Microstructural instabilities occurring in two new generation single crystal nickel-based superalloys containing additions of both Re and Ru have been characterised by means of electron backscatter diffraction (EBSD). Cellular colonies develop in the MC632 experimental alloy at subgrain boundaries with misorientations higher than 10°. The role of these microstructural instabilities in the mechanical behaviour was investigated. After creep at high temperature microcracks are observed at the limit of the cellular colonies but these defects are not more frequent than those initiating from micropores or within the dendrites. As only a low volume fraction of γ-γ' alloy is involved in this transformation, the influence of this phenomenon on the alloy creep strength is negligible. Secondary reaction zones (SRZ) were observed beneath Pt-modified aluminide coating in the MC544 superalloy. EBSD analysis clearly demonstrated the polycrystalline structure of these SRZ. Despite the presence of internal grain boundaries, the SRZ are not preferential sites for premature failure initiation. However, the stress-rupture life is adversely affected by the reduction of the load-bearing section of safe γ-γ' alloy. The decrease of low cycle fatigue (LCF) strength observed at 650°C for coated MC544 compared to the bare alloy was mainly attributed to premature crack initiation within the aluminide coating and not from the SRZ. During LCF tests at 950°C of coated MC544, numerous cracks also initiate from the aluminide coating but then remain restricted to the SRZ area. The failure initiates at internal casting porosities. The slight LCF strength reduction observed in these conditions was attributed to the load bearing section reduction of unaffected γ-γ' alloy.

A Co-Ni-based alloy strengthened by '-(L1 2) precipitates was utilized to investigate the precipitation evolution after various cooling rates and several aging conditions. In this study, the precipitate size and volume fraction have been studied via scanning electron microscopy and transmission electron microscopy. The influence of the precipitation evolution was measured via microhardness tests. The cooling rate study shows a more sluggish ' precipitation reaction compared with that observed in a Ni-based superalloy. Following a rapid cooling rate, the application of appropriate double aging treatments allows the increase of the ' volume fraction as well as the control of the size and distribution of the precipitates. The highest hardness values reach those measured on supersolvus cast and wrought Ni-based superalloys. The observed ' precipitation behavior should have implications for the production, the heat treatment, the welding or the additive manufacturing of this new class of high temperature materials.

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Abstract. Chromium-based alloys are considered as potential candidates for applications in hot sections of aero engine turbines due to their high melting point and their moderate density. Some ternary (Cr-Ni-Al) and quaternary (Cr-Ni-Al-Ti) alloys have been designed with the aim to induce precipitation of ordered strengthening precipitates and to promote the formation of a protective alumina scale. These alloys were cast using various techniques in order to assess their microstructural and mechanical properties and their oxidation resistance.

Abstract. Various solution heat treatment sequences including a hot isostatic pressing step were tested on the MC-NG fourth generation single crystal superalloy with the aim to minimize the total duration of the treatment while obtaining a satisfactory level of microstructural homogeneity and low amount and size of pores. A procedure including the HIP step after full solutioning of the material allows to avoid reopening of the pores but needs a short post-HIP heat treatment followed by air quenching to get the desired γ precipitate size. The total duration of the solution heat treatment has been reduced from 18 to 6 hours as compared with the previous standard procedure.