Abstract
β-relaxation has long been attributed to localized motion of constituent molecules or atoms confined to isolated regions in glasses. However, direct experimental evidence to support this spatially heterogeneous scenario is still missing. Here we report the evolution of nanoscale structural heterogeneity in a metallic glass during β-relaxation by utilizing amplitude-modulation dynamic atomic force microscopy. The successive degeneration of heterogeneity during β-relaxation can be well described by the Kohlrausch–Williams–Watts equation. The characteristic relaxation time and activation energy of the heterogeneity evolution are in accord with those of excess enthalpy release by β-relaxation. Our study correlates β-relaxation with nanoscale spatial heterogeneity and provides direct evidence on the structural origins of β-relaxation in metallic glasses.
Highlights
In this study, we utilize amplitude-modulation dynamic atomic force microscopy (AM-AFM)[20,32,33] to characterize the successive changes of structural heterogeneity in a metallic glass regulated by the sub-Tg enthalpy relaxation
Our enthalpy relaxation is controlled below 0.8Tg at which the sub-Tg relaxation shows irrelevant influence on the glass transition as the differential scanning calorimeter (DSC) traces above Tg are almost identical before and after annealing
The time dependence of the normalized enthalpy release at each annealing temperature can be well fitted by the Kohlrausch–Williams–Watts (KWW) function: DHremain=DHtotal 1⁄4 exp1⁄2 À ðta=tbÞbKWW , where tb is the characteristic relaxation time and bKWW is the stretch parameter for the fitting
Summary
We utilize amplitude-modulation dynamic atomic force microscopy (AM-AFM)[20,32,33] to characterize the successive changes of structural heterogeneity in a metallic glass regulated by the sub-Tg enthalpy relaxation. It is found that the characteristic relaxation time and activation energy of the heterogeneity dynamics are approximately equal to those of b-relaxation measured by excess enthalpy release using a hyper-quenching– annealing–calorimetric scan approach[7]. The quantitative measurements of the dynamics for both spatial heterogeneity and b-relaxation in one system provide direct evidence on the intrinsic correlation between b-relaxation and heterogeneous structure in metallic glasses, which reveals the structural origin of sub-Tg relaxation of metallic glasses
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