Modeling competitive continuous and discontinuous precipitation

A model based on classical kinetic theory has been developed to predict the evolution of microstructure that occurs on ageing in alloy systems where the supersaturated solid solution decomposes by forming either continuous, discontinuous or mixed precipitate morphologies. The model has been applied to make predictions for the Mg–Al system and explore the interaction between precipitation modes. It predicts that the formation of continuous precipitates inhibits discontinuous precipitation by reducing the supersaturation, which is a far stronger effect than boundary pinning. It is demonstrated that once continuous precipitation becomes established, discontinuous precipitation is effectively suppressed. The reverse is not the case; the formation of discontinuous precipitates only strongly suppresses continuous precipitation when a sufficient volume fraction of discontinuous precipitation occurs to substantially reduce the free volume available for transformation.