The formation of precipitate free zones and the growth of grain boundary carbides in the nickel-base superalloy NIMONIC PE16

Abstract

The formation of precipitate free zones (PFZ) along large and small angle grain boundaries has been investigated for bi- and polycrystals of the nickel-base superalloy NIMONIC PE16. This material is strengthened by coherent precipitates of the γ′-phase. The most important result is that the PFZs are caused by the growth of titanium-rich carbides in the grain boundaries. Since titanium is a constituent of the γ′-precipitates, its depletion along the grain boundaries leads to the dissolution of the γ′-particles and hence to PFZs. The former belief that they are due to chromium-rich grain boundary carbides is disproved. The growth rate of the width w of the PFZs is controlled by the volume diffusion of titanium. Until w reaches its final value w final, w is proportional to the square root of the aging time. The growth rate of the PFZs yields the volume diffusion coefficient of titanium. The w final is probably governed by the grain boundaries' limited capacity to accommodate titanium-rich carbides.