NUCLEATION AND GROWTH OF APERIODIC CRYSTALS IN ALUMINUM ALLOYS

The icosahedral and decagonal aperiodic phases dominate the microstructures of rapidly solidified Al-Mn alloys because their nucleation and growth behavior differs substantially from that of the equilibrium phases. Electron beam surface melting can be used to produce a wide range of solidification conditions, in which the different stages of the nucleation and growth processes can be observed. It is found that the icosahedral phase nucleates abundantly in supercooled Al-Mn melts, and that the decagonal phase is subsequently nucleated by the icosahedral phase. Addition of Si to the Al-Mn alloys suppresses formation of the decagonal phase, but in these alloys the hexagonal β phase can grow rapidly