  Incipient plasticity is typically associated with thermally activated events
like the nucleation of dislocations in crystalline solids and the activation of
shear transformation zones in metallic glasses. A widely employed method of
estimating the activation parameters of such mechanisms is to analyze the
statistical distribution of critical loads obtained through a series of
repeated measurements. However, this approach has been observed to produce
activation volumes of the order of atomic volumes in a variety of materials and
experimental setups. Such exceptionally small activation volumes have been
explained by conjecturing non-trivial nucleation mechanisms. Here, we
critically analyze the inherent assumptions of the statistical method and show
that the exceptionally small activation volumes can be a misleading result
emerging simply from the small statistical variance of activation parameters.
To this end, we perform deformation simulations of iron nanopillars under
tensile and compressive loading and measure the resulting yield stresses.
Although the conventional statistical analysis exhibits extremely small
activation volumes, the atomistic simulations indicate a transition pathway
that is physically incommensurate with the statistical result. Using a simple
Monte Carlo scheme and analytical model, we show that even a relatively small
dispersion in activation parameters can suppress the measured activation volume
to a significant extent. It suggests that the ultra-small atomic volumes
reported in the earlier studies do not need exotic mechanisms but can be
explained as the misleading outcome obtained by ignoring the physically
plausible dispersion of activation parameters.
