Discontinuous coarsening of discontinuous precipitates in a Co–6 at.% Mo alloy

The morphology and growth kinetics of the discontinuous coarsening of discontinuous precipitates in a Co–6 at.% Mo alloy have been investigated at temperatures ranging from 773 to 998 K by optical and scanning electron microscopy and X-ray diffraction. At all aging temperatures the alloy was observed to decompose completely by discontinuous precipitation into a fine lamellar structure of b.c.t. Co rich solid solution and an intermetallic compound phase of Co3Mo. This lamellar structure was then decomposed at all aging temperatures by a discontinuous coarsening reaction. This reaction occurred at a much slower rate than the primary discontinuous precipitation reaction and resulted in a much coarser lamellar structure of the same phases. Through the measurements of the growth rates and lamellar spacings for both the discontinuous precipitation and discontinuous coarsening reactions, the grain boundary diffusion coefficients (SδDb) were analyzed using the kinetic model of Petermann and Hornbogen. By comparison with the diffusion data in literature, it was concluded that the reactions were controlled by grain boundary diffusion in the reaction fronts of the discontinuous precipitates.