ABSTRACT
In the context of solid-solid phase transformations, precipitation is the formation by diffusional nucleation and growth of a phase that differs in composition from its matrix phase at all stages of its development. It follows immediately from this definition that the long-range volume diffusion of solute in interstitial alloys and the long-range volume interdiffusion of solute and solvent in substitutional alloys is the ultimate kinetic factor limiting the kinetics of both nucleation and growth. Another consequence of this definition is that whenever the boundary or boundaries of a singlephase field vary in composition with temperature, a precipitation process is possible as long as the homologous temperature is sufficiently high to permit diffusion to take place at detectable rates.
