  Twin boundaries (TBs) are assumed to be obstacles to dislocation motion and
increase the strength of metals. Here, we report the abnormal phenomenon that
TBs reduce the strength of body-centered cubic (BCC) tungsten (W).
[1-11]-oriented W nanowires with (121) twin planes and free of dislocations
were fabricated by chemical vapor deposition. In situ tensile tests within the
transmission electron microscope were performed on single-crystal and twinned W
nanowires. The fracture strength of the twinned W nanowire was 13.7 GPa, 16%
lower than that of the single-crystal W nanowire (16.3 GPa). The weakening
mechanism of the TB was revealed by a combination of atomic-resolution
characterizations and atomistic simulations. Twinned W nanowires failed by the
early nucleation of a crack at the intersection of the TB with the surface. The
standard strengthening mechanism by dislocation/TB interaction was not
operative in W because the high Peierls barrier and stacking fault energy in W
hinder dislocation nucleation and glide. These findings provide a new insight
into the influence of TBs on the mechanical properties of BCC metals.
