PSI - Issue 52

Marie Kvapilova et al. / Procedia Structural Integrity 52 (2024) 89–98 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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microscope equipped with the energy dispersive X-ray analysis (EDS) accessory and NordlysNano EBSD detector. Details of the microstructural and fractographic methods used can be found in our earlier papers (Sklenicka et al. (2018), Kvapilova et al. (2021)). 3. Results and discussion 3.1 . Initial microstructure of the alloy The coarse-grained dendritic microstructure of as-cast GTD 111 superalloy has an average dendritic grain size of ~ (2.5 ± 0.9) mm (Fig. 1(a)). The superalloy has a multiphase microstructure. The major phases in the microstructure are the gamma matrix γ and the bimodal γ´ (Ni 3 (AlTi)) intermetallic phase located within dendrite cores and in the interdendritic regions, which is coherent with the matrix (Fig.1(b )). The primary γ´precipitates are present with 60% volume fraction and have a cubic shape with 800 nm. By contrast, the average size of fine more likely spherical secondary γ´ precipitates was of about 60 nm. The γ´precipitates located wit hin dendrites are smaller in size than the precipitates located in the interdendritic region. The primary γ´precipitates are separated by narrow γ -channels containing small secondary γ´precipitates (Fig.1(c)). Further components of the microstructure are γ - γ´eutectics, bulk carbides with high content of Ta, Ti and W (Fig.1(b)), chromium carbides MC and M 23 C 6 and small volume content of TPC-phases such as sigma, eta and/or Laves phase (Lee et al. (2012), Long et al. (2009), Qin et al. (2008)).

Fig. 1. Cross-sectional SEM images showing the microstructure of the GTD 111 superalloy in the virgin as-cast state: (a) dendritic microstructure, (b) γ phase and γ´precipitates and bulk carbides, and (c) detail of primary and secondary γ´precipitates.

3.2. Microstructure after creep In the course of creep, mild coarsening of primary γ´precipitates occurs and their shapes change from cubic to ellipsoidal and/or spherical. It was observed that the size of primary γ´precipitates increases with increasing temperature and the time of creep exposure. However, coarsening of primary γ´precipitates is not significantly sensitive to the applied tensile stress perhaps due to very small misfit between the matrix/γ´. The growth kinetics, coarsening behaviour, and morphology change of primary γ´ pr ecipitates in GTD 111 superalloy were studied by Berahmand and Sajjadi (2012) and Wang et al. (2015). At testing temperatures higher than 800°C a disappearance of fine secondary γ´ precipitates in γ -channels was observed. In addition, at 950°C there is the onset of coalescence of primary γ´precipitates.