray diffractometer system. - Radiation:Cu-K21,21,5406 - Voltage: 40kV - Current: 30mA - X' Celerator RTMS detector - Filter: Ni filter - Measurement range: 20°≦2Θ≦158.95° - Step width: 0.0167° - Measurement time: 330.2 s (per step width).

[0044] In all five samples, the Mo-V mixed crystal, (Mo,V)3Si, and (Mo,V)5SiB2 were detected. The results of the analysis of MK6-FAST are shown in Figure 1.

[0045] 2. Tissue analysis and density measurement The microstructure and morphology of the powder particles were analyzed using a Philips ESEM (SEM) XL30 scanning electron microscope. Visualization of phase contrast was performed via BSE contrast. The contained phases were assigned by EDX analysis.

[0046] For the preparation of the sample, a small amount of the sample powder was cold-embedded in epoxy resin as follows, followed by wet grinding with SiC abrasive paper having grit sizes of 800 and 1200, and polishing with a diamond suspension. For the SEM investigation, the sample was sputtered with a thin gold layer before embedding.

[0047] The structure of alloy MK6-FAST is shown in binarized form in Figure 2. In this case, the Mo mixed crystal phase is white, and both silicide phases are black.

[0048] The density of MK6-FAST was measured according to Archimedes' principle as 7.8 g / cm 3 .

[0049] C) Rating 1. SEM / EDX analysis The EDX analysis confirmed the results of the XRD measurement. In the structure of all samples, in addition to the Mo mixed crystal, silicide phases (Mo,V)3Si and (Mo,V)5SiB2 were formed. In this case, a higher proportion of vanadium was found in the silicide phases than in the mixed crystal matrix.

[0050] The evaluation of MK6-FAST revealed that it has the highest proportion of silicide phase in its structure compared to heat-treated samples.

[0051] The following table summarizes the percentage proportions (atomic %) of the silicide phase in the individual samples described above.

Table 3

[0052] 2. Microhardness testing The microhardness of mechanically alloyed (ML) samples MK3, MK4, MK5, MK6, and MK6-FAST was measured.

[0053] The minute hardness was measured using the Vickers method with a hardness tester (model MHT-10) from Anton Paar GmbH, integrated with a microscope (model Axiophod 2) from Carl Zeiss Microscopy GmbH: - Test load: 10p - Exam time: 10s - Tilt: 15p / s.

[0054] The sample was prepared for the SEM analysis (see B.2.), but without gold sputtering.

[0055] Fifty indentations were made in each phase and evaluated.

[0056] The results are shown in Figure 3, taking standard deviation into account. The microhardness of the silicide in the FAST sample is significantly higher than that of the mixed crystal phase. The extremely fine and homogeneous distribution of the silicide phase and its proportion of approximately 55% guarantee the high overall hardness of the alloy. The overall hardness of the FAST sample is synthesized from the microhardness of the individual phases, namely the Mo,V mixed crystal phase and the two types of silicide phases.

Claims

1. A molybdenum alloy comprising 5-25 atomic percent silicon, 0.5-25 atomic percent boron, 30-40 atomic percent vanadium, and the remainder being molybdenum, wherein the proportion of molybdenum is at least 40 atomic percent, and optionally containing 0.5-30 atomic percent titanium (Ti), The molybdenum alloy has a molybdenum-vanadium mixed crystal matrix and at least one silicide phase distributed therein, and the density of the molybdenum alloy is 8 g / cm³. ３ It is less than and the at least one silicide phase is (Mo, V) ３ Si, (Mo, V) ５ SiB ２ and (Mo, V) ５ Si ３ The molybdenum alloy is selected from among and has a silicide phase proportion of at least 30%.

2. The molybdenum alloy according to claim 1, wherein silicon is present in an amount of 8 to 15 atomic percent and / or boron is present in an amount of 7 to 20 atomic percent.

3. The molybdenum alloy according to claim 1 or 2, wherein the Ti content is 0.5 to 10 atomic percent.

4. The molybdenum alloy according to any one of claims 1 to 3, wherein the proportion of the silicide phase is at least 46%, preferably at least 50%.

5. Said alloy has a Mo-V mixed crystal matrix and (Mo,V) distributed therein ３ Si and / or (Mo,V) ５ SiB ２ The molybdenum alloy according to any one of claims 1 to 4, which has a structure comprising

6. Furthermore, phase (Mo, V) ５ Si ３ The molybdenum alloy according to claim 5, wherein such an alloy exists.

7. A method for producing a molybdenum alloy according to any one of claims 1 to 6, The method comprising mechanically alloying the starting element in a first step, and then compacting it in a second step by either the FAST (electric field-assisted sintering) method or the hot isostatic press method.

8. Use of the molybdenum alloy according to any one of claims 1 to 6 as a structural material for rotational or flight applications, particularly in aeronautical and space technology.

9. Use of the molybdenum alloy according to claim 8 as a turbine material.

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