Glass formation and primary nanocrystallization in Al-base metallic glasses

Aluminum-rich metallic glasses containing transition metals and rare earth elements have been found to yield finely mixed microstructures of Al nanocrystals embedded in an amorphous matrix and exhibit enhanced fracture strength with several percent strain. Upon primary crystallization of melt spun ribbons, this novel microstructure comprised of a high particle density (>1020 m−3) of Al nanocrystals (20 nm) in an amorphous matrix develops and offers exceptional strength (1500 MPa) and high temperature stability (533 K). Numerical modeling based upon the size distribution of the Al nanocrystals after isothermal annealing is applied to study the nucleation kinetics in the metallic glasses. In addition to the kinetic study of primary nanocrystallization, the glass transition temperature (Tg) has been assessed in Al–7at.% Y–5at.% Fe and Al–8at.% Sm alloys. In usual calorimetric measurements, the thermal response of the primary crystallization often obscures the observation of the signal corresponding to the glass transition. As a result, Tg is often assumed to be near the onset of the primary crystallization reaction (TxAl). However, it has been demonstrated by modulated-temperature calorimetry that this assumption does not apply strictly to the metallic glasses under study. The thermal stabilization of the microstructure by the occurrence of diffusion field impingement allows for the observation of the glass transition of the remaining amorphous phase in the matrix by modulated differntial scanning calorimetry (DSC). The reliable assessment of the glass transition temperature provides not only a fundamental basis for the kinetics analysis, but also an important parameter in designing suitable annealing treatments that allow for the development of desired microstructures to yield optimized properties.