Positron-annihilation studies of free-volume changes in the bulk metallic glass ${\mathrm{Zr}}_{65}{\mathrm{Al}}_{7.5}{\mathrm{Ni}}_{10}{\mathrm{Cu}}_{17.5}$ during structural relaxation and at the glass transition

Volume changes in ${\mathrm{Zr}}_{65}{\mathrm{Al}}_{7.5}{\mathrm{Ni}}_{10}{\mathrm{Cu}}_{17.5}$ bulk metallic glass have been observed by positron annihilation and density measurements. Excess volume of the order of 0.1% is quenched in the bulk glass at cooling rates as low as 1–2 K/s. The isothermal relaxation kinetics below the glass transition temperature obey a Kohlrausch law with β exponents of ≅0.3 between 230 and 290 °C. The effective activation energy for relaxation is $(1.2\mathrm{}\pm 0.2)\mathrm{eV}.$ Stress-strain measurements indicate that structural relaxation is not accompanied by embrittlement. It is shown that the outer surface plays a crucial role in the annealing of excess volume. Free volume can be restored by a heat treatment in the undercooled liquid state. The observed free volume changes contrast with the behavior of perfectly strong glasses. The temperature dependence of the positron lifetime can be explained by thermal detrapping of the positrons from shallow traps.