STRUCTURAL STATE OF HIGH-ENTROPY Fe_40-xNiCoCrAl_х ALLOYS IN HIGH-TEMPERATURE OXIDATION

Abstract

The evolution of the phase composition, mechanical properties, and features peculiar to the formation of oxide layers of Fe_40−хNiCoCrAl_х (х = 5 and 10% at.) alloys in long-term oxidation at 900 and 1000 °C was were studied. In the initial cast state, depending on the aluminum content and the valence electron concentration, the alloys contain only an fcc solid solution (VEC = 8 e/a) or a mixture of fcc and bcc phases (VEC = 7.75 e/a). Thin continuous oxide scales containing Cr₂O₃ and NiCr₂O₄ spinel formed on the surface of both alloys oxidized at 900 °C for 50 h. A further increase in the annealing time to 100 h leads to the formation of aluminum oxide Al₂O₃ in the scale of the Fe₃₀Ni₂₅Co₁₅Cr₂₀Al₁₀ alloy, having high protective properties. An increase in the oxidation temperature to 1000 °C results in partial damage of the protective layer on the alloy with 10% at. Al. Long-term holding at 900 °C (100 h) + 1000 °C (50 h) does not change the phase composition of the Fe₃₅Ni₂₅Co₁₅Cr₂₀Al₅ alloy matrix, being indicative of its high thermal stability. In the two-phase Fe₃₀Ni₂₅Co₁₅Cr₂₀Al₁₀ alloy, the quantitative ratio of solid solutions sharply changes: the amount of the bcc phase increases from 4 to 54 wt.% and its ordering by type B2 is observed. The mechanical characteristics of the starting alloys and those after long-term high-temperature annealing were determined by automatic indentation. The hardness (H_IT) and elastic modulus (E) of the cast Fe₃₅Ni₂₅Co₁₅Cr₂₀Al₅ alloy are equal to 2 and 147 GPa, respectively, and decrease to 1.8 and 106 GPa after a series of long-term annealing operations. The Fe₃₀Ni₂₅Co₁₅Cr₂₀Al₁₀ alloy shows the opposite dependence: Н_ІТ increases from 2.5 in the initial state to 3.1 GPa after annealing and E decreases from 152 to 134 GPa. This indicates that the Fe₃₀Ni₂₅Co₁₅Cr₂₀Al₁₀ alloy is promising as a high-temperature oxidation-resistant and creep-resistant material.