DIRECT EVIDENCE OF FREE-VOLUME RELAXATION AND THE CROSSOVER EFFECT INNI25ZR55AL20 METALLIC-GLASS

The changes in positron annihilation lifetime in a thermally treated N i25Zr55Al20 alloy were observed. The glass transition temperature T-g lies around 700 K. Isothermal annealing was used to treat samples sequ entially in the amorphous (at 640 K) and the undercooled liquid (at 74 0 K) states. This provides insight into the microstructural processes taking place via determination of the evolution of the positron captur e site density and distribution. The observed shifts in the positron l ifetime distribution are analyzed in terms of relaxation of the free v olume V-f, i.e. chemical and topological short-range ordering (CSRO, T SRO) which are known to be influenced in an important manner by the re laxation processes. The upper limits of the activation energies of the two relaxation processes (2.2 and 2.6 eV) are determined. For the fir st time, the ''crossover'' effect in the free-volume distribution has been observed.