Coarsening kinetics of γ′ precipitates in Ni-Al and Ni-Al-Mo alloys

Abstract

The ordered L1₂ γ′ precipitate coarsening kinetics without the influence of an external stress were studied in a Ni-Al(13.5 at%) alloy at 1413 K and a Ni-Al(13.8 at%)-Mo(5.9 at%) alloy at 1443 K. The Ni-Al-Mo alloy has a lattice mismatch of about −0.3% at the ageing temperature while the Ni-Al has a positive lattice mismatch of about 0.25% at the ageing temperature. For both alloys, the precipitates were initially cuboidal. After ageing for 3–10 min, the precipitates in the Ni-Al-Mo alloy split mostly into two parallel plates (doublets) or eight sub-cubes (octets), but the initial cuboidal precipitates in the Ni-Al alloy only showed the tendency to split into doublets. After further ageing, the precipitates in both alloys eventually aligned and agglomerated into groups consisting of many particles separated by a small distance of ∼30 nm, and the distribution of the precipitates became inhomogeneous. There was no linear relationship between the cube of the average precipitate size and the ageing time as predicted by the classical Lifshitz-Slyozov-Wagner theory. Instead, a retardation of the coarsening process is found.