LONG-RANGE ORDERING IN THE EARLY STAGES OF PRECIPITATION - A BRIEF REVIEW

Local order adjusts more rapidly than long-range order for several rea sons, and this local order need not mimic the low-temperature precipit ate (although it does in many cases). This may drastically alter the a ctivation energy for nucleation. Strain energy can play a key role in determining whether precipitation, ordering, or a displacive transform ation occurs and in determining not only the shape but also the compos itions of the phases. Long-range ordering in the matrix may develop pr ior to precipitation of an ordered phase. A plate-like morphology for ordered precipitates can occur for several reasons, e.g. strain, aniso tropic interfacial energy, or planar antiphase boundaries in an ordere d matrix.