Reactive Element Effects in High-Temperature Alloys Disentangled

Abstract

Reactive elements-REs-are decisive for the longevity of high-temperature alloys. This work joins several previous efforts to disentangle various RE effects in order to explain apparently contradicting experimental observations in alumina forming alloys. At 800-1000 °C, "messy" aluminum oxy-hydroxy-hydride transients initially formed due to oxidation by H 2 O which in turn undergo secondary oxidation by O 2. The formation of the transient oxide becomes supported by dispersed RE oxide particles acting as water equivalents. At higher temperatures, electron conductivity in impurity states owing to oxygen vacancies in grain boundaries (GBs) becomes increasingly relevant. These channels are subsequently closed by REs pinning the said vacancies. The universality of the emerging understanding is supported by a comparative first-principles study by means of density functional theory addressing RE(III): Sc 2 O 3 , Y 2 O 3 , and La 2 O 3 , and RE(IV): TiO 2 , ZrO 2 , and HfO 2, that upon reaction with water, co-decorate a generic GB model by hydroxide and RE ions. At 100% RE coverage, the GB model becomes relevant at both temperature regimes. Based on reaction enthalpy ΔH r considerations, "messy" aluminum oxy-hydroxyhydride transients are accessed in both classes. Larger variations in ΔH r are found for RE(III)-decorated alumina GBs as compared to RE(IV). For RE(III), correlation with GB width is found, increasing with increased ionic radius. Similarly, upon varying RE(IV), minor changes in stability correlate with minor structural variations. GB decorations by Ce(III) and Ce(IV) further consolidate the emerging understanding. The findings are used to discuss experimental observations that include impact of co-doping by RE(III) and RE(IV).

High-Temperature Oxidation and Microstructural Changes of Al0.75CoCrFeNi High-Entropy Alloy at 900 and 1100 °C

“…A relatively large difference in thermal expansion between the oxide scale and substrate was the cause of scale spallation. The addition of small amounts of elements of Ce, Hf, Y, or Zr less than 1% is expected and can increase the spalling resistance of the oxide scales [59,60].…”

Microstructure and Hot Corrosion Property of a Si–Co–Y Diffusion Coating Deposited on TiAl–Nb Alloy

“…Once a dense and continuous Al2O3 layer formed in the corrosion product scale, the inward diffusion of corrosive media such as O, S, Cl, etc., will be effectively inhibited. Studies have shown that both Co and Y can promote the selective oxidation of Al to form a continuous Al2O3 protective layer [15,31,32].…”

High-temperature oxidation of AlCrFeNi-(Mn or Co) high-entropy alloys: Effect of atmosphere and reactive element addition

“…Small additions of so-called reactive elements (RE), e.g., Y, Zr, Hf, and Ce, either in elemental form or dispersed RE oxide particles, to enhance the oxidation performance and lifespan of alumina-forming high-temperature alloys have been widely documented and studied [33,41,54,55]. The beneficial effects for RE addition include remarkably improved scale adherence, suppression of convoluted oxide scale morphology and void formation, enhanced selective Al oxidation and altering scale growth mechanism from both anion and cation to predominant anion diffusion.…”