FORMATION AND STABILITY RANGE OF THE G PHASE IN THE AL-MN SYSTEM

The metastable precipitation of orthorhombic Al//6Mn occurs at 400 degree C at a speed which is orders of magnitude faster than the subsequent reaction to form the stable G phase. It proceeds as a classical nucleation and growth reaction that goes to completion in about one hour. Whether or not G nucleated while the supersaturation was high, or only after this metastable reaction, which after all reduces the supersaturation drastically, was not determined. This is not a good model system for a detailed study of metastable sequences, but it does illustrate how rapid solidification processing can be used to produce the equilibrium phase in a system which normally has a persistent metastable structure when processed by conventional processing methods.