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 -y' 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 y' L12-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.