Direct evidence of free-volume relaxation and the crossover effect in Ni25Zr55Al20 metallic glass

The changes in positron annihilation lifetime in a thermally treated Ni25Zr55Al20 alloy were observed. The glass transition temperature Tg lies around 700 K. Isothermal annealing was used to treat samples sequentially in the amorphous (at 640 K) and the undercooled liquid (at 740 K) states. This provides insight into the microstructural processes taking place via determination of the evolution of the positron capture site density and distribution. The observed shifts in the positron lifetime distribution are analyzed in terms of relaxation of the free volume Vf, i.e. chemical and topological short-range ordering (CSRO, TSRO) which are known to be influenced in an important manner by the relaxation processes. The upper limits of the activation energies of the two relaxation processes (2.2 and 2.6 eV) are determined. For the first time, the “crossover” effect in the free-volume distribution has been observed.