WO1995033079A1

WO1995033079A1 - Method of producing intermetallic master alloys

Info

Publication number: WO1995033079A1
Application number: PCT/DE1995/000683
Authority: WO
Inventors: Roland Scholl; Bernd Kieback
Original assignee: Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V.
Priority date: 1994-05-27
Filing date: 1995-05-23
Publication date: 1995-12-07
Also published as: DE4418598A1; DE4418598C2

Abstract

Described is method of manufacturing a high-disperse powder blend by reducing in size, by grinding, at least one element powder and an alloy powder in an oxygen-free atmosphere. The powder blend is intended, in particular, for use as the starting material in the production of articles and layers of difficult to fuse and difficult to sinter materials with closed-pore intermetallic phases. The method is characterized in that the size-reduction/grinding process is discontinued before the formation of the intermetallic phase.

Description

FORMATION OF INTERMETALLIC-LIKE ALLOYS

Process for the production of a highly disperse powder mixture and its use for the production of components and layers made of difficult to sinter materials with intermetallic phases

The invention relates to a method for producing a material from intermetallic phases that are difficult to melt and sinter in the process of comminuting element powders by grinding according to the preamble of the main claim.

Intermetallic phases here include both the phases usually formed from two or more metals, such as TiAl, NiAl, f ^ AI, FeTi, NiTi, MoSi2, TiSi2, as well as the metallic hard materials, e.g. T considered B2, WC, NbC.

Structural and functional materials that are difficult to melt and sinter with internal metallic phases and components made therefrom are used in particular for high-temperature processes in energy generation and energy conversion systems in oxidizing and aggressive media and in the chemical industry. These materials must in particular have a high resistance to temperature changes, corrosion, high

Have fracture toughness, possibly low density and a high specific strength and resistance to high temperature creep. The starting materials for such components are produced in a process of comminution, preferably in a grinding process. The individual elements suitable for the materials are comminuted simultaneously or at different times. Such a method is known, for example, from EP 0 203 311. The starting elements are then ground to an amorphous state and then components are produced from these starting powders by sintering below the recrystallization temperature. However, only components without intermetallic phases can be produced using this method. Components from starting powders that have been crushed in the grinding process and that have intermetallic phases can only be produced by high-temperature sintering and high-pressure sintering. Such a method is disclosed, for example, in EP 0 209 179. Thereafter, a starting powder containing a metal component and a non-metal component is comminuted in a drum mill with high energy input, the powder being mechanically alloyed in the grinding process; This powder can then be used to produce components with intermetallic phases using one of the customary processes, for example hot pressing, hot isostatic pressing, metal powder injection molding or sintering under pressure and temperature.

EP 0 574 440 describes a further method for producing starting materials for components made of materials which are difficult to melt and sinter. The starting components or the individual elements are then ground in a ball mill under a nitrogen or nitrogen-producing atmosphere. This method is preferably suitable for metal nitride composites. Components made of these materials are processed into a solid body by injection molding and heat treatment.

Another method for producing an intermetallic compound is known from DE 40 01 799. The individual elements are then mechanically alloyed in an inert atmosphere, the resulting powder material is then heated and compressed, and the components to be produced from it are produced by hot pressing at a temperature above the phase formation temperature.

Another process for the powder metallurgical production of molded parts from intermetallic compounds is known from DE 39 35 955. Here too, due to the nature of the starting powder, the components can only be produced in one process of the isostatic reaction press or by means of a vacuum. All of the above methods have in common that they are energy-intensive due to the need to use high pressures and high temperatures, since the starting powders for the production of components or material layers with intermetallic phases can only be processed at high temperatures and under high pressures.

The object of the present invention is to provide a method in which a material for the production of components and layers from materials which are difficult to melt and sinter from intermetallic phases is produced and which can be carried out at low temperatures and low pressures.

This object is achieved by the method specified in claim 1. Subclaims indicate advantageous developments of the method according to the invention.

The process according to the invention provides that various element powders are mixed and ground in a grinding process by grinding, but that the grinding process is terminated before the intermetallic phase is formed. The starting element powders are ground in a high-energy ball mill for several hours in an oxygen-free atmosphere, which activates them mechanically. In contrast to the prior art, the process of mechanical alloying is not aimed at phase formation, the milling process is terminated shortly before the formation of the intermetallic phase. The grinding process introduces mechanical energy into the starting powder, as a result of which comminution and homogenization processes take place, which result in the element powders used being homogeneously distributed in the submicrometer range. Typical ranges of homogeneity of the insert components from 10 to 1000 nm occur. As a result of the grinding process being interrupted in good time before the phase formation, agglomerates are obtained which have a homogeneous distribution of the starting components. Compared to the distance between the inner phase boundaries, these have a particle size that is several orders of magnitude larger. Accordingly, the surface of the agglomerates is small compared to the expansion of "internal" phase boundaries. Despite the high internal energy of the highly disperse powder (lattice defects, dislocation densities, Surface energy at the phase boundaries, chemical pontentials), therefore, there is no increased reaction with oxygen. The powders can therefore be processed in air. The powder mixture has a high degree of dispersity. The grain boundaries represent approx. 20 - 50% of the powder mixture.

The powders produced by the grinding according to the invention are further processed in an unpressurized sintering process. In order to determine the required duration of the grinding process, calorimetric measurements are carried out, the temperature range of the phase formation being verified by taking a sample and a calorimetic analysis thereof, and the grinding process being stopped in good time before the phase formation. However, they could also be processed further in any powder-metallurgical process, but the phase formation process which was terminated in the grinding process is made up for in the subsequent sintering process. The recrystallization processes in the metallic component associated with the thermal process and above all the process of phase formation below the melting temperature of the lower melting phase surprisingly lead to an extreme increase in sintering activity (FIG. 1). As a result, molded parts can be produced in the sintering process at lower temperatures than in conventional sintering processes and under lower pressures or without pressure. The fact that no phase formation takes place in the production of the powder mixtures means that the energy expenditure required is lower than in the conventional processes. In addition, the amount of energy required in the production of starting materials or components and layers from these powders is further reduced by the fact that the phase formation taking place in the sintering process takes place below the melting temperature. In the method according to the invention, the production of components or layers with closed porosity is also possible. It is also possible to manufacture complex components without a high expenditure of energy. The fact that the phase formation takes place far below the melting temperature and is coupled with the sintering process means that significantly lower temperatures are required compared to conventional powder metallurgical processes. Furthermore, the use of the powder produced in the process according to the invention and the production of components in the pressureless sintering process give fine-grained materials which are obtained by suitable Reinforcement components have improved mechanical properties in the relevant temperature range from room temperature to the highest application temperatures. The powders produced in the process according to the invention can not only be used in the process of pressure-free sintering, they can also be processed by a capsule-free sintering hot isostatic process; the manufactured parts reach a high density in every process. The density is more than 96% of the theoretical density, preferably 98%. The crystallite size can be set about an order of magnitude smaller than is possible with conventional methods. The in-situ formation of disperse reinforcement components improves the high-temperature behavior of the materials, in particular the resistance to high-temperature creep and the corrosion behavior.

In particular, the method according to the invention is suitable for producing the powder mixture from two or more of the following elements: Mo, Ti, W, Cr, Zr, V, Y, Fe, Ni, Nb, Re, Hf, Ta, Nd, Cn, Co, Sm, AI, Si, B and C insofar as they form intermetallic phases or hard materials. In addition, powder mixtures can be produced by the process according to the invention, a pre-formed reinforcement phase being additionally introduced.

An advantageous embodiment of the method provides that a liquid is added to the regrind 0.1-30% by volume based on the volume of the solid feed materials during the fine comminution process. This improves the comminution behavior and / or provides the active elements necessary for the phase formation of the main, secondary or reinforcement phases. The starting materials can also be pre-formed according to an advantageous embodiment of the method

Reinforcing components or special phases not to be formed are added.

In one embodiment, the process according to the invention provides that the formation of dispersoids (carbides, borides, oxides, nitrides) is also possible in the grinding process, or that these are added to the powder mixture in a preformed form. The use of the powder mixtures according to the invention in a process of pressure-free sintering also leads to sintered bodies with increased density. Components with, for example, 10% of a powder obtained in the process according to the invention and 90% of a non-sinter-active powder can also be produced.

The powders produced according to the invention show a high dispersity in the initial state or during processing, small distances between the phase-forming partners, gradients of the chemical potentials, concentration gradients and increased vacancy densities, caused by recrystallization and phase-forming processes which, in their entirety, lead to sinter-active states with increased geometric and structural Lead activity.

Using the powder mixtures according to the invention, largely dense, ie closed-pore components with fine-crystalline structures in the range from 0.5 to 10 μm are produced, even in the case of high-melting phases, at temperatures of 0.5-0.8 ^* Ts _C melt. Thermal post-treatment allows them to be brought into the coarser structure that is favorable for the application.

In an embodiment, the invention provides that in the case of pressure-free sintering and in the case of an existing residual porosity, capsule-free post-compression can be carried out by hot isostatic pressing.

The powder mixtures produced in the process according to the invention can also be homogeneously added to the conventionally produced, difficult to sinter powders before shaping as a sinter-promoting additive in order to create the pore seal at these at low temperature and thus the prerequisite for capsule-free hot isostatic pressing. The powder mixtures according to the invention are also suitable as filler materials in the powder-metallurgical production of graded materials, in which materials with different sintering behavior are combined. In an advantageous embodiment, the invention provides that the powders produced are used as starting materials for sprayed layers on components and functional materials with special functional properties.

When the powders according to the invention are sintered without pressure, it is advantageous if a partial pressure of one or more elements is set before the phase formation in the case of elements with increased vapor pressure.

The method according to the invention is explained in more detail using an exemplary embodiment.

Example 1: Molybdenum, silicon and carbon (acetylene black) are placed in a planetary ball mill in such a ratio that after complete implementation of the elements, a MoSi2 matrix phase (85% by volume) and a reinforcing component made of SiC (15% by volume) can arise. For this purpose, the necessary powder quantities are ground in a planetary ball mill with a steel grinding container and steel grinding balls (volume ratio of balls to regrind of 10: 1) at 320 rpm for 8 hours in an oxygen-free atmosphere. After this grinding period, no suicides or carbides are detectable by X-ray. The powder mixture obtained is compacted uniaxially up to a compact density of 2.5 g / cm ³ and sintered in vacuo (10 ^"2 mbar) at 1440 ° C. The intensive shrinkage (Fig. 1: Dilatometer and DSC measurement) a pressed MoSi2 sample) preferably starts at a temperature at which the phase formation of the main phase from the elements and / or temporary intermediate phases takes place, a binary or ternary phase being formed in the process of sintering about 1000 ° C (beginning of the complete formation of the MoSi2 matrix phase), the shrinkage maximum is reached at about 1300 ° C. After sintering for one hour at 1440 ° C, a shrinkage of about 25% is achieved. The size of the crystallites is between 2 and 8 μm (FIG. 2: micrograph of a MoSi2 sample produced in the method according to the invention.) In the cut, no SiC reinforcement components are visible under the light microscope, MoSi2 is radiographic tetragonal as main phase and MoSi2 hexagonal and SiC as very weak secondary phase. Other binary suicides or ternary Mo-Si-C phases are undetectable. The Sinteφrobe shows a closed porosity.

Example 2: Titanium and silicon powders (grain size 100μm and 150μm) are filled under a high-purity argon atmosphere (99.998% Ar by volume) in an atomic ratio of 1 to 2 into the grinding container of a planetary ball mill and for 12 h at 250 rpm in one Steel grinding set, ground. The ball-to-ground ratio is 10: 1. After this grinding, only Ti and Si are found by X-ray analysis. The element powder mixture produced in this way is compressed by uniaxial pressing to a density of 2.0 g / cm ³ and sintered at 1350 ° C. for one hour under a flowing argon atmosphere. The density after sintering is 3.95 g / cm ³ (approx. 98% of the theoretical density). The structure is single-phase except for small SiO 2 inclusions. Only TiSi2 was detected by X-ray analysis.

Example 3: Titanium and silicon powders (grain size 100μm and 150μm) are filled into the grinding container of a planetary ball mill in an atomic ratio of 1 to 2 under a high-purity argon atmosphere (99.998 vol.% Ar) and 1 h, 5 h or 12 h at 320 RPM ground in a steel grinding set. The ball-regrind ratio is 10: 1. The element powder mixtures produced in this way are compressed by uniaxial pressing and sintered at 1350 ° C. for one hour under a flowing argon atmosphere. The comparison of the shrinkage behavior of the powder compacts sintered in the dilatometer is shown in FIG. 3 shown. After a short grinding time (1 h) an intensive swelling occurs, a grinding time of 5 hours leads to an optimal shrinkage behavior. Element powder mixtures ground for 12 hours already show TiSi2 phase fractions, which leads to a deterioration in the sintering behavior.

Claims

1. A process for the production of a highly disperse powder mixture in the process of comminution by grinding at least one element powder and one alloy powder in an oxygen-free atmosphere, in particular as a starting material for producing components from materials which are difficult to melt and sinter with intermetallic phases, characterized in that the comminution / The grinding process is stopped shortly before the formation of the intermetallic phase.

2. The method according to claim 1, characterized in that the time of formation of the intermetallic phase is determined by calorimetric measurements.

3. The method according to claim 1 or 2, characterized in that two or more of the elements used Mo, Ti, W, Cr, Zr, V, Y, Fe, Ni, Nb, Re, Hf, Nd, Sm, Al, Si , Ta, Cu and Co are mixed and crushed together or at different times.

4. The method according to claim 3, characterized in that the said elements one of the elements B or C, added and mixed with them and that all elements are comminuted together or at different times.

5. The method according to claim 3 or 4, characterized in that the elements are selected so that elements are at least present to form an amplification phase.

6. The method according to any one of the preceding claims, characterized in that 0.1 to 30% by volume, a liquid, based on the volume of the solid insert elements, are added to the regrind during the comminution process.

7. The method according to any one of the preceding claims, characterized in that pre-formed reinforcing components or special phases not to be formed are introduced into the grinding material

8. The method according to any one of claims 1-7, characterized in that additionally the formation of dispersoids (carbides, bonds, oxides, nitrides) takes place or these are added to the powder mixture in a pre-formed form.

9. Powder mixture produced in the process according to one of claims 1 to 8, characterized in that the lattice condition is disturbed and the sintering activity is high and that the grain boundaries have a high degree of dispersity.

10. Use of the powder mixture produced in the process according to one of claims 1 to 8 for the production of components in a sintering process, characterized in that the sintering process is carried out in an inert atmosphere at temperatures below the melting temperature of the lower melting phase and without pressure.

11. Use according to claim 10, characterized in that in addition to the main phase at least one amplification phase during the

Sintering process is formed.

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12. Use according to claim 10 or 11, characterized in that, in the case of an existing residual porosity following the pressure-less sintering, a capsule-free densification takes place by hot isostatic pressing.

13. Use of the powder mixture produced in the process according to one of claims 1 to 8 in an injection molding process for the production of highly binder-containing components of complicated shape and a subsequent sintering process.

14. Use of the powder mixture produced in the process according to one of claims 1 to 8 as a sinter-promoting additive in the sintering process of powder which is difficult to sinter, the powders being homogeneously added to the other materials before shaping.

15. Use of the powder mixture produced in the process according to one of claims 1 to 8 as an additive material in the powder metallurgical production of graded materials, in which materials with different sintering behavior are combined.

16. Use of the powder mixture produced in the process according to one of claims 1 to 8 as the starting material for sprayed layers on components and functional materials.

17. Use of the powder mixture produced in the process according to one of claims 1 to 8, characterized in that a partial pressure of individual or several elements is set during the thermal treatment, in particular during pressure-less sintering and in particular before the phase formation for elements with increased vapor pressure.

18. Component obtained by using the powder mixture according to one of claims 10 to 17, characterized in that the structures obtained are fine crystalline with a size of 0.5 to 30 microns.

19. Component obtained by using the powder mixture according to one of claims 10 to 17, characterized in that its density is more than 96% of the theoretical density, preferably 98% of the theoretical density.

20. Component obtained by using the powder mixture according to one of claims 10 to 17, characterized in that it has a closed porosity.