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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the trends observed in tensile properties to some extent. In the low-cycle region, direct aged PBF-LB/M IN939 batches exhibit the highest cyclic stresses, and the lowest resistance to cyclic plastic straining. The slopes of the corresponding stress-life curves indicate that there is a crossover; considering vertical specimen orientation, the crossover occurs at ca. 20000 cycles. Resultingly, other PBF-LB/M batches exhibit overall better fatigue performance in the high-cycle region, i.e., when fatigue life is dominated by crack growth. It is thus evident that the direct aged PBF-LB/M variant exhibits low resistance to crack growth due to the reduced ductility. This is confirmed by the fractography (Fig. 5) which does not reveal any signs of stable crack growth area on any of the fracture surfaces of direct aged IN939 variant. Striations were not observed, and a typical decrease in stress amplitude towards the end of life, indicative of the growth of the principal crack, was not observed (Fig. 4c). This altogether indicates that cracking was rather sudden. Cracking occurred in a mixed mode. Occasional cleavage planes can be recognized on fracture surfaces regardless of the specimen orientation, see Fig. 5a. These planes likely correspond either to grain boundaries or cell walls. Vertical specimens exhibit short, intergranular, wedge-type secondary cracks running approximately parallel to the loading and building direction. Horizontally-oriented specimens exhibit a larger tendency to intergranular cracking as alternating ridges and valleys corresponding to the cracking along the elongated grains are commonly occurrent over the fracture surfaces, see Fig. 5b. The tendency to intergranular cracking is likely due to the directional microstructure with a large portion of grain boundaries oriented parallel to the maximum opening stress. Qualitatively similar cracking features were previously observed on horizontally-oriented fully heat-treated PBF-LB/M IN939 specimens (Babinský et al. (2025)).

Fig. 4. Fatigue life curves: (a) Basquin-type stress-life and (b) Coffin-Manson-type plastic strain-life representation. (c) Cyclic hardening/softening curves ( ε a = 0.5%).

Fig. 5. SEM micrographs show representative fracture surfaces of fatigued direct aged PBF-LB/M IN939 superalloy with (a) vertical ( ε a = 0.5%) and (b) horizontal orientation ( ε a = 0.5%). Arrows indicate secondary cracks and cleavage planes.