Enthalpy relaxation in Cu46Zr45Al7Y 2 and Zr55Cu30Ni5Al10 bulk metallic glasses by differential scanning calorimetry (DSC)
Résumé
Structural relaxation process in Cu46Zr45Al 7Y2 and Zr55Cu30Ni 5Al10 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) = ΔH eq1 - 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 Cu 46Zr45Al7Y2 and Zr 55Cu30Ni5Al10 bulk metallic glasses are 72 and 69, respectively, indicating therefore that these alloys are intermediate glasses. Crystallization 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. © 2010 Elsevier Ltd. All rights reserved.
Mots clés
A. Intermetallics
Annealing temperatures
Annealing time
As-cast
B. Glasses
metallic
Bulk metallic glass
Cooling rates
Crystallization process
E. Physical
Enthalpy relaxation
Exponential relaxation
Free volume models
Glass transition temperature
Heating treatments
Kinetic fragility
Kissinger model
Physical aging
Thermal properties
Williams
Activation energy
Aluminum
Annealing
Differential scanning calorimetry
Enthalpy
Experiments
Exponential functions
Glass
Glass transition
Intermetallics
Physical properties
Point defects
Structural relaxation
Zirconium
Metallic glass