Interplay of process kinetics in the undercooled melt in the proximity of the glass transition

The mobility of atomic (molecular) species or groups of atoms in the proximity of the glass transition where various processes can display similar time-scales and occur in competition with each other, is considered. er to describe mobility, the product ηD (with η the viscosity and D a diffusion coefficient) is usually taken as constant at constant temperature (Stokes–Einstein equation). Viscosity is analysed at first in the frame of the strong/fragile classification of liquids. Then, the evidences that the SE equation does not hold both below and above Tg for some molecular and metallic glass-formers are reviewed. Since attachment kinetics in nucleation and growth of crystals is often found to depend on the rate of viscous flow, a possible link in its description with the breakdown of the Stokes–Einstein equation is suggested. The topic is reviewed for fluoride and metallic glass-formers with emphasis on Al-based alloys where a glass to nanocrystal transformation occurs.