Transition from stress-driven to thermally activated stress relaxation in metallic glasses

Abstract

The short-range ordered, but long-range disordered structure of metallic glasses yields strong structural and dynamic heterogeneities. Stress relaxation is a technique to trace the evolution of stress in response to a fixed strain, which reflects the dynamic features phenomenologically described by the Kohlrausch-Williams-Watts (KWW) equation. The KWW equation describes a broad distribution of relaxation times with a small number of empirical parameters, but it does not arise from a particular physically motivated mechanistic picture. Here we report an anomalous two-stage stress relaxation behavior in a Cu 46 Zr 46 Al 8 metallic glass over a wide temperature range and generalize the findings in other compositions. Thermodynamic analysis identifies two categories of processes: a fast stress-driven event with large activation volume and a slow thermally activated event with small activation volume, which synthetically dominates the stress relaxation dynamics. Discrete analyses rationalize the transition mechanism induced by stress and explain the anomalous variation of the KWW characteristic time with temperature. Atomistic simulations reveal that the stress-driven event involves virtually instantaneous short-range atomic rearrangement, while the thermally activated event is the percolation of the fast event accommodated by the long-range atomic diffusion. The insights may clarify the underlying physical mechanisms behind the phenomenological description and shed light on correlating the hierarchical dynamics and structural heterogeneity of amorphous solids.

Spatial correlation of elastic heterogeneity tunes the deformation behavior of metallic glasses

“…38,39 For instance, Qu, et al showed that the brittle titanium-based MG formed one straight shear band while a ductile Pd-based MG formed several curved shear bands in the early stages of plastic deformation. Though our model employs different sample sizes and the relaxation dynamics compared to the experiments, [38][39][40] our simulation results clearly reveal that the spatial correlation of the elastic heterogeneity at the nanoscale can be an important factor that significantly influences the shearbanding process and resultant shear-band patterns, which is consistent with the mechanistic insight provided by several recent works at atomic scale. [41][42][43] The connection between elastic heterogeneity and deformation behaviors of MGs To establish a connection between nanoscale elastic heterogeneity and deformation behaviors of MGs, we perform a statistical analysis of the spatial connectivity of the soft sites, termed soft cluster, in the MGs with different levels of spatial correlation, focusing on the number and size of the soft clusters.…”

Sub-T relaxation times of the α process in metallic glasses

“…In refs. [36] and [37] the stress relaxation curves of several metallic glasses were shown to be composed of two regimes, an initial stress decay assigned to a stress-driven process [36] and a slower stress decay with a thermally activated relaxation time. The two relaxation time scales decouple as temperature decreases [37].…”

Crystals at High Deformation Rates Displaying Glassy Behavior

“…We claim that the KWW behavior for stress relaxation at high strain rate, typical of the relaxation in glasses including metallic ones, is an indication of a “glassy” behavior, and that plays here a similar role to the cooling rate in supercooled liquids. It has been found, indeed, that viscous liquids close to their glass transition exhibit non‐exponential relaxation, and the temporal behavior of the response function (e.g., stress in mechanical tests or polarization in response to an applied electric field) is described by a KWW function with β < 1 .…”