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To reveal the effects of water vapor on the oxidation mechanism of Al–Zr-based materials, the Al2Zr alloy was oxidized in a wet atmosphere. Water vapor achieves the synchronous oxidation of Al and Zr to form amorphous (Al0.67Zr0.33)O1.66 oxide on the Al2Zr surface. The oxidation kinetics of the Al2Zr alloy is significantly retarded by the presence of water vapor. Compared with the oxygen molecules, water molecules are preferentially adsorbed on the surface, thereby providing fewer oxidizing species for wet oxidation than those for oxygen-involved oxidation. This work provides fundamental insights for surficial protection of structural alloys in humid environments. [ABSTRACT FROM AUTHOR]

The high-temperature oxidation of ZrAl2 upon exposure to pure O2 at 800–950 °C was studied in terms of the oxidation kinetics and the formation mechanism of the oxide layer. The alloy followed parabolic oxidation kinetics, and the activation energy of oxidation was 239 ± 14 kJ/mol. During the early stages of oxidation below 850 °C, a single-layer oxide formed due to the crystallization of the initially formed amorphous oxide layer. A multilayer oxide structure developed at higher temperatures, due to the slightly higher affinity of oxygen for Zr than for Al and the oxidation-induced compositional changes. [ABSTRACT FROM AUTHOR]

During thermal oxidation of binary amorphous Al x Zr 1− x solid solutions over wide alloy-composition (0.26 ⩽ x ⩽ 0.68) and oxidation-temperature (350–560 °C) ranges, a single amorphous ternary oxide phase of well-defined stoichiometry, (Al 0.33 Zr 0.67 )O 1.83 , emerges. The composition of this unexpectedly stoichiometric ternary amorphous oxide phase, that develops instead of the crystalline oxide phases Al 2 O 3 and ZrO 2 , under conditions far away from genuine thermodynamic equilibrium, is governed by thermodynamics, as demonstrated by thermodynamic assessment, adopting the amorphous (Al 0.33 Zr 0.67 )O 1.83 phase as a supercooled liquid oxide–oxide (Al 2 O 3 –ZrO 2 ) solution phase. [ABSTRACT FROM AUTHOR]

Refractory high-entropy alloys (RHEAs) are sensitive to thermal oxidation, significantly restricting their service performance and potential applications. However, the wet oxidation behavior of RHEAs has remained poorly understood despite the potential risk of catastrophic oxidation caused by water vapor. In this study, the oxidation mechanism of an equiatomic TiZrHfNbTaV high-entropy alloy (HEA) has been investigated in a wet atmosphere at elevated temperatures. Synchronous oxidation of all elements occurs at temperatures below 800 °C. At temperatures above 800 °C, preferential oxidation of Nb, Ti, and Zr, and internal oxidation of V occur on HEAs, which are thermodynamically less preferred as compared with oxidations of more reactive Hf and Ta. Experimental and theoretical analyses show that the oxide grain boundaries and substrate interdendritic regions of HEAs act as diffusion pathways for V and oxidizing species during preferential and internal oxidation processes. This work provides a fundamental understanding of the oxidation of multicomponent alloys in wet environments, which is helpful in the development of effective oxidation resistance strategies and innovative surface engineering techniques. • Oxidation of TiZrHfNbTaV high-entropy alloy in a wet atmosphere at 600–900 °C was investigated. • Synchronous oxidation of all elements occurs at temperatures below 800 °C. • Preferential oxidation of Nb, Ti, and Zr occurs on RHEAs, with internal oxidation of V at temperatures above 800 °C. • The oxide grain boundary and substrate interdendritic diffusion play essential roles in wet oxidation behaviors of RHEAs. [ABSTRACT FROM AUTHOR]

The oxidation of amorphous Al 0.44 Zr 0.56 alloys upon exposure to pure O 2 (g) at 500 and 560 °C (and p O2 = 1 × 10 5 Pa) was investigated by a combinatorial experimental approach using X-ray diffraction, spectroscopic ellipsometry, transmission electron microscopy and Auger electron spectroscopy. During the early stages of oxidation at 500 and 560 °C, an amorphous (Zr,Al)-oxide layer of homogeneous composition and uniform thickness is formed, which is enriched in Zr with respect to the alloy substrate. At 500 °C, both the alloy substrate and the oxide layer remain amorphous during continued oxidation, whereas at 560 °C, a crystalline tetragonal ZrO 2 ( t -ZrO 2 ) phase nucleates after prolonged oxidation, while the alloy remains amorphous. The nucleation and growth of t -ZrO 2 at 560 °C occurs exclusively close to the interface between the initially formed amorphous (Zr,Al)-oxide layer and the alloy, immediately underneath a region of Al enrichment in the substrate, as triggered by oxidation-induced compositional changes in the alloy below the reacting alloy/oxide interface and a favorable energy of the interface between t -ZrO 2 crystallites and the amorphous alloy matrix. The growing t -ZrO 2 oxide crystallites eventually laterally coalesce to form a continuous layer constituted of branches of dendrite-shaped t -ZrO 2 phase crystallites surrounded by an Al-rich amorphous Al–Zr alloy matrix. The underlying mechanism of the oxidation process is discussed. [ABSTRACT FROM AUTHOR]

• Temperature affects the oxidation behavior of the refractory high-entropy alloy. • Specimens exhibit complex microstructures after oxidation. • TiNb 2 O 7 provides favorable conditions for the inward diffusion of oxygen. • The oxide layers of the TiZrHfNbTaV alloys contain various complex oxides. The elemental effects and oxide layer evolutions of high-entropy alloys during oxidation have remained largely unclear. The high-temperature oxidation of equimolar refractory high-entropy alloy TiZrHfNbTaV in the air at 800–1000 °C for 1 h was investigated in terms of the oxide layer microstructure and formation mechanism. The non-dense oxides formed on the alloy surface at 800–1000 °C mainly contain TiNb 2 O 7 , providing favorable oxygen diffusion conditions. The growth of the oxide layer is influenced by the oxidation temperatures, free energy of oxides, and elemental segregation. The findings offer fundamental insights into the oxidation mechanism of the refractory high-entropy alloys. [ABSTRACT FROM AUTHOR]

Structural disordering leads to outstanding mechanical properties of amorphous alloys. Many properties are strongly related to formation of surface oxide layers. An in-depth understanding of effect of structural disordering of amorphous alloys on oxide evolution is essential for applications. This paper presents a review of recent comparative oxidation studies of Zr-based amorphous and single-phase crystalline counterpart alloys. Three representative model alloy systems are selected: Zr–Cu and Zr–Al representing alloys with components of different and similar oxygen affinities, respectively, and Zr–Cu–Al representing multi-component alloys. In combination with molecular dynamic simulations, the fundamental role of the structural disordering of parent alloys in the oxidation processes is revealed. The oxidation of Zr–Cu–Al–Ni amorphous alloys at the supercooled liquid temperature region is further reviewed to clarify the relationship between oxidation and substrate crystallization. • A focused review of role of structural disordering of amorphous alloys on thermal oxidation. • Three representative alloys with simple compositions were selected as model alloy systems. • Structural disorder of parent alloys influences strikingly oxide evolution and oxidation kinetics. • The disputes on the oxidation kinetics of amorphous and crystalline states were clarified. [ABSTRACT FROM AUTHOR]

The high-temperature oxidation of ZrAl2 upon exposure to pure O2 at 800–950 °C was studied in terms of the oxidation kinetics and the formation mechanism of the oxide layer. The alloy followed parabolic oxidation kinetics, and the activation energy of oxidation was 239 ± 14 kJ/mol. During the early stages of oxidation below 850 °C, a single-layer oxide formed due to the crystallization of the initially formed amorphous oxide layer. A multilayer oxide structure developed at higher temperatures, due to the slightly higher affinity of oxygen for Zr than for Al and the oxidation-induced compositional changes. [ABSTRACT FROM AUTHOR]

The natural oxidation of amorphous Cu x Zr 1- x ( x  = 0.33, 0.5, 0.67 and 0.75) alloys has been studied by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy and cross-sectional transmission electron microscopy. It is revealed that passivation behavior occurs for amorphous Cu x Zr 1- x alloys where x  ≤ 0.67, which provides an effective composition range of amorphous CuZr alloys for long time service. A thin (2–5 nm) ZrO 2 film is formed on top of amorphous alloys, with a Cu-enriched amorphous alloy layer at below. Surprisingly, tetragonal ZrO 2 nucleates between the oxide film and the alloy at room temperature in the naturally oxidized am-Cu 0.33 Zr 0.67 alloy. Amorphous Cu 2 O layer forms on top of the oxide film and surface segregation forming metallic Cu occurs for amorphous Cu x Zr 1- x when x ≥ 0.75. [ABSTRACT FROM AUTHOR]

The effect of (6 wt%) Cr and (6 wt%) W on the oxidation behavior of Ni–8%Al alloy at 1000 °C in air was investigated. The oxidation-affected zones of Ni–8%Al–6%Cr/W alloys consist of outer oxidation zone (NiO layer) and mixed internal oxidation zone. The addition of Cr promotes the formation of the intermediate Ni(Al, Cr) 2 O 4 layer and internal Al 2 O 3 in internal oxidation zone, which provides a blocking effect and reduces the oxidation rate of Ni–8%Al–6%Cr alloy. However, the addition of W element facilitates the development of non-protective NiO and NiWO 4 oxides, which accelerates the oxidation of alloy. As a result, Ni–8%Al–6%W alloy shows poor oxidation performance. • The oxide films of Ni–8%Al–6%Cr/W consist of outer oxidation zone and mixed internal oxidation zone. • The addition of Cr promotes the formation of Ni(Al, Cr) 2 O 4 and Al 2 O 3 , providing a blocking effect. • NiWO 4 in Ni–%8Al–6%W provides diffusion paths for ions and accelerates the oxidation process. [ABSTRACT FROM AUTHOR]

The isothermal oxidation behavior of micro-regions in multiphase Ni 3 Al-based superalloys (high Fe and Cr contents) was investigated at 600 °C in air. The oxide scale is mainly composed of the outer mixed oxide layer (NiO, NiFe 2 O 4 and Al 2 O 3) and the inner single Al 2 O 3 layer. In particular, Cr 2 O 3 is formed in the outer oxide scale of γ' + γ dendrite with enrichment in Cr element. The relative dense continuous Al 2 O 3 layer is formed in the interdendritic region (β phase with high Al content). In addition, γ' envelope (connecting the γ' + γ dendrite and interdendritic β phase) shows preferential oxidation featuring significant cellular bulge (NiO and NiFe 2 O 4 mixed oxides). Unlabelled Image • The micro-regions in multiphase Ni 3 Al-based superalloy present significant different oxidation behaviors at 600 °C. • The γ' envelope shows preferential oxidization featuring obvious cellular bulge (NiO and NiFe 2 O 4 mixed oxides). • Cr 2 O 3 is formed in the outer oxide layer of γ' + γ dendrite enriching in Cr element. • The dense protective alumina layer is formed on interdendritic region (β phase) with relative high Al content. [ABSTRACT FROM AUTHOR]

• The commonly disregarded role of noble constituents on the oxidation of metallic glasses is revealed. • Amorphous Cu-Zr-Al alloys are designed as a model system which contains one noble and two similarly ignoble constituents. • The oxidation for Cu-poor Cu-Zr-Al alloy involves synchronous oxidation of Zr and Al. • Chemical interaction of Cu with Al in the Cu-rich Cu-Zr-Al alloys induces preferential oxidation of Zr over Al. • The study offers a new perspective for rational design of surficial oxide structures. The competition between synchronous oxidation and preferential oxidation of Zr and Al in amorphous Cu-Zr-Al alloys has been investigated. Oxide-layer growth on Cu-poor Cu-Zr-Al alloys occurs by synchronous oxidation of Zr and Al. However, for relatively high Cu alloying contents, synchronous oxidation of Zr and Al becomes disturbed by the increased chemical interaction of Cu with Al, which immobilizes Cu and Al atoms from oxidation. Consequently, only Zr is preferentially oxidized. The findings provide fundamental insights on the role of nobler alloy constituents on the oxidation mechanism of multi-component alloys, which opens new avenues for rational design of surficial oxide structures. [ABSTRACT FROM AUTHOR]

• The fundamental role of atomic structure on thermal oxidation of alloys is revealed. • Al-Zr was selected as the model alloy system to circumvent preferential oxidation effect. • Structural order of parent alloys influences strikingly oxide compositions and oxidation kinetics. • Underlying mechanisms were disclosed by molecular dynamics simulations. • The study offers a new perspective in the field of surface engineering. To reveal the fundamental role of structural order of the parent alloy on the oxidation mechanism, thermal oxidation of amorphous and crystalline Al–Zr alloys with identical compositions were investigated in detail. An (Al 0.68 Zr 0.32)O 1.66 amorphous oxide emerged on crystalline Al 2 Zr, whereas an (Al 0.33 Zr 0.67)O 1.83 amorphous oxide formed on amorphous Al 68at.% Zr 32at.% under the same conditions. Oxidation kinetics was fast and linear for crystalline Al 2 Zr but slow and parabolic for amorphous Al 68at.% Zr 32at.%. The underlying mechanisms of such striking differences were disclosed by molecular dynamics simulations. The findings thus offer new prospects in the field of surface engineering. [ABSTRACT FROM AUTHOR]

• Thermal oxidation of amorphous Cu-Zr alloys with different Cu/Zr ratios has been studied. • High-resolution AES depth profiling and cross-sectional analytic HRTEM analyses were performed. • Cu-enriched nanoparticles emerge due to localized Cu-accumulation by selective oxidation of Zr. • Microstructures of Cu-enriched nanoparticles depend on stabilities of parent alloys. • The composition-dependent microstructure of oxidized Cu x Zr 1− x influences oxidation kinetics. The thermal oxidation of amorphous Cu–Zr alloys with different Cu:Zr ratios was studied by X-ray diffraction, Auger electron spectroscopy, and cross-sectional transmission electron microscopy. It was revealed that amorphous ZrO 2 /Cu-enriched bilayers are formed on the amorphous Cu–Zr alloys at 200–250 °C by the selective oxidation of Zr atoms in the alloys. Amorphous Cu-enriched particles (diameter: ~10 nm) are distributed randomly in the amorphous Cu-enriched sublayer of the am-Cu 33at.% Zr 67at.% alloy, due to the sluggish atomic diffusion in the amorphous Cu-enriched layer. In contrast, crystalline Cu-enriched particles (diameter: ~20 nm) are distributed densely in the amorphous Cu-enriched sublayer of the Cu 50at.% Zr 50at.% alloy. The different states of the Cu-enriched particles in the amorphous Cu-enriched sublayer of the am-Cu 33at.% Zr 67at.% and Cu 50at.% Zr 50at.% alloys are ascribed to thermodynamic reasons. The oxidation rate of amorphous Cu 50at.% Zr 50at.% alloy is faster than that of amorphous Cu 33at.% Zr 67at.% alloy, since the crystalline nanoparticles in the Cu-enriched sublayer provide fast migration pathways for ions, whereas the thick amorphous Cu-enriched sublayer presents as a diffusion barrier in the case of amorphous Cu 33at.% Zr 67at.% alloy. [ABSTRACT FROM AUTHOR]