EP1718777B1 - Method for the production of a molybdenum alloy - Google Patents

Abstract

The invention relates to a method for the production of semi-finished or finished parts made of a molybdenum alloy with intermetallic phase parts, preferably molybdenum silicide, molybdenum boron silicide, selectively also molybdenum boride, phases. The heat compacted material, starting from a mechanically alloyed powder, exhibits superplastic forming behavior. It is thus possible to reduce the forming temperature by at least 300 °C, wherein processing is possible on conventional systems.

Description

The invention relates to a process for the production of semi-finished or finished parts from a molybdenum alloy with intermetallic phase fractions.

Molybdenum and molybdenum alloys are widely used in engineering because of their good mechanical strength properties at high temperatures. A problem of these alloys is their low oxidation resistance at temperatures above 600 ° C. Accordingly diverse are the known measures for improving the oxidation properties. They range from applying superficial protective coatings to alloying measures. Thus, the oxidation resistance can be improved by alloying silicon and boron, as shown in FIG Akinc, M. et al .: Materials Science and Engineering, A261 (1999) 16-23 ; Meyer, MK et al .: Advanced Materials 8 (1996) 8 and Meyer, MK et al .: J. Am. Ceram. Soc. 79 (1996) 63-66 is described.

Also the EP 0 804 627 describes an oxidation resistant molybdenum alloy consisting of a molybdenum matrix and intermetallic phase domains dispersed therein of 10 to 70 vol.% Mo-B silicide, optionally up to 20 vol.% Mo boride and optionally up to 20 vol.% Mo Silicide exists. The alloy comprises, in addition to molybdenum, the elements C, Ti, Hf, Zr, W, Re, Al, Cr, V, Nb, Ta, B and Si in the form that, in addition to the aforementioned phases, one or more elements of the group Ti, Zr , Hf and Al must be present in a proportion of 0.3 to 10 wt.% In the Mo mixed crystal phase.
Alloys according to the EP 0 804 627 Form at temperatures above 540 ° C from a boron-silicate layer, which prevents further penetration of oxygen into the body. Due to the Mo matrix alloys according to the EP 0 804 627 a significantly improved ductility.

The US 5,595,616 describes a method for producing a Mo-Si-B alloy with Mo matrix, embedded in the intermetallic phase components are. The method involves the rapid solidification of a melt, which can be done by atomizing a melt. Subsequently, the rapidly solidified powder is compacted by hot compacting, wherein this process step must be such that no coarsening of the intermetallic phase components occurs. Semifinished products produced in this way can be further processed by hot forming. The disadvantage here is that for the purpose of rapid solidification, the molybdenum alloy must be melted. Due to the high melting point and the chemical aggressiveness of the melt, however, no crucible material is available. It must therefore be melted without a crucible, which makes this process step very expensive. In addition, by this process, alloys having an optimum silicon and boron content (about 4% by weight of Si, about 1.5% by weight of B) with regard to their oxidation resistance can be obtained.
no longer process forming, whereby a compromise between oxidation resistance and process capability are made.
The object of the present invention is then to provide a method which makes it possible to produce oxidation-resistant molybdenum-silicon-boron alloys at low cost using a forming process.

This object is achieved by a method according to claim 1.
The method according to the invention comprises a high-energy milling process in which the powder particles used are mixed into one another in such a way that one can speak of a mechanical alloying. The powder mixture used consists of at least 60% by weight of Mo, 0.5% by weight of Si and 0.2% by weight of B. The powder may be present in elemental, partially prealloyed or completely pre-alloyed form. Elemental powder mixtures are said to be present when the individual particles are in a pure form and the alloy is prepared by mixing just such powders. A powder particle is completely pre-alloyed if it consists of a homogeneous alloy. Partially prealloyed powder consists of particles having different concentration ranges. As systems for mechanical alloying high energy mills, such as attritors, ball drop mills or vibrating mills are suitable. The Meals depend on the used aggregate. The typical process times when using an Attritor are 0.5 to 48 hours.

In order to avoid oxidation of the alloy components, it is necessary to carry out the milling process under a protective gas atmosphere. The use of hydrogen has proven particularly useful. The mechanically alloyed powder can then be further formed by cold compacting, such as die pressing, cold isostatic pressing, metal powder injection molding or slip casting. However, it is also possible to immediately subject the mechanically alloyed powder to a hot compacting process, as is the case, for example, in hot isostatic pressing and powder extrusion. The former has proven itself especially. The milled powder is then filled into a molybdenum or titanium alloy can, vacuum-sealed, and at temperatures typically in the range of 1,000 ° C to 1,600 ° C, preferably 1300 ° C to 1500 ° C, and a pressure of typically 10 to 300 MPa, preferably 150 to 250 MPa, compressed. Alternatively, sintered material with a predominantly closed porosity can be densely hot-isostatically recompressed. Conventional sintering HIP processes, the Ceracon process or the ROC (rapid omnidirectional compacting) process can also be used.
In addition, non-pressurized processes, such as conventional sintering, plasma-assisted sintering or microwave sintering, suitable, in the case of solid phase sintering temperatures of> 1500 ° C are required. If alloying components are added which lower the solidus temperature, it is also possible to achieve a sufficient density at lower temperatures.

It has now surprisingly been found that a molybdenum alloy prepared in this way can be superplasticized at temperatures of 1000 ° C. to 1600 ° C. at deformation speeds ε̇ of 10 ^-6 s ^-1 <ε̇ <10 ° s ^-1 . Both forming processes, such as rolling or pressing, are suitable as forming processes forming processes, such as pressing into a die or deep drawing. The inventive method, it is possible to lower the forming temperatures below 1600 ° C, whereby conventional equipment, in particular heating devices, such as those used for the production of refractory metals, can be used.

However, in order to achieve sufficient creep resistance, it is necessary to subject the superplastic formed molybdenum alloy in a further process step to a heat treatment at a temperature> 1,400 ° C, preferably 1600 ° C to 1900 ° C, preferably in a reducing atmosphere or vacuum. This is documented in the examples.

In principle, it is also possible to deform the molybdenum alloy prior to the superplastic forming step conventionally according to the prior art. This can be advantageous if additional structural refinement and homogenization is desirable, as is the case, for example, when hot compaction takes place by pressureless sintering.

The process according to the invention has proved to be particularly advantageous when the molybdenum alloy contains 2 to 4% by weight of silicon and 0.5 to 3% by weight of boron.
As already mentioned, molybdenum-silicon-boron alloys in this concentration range can be processed only at very high forming temperatures, or can no longer be processed in the high silicon and boron range by forming technology. Molybdenum alloys containing from 2 to 4% by weight of silicon and from 0.5 to 3% by weight of boron contain intermetallic molybdenum-silicide, molybdenum-boron-silicide, optionally also molybdenum-boride phases, and molybdenum or molybdenum mixed crystal , Mo ₃ Si and Mo ₅ SiB _{2 are} to be mentioned as preferred molybdenum silicide or molybdenum boron silicide phases. By means of the method according to the invention, it is also possible to deform alloys that can not be processed by forming technology according to the prior art.

Furthermore, it has been found that when using the method according to the invention molybdenum-silicon-boron alloys containing 0.5 to 30 wt.% Niobium and / or tantalum, both higher ductility and heat resistance values, as alloys containing these alloying components not or to a lesser extent included. This too is explained in more detail in the examples.

Surprisingly, it has also been found that the superplastic forming behavior is not adversely affected even when admixing oxides or mixed oxides which have a vapor pressure at 1500 ° C. of <5 × 10 ^-2 bar. The alloying of oxides or mixed oxides improves the hot or creep resistance without surprisingly the ductility of the material is negatively affected. Particularly suitable oxides are Y ₂ O ₃ , ZrO ₂ , HfO ₂ , TiO ₂ , Al ₂ O ₃ , CaO, MgO and SrO or their mixed oxides.

If the molybdenum alloy added to 0.001 to 5 wt.% Of one or more metals from the group rhenium, titanium, zirconium, hafnium, vanadium, chromium and aluminum, this promotes the formation of a dense boron-silicate layer.

In the following the invention will be described by examples.

example 1

• Molybdän mit einer Korngröße nach Fisher von 4,1 µm,
• Niob, abgesiebt auf < 32 µm,
• Silizium mit einer Korngröße nach Fisher von 4,3 µm,
• Bor mit einer Korngröße nach Fisher von 1,01 µm.

The following powders were used for the production of a molybdenum alloy:

• Molybdenum with a Fisher grain size of 4.1 μm,
• Niobium screened to <32 μm,
• Silicon with a grain size of Fisher of 4.3 microns,
• Boron with a grain size of Fisher of 1.01 microns.

The niobium content was varied with the silicon and boron contents being respectively 3 and 1% by weight. The alloy compositions are shown in Table 1. <b> Table 1 </ b>: Composition of molybdenum-silicon-boron alloys method Mo (% by weight) Nb (% by weight) Si (% by weight) B (% by weight) Alloy 1 inventively 93 3 3 1 Alloy 2 inventively 86 10 3 1 Alloy 3 inventively 76 20 3 1 Alloy 4 State of the art 76 20 3 1 Alloy 5 State of the art 96 0 3 1

Alloys 1, 2 and 3 were made according to the method of the invention, and the fabrication of alloys 4 and 5 followed the state of the art. Powder blends according to alloy compositions 1, 2 and 3 were mechanically alloyed in a stainless steel attritor. 100 kg of steel balls with a diameter of 9 mm were used. The respective powder batch quantity was 5 kg. The grinding took place under hydrogen. The milled powder was filled into a molybdenum alloy pot, vacuum-sealed and hot-isostatically compressed for 4 hours at a temperature of 1,400 ° C. and a pressure of 200 MPa. The thus hot-compacted material showed a pore-free microstructure and a density of> 99% of the theoretical density. For comparative purposes, prior art alloys 4 and 5 were prepared by atomizing sintered bars. The powder was cold isostatically compacted at 200 MPa and sintered at 1,700 ° C for 5 hours under hydrogen. The sintered rods were atomized without crucibles. The powder thus prepared was filled in a titanium can and hot isostatically compacted (1500 ° C, 200 MPa, 4 hours). After hot isostatic pressing, a density of 9.55 g / cm ^{2 was} measured, corresponding to 99% of the theoretical density.

From semifinished products thus produced samples were prepared by wire erosion and turning. These samples were deformed at a temperature of 1300 ° C and strain rates of 10 ^-4 s ^-1 and 10 ^-3 s ^-1, respectively. With semifinished product according to the invention, superplastic behavior could be determined. In Depending on the deformation rate and alloy composition, the measured strains were 60.2 to 261.5% (see Table 2). These properties enable superplastic forming at temperatures below 1,500 ° C, ie on conventional equipment for refractory metal production. A niobium addition of more than 5 wt.% (Alloy 2 and Alloy 3) causes a significant increase in strength while increasing the elongation at break. <b> Table 2: </ b> Properties of Inventive Molybdenum-Silicon-Boron Alloys (Alloys 1 to 3) Compared to the Prior Art (Alloys 4 and 5) description Temperature (° C) Strain rate (s ^-1 ) Maximum voltage (MPa) Strain (%) Alloy 1 1300 10 ^-4 33 161.7 1300 10 ^-3 125 60.2 Alloy 2 1300 10 ^-4 43 210.8 1300 10 ^-3 140 76.5 Alloy 3 1300 10 ^-4 45 281.5 1300 10 ^-3 162 95.3 Alloy 4 1300 10 ^-4 299 11.9 1300 10 ^-3 267 0.1 Alloy 5 1300 10 ^-4 278 15.2 1300 10 ^-3 250 0.1

Example 2

Again, molybdenum-silicon-boron-niobium alloys having the compositions shown in Table 1 were used. The materials according to the invention were filled into a titanium can after the mechanical alloying, which took place in a 250 l attritor under hydrogen, sealed in a vacuum-tight manner and at 1400 ° C. and 200 MPa hot isostatically compacted. The density was> 99% of the theoretical density.
Alloys 4 and 5 were prepared according to Example 1. Semifinished product thus produced was subjected to a heat treatment under vacuum. The temperature was 1.700 ° C with a holding time of 5 hours. Tensile samples were made by eroding and turning. The tensile tests were carried out at a constant strain rate of 10 ^-4 s ^-1 at three different temperatures. The results are shown in Table 3. Specifically Alloy 3 shows a significantly improved heat resistance. <b> Table 3: </ b> Results of tensile tests on heat-treated molybdenum-silicon-boron alloys (alloys 1 to 3 produced according to the invention, in comparison with the prior art, alloy 4) Temperature (° C) Maximum voltage (MPa) Strain (%) 1200 418 16.6 Alloy 1 1300 333 23.2 1400 120 65.1 1200 445 2.1 Alloy 2 1300 358 17.6 1400 153 27.1 1200 528 2.1 Alloy 3 1300 372 17.2 1400 161 35.1 1200 472 3.1 Alloy 4 1300 288 15.4 1400 127 23.9 1200 424 5.1 Alloy 5 1300 267 17.1 1400 108 30.3

Claims (15)

1. Process for the production of semi-finished or finished parts from a Mo alloy having intermetallic phase components, which process comprises at least the following steps:

   - mechanical alloying of a powder mixture containing at least 60 wt.% Mo, at least 0.5 wt.% Si and at least 0.2 wt.% B, wherein the powder mixture can be present in elementary, partially pre-alloyed or completely pre-alloyed form;

   - pressureless and/or pressure-assisted hot compaction at a temperature T, where 1100°C < T < 1900°C;

   - superplastic forming at a forming temperature T, where 1000°C < T < 1600°C, at a forming rateε̇ of 1 x 10^-6 s^-1 < ε̇ < 10⁰ s^-1;

   - heat treatment at a temperature T, where 1400°C < T < 1900°C.
2. Process according to claim 1, characterised in that the Mo alloy contains from 2 to 4 wt.% Si and from 0.5 to 3 wt.% B.
3. Process according to any one of the preceding claims, characterised in that the Mo alloy contains from 0.5 to 30 wt.% Nb and/or Ta.
4. Process according to any one of the preceding claims, characterised in that the Mo alloy contains one or more oxides or mixed oxides having a vapour pressure at 1500°C of < 5 x 10^-2 bar.
5. Process according to any one of the preceding claims, characterised in that the Mo alloy contains at least one oxide or mixed oxide from the group of the metals Y, lanthanides, Zr, Hf, Ti, Al, Ca, Mg and Sr.
6. Process according to any one of the preceding claims, characterised in that the Mo alloy contains from 0.001 to 5 wt.% of one or more metals from the group Re, Ti, Zr, Hf, V, Ni, Co and Al.
7. Process according to any one of the preceding claims, characterised in that the mechanical alloying is carried out in an attritor, a fallirig-ball mill or a vibratory mill with process times of from 0.5 to 48 hours.
8. Process according to claim 7, characterised in that the mechanical alloying is carried out under hydrogen.
9. Process according to any one of the preceding claims, characterised in that the mechanically alloyed powder is subjected to cold compaction before the hot compaction.
10. Process according to any one of the preceding claims, characterised in that the hot compaction is pressure-assisted and is carried out at a temperature T, where 1200°C < T < 1600°C.
11. Process according to claim 10, characterised in that the hot compaction is carried out by hot isostatic pressing, sintering HIP or by powder extrusion.
12. Process according to any one of claims 1 to 9, characterised in that the hot compaction is pressureless and is carried out at a temperature T, where 1600°C < T < 1900°C.
13. Process according to any one of the preceding claims, characterised in that the superplastic forming is carried out at a forming rate ε̇ of 1 x 10^-4 s^-1 < ε̇ < 1 x 10^-2 s^-1.
14. Process according to any one of the preceding claims, characterised in that the superplastic forming is carried out by rolling or pressing.
15. Process according to any one of the preceding claims, characterised in that the heat treatment is carried out at a temperature T, where T 1600°C < T < 1900°C, in a reducing atmosphere or in vacuo.