DE102015114092A1 - Oxidation-resistant vanadium alloys for high-temperature stressed components - Google Patents

Abstract

The present invention relates to a vanadium alloy with 2 to 35 at.% Silicon and 3 to 50 at.% Boron, which is resistant to oxidation even at higher temperatures and is particularly suitable for the production of high temperature stressed components.

Description

The present invention relates to a vanadium alloy, which has excellent oxidation resistance even at high temperatures and is particularly suitable for the production of high temperature stressed components with simultaneous mechanical stress.

Vanadium has a low neutron capture cross-section, making it a promising candidate as a structural material for fusion reactors and for energy conversion applications such as gas turbines.

However, it is disadvantageous that vanadium and vanadium alloys rapidly form oxides at high temperatures, as occur, for example, in fusion reactors and in the presence of oxygen. The oxides can flake off or become liquid or gaseous at correspondingly higher temperatures and thus lead to an uncontrollable loss of material and shape of the component. For this reason, it is not possible to use vanadium for the manufacture of components that are exposed to atmospheric conditions or other oxygenated environments at high temperatures.

Vanadium and vanadium alloys are therefore limited to applications carried out either under protective gas or in low-oxygen media or at low temperatures.

It is known to use vanadium alloys with, for example, Ti, Zr, Ta, Nb and Cr as alloying elements as reactor materials at temperatures up to about 950 K in an oxygen-free or oxygen-poor environment such as an inert gas atmosphere (for example helium) or in water.

However, the formation of voluminous oxide layers has also been observed for vanadium alloys with in particular high Ti and Zr content under the influence of oxygen at higher temperatures, so that these materials are also unsuitable for use in an oxygen-containing environment at higher temperatures.

Thus, V-4Ti-4Cr alloys showed the formation of a significant oxide layer when treated in dry air at 700 ° C (973 K). Significantly thicker oxide layers were observed when minor additions of Si, Al and Y were added to the base alloy ( Fujiwara et al., "Oxidation and Hardness Profiles of V-Ti-Cr-Si-Al-Y Alloys", Journal of Nuclear Materials 283-287 (2000) 1311-1315 ).

It was therefore an object of the present invention to provide vanadium alloys which have improved oxidation resistance even at relatively high temperatures and can therefore also be used at higher temperatures in an oxygen-containing atmosphere. Furthermore, the vanadium alloy should have the best possible mechanical strength even at higher temperatures.

This object is achieved by a vanadium alloy having 2 to 35 at.% Silicon (Si), 3 to 50 at.% Boron (B) and the remainder vanadium (V), wherein the vanadium alloy comprises at least one intermetallic phase of at least vanadium, silicon and boron having.

Preferred content ranges are 5 to 18 at.% For silicon and 22 to 35 at.% For boron. In a preferred embodiment, the alloy contains 5 to 18 at.% Silicon and 22 to 35 at.% Boron.

Preferably, the vanadium alloy according to the invention has a microstructure in which one or more intermetallic phases with the constituents vanadium, silicon and boron are distributed in a matrix of vanadium.

The intermetallic phases of vanadium according to the invention with silicon and boron are collectively referred to hereinafter as "intermetallic V-Si-B phase (s)".

In addition to the at least one intermetallic V-Si-B phase according to the invention, intermetallic phases of a different composition may also be present in the alloy according to the invention, such as phases of V with Si such as V ₃ Si, V ₅ Si _3, etc., as well as intermetallic phases may contain other alloying elements.

Unlike the known vanadium alloys, which can only be used at low temperatures or under protective gas or in low-oxygen media, the vanadium alloys according to the invention are resistant to oxidation even at relatively high temperatures. They are therefore particularly suitable as structural materials for the manufacture of components which are exposed to an oxygen-containing atmosphere at elevated temperatures. Special protective measures such as the provision of a protective gas atmosphere or other low-oxygen media are not required.

The intermetallic V-Si-B phases, which are distributed in the vanadium matrix, form a thin protective layer on the surface at high temperatures under oxygen-containing atmosphere. Through this protective layer, the vanadium material is protected from further oxidation and the Dimensional stability of components which have been produced from the vanadium material according to the invention allows.

The protective effect or oxidation resistance is all the better the higher the volume fraction of inventive intermetallic V-Si-B phase in the vanadium alloy. However, since the intermetallic V-Si-B phase is comparatively brittle, as the intermetallic phase in the vanadium alloy increases, the susceptibility to cracking and breakage of the material increases, that is, the mechanical strength in terms of fracture toughness decreases. On the other hand, it has been found that the vanadium alloys according to the invention have increasing strengths with increasing volume fraction of the intermetallic phases under compressive stress.

In view of a balance between oxidation resistance on the one hand and mechanical resistance on the other hand, the volume fraction of the intermetallic V-Si-B phase (s) in the vanadium alloy should preferably be at least 35 vol.%, In particular at least 50 vol.% And 75 vol.%. do not exceed. These values are values optimized with regard to oxidation resistance and mechanical resistance, which can be exceeded or exceeded depending on the application and requirements. From the point of view of mechanical resistance, in particular fracture toughness, it is particularly advantageous if the intermetallic phases do not interrupt the vanadium matrix.

In general, it can be said that the V-Si-B intermetallic phases improve the oxidation resistance of the alloy and the more ductile and fracture-resistant vanadium matrix ensures good mechanical properties. From the viewpoint of oxidation resistance, therefore, a material of 100% intermetallic V-Si-B phase is advantageous, but such a perfectly intermetallic material is very brittle, so that for use in the production of mechanically stressed components also sufficient mechanical resistance should be respected.

The vanadium alloy according to the invention also has an excellent strength even at temperatures up to 1000 ° C even at pressures of 1 to 2 GPa. The values found are significantly higher than those of materials such as steels, nickel-base alloys or titanium alloys. The vanadium alloys according to the invention thus have a strength level which is high and stable even at the high temperatures which prevail in the intended fields of application.

It shows

1 4 is a diagrammatic sectional view of an embodiment of a vanadium alloy according to the invention with intermetallic V-Si-B phase,

2 a diagram of the oxidation curves at 600 ° C of examples of inventive V alloys in comparison with prior art V alloys,

3 a diagram of oxidation curves of examples of inventive V alloys at 600 ° C and 700 ° C after pre-oxidation, and

4 a diagram showing the results of a compressive stress measurement of examples of inventive V-alloys.

This in 1 The sectional view shown was made on the basis of a microscopic analysis of the alloy according to the invention, wherein the morphology of the phases and the distribution were idealized, but the size of the precipitated phases and their volume fraction of the microscopic template correspond.

The sectional view of the 1 shows the microstructure of a vanadium alloy according to the invention with the composition V-9Si-15B according to embodiment 1, wherein an intermetallic phase having the composition V ₅ SiB _{2 is} homogeneously distributed in the vanadium matrix. The volume fraction of phase V ₅ SiB ₂ of the alloy is about 50% in this example.

The macroscopic phase areas formed can have a multiform shape and are idealized only in a sectional view as a hexagon. Is shown in 1 an optimized alloy with respect to the balance between oxidation resistance and mechanical properties.

The vanadium alloy of the present invention may contain a V-Si-B intermetallic phase as shown in FIG 1 the case is. There may also be two or more V-Si-B intermetallic phases. An example of a particularly preferred V-Si-B intermetallic phase is phase V ₅ SiB ₂ , as described in US Pat 1 is shown in idealized form.

In addition to the intermetallic V-Si-B phase according to the invention, one or more further phases having different compositions may also be present in the vanadium alloy according to the invention, for example intermetallic phases formed from V and Si. In addition, there may be intermetallic phases comprising one or more of the additional alloying elements mentioned below.

As required, the alloy of the present invention may contain one or more additional alloying elements selected from the group consisting of Ti, Fe, Zr, Mg, Hf, Li, Pb, Bi, Cr, Mn, Co, Ni, Cu, Zn, Ge, Ga, Y , Nb, Mo, Ru, Rh, Pd, Ag, Au, Cd, Ca, La, Ta, W, Re, Os, Ir, Pt and Au. These additional alloying elements may typically each be added at a level of from 0.01 at.% To 15 at.%, And preferably to at least 10 at.%.

The above-mentioned additional alloying elements may also be added in the form of their oxides, nitrides and / or carbides in concentrations of up to 15% by volume of the alloy.

For the preparation of the vanadium alloys according to the invention, a powder of supersaturated mixed crystals of the alloy components is produced in a first step.

The formation of an imbalance state in the form of supersaturated mixed crystals is required because otherwise the solubility of Si, B and optionally other alloying constituents is insufficient and in particular the content of Si and B in the mixed crystal powder can become too low.

As a consequence of this, the particularly preferred uniform distribution of the intermetallic phases in the vanadium matrix can no longer be obtained. Instead, increasingly different sized and inhomogeneously distributed intermetallic phase regions would occur. However, an inhomogeneous distribution of the intermetallic phases in the vanadium alloy leads to the fact that the macroscopic properties such as mechanical strength of the vanadium alloy or the components produced therefrom no longer have the desired isotropy. The more uniform the distribution of the intermetallic phases in the vanadium alloy, the more isotropic the resulting macroscopic properties of the vanadium alloy or the components produced therefrom.

In order to achieve the required supersaturation, powder metallurgical processes are used for the production of the mixed crystal powder for the alloy according to the invention, such as gas atomization with rapid cooling (rapid solidification) or mechanical alloying. These methods are known per se.

Next, the resulting pre-alloyed powder is compacted. The compaction is carried out in such a way that in the end result compact structural materials are obtained which have the desired microstructure after the last compaction step. Conventional compaction processes such as cold pressing, sintering, hot pressing or other compaction processes such as Field Assisted Sintering Technology (FAST), Spark Plasma Sintering (SPS), Sintering or Equal Channel Angular Pressing (ECAP) can be used for compaction. The mentioned compaction methods are known per se.

The precipitation of the intermetallic V-Si-B phase or phases according to the invention and the residual porosity of the material can be controlled via the temperature and holding time. Typical temperatures are between 1000 ° C and 1700 ° C. The heating to the desired temperature can be done in one step or in several steps. It is then cooled rapidly to room temperature. Suitable heating or cooling rates are in a range between 50 and 150 K / min.

Suitable holding times are between 15 minutes and several hours, the holding time depending on the method used, the nature and chemical composition of the powder particles and the temperature.

For example, the sintering activity of smaller powder particles is higher than the larger particles, so that the required holding time decreases with decreasing particle size. Also, particles of irregular shape require a longer hold time and / or higher temperature than regularly shaped particles. Another consideration is that the precipitation processes of the intermetallic phases are diffusion-driven processes that require time and temperature. For a fast process, therefore, the temperature must be increased, or at a lower temperature to increase the holding time in order to achieve the same result. Furthermore, the higher the Si and B content, the longer the compaction process should take at high temperature in order to achieve the highest possible density in the compact material.

• – Aufheizen auf 1.100°C mit 100 K/min, Haltezeit 15 min,
• – Weiterheizen auf 1.500°C mit K/min, Haltezeit 15 min,
• – Abkühlen bis auf Raumtemperatur; und

für größere Partikel mit einem höheren Si- und B-Gehalt (10% Si, 15% B):

• – Aufheizen auf 1.1000°C mit 100 K/min, Haltezeit 15 min,
• – Weiterheizen auf 1.300°C, Haltezeit 20 min,
• – Weiterheizen auf 1.600°C, Haltezeit 30 min,
• – Abkühlen bis auf Raumtemperatur.

For example, for small particles with a low content of Si and B (7% Si, 28% B), the compaction by means of FAST method can be carried out as follows:

• Heating to 1,100 ° C at 100 K / min, holding time 15 min,
• - Continue heating to 1,500 ° C with K / min, holding time 15 min,
• - cooling to room temperature; and

for larger particles with a higher Si and B content (10% Si, 15% B):

• - heating to 1.1000 ° C at 100 K / min, holding time 15 min,
• - Continue heating to 1300 ° C, holding time 20 min,
• - Continue heating to 1,600 ° C, holding time 30 min,
• - Cool to room temperature.

The production method (production of the supersaturated mixed-crystal powder and compaction) is preferably carried out in an oxygen-free atmosphere as possible, for example under an inert gas such as argon, a reducing atmosphere, for example hydrogen or a hydrogen-containing mixture, or vacuum.

Hereinafter, the present invention and its production method will be further explained in detail by way of concrete examples.

Example 1: Preparation of V-9Si-15B

As starting materials, pure V, Si and boron powders were mechanically alloyed in the stated quantitative ratio (9 at.% Si, 15 at.% B and the remainder vanadium) in order to obtain a supersaturated mixed-crystal powder. For this purpose, the starting materials were ground in a planetary ball mill for 20 hours at 200 U / min. The resulting supersaturated powder was compacted using the Field Assisted Sintering method. The compaction was carried out by first heating to 1100 ° C and held for 15 minutes. Thereafter, it was further heated to 1500 ° C and held again for 15 minutes. Subsequently, it was cooled down to room temperature. As a result, a vanadium alloy was obtained whose idealized sectional image in 1 is shown.

The vanadium alloy obtained contained the intermetallic phase V ₅ SiB ₂ , which was homogeneously distributed in the vanadium matrix, and had no interruption of the vanadium matrix. The volume fraction of the phase is about 50%.

Example 2: Preparation of V-12.5Si-25B

The alloying constituents in the stated amounts (12.5 at.% Si, 25 at.% B, remainder vanadium) were processed as under 1, with the exception that during the compaction in the second stage the mixture was heated to 1,600 ° C. and the Temperature was kept for 30 minutes.

As a result, a V alloy containing about 90 vol% intermetallic phase V ₅ SiB _{2 was obtained} .

Investigation of oxidation resistance:

The vanadium alloys obtained in Examples 1 and 2 were subjected to an oxidation test as follows. For this purpose, the sample material with about 50% intermetallic phases (Example 1) and about 90% intermetallic phase (Example 2) was heated to 600 ° C, wherein after 2, 5, 10, 20, 50 and 100 hours cooled to room temperature and the mass was determined.

With the same sample material, this cyclic temperature treatment was continued. The obtained oxidation curves are in 2 and show the results of the cyclic oxidation experiments at 600 ° C, showing the mass change with respect to the surface of the starting samples. Comparative values from the literature for V-4Cr-4Ti and V-5Cr-5Ti ( K. Natesan, M. Uz: Fusion Eng. Design 51-52 (2000) 145 As a result, it is noted that the alloys of the present invention have higher oxidation resistance.

For further investigation of oxidation resistance, the vanadium alloys of Example 1 and Example 2 were subjected to pre-oxidation at 1,000 ° C. for one hour under air. This pre-oxidation served to form an oxide protective layer on the surface, which in turn protects against further oxidation. The results are in 3 shown. These results show the stabilizing effect of the intermetallic phase, by their reaction with oxygen, the oxide protective layer could be formed first. Even at a temperature of 700 ° C, the vanadium alloy with about 90% intermetallic phases showed only a slight mass change, whereas a corresponding alloy according to example 1 with about 50% intermetallic phases showed mass reduction during the course of the experiment.

Investigation of compressive strength

The compressive strength of the vanadium alloys according to Examples 1 and 2 was measured at temperatures between 600 ° C and 1000 ° C. The stress-strain-compression curves, which show the strength and deformation potential of the alloys as a function of the temperature and the content of intermetallic phases, are given in FIG 4 shown. The measurement was carried out with a testing machine from Zwick Z100 type, the measurement according to DIN 50106: 1978-12 "Testing of metallic materials; Pressure test " was carried out.

As can be seen from the compressive stress-compression curves, the pressure range limit of the testing machine was achieved in the alloys according to Example 1 at 600 ° C and after Example 2 at 1000 ° C. It follows that the material is significantly stronger at these temperatures than can be measured with the testing machine used. As from the curves for the vanadium alloy according to Example 1 (about 50% intermetallic phase), the strength decreases with increasing temperature and the deformation potential increases. The curves clearly show the dependence of the strength on the proportion of intermetallic phase. Due to the higher concentration of intermetallic phases, the vanadium alloy is about 90% stronger than the vanadium alloy according to example 1 with about 50% intermetallic phase. The courses clearly show that the vanadium alloys according to the invention have an excellent compressive strength even at high temperatures.

The high oxidation resistance in conjunction with the high compressive strength make the vanadium alloys according to the invention an excellent material for the production of components which are exposed to high temperatures with simultaneous mechanical stress.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited non-patent literature

• Fujiwara et al., "Oxidation and Hardness Profiles of V-Ti-Cr-Si-Al-Y Alloys", Journal of Nuclear Materials 283-287 (2000) 1311-1315 [0007]
• K. Natesan, M. Uz: Fusion Eng. Design 51-52 (2000) 145 [0048]
• DIN 50106: 1978-12 "Testing of metallic materials; Pressure test " [0050]

Claims (11)

Vanadium alloy having 2 to 35 at.% Silicon and 3 to 50 at.% Boron, remainder vanadium, wherein the vanadium alloy has at least one intermetallic phase formed of at least vanadium, silicon and boron. The vanadium alloy according to claim 1, wherein the content of silicon is 5 to 18 at.%. A vanadium alloy according to any one of claims 1 or 2, wherein the content of boron is 22 to 35 at.%. Vanadium alloy according to one of the preceding claims, additionally containing one or more alloying elements selected from the group consisting of Ti, Fe, Zr, Mg, Hf, Li, Pb, Bi, Cr, Mn, Co, Ni, Cu, Zn, Ge, Ga, Y, Nb, Mo, Ru, Rh, Pd, Ag, Au, Cd, Ca, La, Ta, W, Re, Os, Ir, Pt and Au. The vanadium alloy of claim 4, wherein the additional alloying elements are each present at a level of from 0.01 at.% To 15 at.%. A vanadium alloy according to any one of the preceding claims wherein the at least one intermetallic phase of at least vanadium, silicon and boron is V ₅ SiB ₂ . A vanadium alloy according to any one of claims 1 to 6, wherein the volume fraction of the at least one intermetallic phase of at least vanadium, silicon and boron in the alloy is at least 35%. Vanadium alloy according to one of the preceding claims, wherein the volume fraction of the at least one intermetallic phase of at least vanadium, silicon and boron in the alloy is at most 75%. Vanadium alloy according to one of the preceding claims, wherein the vanadium alloy is preoxidized. A process for producing a vanadium alloy according to any one of claims 1 to 9, wherein in a first step, a powder of supersaturated mixed crystals of the alloying constituents vanadium, silicon and boron and optionally one or more further alloying elements is prepared, and the resulting powder of supersaturated mixed crystals at a temperature is compacted in a range between 1,000 ° C and 1,700 ° C to form the alloy of the invention. Use of a vanadium alloy according to any one of claims 1 to 8 as a structural material for the production of high temperature stressed components.

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