No abstract

Figure 4. Cross sections of various RHEA after oxidation in air (conditions in brackets): a) surface oxide layer (SOL) of the alloy TiZrNbHfTa(1 h at 700 C), [35] ODZ represents the oxygen diffusion zone. Reproduced with permission. [24] Copyright 2018, Wiley. b) Oxide scale formed on the alloy NbMoCrTiAl (100 h at 1000 C) [21] (mixed oxides consists of Nb 2 O 5 , TiO 2 , Cr 2 O 3 , Al 2 O 3 , and CrNbO 4 ). Reproduced with permission. [26] Copyright 2019, Elsevier. c) Backscattered electron (BSE) image of the alloy TaMoCrTiAl (100 h at 1000 C). [21] Reproduced with permission. [26] Copyright 2019, Elsevier. d) BSE image of an α-alumina scale formed on the alloy Nb1.3Si2.4Ti2.4Al3.5Hf0.4 (100 h at 1200 C). [28] Reproduced under the terms of the CC BY 4.0 license. [32] Copyright 2019, The Authors. Published by MDPI. Figure 3. Oxide scales formed on two equiatomic RHEA after 24 h of oxidation at 1200 C; a) MoCrTiAl and b) TaMoCrTiAl.

Poor oxidation resistance represents one of the main shortcomings of refractory metal based alloys. This work shows an innovative way to intrinsically protect such materials. Our approach relies on the alloying with Cr and Ta allowing the formation of CrTaO4. The CrTaO4 scale formed on the novel refractory equiatomic high entropy alloy Ta-Mo-Cr-Ti-Al reveals a unique combination of the following properties: (i) easy formation in a wide temperature range from 500°C to 1500°C, (ii) excellent adherence after cooling, (iii) low growth rates, and (iv) wide stability range of the stoichiometric composition that suppresses the formation of non-protective oxides.