Abstract
The directed growth of precipitates proceeds in their parent phases at low transformation temperatures. The growth needs the activation energy. The activation energy is usually supplied in the form of thermal energy. However, at low transformation temperatures, the thermal energy is not enough to surmount the activation barrier, and so the strain energy developed in the parent phase assists surmounting the barrier, resulting in the directed growth of precipitates. The strain energy can result from a difference in density between the nucleus and matrix and a lattice mismatch along the nucleus:matrix interface. The fundamental concept of the model is that the maximum growth rate of precipitate is along the direction that generates the maximum strain energy and minimizes the interface energy. In this paper, orientation relationships between ferrite precipitate and parent austenite, between orthorhombic cementite precipitate and parent austenite, between cementite precipitate and parent ferrite, and between hexagonal Mo2C precipitate and parent ferrite have been discussed based on the directed growth model.
