Enthalpy relaxation in Cu46Zr45Al7Y2 and Zr55Cu30Ni5Al10 bulk metallic glasses by differential scanning calorimetry (DSC)

Structural relaxation process in Cu46Zr45Al7Y2 and Zr55Cu30Ni5Al10 bulk metallic glasses during annealing below the glass transition temperature Tg was investigated by differential scanning calorimetry (DSC). The features of enthalpy relaxation are sensitive to both annealing temperature and annealing time. For a given annealing time ta, the results indicated that the relaxation time ta decreases with increasing the annealing temperature Ta, in good agreement with results relative to other bulk metallic glasses. Additionally, the enthalpy relaxation behaviour of the bulk metallic glasses appears independent on the cooling rate used before the physical aging experiments, i.e. on the initial as-cast state. The recovered enthalpy evolution of the bulk metallic glasses is well described by the Kohlrausch–Williams–Watts (KWW) exponential relaxation function as ΔH(Ta) = ΔHeq{1 − exp[−(ta/τ)β]}. Kohlrausch exponent β and enthalpy relaxation time τ are sensitive to the composition of the bulk metallic glasses. Finally, the influence of different heating treatment processes on the enthalpy relaxation in the bulk metallic glasses is presented and shows that this phenomenon is mainly reversible. The structural relaxation behaviour is interpreted by free volume model and quasi-point defects model. Kinetic fragility parameters m in Cu46Zr45Al7Y2 and Zr55Cu30Ni5Al10 bulk metallic glasses are 72 and 69, respectively, indicating therefore that these alloys are intermediate glasses. llization process was also investigated by DSC experiments. According to the Kissinger model, corresponding activation energy is 3.18 eV in Cu46Zr45Al7Y2, and 3.19 eV in Zr55Cu30Ni5Al10, respectively.