Characteristics of rapidly solidified powder particles in Hf-modified nickel-based P/M superalloys

The effects of Hf content on the microstructure, dendrite microsegregation and precipitated phase of FGH96 superalloy powders were investigated by scanning electron microscopy, transmission electron microscopy with energy-dispersive spectrometry, and carbon extraction method. It is found that Hf content can change the proportion of dendrite, cellular and microcrystal structures. The microstructures of the superalloy powders mainly depend on cooling rate and the ratio of thermal gradient to solid/liquid interface velocity. Nb, Ti, Zr and Al elements enrich between the dendrite arms, but Co, Cr, W and Ni elements enrich in the dendrite axis for all FGH96 superalloy powders with different Hf contents. The dendrite microsegregations of Ti, Nb, Zr and Hf elements are the least when the mass fraction of Hf is 0.3%. Hf is more sensitive to the oxygen content than the carbon of the rapidly solidified powder particles, and forms the stable oxide of HfO₂ firstly.