  The existence of a well defined yield stress, where a macroscopic piece of
crystal begins to plastically flow, has been one of the basic observations of
materials science. In contrast to macroscopic samples, in micro- and
nanocrystals the strain accumulates in distinct, unpredictable bursts, which
makes controlled plastic forming rather difficult. Here we study by simulation,
in two and three dimensions, plastic deformation of submicron objects under
increasing stress. We show that, while the stress-strain relation of individual
samples exhibits jumps, its average and mean deviation still specify a
well-defined critical stress, which we identify with the jamming-flowing
transition. The statistical background of this phenomenon is analyzed through
the velocity distribution of short dislocation segments, revealing a universal
cubic decay and an appearance of a shoulder due to dislocation avalanches. Our
results can help to understand the jamming-flowing transition exhibited by a
series of various physical systems.
