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Plasma-Assisted Centrifugal Atomization of Refractory Alloys and Compounds
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. Höganäs Sweden AB - Metasphere, Luleå, Sweden.
2024 (English)Doctoral thesis, comprehensive summary (Other academic)Alternative title
Plasmasmältning och centrifugalatomisering av eldfasta legeringar och föreningar (Swedish)
Abstract [en]

The development of fabrication technologies suitable for the production of fine, high-quality metallic powders of conventional and novel alloys is crucial to the development of the powder metallurgy route. The plasma-assisted centrifugal atomization process was developed by Metasphere Technology AB for the production of spherical cast tungsten carbide, and subsequently acquired by Höganäs AB. In the standard implementation of the process, feedstock material in the form of crushed powder is fed into a rotating crucible, melted by a transferred plasma arc, and atomized in the form of fine, spherical droplets. The capability to melt alloys and compounds with melting temperatures above 3,000 ᵒC, combined with the extremely rapid solidification of the ejected droplets, allows for the processing of metastable refractory alloys that cannot be obtained otherwise.

The main objectives of this work were to (1) better understand the role of the centrifugal atomization mechanism on the microstructure and the mechanical properties of the final powders, and (2) explore the capabilities of a pilot-scale plasma-assisted centrifugal atomization unit for the design and development of novel refractory alloys.

The local mechanical characterization of micron-sized powders of hard and brittle compounds is challenging. The use of three-dimensional topography images to measure the residual imprints of microindentation hardness tests has been proposed, ultimately enabling a reliable comparison among CTC powders fabricated by different methods. The microindentation hardness, the micro-pillar compressive strength, and the resistance to cyclic compressive loading of entire particles were extensively investigated in centrifugally-atomized CTC powders subjected to different heat treatments. 

Suitable processing routes for the fabrication of spherical powders of a compositionally complex Ti-V-Zr-Nb-Mo-Hf-Ta-W refractory high-entropy alloy have been developed. The preparation of pre-alloyed feedstock material by partial sintering followed by cryogenic crushing was considered. Subsequently, the simultaneous melting, alloying, and atomization of a blend of elemental powders was envisaged as an alternative to the time-consuming cryogenic crushing. The microstructure, the indentation hardness, the phase stability, the prospects of consolidation into bulk alloys, and the hydrogenation behavior of the alloys thus produced have been extensively investigated.

Place, publisher, year, edition, pages
Luleå: Luleå University of Technology, 2024.
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
National Category
Metallurgy and Metallic Materials
Research subject
Engineering Materials
Identifiers
URN: urn:nbn:se:ltu:diva-109950ISBN: 978-91-8048-630-9 (print)ISBN: 978-91-8048-631-6 (electronic)OAI: oai:DiVA.org:ltu-109950DiVA, id: diva2:1897017
Public defence
2024-11-06, E632, Luleå University of Technology, Luleå, 09:00 (English)
Opponent
Supervisors
Funder
Swedish Foundation for Strategic Research, ID19-0071Available from: 2024-09-12 Created: 2024-09-11 Last updated: 2024-10-16Bibliographically approved
List of papers
1. Use of AFM topography images to determine microindentation hardness of cast tungsten carbide powders
Open this publication in new window or tab >>Use of AFM topography images to determine microindentation hardness of cast tungsten carbide powders
2022 (English)In: International journal of refractory metals & hard materials, ISSN 0263-4368, Vol. 107, article id 105878Article in journal (Refereed) Published
Abstract [en]

Hardness is defined as the resistance of a material to localized plastic deformation. Owing to their non-destructive nature, static indentation hardness tests are widely used in industry. Hardness testing is particularly useful for the mechanical characterization of materials that cannot be tested otherwise, e.g. powdered materials. In this study, challenges related to Vickers microindentation hardness testing of hard brittle cast tungsten carbide (CTC) powders were extensively investigated. Test load was optimized to obtain sufficiently large crack-free indentations allowing for precise measurement of the diagonal lengths. The influence of the operator and imaging technique on the measured hardness value was evaluated. Topography of residual imprints was investigated using atomic force microscopy (AFM) and a systematic and operator bias-free method to locate the indentation vertexes was developed. Results suggested that measurement variability introduced by AFM scanning and post-processing was as low as 3.1% and 1.3% with respect to the mean hardness value, respectively. Since the variability due to the measuring system can be isolated, the homogeneity of powders can be reliably evaluated from the hardness measurements thus obtained.

Place, publisher, year, edition, pages
Elsevier, 2022
Keywords
Microindentation, Hardness, Vickers, Atomic force microscopy, Image analysis, Cast tungsten carbide
National Category
Metallurgy and Metallic Materials
Research subject
Engineering Materials
Identifiers
urn:nbn:se:ltu:diva-90623 (URN)10.1016/j.ijrmhm.2022.105878 (DOI)000806791500005 ()2-s2.0-85130573927 (Scopus ID)
Funder
The Kempe Foundations, SMK-2546
Note

Validerad;2022;Nivå 2;2022-06-08 (sofila);

Funder:  Swedish Foundation for Strategic Research (ID19-0071)

Available from: 2022-05-12 Created: 2022-05-12 Last updated: 2024-09-11Bibliographically approved
2. Role of the microstructure and the residual strains on the mechanical properties of cast tungsten carbide produced by different methods
Open this publication in new window or tab >>Role of the microstructure and the residual strains on the mechanical properties of cast tungsten carbide produced by different methods
Show others...
2024 (English)In: Journal of Materials Research and Technology, ISSN 2238-7854, E-ISSN 2214-0697, Vol. 30, p. 3640-3649Article in journal (Refereed) Published
Abstract [en]

Cast tungsten carbide (CTC) is a biphasic, pearlitic-like structure composed of WC lamellae in a matrix of W2C. Besides excellent flowability, spherical CTC powders exhibit superior hardness and wear resistance. Nevertheless, the available literature generally fails to explain the physical mechanisms behind such a phenomenon. In the present work, the microstructure and the mechanical properties of the novel centrifugally-atomized spherical CTC have been extensively investigated. This material exhibited an extremely fine microstructure, with WC lamellae of 27-29 nm in thickness and bulk lattice strains of 1.0-1.4 %, resulting in a microindentation hardness of 31.4 ± 1.6 GPa. The results of this study clearly show that centrifugally-atomized CTC is mechanically superior to both spheroidized CTC and conventional cast-and-crushed CTC. In addition, the effect of a series of heat treatments on the bulk fracture toughness and the fatigue life of entire CTC particles was also investigated. The reduction of residual stresses in the bulk of particles upon annealing dramatically increased the indentation fracture toughness, whereas the bulk microindentation hardness remained essentially unaffected. Regarding the fatigue life of entire particles under uniaxial cyclic compressive loading, local phase transformation phenomena at the surface of the particles upon heat treatment were concluded to play the most critical role. Indeed, the cumulative fatigue damage was minimized in surface-carburized CTC powders, where compressive stresses were induced at the outermost surface.

Place, publisher, year, edition, pages
Elsevier, 2024
Keywords
Cast Tungsten Carbide, Microindentation Hardness, X-ray Diffraction, Lattice Microstrains, Dislocation Density, Compression
National Category
Other Materials Engineering
Research subject
Engineering Materials
Identifiers
urn:nbn:se:ltu:diva-105077 (URN)10.1016/j.jmrt.2024.04.067 (DOI)001294655000001 ()2-s2.0-85190595488 (Scopus ID)
Funder
Swedish Foundation for Strategic Research, ID19-0071
Note

Validerad;2024;Nivå 2;2024-05-02 (joosat);

Full text: CC BY License

Available from: 2024-04-14 Created: 2024-04-14 Last updated: 2024-11-20Bibliographically approved
3. Fabrication of spherical Ti-V-Zr-Nb-Mo-Hf-Ta-W refractory high-entropy alloy by a combination of spark plasma sintering, cryogenic grinding, and plasma centrifugal atomization
Open this publication in new window or tab >>Fabrication of spherical Ti-V-Zr-Nb-Mo-Hf-Ta-W refractory high-entropy alloy by a combination of spark plasma sintering, cryogenic grinding, and plasma centrifugal atomization
(English)Manuscript (preprint) (Other academic)
Keywords
High Entropy Alloy, Refractory, Spherical Powder, Plasma Centrifugal Atomization, Spark Plasma Sintering, Cryogenic grinding
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:ltu:diva-109819 (URN)
Funder
Swedish Foundation for Strategic Research, ID19-0071
Available from: 2024-09-11 Created: 2024-09-11 Last updated: 2024-09-11
4. Flexible production of spherical powders of Ti-V-Zr-Nb-Mo-Hf-Ta-W refractory high-entropy alloys by plasma-assisted centrifugal atomization
Open this publication in new window or tab >>Flexible production of spherical powders of Ti-V-Zr-Nb-Mo-Hf-Ta-W refractory high-entropy alloys by plasma-assisted centrifugal atomization
(English)Manuscript (preprint) (Other academic)
Keywords
High-Entropy Alloy, Refractory, Spherical Powder, Plasma Centrifugal Atomization, Superalloy
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:ltu:diva-109820 (URN)
Funder
Swedish Foundation for Strategic Research, ID19-0071
Available from: 2024-09-11 Created: 2024-09-11 Last updated: 2024-09-11
5. Hydrogenation behavior of a fine-grained Ti-V-Zr-Nb-Mo-Hf-Ta-W refractory high-entropy alloy produced by plasma centrifugal atomization and spark plasma sintering
Open this publication in new window or tab >>Hydrogenation behavior of a fine-grained Ti-V-Zr-Nb-Mo-Hf-Ta-W refractory high-entropy alloy produced by plasma centrifugal atomization and spark plasma sintering
Show others...
(English)Manuscript (preprint) (Other academic)
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:ltu:diva-109821 (URN)
Funder
Swedish Foundation for Strategic Research, ID19-0071
Available from: 2024-09-11 Created: 2024-09-11 Last updated: 2024-10-11

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