Transition metal attenuated mechanism for protective alumina formation from first principles

Babic, Vedad; Geers, Christine; Panas, Itai

doi:10.1039/c8ra08195f

Cited by 4 publications

(3 citation statements)

References 44 publications

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“…These observations support RE-induced oxidation slowdown by pinning impurity states in the band gap, cf. [2] and also [32][33][34]. Greater suppression by RE(IV) than RE(III) is consistent with greater stability in case of the former, cf.…”

supporting

confidence: 60%

“…It is achieved by the enhanced electronic conductivity across the scale emerging from the top-of-valence-band to the bottom-of-conduction-band electronic excitations that in turn become facilitated by chemical disruptions in high-angle grain boundaries [32][33][34], and/or by employing oxygen vacancies as generic impurity states in the band gaps of the grain boundaries [2,16,38]. In [2], bipolaron-mediated redox processes among V O sites were shown to drastically enhance V O mobility and thus to facilitate thermal oxide growth. The change in scaling conditions is captured by the superparabolic-cubic model [15]; see Fig.…”

Section: Discussionmentioning

confidence: 99%

“…Greater thermodynamic stabilities of Al 2 O 3 and Cr 2 O 3 as compared to the oxides of the more noble base metal render both to form and enrich at the alloy surface, thereby providing the base metal protection from the harsh environment. When both Al and Cr are present in the alloy, Cr acts as 3rd element in that under oxidizing conditions, transient Cr 2 O 3 inhibits internal oxidation of aluminum and facilitates the formation of the more stable alumina [1][2][3]. The 3rd element effect is most effective at intermediate temperatures, i.e., 800-1000 °C.…”

Section: Introductionmentioning

confidence: 99%

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Reactive Element Effects in High-Temperature Alloys Disentangled

2019

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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).

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confidence: 60%