The influence of iron and aluminum on the kinetics of precipitation in NiTa alloys

Abstract

The metastable γ′ and γ′′ phases can be precipitated independently of each other in NiTa alloys by employing small additions of iron and aluminum. The relatively small coherency strains and surface energies associated with both types of precipitate particles result in a high resistance to particle coarsening in alloys of appropriate composition. The kinetics of the γ′′ precipitation are determined by diffusion-controlled precipitate growth, and coarsening does not begin until after 300 h at 973 K. Although the γ′ precipitates begin to coarsen at earlier times, the rate of coarsening is extremely slow and is controlled by the diffusion of aluminum. The difference in the observed growth rates for the γ′′ phase between the ternary and quaternary alloys has been identified with the influence of aluminum and iron additions on the solubility of tantalum in the matrix phase.