Formation and structure of nanocrystals in an Al86Ni11Yb3 alloy

The formation and structure of the nanocrystalline phase in the Al86Ni11Yb3 alloy are investigated using differential scanning calorimetry (DSC), tran smission electron and high-resolution electron microscopy, and x-ray diffra ction. The nanocrystalline phase is formed upon controlled crystallization of the amorphous alloy prepared by quenching of the melt on a rapidly movin g substrate. It is revealed that the nanocrystalline alloy consists of alum inum nanocrystals (5-12 nm in size) randomly distributed in the amorphous m atrix. The maximum fraction of the nanocrystalline phase does not exceed 25 %. The nanocrystal size substantially increases at the initial stage of iso thermal treatment (at 473 K) and then changes insignificantly. It is found that nanocrystals are usually free of defects. However, some nanocrystals h ave a more complex microstructure with twins and dislocations. The size dis tributions of nanocrystals are determined at several durations of isotherma l treatment. It is demonstrated that the nucleation of nanocrystals predomi nantly occurs through the heterogeneous mechanism. The experimental distrib utions are compared with those obtained from a computer simulation. The act ivation energy of crystallization, the time-lag, and the coefficient of ytt erbium diffusion in the alloy are estimated (C) 2001 MAIK "Nauka/Interperio dica".