Development of nanocrystals in amorphous Al-alloys

Rapidly quenched aluminum-based metallic glasses have been found to undergo a partial transformation upon heating that yields a fine-scaled dispersion of Al-nanocrystals within an amorphous matrix. The origin of the nanocrystallisation process is discussed based upon calorimetric and structural investigations of Al92Sm8 samples prepared using different processing pathways. Additionally, the early stages of the nanocrystal development was monitored by isothermal calorimetry techniques on long time scales. The results strongly indicate that the nanocrystals originate from quenched-in nuclei. Isothermal annealing of Al88Y7Fe5 glasses at 245 °C revealed that α-Al nanocrystals develop at temperatures well below the primary crystallisation onset temperature after short time periods. On annealing, an additional phase evolves, which gives an exothermic signal in DSC at about 330 °C. XRD measurements have been performed to study the phase formation sequence at high temperature. The formation of additional phases is related to a modification of the matrix composition near the nanocrystal/amorphous interface due to a pronounced concentration gradient as a result of the unequal component diffusivities.