Cellular precipitation and precipitate coarsening in a Mg-Al alloy

Abstract

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 metastable Mg-7 at.% Al alloy was observed to decompose completely via a process which has been termed “cellular precipitation” into a lamellar structure consisting of the δ and γ phases at all aging temperatures 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 have indicated that the depleted matrix of the δ phase associated with the initial cells was richer in solute than the equilibrium solvus, δ/(δ+γ) . The solute concentration in the depleted matrix associated with the coarsened material was less than the published equilibrium solvus. Analysis of the growth kinetics of both the primary cellular reaction, and its subsequent coarsening stage, has indicated that the transformation is controlled by diffusion of aluminum through the cell boundaries.