R. Molins

Intergranular crack tip oxidation mechanism in a nickel-based superalloy

Abstract This paper is concerned with the intergranular crack tip oxidation mechanism in alloy 718 at elevated temperatures. The basic concept is based on the ability of the oxygen partial pressure to control the preferential formation of oxide layers at the crack tip. The time required to complete the build-up of the protective oxide type at the metal-oxide interface is considered a measure of the limits of the oxidation process. Identification by transmission electron microscopy of oxide scale formed along fracture surfaces during a low frequency fatigue crack process in alloy 718 at 650°C supports the proposed model concepts. An experimental program was carried out to investigate the role of passivation time in controlling the progressive process of crack tip oxidation. This was achieved by testing the influence of oxide buil-up during hold time at minimum load, as well as the effect of a minor high frequency cycle imposed on the hold time period. It was established that an increase in fatigue crack growth rate accompanies the increase in passivation time period. These results were interpreted on the basis of the oxidation formation concepts.

Microstructural and analytical characterization of Al2O3(AlMg) composite interfaces

Abstract Mechanical properties of composite materials depend on the reactions taking place at the ceramic-matrix interface. This study was devoted to the identification of the mechanisms of interaction for a Saffil-(AlMg) composite during squeeze casting and heat treatment. Transmission electron microscopy characterization and microanalysis clearly show migration of magnesium to the interface and reactions during processing of the composite and appropriate heat treatments allowed MgO and MgAl 2 O 4 clusters to be identified along the interface. Additionally, structural hardening of matrix through Mg 2 Si precipitation was made clear. Some evidence of macroheterogeneity in composition is also given.

Interlamellar boundary characterization in Ni-based alloy thermally sprayed coating

The use of thermal spraying process for corrosion protection requires a high level of knowledge concerning the material modifications during coating deposition. The effect of the high velocity oxygen fuel process on inconel 718 thermally sprayed coating microstructure was studied by XRD, transmission electron microscopy and STEM analyses. Thermally sprayed coating exhibits an interlamellar oxidation related to the in-flight and cooling particles. The in-flight oxidation is characterised by a globular oxide which results from convective motion in the liquid phase of the molten particles before impact on the substrate. The oxide was identified as CrNbO4. The cooling step on the substrate induces a duplex oxide with an outer layer composed of columnar spinel oxide (Ni, Fe)Cr2O4 and an inner oxide corresponding to a nanocrystalline CrNbO4 oxide, which is related to the rapid cooling of the flattened particles. # 2002 Elsevier Science B.V. All rights reserved.

Yttrium segregation and intergranular defects in alumina

Intergranular segregation of yttrium is investigated in a rhombohedral twin grain boundary of alumina. The deviation from the twin orientation is compensated by a periodic arrangement of intergranular dislocations. TEM tools (CTEM and HREM, EDXS, EFTEM and EELS/ELNES) have been used to characterize changes in chemical and electronic environments along this twin. Within the experimental limits, no Y is detected in the perfect twin parts. On the other hand, Y segregation occurs very locally in the dislocation cores on about four atomic planes perpendicularly to the GB, which in fact corresponds to the step height associated with the interfacial dislocations. By comparison with an undoped bicrystal, both the dislocation distribution and the Y segregation localization confirm the influence of Y on the dislocation mobility. Furthermore, in the Y-rich defects, high spatial resolution analysis of the energy loss near edge structures (ELNES) of the Al-L23 absorption edge brings some enlightenments about the Al3+ cation environment, provided the effects of local radiation damage are fully considered and under control.

Stress Corrosion Cracking of Alloy 600 in PWR Primary Water : Influence of Chromium, Hydrogen and oxygen Diffusion

Alloy 600, a nickel base alloy containing 15 % chromium, is used in the primary circuit of Pressurized Water Reactor (PWR). This alloy is well-known to be susceptible to Stress Corrosion Cracking (SCC) in PWR primary water. Despite the fact that many laboratory studies have been performed and that many models are proposed in the literature, the mechanisms involved are still not well-known. In some models, the transport of species (oxygen, hydrogen and chromium) has a key role. Therefore, experiments and calculations have been performed to study the transport of chromium, hydrogen and oxygen in Alloy 600 and in model microstructures. The results lead to the conclusion that the transport of oxygen and hydrogen cannot be considered as the rate-controlling steps. The asymmetric aspect of the crack tip and of the chromium depletion ahead of the crack lead to the conclusion that chromium diffusion could play a significant role in the mechanism.

Advanced nano-secondary ion mass spectrometry analysis of intergranular oxide penetration of Alloy 600 in a pressurized water reactor primary water environment

Advanced nano-secondary ion mass spectrometry analyses were used to investigate intergranular attack and improve the understanding of stress corrosion cracking in Alloy 600. Observations and analyses carried out on cross-sectioned samples without applied stress revealed penetrative intergranular attacks. Oxygen tracer evidence shows that oxide does not constitute a barrier against oxygen diffusion in oxide. This result allows us to investigate a new mechanistic model. Oxide growth seems to depend on oxygen diffusion in porous oxide associated with chromium diffusion in strained alloy to the grain boundary located beyond the crack tip. Strain should increase transport kinetics and promote oxide formation by increasing rates of defects such as dislocations.