  Graphene is well known for its extraordinary mechanical properties combining
brittleness and ductility. While most mechanical studies of graphene focused on
the strength and brittle fracture behavior, its ductility, plastic deformation,
and the possible brittle-to-ductile transition, which are important for
high-temperature mechanical performance and applications, still remain much
less understood. Here the mechanical response and deformation dynamics of
graphene grain boundaries are investigated through a phase field crystal
modeling, showing the pivotal effects of temperature and local dislocation
structure. Our results indicate that even at relatively high temperature
(around 3350 K), the system is still governed by a brittle fracture and
cracking dynamics as found in previous low-temperature experimental and
atomistic studies. We also identify another type of failure dynamics with
low-angle grain boundary disintegration. When temperature increases a
transition to plastic deformation is predicted. The appearance of plastic flow
at ultrahigh temperature, particularly the phenomenon of jerky plasticity, is
attributed to the stick and climb-glide motion of dislocations around the grain
boundary. The corresponding mechanism is intrinsic to two-dimensional systems,
and governed by the competition between the driving force of accumulated local
stress and the defect pinning effect, without the traditional pathways of
dislocation generation needed in three-dimensional materials.
