## 1 Introduction  
For nearly 80 years, nickel-based superalloys have served as the preferred materials for the hottest parts of gas turbines. However, an ongoing need to increase the performance of these systems requires materials that can operate at higher temperatures. Among the potential candidates, refractory high-entropy alloys (RHEAs) are considered the most promising, because of their excellent strength at ultra-high temperatures and phase stability [1; 2; 3; 4]. However, for high-temperature service, oxidation resistance is essential. It is known that most refractory metals and alloys are vulnerable to oxidation at T \(>\) 500 \({}^{\circ}\)C due to the formation of volatile oxides (e.g., MoO\({}_{3}\), WO\({}_{3}\), V\({}_{2}\)O\({}_{5}\)). While the oxidation resistance of some RHEAs is similarly inferior [5; 6; 7; 8; 9; 10], certain RHEAs can withstand oxygen attack up to 1500 \({}^{\circ}\)C thanks to complex oxides [11; 12; 13]. For instance, Al-Cr-Nb-Mo-Ta-Ti system RHEAs tend to the form a protective rutile-type CrTaO\({}_{4}\) oxide stemming from a reaction between Cr\({}_{2}\)O\({}_{3}\) and Ta\({}_{2}\)O\({}_{5}\) oxides [12; 14; 15; 16]. Unlike traditional Al\({}_{2}\)O\({}_{3}\) and Cr\({}_{2}\)O\({}_{3}\) oxides that appear as dense external layers, CrTaO\({}_{4}\) nucleates beneath a mixture (Al\({}_{2}\)O\({}_{3}\) + Cr\({}_{2}\)O\({}_{3}\) + TiO\({}_{2}\)) of discontinuous scales and does not prevent internal oxidation, yet effectively blocks the outward diffusion of metal cations [15]. Another rutile-type CrNbO\({}_{4}\) oxide has been found to be responsible for good oxidation resistance of Al-Cr-Nb-Ti-Zr system RHEAs [17; 18; 19; 20; 21]. Occasionally, Cr alloying is more attractive for improvement of the oxidation resistance of RHEAs compared to Al additions [22]. However, in some Cr-free RHEAs, a fair protectiveness is provided by AlNbO\({}_{4}\), Ti\({}_{2}\)ZrO\({}_{6}\), Ti\({}_{2}\)Al\({}_{2}\)O\({}_{15}\), and Ta\({}_{2}\)Nb\({}_{4}\)O\({}_{15}\) oxides [23; 24]. Unfortunately, most studies of the oxidation behaviour of RHEAs are performed in isothermal conditions [11; 13; 17; 18; 20; 21; 23; 24; 25; 26; 27; 28; 29]. This testing scheme is suitable for establishing the oxidation kinetics and oxidation mechanisms, but it does not allow for estimating the response of the alloy to repeated heating-cooling cycles that usually occur during service life. Cyclic testing can induce the spallation of oxide scale and even disintegration of the specimen due to the accumulation of thermal stresses [30]. A recent report [31] revealed the trade-off between the oxidation and spallation/disintegration resistances in RHEAs, which was highly dependent on the chemical and phase compositions. Specifically, an increase in Al concentration deteriorated the spallation/disintegration resistance due to a large volume fraction of brittle Laves phase despite lower mass gains at early stages of oxidation compared to alloys with reduced Al content. Existing studies focus on RHEAs consisting exclusively of refractory metals with additions of Al, Cr, or Si [15]. In contrast, the recently introduced RHEAs, containing late transition metals, open new directions for designing high-performance alloys [32; 33; 34; 35; 36; 37; 38; 39]. One of the examples is a series of (H\(\mathrm{{\mathrm{C}o}}\))\({}_{100\mathrm{-}x}\)(Nb\(\mathrm{{\mathrm{Mo}}}\))\({}_{\mathrm{x}}\) alloys that have demonstrated good room-temperature plasticity while retaining high strength at 1000 \({}^{\circ}\)C [34; 36]. Such a combination of properties was achieved in dual-phase alloys comprised of a \"soft\" (Hf, Co)-rich B2 matrix (hardness of 6.5-8.4 GPa) with embedded \"hard\" (Nb, Mo)-rich particles (hardness of 7.5-9.3 GPa). This bcc+B2 structure was stable after 700 h of annealing at 1000 \({}^{\circ}\)C [40], thereby outperforming the temperature limit for microstructural degradation of many RHEAs [41; 42; 43; 44; 45]. However, the oxidation behaviour of RHEAs containing late transition metals is yet to be investigated.