Discontinuous Coarsening of Lamellar Cellular Precipitate in An Ni-8 AT.% Sn Alloy

The morphology and growth kinetics of cellular precipitation and discontinuous coarsening of the cellular precipitate in a Ni-8 at.% Sn alloy has been investigated at temperatures ranging from 575 to 741°c by light and electron microscopy. At all aging temperatures the alloy was observed to decompose completely by a cellular precipitation reaction which resulted in a fine lamellar structure of nickel rich a solid solution and Ni 3Sn intermetallic compound. The first cell lamellar structure was subsequently decomposed at all aging temperatures by a second cellular or discontinuous coarsening reaction. The discontinuous coarsening reaction occurred at a much slower rate than the first cellular reaction and resulted in a much coarser lamellar structure. At 659°c the products of the first cellular reaction would be completely decomposed by the second cellular reaction and followed by the third cellular reaction. Several models for discontinuous precipitation and discontinuous coarsening have been used to evaluate the growth kinetics. The Fournelle model appears to describe the results of discontinuous coarsening better than the Livingston and Cahn model. Finally, analysis of the growth kinetics suggests that both first and second cellular reactions are controlled by cell boundary diffusion in the whole aging temperature range