On the Shear Modulus and Thermal Effect During Structural Relaxation in a Model Metallic Glass: Correlation and Thermal Decoupling

Abstract

Pd40 Ni40P20 (at.%) samples with different enthalpy states and relaxation behaviors were fabricated through high-pressure torsion or sub-Tg annealing of the as-cast state. Subsequently, the underlying structural relaxation was studied by investigating the modulus and thermal characteristics using in-situ shear modulus measurement and modulated differential scanning calorimetry. The results show that high-pressure torsion leads to shear modulus softening and an increase of the irreversible exothermic enthalpy, indicating a significant structural rejuvenation, while sub-Tg annealing causes shear modulus hardening and a decrease of the irreversible exothermic enthalpy. Besides, the reversible endothermic effect which reflects the heat capacity was found to be almost identical for all samples, independent on deformation or thermal history. The total heat flow can be well correlated to the shear modulus within the framework of interstitialcy theory. Furthermore, we demonstrate that the structural relaxation below Tg decouples into the internal stress relaxation and β-relaxation. The former is an irreversible process of releasing internal stress, accompanied by an exothermic effect and modulus hardening. The latter is a complex process involving kinetic and thermodynamic components, accompanied by an endothermic effect and modulus softening. Shadow glass transition and glass transition overshoot are related to the activation (cage-breaking) processes in the kinetics of β-relaxation and α-relaxation, respectively. This work indicates that β-relaxation and α-relaxation are kinetically and thermodynamically identical but occur in distinct temperature or frequency domains. Internal stress relaxation as a universal mechanism plays a significant role in the structural relaxation, and simultaneously modulates the diffusive relaxation spectrum.