ICMCTF2013 Session AP: Symposium A Poster Session

Commercial, regulatory and social pressures have led the energy industry to improve power plant efficiency by increasing the operating temperatures and pressures of combustion plant, moving from conventional plant, with operating steam temperatures and pressures of 540 °C and 180 bar, to advance ultra-supercritical plant with steam temperatures >700 °C and pressures > 300 bar. Such an increase in operating conditions would increase the plant efficiency from 42% to an estimated 55%. These increased operating conditions place current materials in extremely aggressive environmental conditions. Current ferritic-martensitic steels do not possess the high temperature capability, with a temperature limit of ~620 °C. There is a need therefore to move towards using more austenitic alloys. Experience of austenitic materials in the US has shown that some alloys are susceptible to early spallation under steam oxidation conditions leading to rapid breakaway oxidation. One method to prevent this is the use of high temperature protective coatings. Aluminide coatings have been developed for use on ferritic steels with encouraging results. These coatings combine good high temperature oxidation resistance through the growth of an Al₂O₃ scale and, in the case of Al slurry, the possibility to apply the coating on an industrial scale at moderate cost. However, there is little information and understanding of the behaviour of the same coatings on austenitic alloys. The purpose therefore of the work reported here was to examine and generate the necessary data on the microstructural evolution of Al slurry coated austenitic alloys subjected to oxidising atmosphere of 100% flowing steam at 700 and 750 °C for times up to 5000 hours at atmospheric pressure. The coating prevented breakaway oxidation on samples exposed at 700 °C and delayed the onset of breakaway until 3000 hours at 750 °C showing the protective potential of the coating. Measurement of the Aluminium diffusion showed that it diffused outwardly to form a protective Al₂O­₃ layer and inwardly into the substrate to form an Aluminium diffusion zone and precipitates of AlN and Ni-Al. Despite differences in Al diffusion behaviour between the three austenitic alloys, indicating an influence of initial composition and grain size, the time at which breakaway oxidation occurred was identical, suggesting that the Al slurry coating provided the same level of protection on the three alloys.

This paper presents the steam oxidation results for the three coated alloys comparing the oxidation behaviour between the three and also referencing back to the performance of the uncoated alloys.

The paper concerns behavior of Rene N5 2nd generation single crystal Ni superalloy during isothermal oxidation in dry oxygen at 1050°C, 1100°C and 1150°C for 100h. The oxidation tests were performed using Mettler Toledo apparatus for isothermal oxidation tests with high accuracy mass control. The mass change curves of the samples oxidized at 1050°C, 1100°C and 1150°C are presented along with corresponding microstructures. High resolution CTEM and S/TEM techniques were applied for a detailed characterization of oxide scales formed after 100h of oxidation at 1050°C, 1100°C and 1150°C for a direct comparison of the phase composition and chemistry of the oxides and the interfaces between them. High resolution S/TEM EDS elemental mappings were performed in order to study segregation of elements to grain boundaries of the alumina scales. The samples for S/TEM analysis were prepared using FIB (Focused Ion Beam).

Progresses in the field of gas-turbine engine for aircraft are controlled by the availability of structural materials able to withstand higher-temperature hostile environments (very significant flow conditions containing aggressive elements such as water vapor, at more than 1150°C). The efficiency of intermetallic silicides Ti₃X₃CrSi₆ (M₇Si₆-TiX with X=Fe,Co or Ni) as protective coatings (bond coats) for niobium alloys against oxidation was demonstrated through many works. These compounds are manufactured by halide activated pack-cementation technique. During oxidation tests, these coatings develop a duplex protective chromia /silica oxides scale. These refractory silicides present a ductile brittle temperature transition around 900-1000°C that led sometimes to the formation of cracks throughout the coatings. This behavior is mainly observed when the coated pieces endure rapid change of temperature, e.g, oxidation in cyclic conditions. Therefore boron was added in these coatings by the cementation way in order to increase the fluidity of silica and to obtain self-healing oxides scale.

Whatever the nature of the matrix around, boron is generally difficult to characterize both qualitatively and quantitatively using SEM and its associated analyses techniques (EDS and WDS spectrometry) based on X-ray emission because of the low rate of emission of this light element. In the present case, the complex chemical composition of the substrate (Nb, Ti, Hf, Fe, Cr, Si) renders still more difficult the characterization of boron because of the spectral interference which exists between the Nb (M_z) and B (K_α) X-ray. To separate both contributions, those of boron and niobium, a coupled WDS-EDS system was employed. Elaboration of boron containing standards was also required.

The present paper aimed at presenting the optimization of the analytical conditions that allowed the location of boron in the coating both post-manufacturing and post-oxidation tests.

Layered thermal barrier coatings are widely used to protect underlying superalloy components from high temperature oxidation. Commercial systems are typically composed of a yittra-stabilized zirconia (YSZ) top coat, a thin thermally grown oxide (TGO), and an intermetallic bond coat on top of the superalloy substrate. Interfacial delamination between the top coat and the bond coat are often observed after cyclic thermal exposure, due in part to the stresses that arise from the mismatch of thermal expansion coefficients of the materials. To evaluate the thermally induced residual stress and predict resulting interfacial failure, the Young’s modulus of the top coat is of primary interest. In this study, micro-beam bending techniques for both attached and free-standing YSZ coatings has been developed to assess the as-deposited top coat modulus as a function of substrate geometry. A tension/compression asymmetrical behavior of top coat was observed, and finite element analysis was carried out to quantify the modulus values.