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Elemental Distribution in CrNbTaTiW-C High Entropy Alloy Thin Films
Chalmers Univ Technol, Dept Phys, SE-41296 Gothenburg, Sweden.
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
Chalmers Univ Technol, Dept Phys, SE-41296 Gothenburg, Sweden.ORCID iD: 0000-0002-6097-6895
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
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2019 (English)In: Microscopy and Microanalysis, ISSN 1431-9276, E-ISSN 1435-8115, Vol. 25, no 2, p. 489-500Article in journal (Refereed) Published
Abstract [en]

The microstructure and distribution of the elements have been studied in thin films of a near-equimolar CrNbTaTiW high entropy alloy (HEA) and films with 8 at.% carbon added to the alloy. The films were deposited by magnetron sputtering at 300 degrees C. X-ray diffraction shows that the near-equimolar metallic film crystallizes in a single-phase body centered cubic (bcc) structure with a strong (110) texture. However, more detailed analyses with transmission electron microscopy (TEM) and atom probe tomography (APT) show a strong segregation of Ti to the grain boundaries forming a very thin Ti-Cr rich interfacial layer. The effect can be explained by the large negative formation enthalpy of Ti-Cr compounds and shows that CrNbTaTiW is not a true HEA at lower temperatures. The addition of 8 at.% carbon leads to the formation of an amorphous structure, which can be explained by the limited solubility of carbon in bcc alloys. TEM energy-dispersive X-ray spectroscopy indicated that all metallic elements are randomly distributed in the film. The APT investigation, however, revealed that carbide-like clusters are present in the amorphous film.

Place, publisher, year, edition, pages
CAMBRIDGE UNIV PRESS , 2019. Vol. 25, no 2, p. 489-500
Keywords [en]
atom probe tomography, carbon clustering, high entropy alloy, segregation, thin film
National Category
Materials Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-385570DOI: 10.1017/S1431927618016264ISI: 000466756600025PubMedID: 30712522OAI: oai:DiVA.org:uu-385570DiVA, id: diva2:1327060
Conference
Atom Probe Tomography and Microscopy (APT and M) Conference, JUN 10-15, 2018, Gaithersburg, MD
Funder
Swedish Research Council, 621-2012-4359Swedish Research Council, 622-2008-405Knut and Alice Wallenberg FoundationSwedish Foundation for Strategic Research , RMA11-0029Available from: 2019-06-19 Created: 2019-06-19 Last updated: 2020-02-28Bibliographically approved
In thesis
1. Microstructure and Mechanical Properties of Magnetron Sputtered Refractory Metal Thin Films
Open this publication in new window or tab >>Microstructure and Mechanical Properties of Magnetron Sputtered Refractory Metal Thin Films
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The design and development of new multifunctional materials that exhibit a combination of high hardness and ductility, as well as a high corrosion resistance and thermal stability, is one of the key challenges in the field of material science. The focus of this thesis is on the development of novel multifunctional magnetron sputtered CrNbTaTiW–C based thin films. Carbon was selected as an alloying element to investigate if it could modify the microstructure (via grain refinement) and improve the properties (e.g. the hardness and ductility).

TaW-rich and near-equimolar high entropy alloys in the CrNbTaTiW system were selected as starting points for this study. The latter alloys were predicted, based on empirical design rules, to form a single-phase solid solution. In contrast, thermodynamic calculations showed that the films at equilibrium should be composed of a mixture of several phases at temperatures below 1100 °C.  Experimentally, however, a single-phase bcc structure was observed for the deposited films and it was concluded that the films were kinetically and not entropy stabilised. A hypothesis is that the kinetics during sputtering allow a ’direct’ phase selection by tuning the process parameters and evidence of this was found in the HfNbTiVZr alloy system.

The CrNbTaTiW–C system is, however, complex and additional studies were carried out on the W–C and TaW–C systems. All metallic films crystallised in a bcc structure with a <110> texture and the column width of these films varied between 25 nm and 80 nm. The films were very hard (~ 13 GPa), which was explained by the small grain size. A single-phase bcc structure was also obtained upon the addition of 5-10 at.% carbon for all compositions except the near-equimolar CrNbTaTiW. X-ray diffraction indicated a unit cell expansion, which was attributed to the formation of a supersaturated solid solution. Additional atom probe tomography (APT) studies on selected samples confirmed the formation of such solid solutions. The supersaturated solid solution is not thermodynamically stable and an annealing study showed that heat treatment yielded segregation and clustering of carbon at the grain boundaries. The addition of carbon had a grain refining effect in the W–C system and the multicomponent CrNbTaTiW–C system. In general, the addition of carbon increased the hardness, which was mainly caused by a reduced grain size in line with the Hall-Petch relationship. Excellent mechanical properties of carbon supersaturated films were further confirmed in pillar tests on W–C films, which showed very high yield strength (~ 9 GPa) and no brittle fracture. The results show that carbon can be used as a chemical approach to control the grain size and properties of these films. 

Multicomponent carbides with a B1 structure were formed at high carbon concentrations (~ 40 at.%). The microstructure of these films depended strongly on the process parameters and a higher deposition temperature was found to increase the film density and hardness. The TaW-rich carbide exhibited a very high hardness of ~ 35 GPa and excellent corrosion resistance.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2020. p. 73
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1909
Keywords
thin films magnetron sputtering, refractory metals, high entropy alloys, mechanical properties, transition metal carbides
National Category
Engineering and Technology
Research subject
Chemistry with specialization in Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-405323 (URN)978-91-513-0884-5 (ISBN)
Public defence
2020-04-17, Ångströmslaboratoriet Siegbahnsalen, Lägerhyddsvägen 1, Uppsala, 09:15 (English)
Opponent
Supervisors
Available from: 2020-03-26 Created: 2020-02-28 Last updated: 2020-03-26

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Fritze, StefanMalinovskis, PauliusRiekehr, LarsJansson, Ulf

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