PSI - Issue 82

Tomáš Babinský et al. / Procedia Structural Integrity 82 (2026) 162–168 Tomáš Babinský et al. / Structural Integrity Procedia 00 (2026) 000–000

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JEOL JEM-2100F. TEM observations were performed at 200 kV in a scanning mode (STEM) using a bright field (BF) and high angle annular dark field detector. Image analysis aiming at estimating precipitate size, precipitate fraction and cell size was done using ImageJ software. 3. Initial State Relative density of 99.14±0.04% was estimated before the heat treatment using the Archimedes method, considering the reference density of 8200 kg/m 3 . Direct ageing at 800 °C/8 h does not significantly alter grain texture and hierarchical character of microstructure present in the as-built PBF-LB/M IN939 (see Šulák et al. (2023) and Babinský et al. (2025)), as this temperature is not sufficiently high to impose overall recrystallization (Dogu et al. (2023)). Grains are elongated along the building direction with a moderate <001> texture (MUD 3.5–5). The granular structure is highly irregular with irregular shapes and a wide distribution of grain sizes (see Fig. 2a) with small, few-μm-sized grains at the meltpool boundaries, and large grains, some sized over 100 μm, located close to the meltpool centres (Pham et al. (2020)). Fig. 2b and Fig. 3a show intragranular structure consisting of dislocation cells elongated along a <001> direction, which are sometimes clustered in subgrains. Typical misorientation across a cell boundary and subgrain boundary is around 0.5° and 2°, respectively. The resulting honeycomb structure shown in Fig. 3a occurs in many PBF-LB/M fcc metals (see e.g. Wan et al. (2018) and Bean et al. (2022)). No cell coarsening was observed as a result of the direct ageing treatment as the observed differences fall within statistical error (0.55±0.07 μm in the as-built state and 0.58±0.06 μm in the direct aged state). As a reference, the cell size following the full heat treatment coarsens to 0.79±0.17 μm. Temperature was, however, sufficiently high to promote precipitation of new phases, both desirable and undesirable. Fig. 3b shows a unimodal distribution of (desirable) γ’ precipitates which contrasts with a bimodal precipitate size distribution in the fully heat-treated PBF-LB/M IN939 (Babinský et al. (2025)). Precipitates with an average size of 26.0±5.4 nm observed after direct ageing are slightly larger than the fine precipitates created by the full heat treatment (21.9±3.7 nm). Complementing the dispersion of fine submicron MC carbides present in the as built state, further precipitation of fine submicron carbides (likely M 23 C 6 ) was observed mostly at grain boundaries. Unlike in Kanagarajah et al. (2013), no TCP or Laves phases were observed. However, an undesirable γ/γ’ eutectics with a fan-like morphology were observed at internal interfaces, preferentially at grain boundaries and at dislocation cell walls such as in Fig. 2b. Such uncontrolled nucleation of γ/γ’ eutectics is linked with deterioration of high temperature properties by promoting hot cracking as shown by Li et al. (2016). The presence of γ/γ’ eutectics is most likely the consequence of omitting the solutioning heat treatment, which reduces chemical heterogeneity in the material. No eutectics whatsoever were observed in the fully heat-treated state by Babinský et al. (2025).

Fig. 2. SEM ECCI micrographs show the microstructure of direct aged PBF-LBM IN939 superalloy, focusing on (a) elongated grains and (b) γ/γ' eutectics at internal interfaces.