Experimental and Theoretical Measurements of the Evolution of Embryos Before and During the Nucleation Stage

The early precipitation of a Cu-Ni-P alloy during aging for 100 ks at 523 K and 623 K (250 A degrees C and 350 A degrees C) after solution treatment has been characterized using a three-dimensional atom probe (3DAP) and transmission electron microscopy (TEM). It is shown that the particles have a wide range of Ni/P ratios when they are relatively small, whereas larger ones exhibit a narrow distribution of the Ni/P ratio, approaching the ratio of approximately two. The threshold radii that show the steady Ni/P ratio are around 1.5 nm and 2.0 nm for the materials aged at 523 K and 623 K (250 A degrees C and 350 A degrees C), respectively. These values are in a reasonably good agreement with the critical nuclei radius estimated from classic nucleation theory. It is suggested that the particles with steady Ni/P ratios of approximately two are considered to be the equilibrium precipitates formed through nucleation, whereas the extremely fine particles with varying Ni/P ratios, detected by the 3DAP experiments, indicate subcritical clusters or embryos.