Differential thermal analyses of an AlSi6Cu4Fe2 alloy at various cooling rates from the liquid state were performed which showed: i) possible discrepancies in the first solidification steps and; ii) solidification ending with multiple peaks at low cooling rate. Metallographic observation of the samples gave hint for understanding the discrepancies while the presence of rounded multi-phase pools or so-called rosettes - entrapped within dendrite arms explained the multiple peaks because they solidified independently of each other. The very fine eutectic microstructure in these rosettes was investigated by scanning electron microscopy which showed the precipitation of a four-phase eutectic. Focused ions beam milling was then used to investigate the topology of the distribution of the four phases. Electron backscattered diffraction analysis was carried out to investigate their local crystallographic orientations and transmission electron microscopy analysis was performed to identify the very fine crystalline structures. This eutectic appears as one of the very few regular four phase eutectics reported in the literature.

The cyclic oxidation of NiCoCrAlYTa-base thermal barrier coating systems was investigated at 1100 °C. The influence of the NiCoCrAlYTa deposition process, the coating modification by a Pt-overlayer, and the surface preparation steps were studied. Thermal cycling results showed that the addition of a Pt-overlayer, a dense and oxide-free bond-coating microstructure, together with a smooth NiCoCrAlYTa surface prior to Pt deposition and a suitable surface preparation before thermal barrier deposition all increase the lifetime. Degradation mechanisms are proposed to explain how coating defects develop during thermal cycling and how the fabrication process influences both failure and lifetime.

The spalling/cracking behaviour, at room temperature, of thermally grown oxide scales under tensile stress was investigated using SEM in-situ tensile testing for two austenitic stainless steels with close composition except their S content. A correlation between damage patterns, microstructure, mechanical and adhesion properties of the oxide scales is proposed. The difference in microstructure evolution during oxidation between the two steels is explained in relation with the volume fraction of MnS inclusions in the substrate (i.e. S content). Although a direct effect of S content on the oxide scale adhesion is not evidenced, the metal/oxide toughness seems strongly affected by oxides features such as scale thickness, Fe content and location of internal oxides (SiO2 along the metal/scale interface or at the grain boundaries of the underneath substrate).

Two austenitic stainless steels, AISI 304L and AISI 303, were submitted to cyclic oxidation and to static mechanical loading after isothermal oxidation at 1000 degrees C. Alloy 303 contains ten times more S than 304L and some Mn addition. During the steel process, it formed manganese sulfides that lead to the formation of a less resistant oxide scale. Both alloys showed similar behavior during thermal cycling but breakaway oxidation and intensive spallation occurred much sooner for alloy 303 than for alloy 304L. A correlation could be drawn between tensile test on preoxidized samples, isothermal and cyclic oxidation. (C) 2015 Elsevier Ltd. All rights reserved.

Most industrial heat-resistant stainless steels contain silicon as a minor constituent. At high temperature, the internal formation of amorphous silica reduces oxidation rates but decreases the metal/oxide interface toughness. Tensile testing experiments performed on AISI 304L previously oxidized in synthetic air for 50 h at 900 or 1000 °C showed a relation between the silica morphology and location and the crack patterns. A micromechanical modeling using cohesive zone models to describe interfaces fracture behavior is proposed to investigate relevant parameters controlling the silica/alloy interface debonding. Calculations carried out using the finite elements method have shown that location of silica inclusions and silica/metal interface toughness are key parameters determining the cracks pattern morphology and the critical strain at failure.

This work focuses on the effect of alloyed sulphur as MnS inclusions in austenitic stainless steels. AISI 304L and AISI 303 were oxidized at 1000 degrees C in synthetic air. The high sulphur grade, AISI 303, presents a breakaway oxidation with formation of nodules with an inner part composed of alternated layers of Fe-rich and Cr-rich oxides. During oxidation, MnS inclusions near the metal/oxide interface are partially dissociated. Manganese is incorporated in the oxide. The sulphur is trapped in Cr,Mn-oxysulphide aggregates which formation leads to a local decrease of the Cr concentration and consequently to the formation of non-protective Fe-rich oxide. (C) 2015 Elsevier Ltd. All rights reserved.

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. High temperature oxidation of austenitic stainless steels: effect of sulfur content on scale adhesion V. Parry, E. Fedorova, C. Pascal, M. Braccini, M. Mantel, D. Oquab, D. Monceau, Y. Wouters

The L10 martensite formed in a Pt-modified NiCoCrAlYTa bond coating has been investigated by atom probe tomography. It was found that obvious segregation of Co and Cr occurred in the micro-twins zone inside the martensite lath. Based upon the compositional analysis, it is known that Pt destabilizes the β phase and Co and Cr act as β stabilizers with respect to the β→L10 martensitic transformation. In addition, some α-Cr particles precipitated inside the martensite lath.

The scratch test technique was used to characterise the adherence of multi-layered oxide scales formed during the high temperature isothermal oxidation (1100°C) of a single crystal Ni-based superalloy AM1 with varying sulfur concentrations (0.22 – 3.2 ppmw). Results were discussed in relation to cyclic oxidation behaviour and microstructural evolution of oxidised samples. The most commonly used elastic energy-based models were applied to calculate the work of adhesion. The obtained values of the work of adhesion were then analysed with regard to the mode-dependence of the fracture behaviour and the possible sources of errors in the scratch test results interpretation. This analysis allowed us to understand the limits of validity of these models and of the experimental value of the work of adhesion.

In the field of advanced ceramics, Spark Plasma Sintering (SPS) is known to be very efficient for superfast and full densification of ceramic nanopowders. This property is attributed to the simultaneous application of high density dc pulsed current and load, even though the sintering mechanisms involved remain unclear. In the first part of the paper, the mechanisms involved during SPS of two insulating oxide nanopowders (Al2O3 and Y2O3) are discussed while in the second part illustrations of the potential of SPS will be given for (i) Consolidation of mesoporous or unstable nanomaterials like SBA-15 or biomimetic apatite, respectively; (ii) Densification of core (BT or BST)/shell (SiO2 or Al2O3) nanoparticles with limited or controlled reaction at the interface. (iii) In-situ preparation of surface-tailored Fe–FeAl2O4–Al2O3 nanocomposites, and finally (iv) One-step preparation of multilayer materials like a complete thermal barrier system on single crystal Ni-based superalloy.

The oxidation behaviour of a thermal barrier coating (TBC) system is a major concern as the growth of the thermally grown oxide (TGO) layer on the bond-coating creates stresses that greatly favour the thermal barrier spallation. To delay the loss of the thermal protection provided, research has focused on the bond-coating composition and microstructure as well as on the parameters required for a suitable pre-oxidation treatment before the deposition of the ceramic top coat. Platinum is known to enhance the oxidation/corrosion resistance of MCrAlY coatings. The effect of Pt on the oxidation behaviour of a NiCoCrAlYTa coating was assessed in this study. In addition, pre-oxidation treatments were conducted to determine if the oxidation behaviour of the modified NiCoCrAlYTa coating could be further improved.

In the last decade, an increasing interest was given to Pt-rich γ–γ′ alloys and coatings as they have shown good oxidation and corrosion properties. In our previous work, Spark Plasma Sintering (SPS) has been proved to be a fast and efficient tool to fabricate coatings on superalloys including entire thermal barrier coating systems (TBC). In the present study, this technique was used to fabricate doped Pt-rich γ–γ′ bond coatings on AM1® superalloy substrate. The doping elements were reactive elements such as Hf, Y or Zr, Si and metallic additions of Ag. These samples were then coated by electron beam physical vapour deposition (EBPVD) with an yttria partially stabilized zirconia (YPSZ) thermal barrier coating. Such TBC systems with SPS Pt rich γ–γ′ bond coatings were compared to conventional TBC system composed of a β-(Ni,Pt)Al bond coating. Thermal cycling tests were performed during 1000-1h cycles at 1100°C under laboratory air. Spalling areas were monitored during this oxidation test. Most of the Pt rich γ–γ′ samples exhibited a better adherence of the ceramic layer than the β-samples. After the whole cyclic oxidation test, cross sections were prepared to characterize the thickness and the composition of the oxide scales by using scanning-electron microscopy. In particular, the influence of the doping elements on the oxide scale formation, the metal/oxide roughness, the TBC adherence and the remaining Al and Pt under the oxide scale were monitored. It was shown that RE-doping did not improve the oxidation kinetics of the studied Pt rich γ–γ′ bond coatings, nevertheless most of the compositions were superior to “classic” β-(Ni,Pt)Al bond coatings in terms of ceramic top coat adherence, due to lower rumpling kinetics and better oxide scale adherence of the γ–γ′-based systems.

The kinetics of isothermal oxidation (1100°C) of commercial nickel-based alloys with different content of sulfur (0.22–3.2 wt ppm) is studied. The adhesion strength in a metal/oxide system is estimated as a function of sulfur content and duration of high-temperature exposure. The scratch-test technique is proposed to quantitatively estimate the work of adhesion of resulting oxide films. It is found that the film microstructure is composed of an inner α-Al2O3 layer and an outer NiAl2O4 spinel layer, which are separated by discrete inclusions of TiO2. Residual stresses in the oxide film are experimentally determined by X-ray diffraction.

Cyclic and isothermal oxidation behaviors of first and fourth-generation superalloys AM1 and MCNG were investigated to evaluate the ability of the scratch test to quantify the adhesion of multi-layered oxide scales. Effects of sulfur content and of scale thickness were studied independently. Available models lead to large discrepancies in the calculated work of adhesion values with the evaluation of the residual stress being the largest source of error. Nevertheless, models can assess the effect of sulfur content and the scratch test can be used to correlate the long-term cyclic oxidation behavior and the adhesion of oxide scales. (C) 2010 Elsevier Ltd. All rights reserved.

Thermal barrier coatings (TBC) are commonly used in modern gas turbines for aeronautic and energy production applications. The conventional methods to fabricate such TBCs are EB-PVD or plasma spray deposition. Recently, the spark plasma sintering (SPS) technique was used to prepare new multilayered coatings. In this study, complete thermal barrier systems were fabricated on single crystal Ni-based superalloy (AM1®) substrate in a one-step SPS process. The lifetime of TBC systems is highly dependent on its ability to form during service a dense, continuous, slow-growing alumina layer (TGO) between an underlying bond coating and a ceramic top coat. In the present paper, we show that such kind of layer (called proto-TGO in the following) can be in situ formed during the SPS fabrication of TBC systems. This proto-TGO is continuous, dense and its nature has been determined using TEM-EDS-SAD and Raman spectroscopy. This amorphous oxide layer in the as-fabricated samples transforms to α-Al2O3 during thermal treatment under laboratory air at 1100°C. Oxidation kinetics during annealing are in good agreement with the formation of a protective α-Al2O3 layer.

A mixture of CoNiCrAlY and talc powders is considered as a new candidate composition for abradable seal coating applications. Dense specimen having the composition of 1:20 weight ratio of talc with respect to CoNiCrAlY was prepared using the Spark Plasma Sintering (SPS) technique. The aim of the present article is to investigate the reactivity and microstructure evolution of the β/γ-CoNiCrAlY based cermet. The resulting microstructures were analysed and their compositions determined using standard analytical techniques such as SEM, TEM and X-ray diffraction. After fabrication, the bulk of the material is shown to contain a continuous oxide layer of MgAl2O4 at the periphery of metallic particles, resulting from the reaction between aluminium, which has diffused from the bulk of CoNiCrAlY grains, with magnesium and oxygen delivered during the high temperature decomposition of the talc phase. Thermodynamic calculations results are found to be consistent with the experimental observations. The oxidation behaviour at a high temperature of this cermet was also investigated. It was shown that at its external surface a continuous double layer is formed — one external film at the surface of the sample made of MgAl2O4 and the second one more internal in between the later and the cermet made of α-Al2O3. The oxide scale is protective with low oxidation kinetics typical of alpha alumina growth (kP=1.8⋅10−7mg2⋅cm−4⋅s−1 at 1050°C in flowing dry air).

A new promising and versatile process based on the sot gel transformation has been developed to deposit yttria-stabilised thermal barrier coatings. The non-oriented microstructure with randomly structured pore network, resulting from the soft chemical process, is expected to show satisfactory thermo-mechanical behaviour when the TBC is cyclically oxidized. First stage of the research consists of optimizing the processing route to generate homogeneous microstructure and controlled surface roughness. The objective is to reduce, as much as possible, the size and depth of the surface cracks network inherent to the process. Indeed, the durability of the TBC when cyclically oxidized strongly depends on the sharpness of those cracks that concentrate thermo-mechanical stresses and generate detrimental propagation resulting in spallation. Cyclic oxidation tests are performed using a cyclic oxidation rig instrumented with CCD cameras to monitor in a real time basis the mechanism of crack propagation and spallation. The impact of various parameters either directly related to the processing route, e.g. the intimate microstructure of the TBC and the TBC thickness, or to the thermal loading, e.g. the oxidation temperature and the cumulated hot time, on the durability of the TBC is investigated. (C) 2010 Elsevier B.V. All rights reserved.

Few studies have already shown that Pt influences the diffusion of aluminium and therefore the microstructure of β-NiAl or γ-Ni/γ′-Ni3Al materials. Besides, several works have revealed that the addition of Pt to MCrAlY (M=Ni and/or Co) improves the oxidation/corrosion behavior of the material. Nevertheless, very few data have been published on the microstructure of such modified MCrAlYs. Then, the present work deals with the addition of Pt to two NiCoCrAlYTa coatings that differ by their manufacturing process. Characterization is carried out in order to understand the influence of Pt diffusion but also the effect of the manufacturing process on the final microstructure. The collected data from XRD, SEM, EDS and TEM analyses reveal that an Al uphill diffusion occurs during heat treatment due to the presence of the Pt layer. The Al diffusion from the NiCoCrAlYTa bulk to the Pt-rich surface is so extensive that no more β-phase remains within the core of the coating. Pt may also dissolve TaC, precipitates largely present in the non-modified NiCoCrAlYTa coatings. In addition to Pt, the microstructure of the NiCoCrAlYTa prior to Pt deposition and heat treatment, dependent on the NiCoCrAlYTa manufacturing process, greatly influences the final microstructure.