Oxidation study of CoCrCuFeNiSix high entropy alloys

Abstract

Oxidation studies of Co-Cr-Cu-Fe-Ni-Si X=(0,0.3,0.6,0.9) high entropy alloys (HEA) at 600, 700 and 800°C for 30 h are performed in the present work. Isothermal oxidation confirmed that the oxidation resistance of Co-Cr-Cu-Fe-Ni-Si X=(0,0.3,0.6,0.9 ) high entropy alloy increases with Silicon (Si) content up to 0.6%, beyond that it decreases. Oxidation rate kinetics study of all the alloys at the three designated test temperature represented the typical cubic curve. X-ray diffraction (XRD) analysis revealed the evolution of Cu oxide (CuO) along with existing phases in all the HEA samples. Microstructural observation after oxidation test exhibited an increase in oxide formation with the oxidation temperature. To comprehend the kinetics of oxidation more clearly, Arrhenius plots were drawn for all the HEA samples and activation energies are calculated.

Insight on passivity of high entropy alloys: Thermal stability and ion transport mechanisms in the passive oxide film on CoCrFeMnNi surfaces

“…Despite the high mass resolution (M/dM=7500) that allows us to locate the maximum intensity of each single peak, the characteristic ion of newly formed chromium oxide (Cr 18 O2 -, 87.93 amu) slightly overlaps in the mass spectrum with the tail of characteristic ion of the pre-formed iron oxide (Fe 16 O2 -, 87.92 amu). To avoid this problem, Cr 18 O3 -(105.93 amu) and Fe2 16 O3 -(159.85 amu) ions (that do not suffer from overlapping) were used to characterize the oxide growth during reoxidation in 18 O2 of the oxide pre-formed in H2SO4. It should be noted that the selected ions do not reflect the real stoichiometry of the species constituting the sample but are the appropriate markers of the studied species.…”

“…The specimen was then heated up to a temperature of 170 ± 1 °C (an IONTOF heating stage was used). A precision leak valve was then used to introduce a constant and low 18 O2 pressure into the analysis chamber, such that the partial pressure p( 18 O2) was constant at 1×10 −5 mbar. After the designated oxidation time had elapsed, the leak valve was closed while the sample temperature was maintained at 170 ± 1 °C, and the chamber immediately pumped down to the base pressure (10 −9 mbar), in order to record the ToF-SIMS depth profiles.After depth profiles analysis, oxygen was then introduced at the same partial pressure for further oxidation and the analytical protocol repeated for each subsequent oxidation step.…”

“…3(a)), the metal/oxide interface, which is defined by the maximum intensity of the Ni2signal, is located at 125s of sputtering time, which corresponds to an equivalent thickness of 1.9 nm. By examining the signals of the newly formed oxides, including the Cr 18 O3 -, Fe 18 O2 -, Mn 18 O3and Co 18 O2ions, one can locate the newly formed oxides (grown by re-oxidation in 18 O2 gas) at the surface of the pre-formed passive film. Thus, the main ion transport mechanism for the passive film pre-formed at RT in sulfuric acid solution on CoCrFeMnNi HEA surfaces is outward cation diffusion.…”

“…Although several studies have investigated oxide film composition and the associated oxidation kinetics at different temperatures [6,[11][12][13][14][15][16][17][18][19], the ion-transport process that governs oxide growth on such complex alloys is rarely elucidated. ToF-SIMS analysis can make use of the isotopic tracer 18 O in order to identify the predominant ion transport during surface oxidation, by exposing the sample to a 'labelled' 18 O2 atmosphere and depth profiling the oxidized surface. Using this analytical approach, Poulain et al [20] investigated ion transport for Cr2O3 growing on a pure Cr substrate at 300 °C and under low oxygen pressure.…”

“…Using this analytical approach, Poulain et al [20] investigated ion transport for Cr2O3 growing on a pure Cr substrate at 300 °C and under low oxygen pressure. In addition, the ion transport mechanisms in oxide films pre-formed in varying conditions can also be studied using ToF-SIMS depth profiling labelled with 18 O isotope. On Ni-based alloy, the results revealed that inward anion diffusion governs the oxide growth on the surface pre-oxidized in high temperature water [21].…”