Abstract
High-entropy alloys (HEAs) synthesized using refractory elements are being strongly considered as candidates for high temperature structural applications. The role of compositional changes of HEA surfaces due to oxidation is crucial to sustain the material properties, but a detailed description of the thermodynamic mechanism driving the adsorption of oxygen on such complex surfaces is absent. We examine and explain the reaction process of oxygen on a representative refractory HEA surface using first principles and atomistic thermodynamic models. The HEA surface is highly reactive to oxygen yielding a full monolayer coverage at temperatures between 300 and 1500 K. The preferential adsorption of oxygen to specific sites of the HEA surface is attributed to the electronic configuration of the bonding shells of the constituent surface atoms. On further oxygen addition, the oxygen atoms diffuse into the bulk regions of the alloy. Manipulation of temperature and oxygen pressure reveals that it is difficult to rid the alloy surface of oxygen even at extremely low pressures of 10−9 bar at 2000 K.
Highlights
High-entropy alloys (HEAs), a subset of multi-principal element alloys (MPEAs), contain N principal elements typically in near equiatomic concentrations with N ≥ 5.1 For the majority of the HEAs investigated to date, the predominant phases formed are disordered solid solutions primarily as face-centered cubic (FCC) or body-centered cubic (BCC) crystals, and some hexagonal-closed packed (HPC) phases
These disordered phases are favored over the ordered phases due to many competing parameters such as the enthalpy of mixing, the valence electron concentration, atomic radii, lattice structure and electronegativity of the constituent elements as well as the high ideal configurational entropy of mixing, ΔSconf = − R∑Xiln(Xi) that is equal to or greater than 1.5R,2,3 where Xi represents the atomic fraction of element i and R is the molar gas constant
Several potential refractory HEAs7,10–14 have been recently identified based on contemporary approaches that involve manipulation of valence electron concentrations, entropy of mixing, the Hume-Rothery rules among others.[1,15,16]
Summary
High-entropy alloys (HEAs), a subset of multi-principal element alloys (MPEAs), contain N principal elements typically in near equiatomic concentrations with N ≥ 5.1 For the majority of the HEAs investigated to date, the predominant phases formed are disordered solid solutions primarily as face-centered cubic (FCC) or body-centered cubic (BCC) crystals, and some hexagonal-closed packed (HPC) phases. Several potential refractory HEAs7,10–14 have been recently identified based on contemporary approaches that involve manipulation of valence electron concentrations, entropy of mixing, the Hume-Rothery rules among others.[1,15,16] These HEAs have been reported to provide promising alternatives to Ni-based superalloys These alloys are composed of three or less phases, with the BCC phase being predominant as the primary phase.[4] Limited literature on the degradation mechanisms of some HEA surfaces due to oxidation and other physical effects are available,[17] while the corrosion resistance of several HEAs has been determined both experimentally and computationally primarily focusing on anodic and cathodic reactions.[18,19] to the best of our knowledge, this report is the first study that describes the fundamental thermodynamic mechanisms analyzing how oxygen adsorbs on complex surfaces, such as those of HEAs. Recently, Singh et al.[20] proposed refractory Mo-W-Ta-Ti-Zr HEAs with compositions that exhibited greatly enhanced modulus of elasticity (3 × at 300 K) over the near equiatomic counterparts.
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