THE MORPHOLOGICAL-CHANGES OF GAMMA'-PRECIPITATES IN A NI-8AL (WT PCT) ALLOY DURING THEIR COARSENING

Abstract

When a Ni-8Al (wt pct) alloy is aged at 1020-degrees-C (30-degrees-C below the solvus temperature of about 1050-degrees-C) by cooling directly from the solution-treatment temperature, the gamma' precipitates undergo Ostwald ripening from the early stage of the aging treatment because the nucleation rate is relatively high. The gamma' precipitates are initially spheres and, as they coarsen, become cubes which then split into eight smaller cubes (octets) or clusters with square and rectangular faces. The observed critical sizes for these shape transformations are fairly close to those predicted by Khachaturyan et al. on the basis of the elastic strain effect arising from lattice misfit if the lattice parameters measured by Hornbogen and Roth are used. As the octets coarsen, they appear to transform into doublets of plates. Because shapes other than those considered by Khachaturyan et al. are also observed, only the sphere-cube and cube-octet transformations can be compared with the theoretical prediction without ambiguity. At temperatures above 1020-degrees-C, dendritic growth occurs, and at low temperatures, the precipitate number density is so high that apparently the elastic fields of the precipitates overlap, rearranging the precipitates into aligned cubes or plates. These observations show that the shape evolution of the isolated precipitates can be observed only in a narrow aging temperature range where the nucleation rate is moderately high but where the precipitate volume fraction is relatively low.