CELLULAR PRECIPITATION AND PRECIPITATE COARSENING IN A MG-AL ALLOY

The morphology and growth kinetics of the cellular precipitate as well as its discontinuous coarsening have been studied in the temperature range 440-580 K. Optical microscopy and X-ray diffraction were used to characterize the cellular transformation. A rapidly solidified metast able Mg-7 at.% Al alloy was observed to decompose completely via a pro cess which has been termed ''cellular precipitation'' into a lamellar structure consisting of the delta and gamma phases al all aging temper atures used in this investigation. The fine lamellar structure of the primary cells subsequently decomposed into a coarse lamellar structure consisting of the same two phases, Lattice parameter measurements hav e indicated that the depleted matrix of the delta phase associated wit h the initial cells was richer in solute than the equilibrium solvus, delta/(delta+gamma) The solute concentration in the depleted matrix as sociated with the coarsened material was less than the published equil ibrium solvus. Analysis of the growth kinetics of both the primary cel lular reaction, and its subsequent coarsening stage, has indicated tha t the transformation is controlled by diffusion of aluminum through th e cell boundaries.