Microstructure and devitrification behavior of melt-spun Al-rich metallic glasses and nanostructured composites

Melt-spun Al-rich metallic glasses and nanostructured composites were synthesized under various quenching rates. Differential scanning calorimetry (DSC), X-ray diffraction and transmission electron microscopic techniques were employed to study the microstructure and devitrification of these samples in more details. The microstructure obtained is strongly sensitive to the quenching rates used. A reduction of the quenching rate from 50 to 9 m/s changed the structure from an amorphous single phase to the coexistence of α-Al nanocrystals and amorphous phase, and finally to a combination of α-Al, Al₃(Ni,Fe), Al₁₁Ce₃ and amorphous matrix. Upon devitrifying, a two-stage crystallization reaction was observed but the glass transition was not detected in these samples. The absence of glass transition was presumably due to the formation of pre-existing nuclei. Isothermal and non-isothermal DSC measurements indicated that the first crystallization was associated with the growth of α-Al nanocrystals controlled by the volume diffusion with a small activation energy of 1.3-1.7eV. The second crystallization was derived from the growth of Al₃(Ni,Fe) and Al₁₁Ce₃ nanophases.