Probing Shear-Band Initiation in Metallic Glasses

Abstract

In situ acoustic emission monitoring is shown to capture the initiation of shear bands in metallic glasses. A model picture is inferred from stick-slip flow in granular media such that the origin of acoustic emission is attributed to a mechanism of structural dilatation. By employing a quantitative approach, the critical volume change associated with shear-band initiation in a metallic glass is estimated to be a few percent only. This result agrees with typical values of excess free volume found in the supercooled liquid regime near the glass transition temperature.

Direct atomic-scale evidence for shear–dilatation correlation in metallic glasses

“…The MD findings are further clarified by DFT athermal quasi-static shear calculations. The atomistically deduced atomic dilatation accompanying shear band initiation reaches a few percent, which agrees with experimental observations [30,45,28]. The atomic-scale information for the shear-dilatation correlation presented here bridges the STZ and free volume model for deformation of MGs.…”

“…A close look indicates the mean atomic volumetric strain reaches 4%-7% during the initial shear band formation. The calculated dilatation is consistent with experiments where a few percent volume change is associated with shear band initiation [30,45,28]. Note that the shear-induced dilatation could be probably enhanced in brittle glasses, where the dilation can be as much as 10% which may eventually lead to fracture via cavitation (or TTZ) mechanisms [35,31].…”

“…The microscopic scenario is that the STZ operations redistribute stress spatially which usually leads to the creation of free volume via atomic-scale dilatation [21,1]. So that local structural dilatation [19,30,31], density change [27,28], and even nanovoids [32,33,21,34] have been observed within the shear bands of MGs. Although the concept of shear-dilatation correlation is widely accepted in the glass community, and various experiments and simulations have indicated STZ and cavitation as important deformation and fracture mechanisms of glassy alloys [33,35,31,18,17,34], there still lacks a direct microscopic evidence and no quantitative relationship established for such an intimate correlation.…”

Shear-band arrest and stress overshoots during inhomogeneous flow in a metallic glass

“…19,20 This naturally emerges towards a picture in which the material in an active shear band is believed to be in a state of lower viscosity relative to the surrounding bulk matrix. Viscosity estimates derived from shear rates within the shear band assuming Couette flow 21 and in-situ acoustic emission experiments 22 are in agreement with this, yielding in the former case viscosity values close to those measured at the glass transition and in the latter case volume expansions during shear-band initiation equivalent to those measured in the undercooled liquid regime. Ultrahigh strain rate molecular dynamics simulations also indicate this picture, deriving that the initiation of flow can be viewed as a stress-induced glass transition confined to the shear band.…”

“…This assumption is well supported by recent results investigating shear-band properties in bulk metallic glasses. 22,30 A trend of increasing s n can be observed as the temperature decreases, as further discussed below and also displayed in Fig. 4.…”