Coarsening characteristics of intergranular γ′ in Nimonic PE 16

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Abstract
The increase in particle radius r during the coarsening of γ′ (Ni3(Ti, Al)) located at grain boundaries in Nimonic PE 16 has been found to vary as r3·6 with time. Models for coarsening of intergranular and intragranular precipitates predict a dependence on grain boundary and lattice diffusion, respectively. The case investigated here, where coarsening is preferential to grain boundaries from precipitates in the matrix, is more complex since it will depend upon more than one diffusion regime. From the experimentally determined diffusion coefficient for the process, it is apparent that the out-diffusion of aluminium from dissolving precipitates is the rate controlling step. Convergent beam electron diffraction has been used to infer compositional changes within precipitates, particularly with respect to the boundary position. These results are used to argue that addition of atomic species to intragranular precipitates is via the grain boundary, rather than across the coherent precipitate/matrix interface. MST/615