Relationships Between Microstructural Instabilities and Mechanical Behaviour in New Generation Nickel-Based Single Crystal Superalloys

Abstract

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.

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“…6. From the micrograph of the longitudinal cross-section of specimen LSP1-4, the emergence of two varied failure modes can be obtained: (i) the slip bands with crystallographic orientations [14][15][16] and (ii) the subgrain boundaries perpendicular to the external loading [17]. It is noteworthy that one can see void formation frequently occurring along the subgrain boundaries inside the material, see Fig.…”

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“…A noter en particulier que pour les superalliages les plus récents (dits de 4`e me génération), une zone de réaction secondaire peut se développer par interaction avec la couche de liaison [19]. En conséquence, la section utile du superalliage peut s'en trouver réduite, ce qui peut être néfaste à l'endroit de sections minces.…”

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