PSI - Issue 83

Daniele Cortis et al. / Procedia Structural Integrity 83 (2026) 3–13

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structure, but it promoted the precipitation of additional and more homogeneously distributed secondary Al ₃ (Sc,Zr) nano particles within the α (Al) matrix, even with a low content of Sc (Table 1) compared to the more common SCALMALLOY ® [Belelli et al. (2022)]. The PBF-LB process produces a high density of dislocations and many grain boundaries, that promote the diffusion of Sc and Zr and the growth of Al 6 Mn and Al ₃ (Sc,Zr) nano particles during the aging [Tang et al. (2024)]. Moreover, Al 3 (Sc,Zr) nano particles possess a cubic symmetry L1 2 structure, where the Sc and Zr occupy the corners of the cube, and Al atoms the face-centered positions. Because of the different diffusion rates between Sc and Zr (i.e., Zr diffuses much more slowly than Sc), the nano precipitates form a core-shell structure with an Sc-rich core surrounded by a Zr-rich shell. If the Al 3 (Sc) precipitates tend to grow above 300-350 °C, the Al 3 (Zr) shell protect the core keeping the nanophase stable [Tolley et al. (2005)]. 3.3. Fracture Fractographic analysis was carried out by means SEM (i.e., Hitachi S-2500) to investigate the fracture behavior at different temperatures. Observing the fracture surfaces of the round cross-section test piece tested at -100 °C, Fig. 8 (a), it is possible to appreciate a mixed fracture mode, where ductile features are predominant compared to fragile, as highlighted by Fig. 8 (b). At low temperature, the material seems to be more sensitive to metallurgical defects, which act as stress concentrators and promote premature fracture. Also, the fracture surfaces revealed the presence of lack of-fusion defects within un-melted powder, as shown in Fig. 8 (c).

(a)

(b)

(c)

Mixed fracture mode

Lack of fusion

Fig. 8. SEM micrographs of different fracture surfaces after tensile test at -100 °C.

At room temperature (+22 °C), the fracture surfaces still exhibited a sensitivity to lack-of-fusion defects visible in Fig. 9 (a), although most of the fracture surface showed a ductile morphology with extensive dimples formation Fig. 9 (b). Fracture surfaces at high temperatures (+100 °C and +200 °C) highlighted the pronounced ductility of the material, as confirmed by the presence of a fully dimpled fracture morphology, Fig. 10 (a) and Fig. 11 (a). Examining the SEM micrographs, it is evident that the increase in temperature leads to a higher density of very fine dimples, corresponding to the more pronounced plastic deformation of the alloy. Also, a careful examination reveals that no intermetallic precipitates or inclusions are present within the dimples. Consequently, at higher temperatures, the mechanical response of the alloy is primarily dominated by its microstructural texture. Further observation revealed the occasional presence of un-melted powder particles within the dimples, Fig. 10 (b) and Fig. 11 (b), due to the local lack of fusion during the part production process. These experimental observations are in agreement with the results of the tensile tests.