NUCLEATION AND FORMATION OF QUASI-CRYSTALLINE PHASES IN UNDERCOOLED AL-MN MELTS

The nucleation and the formation of metastable quasicrystalline icosahedral and dodecagonal phases, as well as of the stable crystalline Al6Mn intermetallic phase, have been analysed for undercooled Al88Mn12 melts on the basis of classical nucleation theory. In detail, the difference of free energy between the undercooled liquid and the solid state, the activation energy for the formation of postcritical nuclei, the nucleation frequency and the crystal growth velocities have been calculated as a function of temperature for the various phases using experimentally determined thermodynamic parameters. These calculations lead to temperature-time-transformation curves capable of describing experimental findings on containerless processed droplets, provided homogeneous nucleation of the quasicrystalline phases is assumed.