Régine Molins

Oxidation Behavior of ODS Fe–Cr–Al Alloys: Aluminum Depletion and Lifetime

The oxidation kinetics of two ODS Fe–Cr–Al alloys, PM 2000 and MA 956, were studied in oxygen and in air under isothermal conditions from 1000 to 1300°C. They both form an α-alumina scale and have good oxidation resistance, without any mass loss. Although the aluminum content in these alloys is higher than the minimum Al content necessary to ensure the growth of a continuous alumina scale, an aluminum depletion occurred in the substrate. This depletion allows the determination of aluminum diffusion coefficients in the ODS alloy. This method is very original and interesting as no Al-stable isotope is available. Moreover, the evolution of the aluminum concentration in the substrate allows one to determine the lifetime of these alloys: indeed, when the aluminum content decreases and becomes lower than a critical value, alumina can no longer form, and less-stable oxides grow very rapidly compared to alumina.

Characterization of the Oxide Films Formed at the Surface of Ni-Base Alloys in Pressurized Water Reactors Primary Coolant by Transmission Electron Microscopy

The oxide film formed on nickel-based alloys in Pressurized Water Reactors (PWR) primary coolant conditions (325°C, aqueous media) has been investigated by Transmission Electron Microscopy (TEM). TEM observations revealed an oxide layer divided in two parts. The internal layer was mainly composed of a continuous spinel layer, identified as a mixed iron and nickel chromite (Ni(1-x)FexCr2O4). Moreover, nodules of Cr2O3 were present at the interface between this spinel and the alloy. The external layer is composed of large crystallites corresponding to a spinel structure rich in iron (Ni(1-z)Fe(2+z)O4) resulting from precipitation phenomena. The influence of alloy surface defects was also studied underlining two main consequences on the formation of the passive film e.g. the internal layer. On one hand, the growth kinetics of the internal spinel rich in chromium increased with the surface defect density. Besides that, when the defect density increased, the oxide scale became more finely crystallized. This result agrees with a growth mechanism due to a rate limiting process of diffusion through the grain boundaries of the oxide. On the other hand, the quantity of Cr2O3 nodules increased with the number of surface defects, revealing that the nodules nucleated preferentially at defect location.

Study of the Sulfur Segregation for a TBC System

The degradation of thermal barrier coatings is closely linked to their spalling resistance, which depends on the stability of the protective oxide scale produced by oxidation of the bond coat. In this context, sulfur is well known to reduce the adherence of the thermally grown oxide on the boundary layer in the TBC systems. So, the aim of the present work is to report experimental results concerning the chemical changes at Al2O3/bond coat layer and Al2O3/superalloy interfaces, namely the effect of interfacial sulfur segregation as a function of the oxidation time, the initial S content and the presence of voids at the alumina/ bond coat interface. Sulfur segregations were identified by SIMS depth profiles and the effect of these segregations discussed according to the types of alloys and heat treatments. SIMS depth profiles and TEM results allowed us to localize the S segregation at the alumina/bond-coat interface and in the bulk of the bond-coat and to differentiate sulfur coming from the superalloy and sulfur trapped in the NiAlPt bond-coat

Thermal cycling behaviour of thermal barrier coating systems based on first- and fourthgeneration Ni-based superalloys

Abstract This study deals with the cyclic oxidation behaviour of thermal barrier coating systems. The systems consist of an yttria-stabilised zircona ceramic top coat deposited by EB-PVD, a β-(Ni,Pt)Al bond coat and a Ni-based superalloy. Two different superalloys are studied: a first-generation one and a fourthgeneration one containing Re, Ru and Hf. The aim of this work is to characterise the microstructural evolution of those systems and to correlate it to their resistance to spallation. Thermal cycling is carried out at 1100°C in laboratory air, with the number of cycles ranging between 10 and 1000. Each cycle consists of a 1 h dwell followed by forced-air cooling for 15 min down to room temperature. Among the main results of this work, it is shown that the MCNG-based system is significantly more resistant to spallation than the AM1-based one. Up to 50 cycles, both systems exhibit similar oxidation rate and phase transformations but major differences are observed after long-term ageing. In particular, a Ru-rich β-phase is formed in the bond coat of the MCNG-based system while the AM1- based one undergoes strong rumpling of the TGO/bond coat interface due to the loss of the thermal barrier coating.

Adhesion of thermal barrier coatings systems after long term oxidation : influence of preoxidation temperature and surface state of the bond coat

Abstract The aim of this study was to determine the effect of the pre-oxidation temperature and surface state of the bond coat on the microstructure of the oxide scale formed at the first stages of oxidation and on its adhesion after subsequent long-term oxidation (1000 h) at 1100–C. Short-term isothermal oxidations of 1 h were performed at several temperatures (900–C, 1100–C) on a Pt-modified NiAl bond coat with two different surface states (as-aluminized or grit-blasted) deposited on a superalloy (AM1). The adherence of the different systems after an additional isothermal ageing treatment in air at 1100–C for 1000 h, was compared in order to deduce the initial oxide scale leading to the best resistance to spallation. Characterization was performed using SEM and analytical TEM. The crystalline structure and the morphology of the as-formed oxide scale were studied as a function of the different parameters.

Oxidation behaviour in air of thin alloy 601 fibres

Abstract The oxidation behaviour in air of 12 μm diameter continuous alloy 601 fibres has been studied using thermo-gravimetric analysis (TGA) for kinetics identification and transmission electron microscopy (TEM) for determination of the nature of the oxide layers. The TGA allows two stages in the formation of the oxide layer to be distinguished: the first stage corresponds to the growth of a continuous layer of NiO above a discontinuous sub-layer of Cr2O3 whereas the second stage is attributed to the parabolic growth of the Cr2O3 sub-layer, from the time it becomes continuous. A third stage can be observed for high oxidation temperatures. The TEM observations of oxide layers formed after 30 min at 650, 750 and 900°C confirm these results. One common characteristic of these 3 oxidation conditions is the appearance of large cavities under the oxide layer. These cavities seem to be the consequence of the oxidation mechanism of Cr and of the particular morphology of the material (i.e. small diameter cylinders).

Effect of impurities on the oxidation mechanism of nickel at 800°C

Abstract The effect of impurities on the oxidation mechanism of nickel was studied on commercial nickel grades compared to a pure nickel. On the basis of oxidation kinetics, SEM and STEM microstructural and analytical investigations allowed us to identify the oxidation mechanism for both types of nickel at 800°C. The morphology of the oxide scale notably differs according to the purity of the nickel. For oxidised commercial grades, a duplex structure was observed with an outer columnar layer and an inner layer made of equiaxed grains. The inner NiO/outer NiO interface is planar without any segregation, while the NiO/Ni interface is convoluted with large cavities. Mn, Ti and sometimes also silicon impurities were detected at this latter interface. Below the NiO/Ni interface, in the underlying nickel, large internal oxidation was observed. The observed microstructure was quite different for the pure nickel. A single porous NiO layer, composed of equiaxed grains, was observed. The NiO/Ni interface was facetted and no porosity was detected. The presence and localisation of impurities, as well as morphological changes through the scale in the nickel grades, were taken into account to explain the modification of oxidation kinetics with substrate purity.

Which tool to distinguish transient alumina from alpha alumina in thermally grown alumina scales

Abstract Alumina scales constitute excellent protective barriers when they form on alumina-forming steels. If they keep tightly adherent to the underlying substrate, they isolate it from the surrounding aggressive atmosphere at high temperature. The protectiveness of the alumina scale is highly dependant upon its growth mechanism. The nucleation and transformation of transient alumina (mainly γ-Al2O3 and θ-Al2O3) is known to play an important role on alumina scale formation. It is therefore fundamental to characterise these transient alumina especially during the early stages of the oxidation process. The morphology of the transient alumina was observed by scanning electron microscopy (SEM), their crystallographic phases determined by X-ray diffraction (XRD) and transmission electron microscopy (TEM). X-ray photoelectron spectrometry (XPS) analyses were performed on reference samples and then compared to the alloys oxidised 5 and 30 minutes at 850, 900 and 950°C.

Strengthening of Al/Ni-based composites by in situ growth of intermetallic particles

Squeeze cast Al matrix composites reinforced with continuous fibers of Inconel 601 were submitted to different annealing treatments aiming at tuning the amount of reaction at the fiber/matrix interface. The reaction develops in the form of intermetallic nodules growing onto the fibers. The tensile flow stress of the composites increases with increasing nodule volume fraction at the expense of a progressive loss of ductility. This loss of ductility is due both to the low cohesion of the oxide layer separating the matrix from the nodules and to brittle cracking at the root of attachment of the nodules onto the fibers. Damage development is evaluated from the evolution of strain hardening. The nodules grow underneath the oxide barrier layer that protects the fibers from reacting with Al during squeeze casting. Their mechanism of growth involves the partial reduction of the oxide layer by Al, followed by diffusion of Al and Ni through the Cr-rich oxide layer

Effect of Material and Environmental Parameters on the Microstructure Evolution and Oxidation Behavior of a Ni-Based Superalloy Coated with a Pt-Modified Ni-Aluminide

The present work, performed on nickel aluminides deposited on a single Ni-based superalloy AM1, focuses on the effect of the following several parameters on the microstructural and chemical changes occurring during isothermal heat treatment at 1100°C for 50h : -oxygen pressure by comparing heat treatment under ambient air (PO2 = 0.2 bar) and under secondary vacuum (PO2 = 0.2x10-6 bar). -cooling rate after isothermal heat treatment by comparing furnace cooling (3°C/min) and water quenching (500°C/min). -Pt addition in the coating by comparing NiAl and NiPtAl coatings. Characterizations were performed using SEM, analytical TEM and electron microprobe analyses. The results show that these parameters have a strong influence on both the microstructural evolution and the oxidation of the thermal barrier coating (TBC) system. Appropriate heat treatments are essential to improve interfacial resistance and increase the durability of TBC systems.