  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.
