Technical field

The invention relates to a method of consolidating oxide dispersion strengthened powders by volumetric forming, particularly rotary swaging, the presented conditions and method of preparation provide alloys highly mechanically resistant at very high temperatures.

Background of the invention

Powder metallurgy is an area dealing with methods of consolidating powder mixtures based on metals, non-metals, oxides, and carbides, or their mixtures. An advantage of these methods is their relatively high production accuracy and the possibility of creating mixtures that are not possible to be created by conventional casting. Compared to other methods of metal processing and production, the resulting products usually achieve lower mechanical strength.

One of the widespread methods of powder metallurgy is the so-called swaging used e.g. for reducing the diameter of metal cylinders, where they are forged by mechanical hammers along their entire circumference at the same time. In powder metallurgy, this method is applied by e.g. the document GB 981 ,065 A. The document describes a method of producing metal pipes or rods based on zirconium, niobium, or their alloys. The alloy powder is placed in a metal container, heated to a temperature of approximately 1000 °C, and then subjected to swaging. Then the metal container is removed and the final product can be used. However, the presented method is not applicable universally to all types of metal powders, as it is always necessary to choose a suitable set of parameters, which may differ even by adding a fraction of an additive to the powder mixture.

Furthermore, a similar method is rotary swaging described e.g. in the document EP 0 238 186 B1. Powder materials based on aluminum, aluminum compounds, copper, iron, steel, or bronze with grains up to 50 pm in size are placed in a working container and tamped to obtain a mixture of the highest possible density. Subsequently, the material is subjected to rotary swaging, where it is acted upon by a hammer both rotating around its own axis and further performing a processional movement around the axis of the container in which the consolidated powder is closed. However, the method presented in this document does not allow creating long cylindrical products based on metal powders.

Another method used for forming carbide-based powders is described in the document EP 1 231 014 B1. The method describes creation of a powder mixture of carbide and metal additives, a binder, and other additives affecting the properties of the resulting material. The powder is placed in a metal container, which is then subjected to isostatic pressure such that the resulting product acquires a substantially higher density than the initial powder.

Current research in the field of powder metallurgy is aimed at developing advanced materials that show high oxidation and creep resistance, especially at very high temperatures. Nickel-based compounds can be used at temperatures of up to approximately 1100 °C, CDS (Oxide Dispersion Strengthened) alloys at temperatures of up to approximately 1300 °C, and tungsten alloys at temperatures of up to 1500 °C. However, the preparation of these materials is relatively demanding, as it is necessary to choose suitable process parameters, e.g. a suitable fraction of aluminum in the powder mixture significantly affects the oxidation resistance of the resulting material, and on the contrary, a too high fraction of it results in a change in the internal structure of the material, i.e. different mechanical properties and sometimes lower ductility. In the case of CDS alloys, an iron-based crystal lattice allows sufficient aluminum enrichment, which guarantees excellent oxidation resistance of the material.

ODS-based alloys are usually relatively costly in terms of low-volume and high- volume production due to the relatively complex technological process of obtaining the resulting material, which leads to a high price of the final products. of the Invention

The above shortcomings are eliminated to a substantial extent by a method of consolidating powders by volumetric forming. The metal powder mixture is composed of a metal matrix and at least one type of a hardening metal oxide. The powder is closed in a deformable and airtight working container, from which air is subsequently pumped out and a vacuum is created. The volumetric forming takes places at temperatures of 700- 1300 °C. An advantage of the present method is a high fraction of the hardening oxide in the resulting material, which is 1-10 %.

Volumetric forming is intended to mean rotary swaging or rolling, particularly biaxial rolling. The volumetric forming takes place preferably at temperatures of 700-1000 °C.

The consolidated powder comprises atoms of iron and aluminum or other metals and, furthermore, a hardening oxide, which is preferably yttrium oxide. Preferably, the metal powder further comprises at least one alloying element, e.g. chromium or manganese, or another additional material influencing the resulting properties of the obtained consolidated body. The presence of the hardening oxide ensures a high hardness of the obtained material, which, after the volumetric forming, is usually annealed over a period of 0.5-24 hours at a temperature of 1000-1400 °C. By annealing, secondary recrystallization of the obtained material is ensured, creating elongated grains along the rotary swaging axis, thereby improving the mechanical properties of the material at very high temperatures and eliminating local defects in the crystal structure. To ensure optimal processing, the volumetric forming is performed in three steps.

Before the consolidation itself, the powder is prepared by mechanical alloying.

The consolidated material obtained by the above-described method shows a 30 % higher creep strength than ODS-based materials obtained by similar methods, particularly at application temperatures of 1100-1300 °C. The above consolidated material comprises an iron- and aluminum-based metal matrix and a hardening oxide, which is, for example, yttrium oxide. Furthermore, the consolidated material preferably comprises at least one alloying element, e.g. manganese or chromium. The obtained material has a creep strength of 70-120 MPa and a ductility of 1 -10 % at a temperature of 1100 °C. Preferably, the fraction of the hardening oxide in the obtained material is 4-6 %, i.e. about 10 times higher than in a material obtained in a similar manner. The presence of the high fraction of aluminum in the material provides a high oxidation resistance thanks to the presence of a thin alumina surface film on the material due to reaction with air.

Description of Drawings

A summary of the invention is further clarified using exemplary embodiments thereof, which are described with reference to the accompanying drawings, in which:

Fig. 1 - schematically shows the presented method of consolidating powders by volumetric forming;

Fig. 2 - schematically shows a rotary swaging device;

Fig. 3 - schematically shows a rolling device;

Fig. 4 - is an image of microstructure of a sample obtained by the presented rolling method; the picture was taken using a scanning electron microscope;

Fig. 5 - is an image of microstructure of a sample obtained by the presented rotary swaging method; the picture was taken using a scanning electron microscope;

Fig. 6 - shows a graphical comparison of the strength of samples obtained by the presented method during rolling, rotary swaging, and by the commercially produced ODS alloy MA 956.

Exemplary Embodiments of the Invention

The invention will be further described using exemplary embodiments with reference to the respective drawings, which, however, have no limiting effect from the point of view of the scope of protection.

The presented method of consolidating powders schematically shown in Fig. 1 uses powders with an iron-, aluminum-, or titan-based metal matrix and further comprising a hardening oxide. This powder may be, for example, Fe-10AI-3Y2O3-1 Ti (in wt. %). The powder to be consolidated is filled in a deformable and airtight working container and tamped. Subsequently, the pressure in the working container is decreased to a value of 1-10 Pa, and the container is closed and sealed. The closing and sealing may be performed by, for example, welding. In the subsequent step, the powder closed and sealed in the working container is subjected to volumetric forming, which takes place within a temperature range of 800-1300 °C, more preferably the temperature interval is limited to 800-1000 °C, wherein the volumetric forming takes place at a temperature of, for example, 900 °C. Volumetric forming is intended to mean particularly rotary swaging or rolling, particularly biaxial rolling. During the rotary swaging, the force of pistons 2 acts on the processed body 1 along the entire circumference thereof at the same time, as shown in Fig. 2, simultaneously, it is rotated during the rotary swaging either by a device performing the rotary swaging or by the processed body 1, a workpiece, or a working container, which, in this exemplary embodiment, is of a cylindrical shape, wherein the rotary swaging is performed in a direction perpendicular to the central axis of the cylinder. After the rotary swaging, the volume of the processed material is decreased by the volume of the space between the grains of the consolidated powder. Due to the action of the rotary swaging, the diameter of the processed cylinder, or working container, is decreased. After the rotary swaging, the diameter of the working container is 1.5x-5x smaller than before the rotary swaging. During rolling, the working container with the powder to be consolidated is subjected to the pressure of cylinders acting from two opposite sides, see Fig. 3. Rolling corresponds to forming the processed body 1 that passes through the space between two rotating cylinders 3, where the gap between the cylinders 3 is smaller than the height of the input processed body 1. For example, the consolidated material can be processed by biaxial rolling, where the axis of action of the forming pressure remains the same but the orientation of the processed material changes, usually by 90°. Rolling therefore leads to a 1.5x-10x decrease of the diameter of the working container in the case of the cylindrical container, or the height of the parallelepiped in the case of another shape of the working container. Preferably, the volumetric forming of the consolidated powder in the working container is performed at least three times. After the volumetric forming, the obtained material is annealed over a period of 0.5-24 hours at a temperature of 1000-1400 °C. In an exemplary embodiment, the annealing takes place over a period of 4 hours at a temperature of 1200 °C. Thanks to this, secondary recrystallization of the grains comprising the obtained material is ensured. In an exemplary embodiment, the method of consolidating powders by volumetric forming in question is preceded by a step of preparing the powder to be consolidated. A highly homogeneous powder comprising a metal matrix and nanoparticles of yttrium oxide Y2O3 is created by mechanical alloying. The metal matrix can also comprise oxides of other metals or other alloying elements, such as chromium and manganese. Another step in the preparation of the powders can be, for example, thermomechanical processing for the purpose of reducing the porosity of the materials and increasing their strength. Thermomechanical processing is intended to mean e.g. hot biaxial rolling at an increased temperature (hot cross rolling). After the initial processing, the powder shows a high degree of homogeneity, where oxygen originating from the yttrium oxide and oxidized metal matrices is completely dissolved in the powder mixture and trapped at defects of crystalline powder base, particularly dislocations and vacancies. A powder prepared in this manner is subsequently closed in the metal working container and subjected to consolidation at an increased temperature. This ensures elimination of porosity and initialization of dynamic recrystallization. The dynamic recrystallization then results in the formation of ultra-fine grained microstructures. The ultra-fine grained microstructures are stabilized by the presence of nano-oxides of a size in the order of 5 nm at temperatures within a range of up to 1000-1100 °C. If the critical temperature of 1200 °C is exceeded over a period of several hours, for example, within an interval of 0.5-24 hours, the secondary recrystallization occurs. As a result, the final microstructure comprises coarse grains of a size in the order of 100 pm strengthened by homogeneous dispersion of yttrium nano-oxides of a size of approximately 20 nm.

In an exemplary embodiment of the invention, the powder Fe-10AI-3Y2O3-1 Ti made of powders of iron, aluminum, yttrium oxide, and titanium is used for the presented method, where the purity of the powders is at least 99.9 %. The individual powders comprising the resulting metal powder are closed in an airtight ball mill that comprises, for example, bearing balls of a size of 40 mm made of low-alloy steel. In the ball mill, mechanical alloying of the powders is subsequently performed by rotation thereof around the horizontal axis. This process is performed, for example, in a vacuum at a pressure of 1-10 Pa over a period of 2-3 weeks. After the mechanical alloying, the properties of the powder are saturated, wherein the powder particles comprise a significant number of defects, e.g. vacancies and dislocations. The material obtained by the above-described method by rolling and secondary recrystallization shows, at a temperature of 1100 °C and strain rate of 10’ ⁶ s’ ¹ , a strength of 75 MPa with a deviation in the order of 5 % and ductility within the value interval of 1 - 10 %, typically 1-2 %. The microstructure of the rolled material shows a high amount of ultra-fine grains with an approximately equiaxial shape, in Fig. 4, an image of the resulting material obtained by back scattered electron analysis in a scanning electron microscope is shown.

The material obtained by the above-described method by rotary swaging and secondary recrystallization shows, at a temperature of 1100 °C and strain rate of 10’ ⁶ s’ ¹ , a strength of 1 15 MPa with a deviation in the order of 5 % and ductility within the value interval of 1-10 %, typically 1-3 %. The microstructure of the material obtained in this manner after the rotary swaging shows, compared to rolling, a higher grain size and elongated grains. In Fig. 5, an image of the resulting material obtained by back scattered electron analysis in a scanning electron microscope is shown.

Fig. 6 shows a graphical comparison of the creep strength of the samples obtained by the above-described methods at a temperature of 1100 °C. Specimen 1 represents a sample obtained by the above-described method when using rolling, Specimen 2 represents a sample obtained by the above-described method when using rotary swaging. MA 956 is a commercially available ODS alloy. The horizontal axis contains the values of the applied stress and the vertical axis shows the time to rupture of the sample. A sample obtained by rotary swaging then shows a much higher strength compared to a commercial ODS alloy or rolled material. Creep strength is a property of a sample tested in the temperature range of use of the given material. In the case of the samples obtained by the above-described method, a creep strength of 70-100 MPa is achieved at the applied temperature of 1100 °C and required time to rupture of 1000.

Industrial Applicability

The presented methods of consolidating powders by volumetric forming produces approximately 30% stronger materials compared to conventional methods of consolidating ODS-based powders. These materials become available for use at temperatures of 1100-1300 °C.

List of Reference Numerals

1 - Processed body

2- Piston 3- Cylinder