Free volume change in crystallization process of Zr–Cu–Al metallic glass studied by positron annihilation techniques

Positron annihilation lifetime (PAL) and coincidence Doppler broadening (CDB) experiments have been performed to study free volume changes and atomic relaxation behavior in Zr–Cu–Al bulk metallic glass. Zr50Cu40Al10 metallic glass was annealed isothermally at 673 K, which is below the glass transition temperature (Tg). During this annealing, PAL was observed to decrease, indicating a decrease in free volume; whereas, no significant change in CDB spectra was detected. This observation implies that the relaxation below Tg takes place without a change in atomic configuration. As for specimens annealed at 723 K (>Tg) for 100 s and subsequently annealed isothermally at 673 K for 18,000 s, PAL decreased similarly, suggesting a decrease in free volume. In addition, CDB spectra showed a systematic fluctuation of electron momentum distribution, especially in a high momentum region. This indicates that a chemical reordering took place during crystallization process.