Comparing the dynamic tensile response of supercooled Pd₄₈Ni₃₂P₂₀ and Pt₆₀Ni₁₅P₂₅

Dynamic measurements of the tensile storage and loss moduli of a bulk glass-forming metallic alloy, Pd₄₈Ni₃₂P₂₀, have been performed below and close to the calorimetric glass transition temperature. These results were compared with the previously measured tensile relaxation behaviour in Pt₆₀Ni₁₅P₂₅ which is a non-bulk metallic glass former. The measurements for the Pd-alloy indicate a higher tensile viscosity and a more gradual change of relaxation times with temperature near the glass transition compared to the Pt-alloy. This is consistent with the fact that Pd₄₈Ni₃₂P₂₀ has a lower fragility parameter than Pt₆₀Ni₁₅P₂₅. From the master curve of the elastic moduli obtained by time-temperature superposition, a discrete spectrum of relaxation frequencies have been computed for both alloys. Unlike the distribution for Pt₆₀Ni₁₅P₂₅ which has the more common features of a single broad and asymmetric peak that is skewed towards high frequencies, the relaxation spectrum for Pd₄₈Ni₃₂P₂₀ exhibits a secondary low amplitude peak on the high frequency side of a main maximum. This suggests that there is a separate concentration of some intrinsic relaxation processes in a finite region of the relaxation spectrum at higher frequencies than the microscopic processes governing the main relaxation event. The fairly wide relaxation frequency distributions for both alloys indicate a dynamically heterogeneous environment near the glass transition.