MICROSTRUCTURAL CHARACTERIZATION OF NI3AL PROCESSED BY REACTIVE ATOMIZATION AND DEPOSITION

Reactive atomization and deposition (RAD) is a new processing techniqu e that has been developed to synthesize dispersion-strengthened materi als. In this process, atomization, in situ reaction, and consolidation are combined into a single step by spray atomization and deposition w ith a reactive gas. The matrix material selected for this study is an Ni3Al + Y + B alloy in combination with N-2-O-2 atomization gas. The a s-deposited microstructures reveal a spheroidal grain morphology, a ba nded structure, and a gamma + gamma' mosaiclike structure. The formati on of the gamma + gamma' mosaiclike structure is attributed to an anne aling effect during deposition. Matrix-lattice mismatches of 0.5 to 1 pct at the gamma/gamma' interface and {100} growth orientations of gam ma' phase are deduced from microscopic observations. The formation of the banded structure is attributed to the high cooling rate that is in herent to RAD processing. Anticipated dispersoids, such as gamma(2)O(3 ), Al2O3, and Y3Al5O12 are identified using transmission electron micr oscopy (TEM). Dislocation pileups and grain boundary pinning are obser ved in the vicinity of oxide dispersoids. The origin and movement of d islocations in the as-deposited materials may be attributed to the res idual stresses that originate from thermal gradients and the large amo unt of deformation experienced by the solid and semisolid droplets dur ing impact. The preliminary results and analyses reported here suggest that the high thermal stability of the RAD processed Ni3Al using N-2- 15 pct O-2 may be attributed not only to the hindering effect of oxide dispersoids on grain boundary migration, but also to the high cooling rate experienced by the droplets during atomization and the short ann ealing effect experienced by the material during deposition.