Oxidation ahead of a crack tip in an advanced Ni-based superalloy

Oxide intrusions which formed ahead of the crack tip in an advanced nickel-based superalloy after exposure to air at 650 °C have been investigated by (scanning) transmission electron microscopy equipped with energy-dispersive X-ray spectrometry. Three different cases were considered: (i) a fast growing crack; (ii) a stationary crack exposed to a 5 h period under load; and (iii) a stationary crack exposed to a 5 h period but held at zero mechanical applied load. The intrusions formed during both hold periods were much longer than those present ahead of the (growing) dynamic crack (～1.5 μm) and consisted of layered oxides in the thermodynamic sequence of CoO, NiO, Cr₂O₃, TiO₂ and Al₂O₃. The last three of these oxides appeared to form protective layers along the flanks and, for the stationary cracks, at the intrusion tip. Calculations of the oxygen partial pressure both across the width and along the length of the oxide intrusions have been undertaken assuming such protective conditions. Significant quantities of oxygen are thus not expected to exist in the alloy ahead of intrusions which are sealed by protective oxide layers.     

The effect of nickel and silicon addition on some oxidation properties of novel Co-based high temperature alloys

Isothermal oxidation was carried out on γ′-strengthened Co–Al–W–B–Ni and Co–Al–W–B–Si superalloys at 800, 900, and 1000 °C. Ni-addition to Co–Al–W–B alloys leads to an absence of B within the inner oxide layer and at the grain boundaries, as indicated by ToF-SIMS. As a result, also inner Al₂O₃-formation is inhibited, which leads to inferior oxidation properties compared to Ni-free alloys. In contrast, Si-addition enhances formation of protective Al₂O₃ at 900 and 1000 °C due to enhanced selective Al-oxidation. Si-containing phases at the oxide/alloy interface, at the grain boundaries, and within precipitates are of subordinate importance but may further improve oxidation resistance.     

A comparison of the oxidation behaviours of Al2O3 formers and Cr2O3 formers at 700 °C – Oxide solid solutions acting as a template for nucleation

The oxidation behaviour of a number of ferritic iron based commercial steels and model alloys containing 6 and 9 wt% Cr and 0–2.5 wt% Al have been studied at 700 °C. The oxidation time ranged from 5 min to 500 h and the atmosphere consisted of flowing dry synthetic air. The oxide layers formed were analysed by SEM, GI-XRD and ToF-SIMS. The material without Al formed a (Cr,Fe)₂O₃ film with an Fe enrichment in the outer part of the layer. The Al containing alloys showed more complex oxidation behaviour. The oxidation started initially by formation of (Cr,Fe)₂O₃ with an Cr enriched inner part. With time Al was oxidized and dissolved in the inner Cr rich part of the oxide. This process continued until it eventually was transformed into α-Al₂O₃ with minute amount of Fe in the outer and Cr in the inner part of the oxide. The thickness of all oxide films ranged from 20 to 400 nm apart from the material that contained 9% Cr and no Al, which experienced breakaway oxidation after 500 h at 700 °C. This means that materials alloyed with small amounts of Al must also be considered to be protective at 700 °C, as the thicknesses of the Al₂O₃ oxides was comparable with the ones not containing Al, and as they do not experience breakaway corrosion.     

Preparation and high temperature oxidation behavior of refractory disilicide coatings for γ-TiAl intermetallic compounds

Novel refractory disilicide layers were applied to γ-TiAl to enhance oxidation resistance at 1050 °C. NbSi₂ and MoSi₂ layers were prepared by joining thin Nb and Mo foils to γ-TiAl surfaces, and siliconizing the combinations (Nb/γ-TiAl, and Mo/γ-TiAl) using molten salts. The coatings and their oxidation behavior were characterized using X-ray diffraction, scanning electron microscopy, and energy dispersive X-ray spectroscopy techniques. Isothermal oxidation tests showed that the oxidation resistance of uncoated γ-TiAl at 1050 °C in air was insufficient, and scale spallation occurred. NbSi₂ coatings were formed and adhered firmly to the γ-TiAl substrate, whereas Mo film detached from the substrate surface causing failure of the MoSi₂ coatings. Oxidation of the NbSi₂-coated γ-TiAl (NbSi₂/Nb/γ-TiAl) at 1050 °C in air showed improved oxidation resistance at exposure times up to 100 h. Microstructural and compositional developments of the coating at prolonged time were discussed. The NbSi₂ coatings provided sufficient oxidation resistance for γ-TiAl at 1050 °C in air, and have potential use in high temperature applications.     

XPS study of the initial stages of oxidation of α2-Ti3Al and γ-TiAl intermetallic alloys

The initial stages of oxidation of α₂-Ti₃Al and γ-TiAl alloys at 650 °C under low oxygen pressure are shown to be characterized by three stages using XPS. Stage I is a pre-oxidation stage characterized by the adsorption and absorption of oxygen species. When the subsurface is saturated with dissolved oxygen (16 and 2 at.% on α₂-Ti₃Al and γ-TiAl, respectively), the selective oxidation of the alloy leads to the nucleation and growth of ultrathin (∼0.5 and ∼1.2 nm on α₂-Ti₃Al and γ-TiAl, respectively) alumina layers (stage II). The growth kinetics of the alumina layers is limited by the transport of Al in the alloy, leading to Al-depletion in the metallic phase underneath the oxide. When a critical concentration is reached (Ti₈₂Al₁₈ and Ti₇₅Al₂₅ on α₂-Ti₃Al and γ-TiAl, respectively), titanium oxidation occurs (stage III). Ti(III) and Ti(IV) oxide particles are formed at the surface of the still growing alumina layer.     

Oxygen adsorption on γ-TiAl surfaces and the related surface phase diagrams: A density-functional theory study

We have constructed the surface phase diagrams for oxygen adsorption on γ-TiAl low-index surfaces using density-functional theory calculations. From these surface phase diagrams, the selective oxidation behaviors of the γ-TiAl surfaces and the corresponding polycrystalline systems can be easily understood and predicted. For the (1 0 0) surface, complete selective oxidation of titanium is favored and a titanium oxide layer may be produced at the initial stage of oxidation. For the (1 1 0) and (0 0 1) surfaces, only titanium oxides may form. For the γ-TiAl polycrystalline system, O may induce complete Ti and Al surface segregations on the Ti-rich and Al-rich conditions, respectively. In addition, the microscopic oxidation mechanisms are identified and the experimental results are successfully explained. More importantly, by comparing the different TiAl surface orientations, a comprehensive surface phase diagram is constructed to study the oxidation behaviors of polycrystalline γ-TiAl. This method can also be applied to other polycrystalline materials.     

Corrosion properties of novel γ′–strengthened Co-base superalloys

Electrochemical properties of γ′-strengthened Co-base superalloys with the composition Co–Al–W–B and Co–Al–W–B–Y are studied in comparison with pure Co at ambient temperature in 0.5 M NaCl aqueous solution (pH 5.8). The different materials exhibit comparable corrosion resistance with a limited initial passivation followed by severe pitting corrosion at higher potentials. Oxide layers, formed under isothermal oxidation at 800 and 900 °C, provide exceptional protection against pitting corrosion. No breakdown events are observed, even for thin layers formed during short-term oxidation (1 h at 900 °C). Si as alloying element further improves the corrosion behaviour of the oxidised alloy in NaCl.     

Superior long-term oxidation resistance of Ni–Al coated TiAl alloys

A nickel aluminide coating, developed on γ-TiAl alloy by electroplating a Ni film followed by a high Al activity pack cementation, has a duplex layer structure with an outer δ-Ni₂Al₃ layer and an inner TiAl₃/TiAl₂/TiNiAl₂ layer. The coated γ-TiAl was oxidized in air for up to 36,000 ks (10,000 h) under thermal cycling between room temperature and 1173 K. A protective Al₂O₃ scale formed with little oxide exfoliation and the average oxidation amount was 37 g/m² after the 36,000 ks oxidation. During oxidation at 1173 K the outer δ-Ni₂Al₃ changed to β-NiAl with voids and then to TiNiAl₂, and the inner TiAl₃/TiAl₂/TiNiAl₂ layers to TiAl₂ and TiNiAl₂ layers and then to TiAl₂ and τ₃ layers. The voids in the outer layer were formed by the phase transformation from the δ-Ni₂Al₃ to β-NiAl during oxidation. It was found that after the 36,000 ks oxidation the higher Al contents in the inner layers were better retained than that in the outer layer.     

Microstructure study of the hardening mechanism of Cr–Ni alloy deposits after flame heating for a few seconds

Cr–Ni alloy deposits with different chemical compositions could be obtained from a bath with trivalent Cr and divalent Ni ions. With a plating current density above 20 A dm^? 2, amorphous Cr-rich alloy deposits were obtained, and crystalline Ni-rich deposits were achieved with an electroplating current density lower than 15 A dm^? 2. The hardness of the Ni-rich deposit decreased gradually with increasing flame-heating time. On the contrary, the hardness of Cr-rich alloy deposit could be significantly increased from 550 Hv to 1450 Hv after flame heating for 3 s. The results of a microstructure study show that the precipitation of nano-sized carbon-related particles, possibly diamond-like particles, could be the main hardening mechanism of flame-heated Cr-rich alloy deposit. These particles have particularly high hardness values, and they distort the nearby Cr-rich lattice, which leads to obvious strain fields in the flame-hardened Cr-rich alloy deposits.     

Quench sensitivity of toughness in an Al alloy: Direct observation and analysis of failure initiation at the precipitate-free zone

An analysis of toughness in 6156 Al–Mg–Si–Cu sheet was performed using enhanced Kahn tear tests on samples quenched at different rates. Crack initiation energies were hardly affected by changing the water quench temperature from 20 °C to 60 °C; however, a significant reduction was evident on air cooling. Crack propagation energy was reduced for both 60 °C water-quenched and air-cooled materials. Observation of failure initiation through synchrotron radiation computed tomography for the 60 °C water-quenched material revealed failure ahead of the crack tip of grain boundaries oriented at 45° to the main loading axis, and crack “tongues” extending into the material ahead of the main crack. Failure was predominantly intergranular. Fractographic assessment revealed predominantly voiding and shear decohesion in the 20 °C water-quenched material. With the aid of the new findings, past models on the influence of precipitate-free zone parameters on toughness were revised.