Effect of HfO₂ dispersoids on the microstructure of a Ni-Cr-Al-Ti superalloy processed by laser-based powder-bed fusion

The effects of 1 wt% HfO₂ nano-dispersoid addition on the microstructure of a high-γ' Ni-8.5Cr-5.5Al-1Ti (wt%) model superalloy are investigated after manufacturing via laser-based powder-bed fusion (PBF-LB). Despite their very high melting point, HfO₂ dispersoids are not fully stable during their short stay in the melt pool. At the nanoscale, the superalloy grains contain various Hf-Al-O-, Hf-O- and Hf-S-rich nano-dispersoids, as well as γ' L1_2[sbnd]Ni₃Hf nano-precipitates, reflecting reaction of HfO₂ dispersoids in the melt. At the meso‑scale, Hf-Al-O-rich slag inclusions are embedded in the metallic matrix, exhibiting a two-phase HfO₂-Al₂O₃ eutectic structure. At the macroscale, millimeter-long cracks form at the boundaries of the elongated, highly (100) textured grains, indicative of solidification cracking. The critical role of Al and O in the superalloy melt in reacting with oxide nano-dispersoids during PBF-LB manufacturing, previously reported for Y₂O₃ nano-dispersoids, is observed to occur here for HfO₂ dispersoids as well, despite their higher stability, melting point and density and their lack of reactivity with Al₂O₃.