Relaxation of free volume in Zr₅₀Cu₄₀Al₁₀ bulk metallic glasses studied by positron annihilation measurements

Relaxation of free volume in Zr₅₀Cu₄₀Al₁₀ bulk metallic glass during isothermal annealing below T_g (glass transition temperature) has been investigated by using positron annihilation lifetime and coincidence Doppler broadening (CDB) techniques. The mean positron lifetime decreases with increasing annealing time at each annealing temperature. The decrease in lifetime is due to shrinkage and annealing out of the free volume. The relaxation kinetics of free volume obeys the stretched exponential function (KWW: Kohlrausch-Williams-Watts law). An adjustable parameter of the KWW exponent β depending on temperature was determined. The electron momentum distribution around free volume derived from a CDB spectrum during annealing showed no appreciable change at each temperature. These results suggest that long-range atomic diffusion and rearrangement, particularly around the free volume, do not occur essentially during relaxation below T_g.