Probing Shear-Band Initiation in Metallic Glasses

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.

Figure 1

(a) Time-resolved stress data and AE stream showing serrations and AE bursts to coincide. (b) Focus on a single load drop with AE activity as a precursor. Inset in (b): Magnification of the AE burst showing rise time $t_r$ measured from the first crossing of the threshold level to peak amplitude $U_p$ within an error of $\pm 5\mu \mathrm{s}$.

Figure 2

Simplified AE setup with the AE source (shear band) oriented at 45° relative to the sensor.

Figure 3

Histogram of values of shear-band dilatation calculated from 76 AE bursts. Associated apparent equivalent temperatures from Eq. (6) reveal that most values lie in the supercooled liquid regime close to the glass transition.