New ODS steel structure for extreme environments using the ultrasonic dispersion of nano-oxides in combination with SLM and PPS

Project financed within the framework of the LAP cooperation under the
Weave initiative OPUS-22 (LAP)

Three years project started 1.1.2023 – 31.12.2025, Slovenia, Poland and Cheh republic
 

Acronym: NanoPowder
Number of the project: N2-0276
 

prof. dr hab. inż. Jarosław Mizera
WARSAW UNIVERSITY OF TECHNOLOGY
dr Jiri Kubasek
UNIVERSITY OF CHEMISTRY AND TECHNOLOGY, PRAGUE
prof. dr Matjaž Godec
INSTITUTE OF METALS AND TECHNOLOGY
 

Abstract

Oxide dispersion-strengthened (ODS) steels are known because of their superior mechanical properties at high temperatures and increased resistance to neutron irradiation embrittlement which favours them for future applications as a fuel cladding for the next generation nuclear systems and blanket materials for fusion power systems. The scientific goal of the project is to examine whether an ultrasonically gas-atomised precursor steel powder or a steel powder with smart surface oxidation in combination with one of two new consolidation techniques – selective laser melting (SLM) and pulse plasma sintering (PPS) – can impart ODS steel with better performance in extremely harsh, high-temperature conditions. The results will be benchmarked against conventionally produced, mechanically alloyed powder that is consolidated using spark-plasma sintering (SPS).

The current best-performing ODS steels have tensile strengths up to almost 800 MPa at 600 °C, compared to over 1200 MPa at room temperature. The rapid fall of in performance as things get hot can be traced to the uneven distribution and clustering of the dispersed oxides in the microstructure of the consolidated steel component. What our experience with ODS steels teaches us, is that the final properties of the consolidated component are closely connected with the precursor powder and homogenously distributed nanoparticles. It is for this reason that we are focusing our attention on new ways to produce the powders using technologies not available until now. Then, after the powder is consolidated by SPS and PPS we will relate the microstructure, corrosion, and mechanical properties back to the characteristics of the powders. As a proof of concept, we have selected austenitic 316L stainless steel, which has good ductility, workability, and mechanical integrity at high temperature. Y2O3 has been chosen as the nano material to add to the matrix, as it has a higher ultimate tensile strength than other powders and improves oxidation resistance. As the project progresses, and the strengthening mechanisms associated with the nano oxide dispersion will be known, TiB2 nanoparticles (with a higher melting point) will also be employed.

Within the project realization we would like to test the following hypotheses:

1. The application of ultrasonics during the process of gas atomisation will lead to increased dispersion and reduced levels of clustering for Y2O3 and TiB2 that are co-melted with steel in a gas-atomisation process to provide a powder precursor.

2. The surfaces of the gas atomised powders can be partially oxidised in such a way that subsequent consolidation leads to a microstructure with highly dispersed precipitates.

3. The re-melting and/or partial remelting of the powder precursor during consolidation by SLM and PPS leads to microstructural changes compared to the precursor and a re-arrangement of the precipitates and changes to the pattern of clustering.

4. The combination of ultrasonically assisted gas atomisation and SLM and/or PPS can produce ODS steel that performs better in extreme conditions compared to conventionally produced components.

To approach this, the proposed project is divided into 5 working packages, namely: WP1 powder preparation, WP2 component consolidation, WP3 Characterization of the powders and the mechanical and corrosion properties of the components, WP4 Management of the project and WP5 Dissemination, communication, and exploitation. The proposed research project is extremely relevant from the basic research point of view, as it will bring about a much better fundamental understanding of the parameters influencing the manufacture of ODS steels using powder metallurgy routes. SLM is a process that will allow us to re-melt the feedstock powders, build a geometrical structure, during which the rapid melting and solidification will ensure the precipitates will appear. We believe that a very homogeneous distribution of oxides will result from the local mixing of the melt and the subsequent rapid solidification. Moreover, we will learn a great deal more about manufacturing uniform microstructures with SPS and PPS.

1) Scientific goal of the project

The scientific goal of the project is to examine whether an ultrasonically gas-atomised precursor steel powder or a steel powder with smart surface oxidation in combination with one of two new consolidation techniques – selective laser melting (SLM) and pulse plasma sintering (PPS) – can impart oxide-dispersionstrengthened (ODS) steel with better performance in extremely harsh, high-temperature conditions. Our aim is to disperse precipitates of Y2O3 and TiB2 much more evenly in the precursor powders by tuning the ultrasonic parameters of the gas-atomisation process to create the maximum extent of dispersion prior to consolidation. The powders will then be either selectively laser melted or pulse-plasma sintered, before being assessed microstructurally and for their mechanical resilience and resistance to corrosion at very high temperatures. The results will be benchmarked against conventionally produced, mechanically alloyed powder that is consolidated using spark-plasma sintering (SPS).

The project is set up to answer 6 important research questions, listed below:

• To what extent is the application of ultrasonics during the gas atomisation of precursor steel powders containing Y2O3 able to reduce the agglomeration and cohesion of the oxides, leading to greater dispersion of precipitates and reduced levels of precipitate clustering?

• Can the techniques of SLM and PPS maintain the enhanced dispersion during consolidation of the powder and restrict the tendency for the precipitates to migrate and coalesce under the conditions of high temperatures, high thermal gradients and, in the case of PPS, applied pressures.

• To what degree does the re-melting of the gas-atomised powder during the SLM procedure lead to a redistribution of the precipitates in comparison with the partial re-melting of the powder during pulse plasma sintering?

• Is it possible to partially oxidise the surfaces of gas-atomised powder (smart surface oxidation) in such a way that the surface oxides become highly dispersed during subsequent consolidation with SLM and PPS.

• How does the enhanced dispersion and the reduced cluster size impact on the elevated-temperature performance of consolidated components produced by SLM and PPS?

• What are the ultimate limits of dispersion using the combination of ultrasonics during the gas-atomisation procedure, smart surface oxidation, and SLM or PPS?

The project will test 4 hypotheses, listed below:

• The application of ultrasonics during the process of gas atomisation will lead to increased dispersion and reduced levels of clustering for Y2O3 and TiB2 that are co-melted with steel in a gas-atomisation process to provide a powder precursor.

• The surfaces of the gas atomised powders can be partially oxidised in such a way that subsequent

consolidation leads to a microstructure with highly dispersed precipitates.

• The re-melting and/or partial remelting of the powder precursor during consolidation by SLM and PPS leads to microstructural changes compared to the precursor and a re-arrangement of the precipitates and changes to the pattern of clustering.

• The combination of ultrasonically assisted gas atomisation and SLM and/or PPS can produce ODS steel that performs better in extreme conditions compared to conventionally produced components.

2) Significance of the project

State of the art: Powder metallurgy techniques including mechanical alloying followed by hot isostatic pressing, extrusion and rolling are used to form oxide dispersion-strengthened (ODS) steels materials [1–3]. During the mechanical alloying, Y2O3 and Ti are added as dispersed particles and dissolved into the matrix. Subsequent consolidation or heat treatment leads to the formation of complex nanoclusters of Y-Ti-O with a sizes of 2–100 nm. Various powders of austenitic steels including 304, 310, 316 and 316L have been successfully prepared in this way [3,4]. Besides Y-Ti-O, other nanoclusters can be formed. These include Ti-N-, Y-Al-O-, and Y-Si-Ti-Obased phases, where the additional elements are derived from steel (Al, Si, N) or even Y-Zr-O- and Y-Hf-O-based phases, with Hf or Zr added during mechanical alloying [5–7]. Unfortunately, the production of austenitic stainless ODS steel by the presented processes is complicated due to the higher ductility of austenite [8,9]. The production of ODS steels includes long milling times (10s of hours) and demanding compaction techniques like HIP or rolling and extrusion at above 1000°C. As a result, alternatives have been searched for. Techniques like SPS, field-assisted sintering technique (FAST) and additive manufacturing (AM) were investigated [10–14]. It was shown that SPS and FAST can form extremely fine-grained materials with almost theoretical density in tens of minutes [9]. Justification for tackling a specific scientific problem: Many applications are limited by the properties of materials – solar cells, electric vehicles and wind turbines are popular examples – but metallic materials that can remain intact and ensure safety in extreme conditions (particularly high temperatures) are undoubtably an example of where effort should be focused, since breakthroughs in this area will have wide-ranging benefits. We have chosen to tackle this problem because we understand its importance, and because we have the expertise and ideas that can make it happen. The best route is clearly a microstructure that contains a high density of small oxide precipitates (the best candidates being Y2O3 and similar) dispersed in an austenitic matrix, and this is what we plan to achieve. Justification for the pioneering nature of the project: The pioneering nature of the project is to alter powder preparation using various atomization processes, e.g., atomization with or without ultrasounds and adding nano particles during melting, as well as partial oxidation of the atomised powder. The project is strong on innovation, with new technologies, new methods and new ideas for how to achieve specific types of microstructures. The project is also interdisciplinary, bringing together researchers from the fields of metallurgy, physics, and mechanical engineering from three different European countries. The impact of the project results for the development of the research field and scientific discipline:

• The project’s results will lead to breakthroughs in mechanical and corrosion properties by creating a new microstructure for ODS steels prepared by SLM and PPS. The results will provide us with new knowledge about how the microstructure forms in ODS steels.

• The project is about powder preparation and nano oxide precipitations, which is related to new  parameters and conditions of manufacturing, and themicrostructure-dependent properties of ODS steels. The results of the project will enhance our understanding of the complexity of the microstructure-dependent mechanical and corrosion properties of ODS steels and lead to them exhibiting better performance under extreme conditions.

• The project will also impact on educational programmes at the universities of the collaborating partners, where some new courses on steels, powder metallurgy and additive manufacturing will be delivered.

• The results of the proposed project will be disseminated at international conferences which will promote the project’s results and heighten awareness of the research programmes.

3) Concept and work plan

General work plan:

The Project has five WPs and they are schematically shown in Fig. 1. Namely: WP1 Powder preparation, WP2 Component consolidation, WP3 Characterization of the powders and the mechanical and corrosion properties of the components, WP4 Management of the Project and WP5 Dissemination, communication, and exploitation.

Figure 1. General work plan of the project

Specific research goals:

1. To control the microstructure formation and oxide precipitation during consolidation using SLM and PPS, and to benchmark them against Spark-plasma sintering (SPS)

2. To determine how the concentration and dispersion of nanoparticles (Y2O3 and TiB2) will affect the microstructure formation during SLM, SPS and PPS, and learn about the main mechanism of the nano oxides’ formation.

3. To discover which microstructural factors affect the build up of dislocations, and how this impacts on the strengths of the steels manufactured using SLM, SPS and PPS.

4. To reveal the strengthening mechanisms and thermal stability of the material under extreme conditions, particularly high temperatures.

Figure 2. Graphical abstract of the project: WP1, three powder-fabrication routes; WP2, three consolidation steps; WP3, three test regimes.

The project is set up to answer 6 important research questions, listed below:

• To what extent is the application of ultrasonics during the gas atomisation of precursor steel powders containing Y2O3 able to reduce the agglomeration and cohesion of the oxides, leading to greater dispersion of precipitates and reduced levels of precipitate clustering?

• Can the techniques of SLM and PPS maintain the enhanced dispersion during consolidation of the powder and restrict the tendency for the precipitates to migrate and coalesce under the conditions of high temperatures, high thermal gradients and, in the case of PPS, applied pressures.

• To what degree does the re-melting of the gas-atomised powder during the SLM procedure lead to a redistribution of the precipitates in comparison with the partial re-melting of the powder during pulse plasma sintering?

• Is it possible to partially oxidise the surfaces of gas-atomised powder (smart surface oxidation) in such a

way that the surface oxides become highly dispersed during subsequent consolidation with SLM and PPS.

• How does the enhanced dispersion and the reduced cluster size impact on the elevated-temperature performance of consolidated components produced by SLM and PPS?

• What are the ultimate limits of dispersion using the combination of ultrasonics during the gas-atomisation procedure, smart surface oxidation, and SLM or PPS? The project will test 4 hypotheses, listed below:

• The application of ultrasonics during the process of gas atomisation will lead to increased dispersion and reduced levels of clustering for Y2O3 and TiB2 that are co-melted with steel in a gas-atomisation process to provide a powder precursor.

• The surfaces of the gas atomised powders can be partially oxidised in such a way that subsequent consolidation leads to a microstructure with highly dispersed precipitates.

• The re-melting and/or partial remelting of the powder precursor during consolidation by SLM and PPS leads to microstructural changes compared to the precursor and a re-arrangement of the precipitates and changes to the pattern of clustering.

• The combination of ultrasonically assisted gas atomisation and SLM and/or PPS can produce ODS steel that performs better in extreme conditions compared to conventionally produced components