Department of Magnetic Reseach Seminar
Microsoft Teams
The physics of high-entropy alloys: From superconductivity to magnetism
dr Primož Koželj
Instytut Maxa Plancka Fizyki Chemicznej Ciał Stałych (MPI CPfS) w Dreźnie w Niemczech
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Streszczenie:
High-entropy alloys (HEAs) are a recent alloy design strategy [1], based on the fact that a high enthalpy of mixing will stabilize equimolar or near-equimolar solid solutions of four or more principal elements on very simple lattices, e.g. bcc or fcc. The popularity of the concept can be attributed to the very simple synthesis methods required (e.g. arc melting), the large number of options to try and design alloys for certain properties (adding, removing element, varying concentrations, thermal treatment) and from a basic science viewpoint to the interesting problems these systems pose as they contain interactions between four or more atomic species as well as micro- or nanostructural features.
The aim of this talk will be to give some examples of interesting physical properties in HEAs and also to discuss the care that must be taken to properly interpret the results. We will start where my involvement in HEAs started – with the discovery of the first supeconducting HEA, Ta34Nb33Hf8Zr14Ti11 [2]. Expanding one's view to other Ta–Nb–Hf–Zr–Ti alloys [3] one notices that the thermal treatment seems to play a role in the superconducting properties of HEAs. The effect of the thermal treatment will be even more apparent in FeCoNiPdCu [4], where through proper thermal treatment one can induce a nanocomposite structure composed of magnetic FeCoNi-based domains and non-magnetic PdCu-based spacers, which via exchange averaging of magnetic anisotropy results in an excellent soft magnetic material. We will conclude with experiments on the Co-Cr-Fe-Mn-Ni HEA [5], which is a concentrated mixture of five types of magnetic atoms, where randomness and frustration lead to a spin glass phase below about 20 K, which we will illustrate via the non-ergodic phenomena of ultraslow decay of thermoremanent magnetization and the thermal memory effect (isothermal aging causes the system to remember the magnetic state and this can be later retrieved during an upward temperature cycle).
[1] J.-W. Yeh et al., Adv. Eng. Mat. 6, 299 (2004).
[2] P. Koželj, et al., Phys. Rev. Lett. 113, 107001 (2014).
[3] S. Vrtnik, P.Koželj, et. al., J. Alloys. Compd. 695, 3530 (2017).
[4] P. Koželj et al., Adv. Eng. Mat. 21, 1801055 (2019).
[5] P. Koželj et al., J. Magn. Magn. Mater., in publication.
