The Effect of Sulfur Segregation on the Adherence of the Thermally-Grown Oxide on NiAl—I: Sulfur Segregation on the Metallic Surface of NiAl(001) Single-Crystals and at NiAl(001)/Al2O3 Interfaces

The aim of this study was to improve the understanding of the deleterious effect of sulfur impurities on the adherence of the thermally-grown oxide on the boundary layer in thermal-barrier-coating systems. In Part I, the sulfur segregation on the free surface of NiAl(001) and at different interfaces between metal and transient alumina scales has been characterized by AES, XPS and LEED. The sulfur diffusion coefficient in the alloy has been determined (D = 0.15 exp(–218,000/RT) cm²/s). It is by three orders of magnitude larger than the nickel and aluminum self-diffusion coefficients. It has also been observed that the sulfur de-segregates upon Al enrichment of the metallic surface. The saturation of the metallic surface with an amorphous alumina layer formed at room temperature blocks the segregation of S. However, in the initial stages of oxidation where the transient -alumina grows by cationic transport and inject vacancies at the interface, S segregates at the interface between the alumina thin films and the metallic substrate.     

Self-assembling of atomic vacancies at an oxide/intermetallic alloy interface

Oxide layers grown on the surface provide an effective way of protecting metallic materials against corrosion for sustainable use in a broad range of applications. However, the growth of cavities at the metal/oxide interface weakens the adherence of the protective layer and can promote its spallation under service conditions, as observed for alumina layers formed by selective oxidation of aluminide intermetallic alloys used in high-temperature applications. Here we show that direct atomic-scale observations of the interface between an ultrathin protective oxide layer (alumina) grown on an intermetallic titanium aluminide substrate (TiAl) can be performed with techniques sensitive to the topmost atomic layers at the surface. Nanocavities resulting from the self-assembling of atomic vacancies injected at the interface by the growth mechanism of the protective oxide are observed for the first time, bringing new insight into the understanding of the fate of injected cavities in oxidation processes.     

High-temperature oxidation of silicon carbide in simulated atmospheric re-entry conditions

Oxidation tests on silicon carbide samples were carried out in a facility specifically developed at ONERA partly to simulate atmospheric re-entry conditions. Thermogravimetric data and surface analyses have shown the effect of low pressure and dissociated species on the oxidation of SiC. The transition between active and passive oxidation is well related to thermodynamic equilibrium between SiC and SiO₂ and does not seem influenced by low dissociated oxygen. On the contrary, atomic oxygen has a drastic effect on the kinetics of formation as well as the structure of the oxide layer and the phenomenon of silica vaporization.     

The Effect of Sulfur Segregation on the Adherence of the Thermally-Grown Oxide on NiAl—II: The Oxidation Behavior at 900°C of Standard,Desulfurized or Sulfur-Doped NiAl(001) Single-Crystals

In Part II of this study, the characterization by TGA, SEM and AFM of the oxidation behavior at 900°C of NiAl(001) samples with various sulfur concentrations is reported. The formation of interfacial cavities is observed for all samples. A constant ratio of the oxide thickness to cavity depth is found showing that the formation of cavities at the metal–oxide interface is not due to sulfur but only to vacancy injection during the cationic growth of transient -alumina. It is also observed that the presence of sulfur in the alloy decreases the oxidation rate of the nickel aluminide and, consequently, lowers the formation of interfacial cavities. This effect is interpreted as an indirect evidence of the control of the transient-alumina growth by the aluminium diffusion in the alloy, also advanced as an explanation for the interfacial nucleation of alpha alumina. These results are combined with those presented in Part I to propose a model that explains how sulfur, present in small quantities in the alloy, has a deleterious effect on the oxide adherence. The indirect role of the cavities formed during the growth of the transient alumina is to create transitory conditions for the rapid segregation of sulfur at the interface. The segregated sulfur remains as a vestige of the initial stages of growth after the transformation of the scale into mature alumina and weakens its adherence.     

Fibrosing mediastinitis causing pulmonary arterial hypertension without pulmonary venous hypertension. Clinical and necropsy observations

Clinical and morphologic observations are described in two patients with severe pulmonary arterial hypertension without pulmonary venous hypertension from fibrosing mediastinitis. In one patient, both main pulmonary arteries and one major pulmonary vein were severely narrowed by dense fibrous tissue; in the second patient, only the right main pulmonary artery was severely narrowed. Both patients had normal intrapulmonary arteries and normal pulmonary parenchyma. Of nine previously described necropsy patients with pulmonary hypertension due to fibrosing mediastinitis, seven had severe narrowing of multiple large pulmonary veins and in six of them the pulmonary hypertension was entirely due to pulmonary venous obstruction. In one other patient, the pulmonary hypertension was due to obstruction of one main pulmonary artery and several large pulmonary veins. Each of these seven previously described patients had severe changes in the small intrapulmonary arteries. Of the other two previously described patients with pulmonary hypertension from fibrosing mediastinitis, one had severe narrowing of only the main right pulmonary artery, and the other, of both main pulmonary arteries. Thus, although pulmonary arterial hypertension in patients with fibrosing mediastinitis is usually due to obstruction of multiple large pulmonary veins and to severe secondary changes in small intrapulmonary arteries, fibrosing mediastinitis can cause severe pulmonary hypertension by obstructing the right or both main pulmonary arteries.     

Influence of an oxidation protective coating upon hot corrosion and mechanical behaviour of Ti–48Al–2Cr–2Nb alloy

48-2-2 specimens were coated with Au using electrochemically process and vacuum heat treatment. Au diffused and the coating was composed of TiAlAu₂ and TiAlAu layers from the surface to the bulk alloy. A transition zone between the coating and the sound alloy composed of Au-enriched TiAl is also formed. The oxidation and NaCl salt corrosion behaviours of the coated specimen were investigated through cyclic tests and the effects of coating on mechanical properties of TiAl were assessed by means of creep properties. Au coating was effective in improving the oxidation and NaCl salt corrosion resistances of the coated specimens at 800 °C and 600 °C respectively. This good resistance is attributable to the formation of an Al₂O₃ scale on the surface of the coated specimen. During oxidation, or NaCl salt corrosion, the upper TiAlAu₂ layer of the coating transformed in an Al₂O₃ layer on a TiAu₂ layer. The slightly lower creep properties exhibited by the coated specimens are presumably linked to Cr-enriched phases that segregate at grains boundaries during the vacuum heat treatment. The degradation of creep properties after coating and corrosion is believed to be brought about either by the formation of the brittle TiAu₂ phase or by corrosion or oxygen diffusion through the upper scale.     

Effect of Platinum on the Growth Rate of the Oxide Scale Formed on Cast Nickel Aluminide Intermetallic Alloys

Thermo-Gravimetric Analysis (TGA) and Scanning Electron Microscopy (SEM) data of the initial stages of oxidation of Ni₅₀Al₅₀ and Ni₄₀Pt₁₀Al₅₀ alloys of low and high sulfur content at 900°C and 1100°C are reported. The results show that the addition of Pt promotes the growth of the transient θ-Al₂O₃ oxide scale. This effect is particularly sensitive in the initial stages of oxidation at 1100°C where Pt considerably increases the total mass gain. It is attenuated in the presence of a high sulfur content in the alloy, indicating a competitive effect of Pt and S on the segregation of Al. The slower θ-to-α transition observed in the presence of Pt leads to an extended lifetime of the θ phase layer, which is proposed to be beneficial to the relaxation of the stresses created by the growth of α-Al₂O₃.