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

·Despite HfO2 high stability, the dispersoids react with the molten alloy during AM.·This reaction leads to the formation of a multitude of Hf-rich nanophases.·Numerous large Hf-Al-O-rich slag inclusions are distributed throughout the part.·The dissolved Hf and its local enrichment along grain boundaries promote cracking. 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' L12Ni3Hf 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. [Display omitted]