Nanocrystal formation in gas-atomized amorphous Al₈₅Ni₁₀La₅ alloy

Abstract

An Al₈₅Ni₁₀La₅ amorphous alloy, produced via gas atomization, was selected to study the mechanisms of nanocrystallization induced by thermal exposure. High resolution transmission electron microscopy results indicated the presence of quenched-in Al nuclei in the amorphous matrix of the atomized powder. However, a eutectic-like reaction, which involved the formation of the Al, Al₁₁La₃, and Al₃Ni phases, was recorded in the first crystallization event (263°C) during differential scanning calorimetry continuous heating. Isothermal annealing experiments conducted below 263°C revealed that the formation of single fcc-Al phase occurred at 235°C. At higher temperatures, growth of the Al crystals occurred with formation of intermetallic phases, leading to a eutectic-like transformation behaviour at 263°C. During the first crystallization stage, nanocrystals were developed in the size range of 5 ∼ 30 nm. During the second crystallization event (283°C), a bimodal size distribution of nanocrystals was formed with the smaller size in the range of around 10 ∼ 30 nm and the larger size around 100 nm. The influence of pre-existing quenched-in Al nuclei on the microstructural evolution in the amorphous Al₈₅Ni₁₀La₅ alloy is discussed and the effect of the microstructural evolution on the hardening behaviour is described in detail.

Keywords:

• amorphous alloys
• crystallization
• eutectic
• Al-Ni-La alloy

Acknowledgements

The authors acknowledge the Army Research Office for financial support under Grant No. W911NF-06-1-0230 with Dr William Mullins as the program officer. Particular thanks go to http://ncem.lbl.gov/frames/staff.htm Velimir Radmilovic at National Center for Electron Microscopy (NCEM) at Lawrence Berkeley Laboratory for high-resolution electron microscopy support.