In situ TEM study of twin boundary migration in sub-micron Be fibers

TL;DR

This study uses in situ TEM to observe twin boundary migration in sub-micron beryllium fibers, revealing surface nucleation as the controlling mechanism and quantifying the activation volume for twin propagation.

Contribution

It provides direct in situ observations of twin boundary migration in small-scale Be fibers and quantifies the activation volume associated with this process.

Findings

Twin boundary migration occurs via surface nucleation of dislocations.

Twin boundary speed correlates with stress relaxation measurements.

Activation volume for twin propagation is consistent with surface nucleation theory.

Abstract

Deformation twinning in hexagonal crystals is often considered as a way to palliate the lack of independent slip systems. This mechanism might be either exacerbated or shut down in small-scale crystals whose mechanical behavior can significantly deviate from bulk materials. Here, we show that sub-micron beryllium fibers initially free of dislocation and tensile tested in-situ in a transmission electron microscope (TEM) deform by a $\{ 10\bar{1}2 \}$ $\langle 10\bar{1}1 \rangle$ twin thickening. The propagation speed of the twin boundary seems to be entirely controlled by the nucleation of twinning dislocations directly from the surface. The shear produced is in agreement with the repeated lateral motion of twinning dislocations. We demonstrate that the activation volume ($V$) associated with the twin boundary propagation can be retrieved from the measure of the twin boundary speed as the stress decreases as in a classical relaxation mechanical test. The value of $V \approx 8.3 \pm 3.3 \times 10^{-29}m^3$ is comparable to the value expected from surface nucleation.