Fragility parameter and nanocrystallization of metallic glasses

Prerequisite for nanocrystallization are high nucleation rates combined with slow growth. The resulting mean (quasi)crystal diameter in so-called bulk Zr-based metallic glasses was observed to decrease with the annealing temperature. In a typical metal-metalloid glass (for example in Fe-Ni-B), the finest microstructure was observed at a temperature very close to the maximum in the nucleation rate. The reason for this differences is probably due to the different temperature dependence of the atomic mobility, i.e. the viscosity, as shown in an Angell plot. In fragile glasses (e.g. Fe-Ni-B) viscosity is known to change very strongly with the temperature; strong glasses (e.g. Zr-Cu-Ni-Al) exhibit a temperature dependence of the viscosity very similar to that of the diffusivity below the glass transition. Among other parameters leading to nanocrystallization are very low interfacial energies, e.g. for the quasicrystalline phase in Zr-Cu-Ni-Al, which allows the high nucleation rates necessary for the formation of a nanocrystalline structure, as well as phase separation into two amorphous phases either spinodal or by nucleation and growth prior to any crystallization. The influence of these parameters will be discussed for a number of metallic glasses