Critical behavior of the fluctuation heat capacity near the glass transition of metallic glasses

The high-frequency shear modulus of five Zr-, Pd-, Cu-based conventional and two high-entropy bulk metallic glasses was measured in a wide temperature range up to the beginning of crystallization. Using these data and general thermodynamic relations, the "fluctuation" heat capacity $ΔC_f$ determined by local structural fluctuations in the defect regions is introduced and calculated. It is found that $ΔC_f$ temperature dependence for all metallic glasses has a large peak located slightly below or above the glass transition temperature but clearly lower than the crystallization onset temperature. The form of this peak resembles the characteristic $λ$-peak typical for order-disorder phase transitions. It is suggested that this $ΔC_f$-peak reflects certain underlying critical phenomenon. The critical temperature $T_0$ (peak temperature) and corresponding critical index $α$ are determined. Averaged over all seven metallic glasses under investigation in the initial and relaxed states, the critical index $α=0.26$. The results obtained indicate that the fluctuations of thermal energy near the glass transition bear the marks of a continuous phase transition. However, the derived critical index is between those corresponding to a second-order phase transition ($α\approx 0.1$) and a critical transition characterized by a tricritical point ($α\approx 0.5$).