Microstructural characterization of Ni₃Al processed by reactive atomization and deposition

Abstract

Reactive atomization and deposition (RAD) is a new processing technique that has been developed to synthesize dispersion-strengthened materials. In this process, atomization,in situ reaction, and consolidation are combined into a single step by spray atomization and deposition with a reactive gas. The matrix material selected for this study is an Ni₃Al + Y + B alloy in combination with N₂-O₂ atomization gas. The as-deposited microstructures reveal a spheroidal grain morphology, a banded structure, and a γ + γ′ mosaiclike structure. The formation of the γ + γ′ mosaiclike structure is attributed to an annealing effect during deposition. Matrix-lattice mismatches of 0.5 to 1 pct at the γ/γ′ interface and {100} growth orientations of γ′ phase are deduced from microscopic observations. The formation of the banded structure is attributed to the high cooling rate that is inherent to RAD processing. Anticipated dispersoids, such as Y₂O₃, Al₂O₃, and Y₃Al₅O₁₂ are identified using transmission electron microscopy (TEM). Dislocation pileups and grain boundary pinning are observed in the vicinity of oxide dispersoids. The origin and movement of dislocations in the as-deposited materials may be attributed to the residual stresses that originate from thermal gradients and the large amount of deformation experienced by the solid and semisolid droplets during impact. The preliminary results and analyses reported here suggest that the high thermal stability of the RAD processed Ni₃Al using N₂-15 pct O₂ may be attributed not only to the hindering effect of oxide dispersoids on grain boundary mi-gration, but also to the high cooling rate experienced by the droplets during atomization and the short annealing effect experienced by the material during deposition.