Ponente
Descripción
The two processes relevant when testing a material for fusion environments are its response to (1) displacement damage and (2) transmutation reactions [1] can be mirrored by ion beam irradiation by heavy ion irradiation, and by helium implantation. The best effect can be achieved when these two types of ion irradiations are applied simultaneously in the dual-beam irradiation setup, like the Dual-Beam Ion Irradiation Facility for Fusion Materials (DiFU) at Ruđer Bošković Institute [2], which is one of only a dozen worldwide. DiFU was built at RBI as support facility for pre-selection of materials for DONES, so the chosen materials will be prepared as small bulk samples for DONES study.
For fusion applications, W and W-based alloys, especially refractory high entropy alloys (RHEA), are known as high heat flux (about 10 MW/m2) materials for plasma facing components e.g. first wall and diverter [3]. To prepare and study larger number of different compositions of alloys as candidates for fusion materials, magnetron sputtering is shown as method of choice, so we used this method to deposit WTaVCr thin films. Their structural properties and chemical composition on atomic scales were studied before and after irradiation in DiFU by dual or single beam.
WTaVCr films were deposited on a silicon substrate. Films were irradiated in DiFU with dual beam of 8 MeV Cu3+ and 200 keV He+ to dose 1 dpa and 6 dpa (for Cu3+), and with single beam of 8 MeV Cu3+ to dose 1 dpa. The films were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) and atomic resolution scanning transmission electron microscopy (STEM), both before and after irradiation. Moreover, a highly-performant machine-learned interatomic potential [4] was employed to perform atomistic simulations of bulk and surface WTaVCr.
As-deposited WTaVCr film dominantly contains body-centered cubic (bcc) structure with a unit-cell parameter a=3.191(2) Å and interplanar distance d011=2.25 Å, and a second, minority bcc phase with unit-cell parameter a=3.126(3) Å and interplanar distance of d011=2.21 Å. While the appearance of the second phase indicates a possible phase segregation of lighter element (V or Cr), this assumption has been disproved by atomic resolution STEM imaging in combination with EDS which show that all the elements (W, Ta, V and Cr) are homogenously distributed in all the crystal grains. These results led to the conclusion that both bcc phases were the high entropy alloys lattices containing all four metals. After irradiation with dual beam in DiFU, narrower bcc phase with a unit-cell parameter a=3.126 Å becomes dominant crystalline phase, although the bcc with a=3.191 Å can still be observed. Moreover, after irradiating as-deposited sample of thickness with single beam of 8 MeV Cu ions in dose of 1 dpa we observed that it caused complete transition of the film in the bcc phase with the smaller unit cell (a=3.126(3) Å), initially found in the minority of the as-deposited samples. These results indicated that irradiation induce ordering of the crystal lattice instead of an expected increase of the defects. To elucidate the observed phenomena and confirm whether irradiation really induce the relaxation of crystal lattice, theoretical simulations of bulk and surface WTaVCr were performed. Probability of WTaVCr to form a bcc bulk crystal of unit cell length "a" was calculated using the tabGAP [5] machine-learning interatomic potential. The simulation indicated that 3.12 is the global minimum of WTaVCr BCC crystal lattice. This statement has been approved experimentally by atomic resolution STEM imaging in combination with EDS which confirmed that all the elements are homogenously distributed in bcc crystal lattice of WTaVCr. Moreover, the Rietveld refinements of XRD measurements show that the micro-strain value ε in as deposited samples was significantly larger compared to the micro-strain present in the irradiated WTaVCr having smaller unit-cell. Observed decrease in micro-strain indicates that the irradiation causes the significant reduction in number of distortions and the relaxation of deformations originally present within the as-prepared crystal lattice finally leading to the observed decrease of unit-cell parameters.
From experimental observations and theoretical analysis, we conclude that the system, as initially deposited, is located in a substrate-induced local minimum, while irradiation of the quaternary W-based alloy WTaVCr induce relaxation in thermodynamically stabile phase.
Therefore, the irradiation‑induced ordering observed in WTaVCr under dual‑beam conditions in DiFU provides a clear basis for selecting this alloy system as a promising candidate for neutron exposure in IFMIF‑DONES.
The DiFU results can identify which compositions exhibit structural stability and reduced micro‑strain under displacement damage and helium implantation, making them suitable for further investigation under fusion‑relevant neutron spectra. Based on these findings, we propose the preparation of miniature mechanical‑test specimens that would enable the assessment of the materials properties following their irradiation in IFMIF‑DONES (small scale mechanical testing samples: for tensile testing, nanoindentation etc.). These small‑scale samples, linking post‑irradiation mechanical characterization with DiFU pre‑screening and DONES neutron‑induced material evolution would ultimately support the development of W‑based RHEAs for plasma‑facing applications.
Acknowledgment: The authors acknowledge financial support of Croatian Science Foundation under the project IP-2025-02-7971.
References:
[1] G. S. Was, Fundamentals of radiation materials science: metals and alloys, Springer (2007).
[2] T. Tadić, et. al. Materials 16 (3) (2023) 1144.
[3] G. Pintsuk, et. al., Fusion Engineering and Design 174 (2022) 112994.
[4] J. Byggmästar, et. al., Acta Materialia (2025) 121276.
[5] J. Byggmästar, K. Nordlund, F. Djurabekova, Phys. Rev. Materials 6 (2022) 083801.