Microstructure of atomized Ni3Al–B powder

Publisher Website
Google Scholar
Abstract
The microstructure of boron-doped Ni3Al powder has been characterized by optical and transmission electron microscopy. Remarkable variations in the solidification morphology and phase reaction were observed as a function of powder size. The degree of boron segregation appeared to be reduced significantly, as the formation of M23B6 boride was suppressed in powder below ∼30 μm diameter. The primary solidification phase was disordered γ–Ni, which underwent a solid-state ordering transition to produce fine γ′–Ni3Al antiphase domains. A bi-modal distribution of domain sizes was observed due to Al segregation. The solidification morphology was dendritic for particles above ∼20 μm, and it changed to cellular below. In powder less than 40 μm diameter, second-phase particles of β–NiAl were found in intergranular regions, likely due to a metastable eutectic reaction of L ⇉ γ + β. The β phase was suppressed only for powder sizes below ∼2 μm. Judging from the tendency of second-phase formation, the Ni3Al–B powder appeared to cool slower than arc-hammer foil and melt-spun ribbon.