  It is well known that the twin boundary (TB) spacing plays an important role
in controlling the strength of twinned metallic nanopillars. One of the reasons
attributed to this strengthening behaviour is the force exerted by the TBs on
dislocations. Since the TBs exert repulsive force on dislocations and the
plasticity in nanopillars is surface controlled, it is interesting to know
whether the TB position from the nanowire surface has any effect on the
strength of twinned nanopillars. Using atomistic simulations, here we show that
the TB position significantly influences the strength of twinned nanopillars.
Atomistic simulations have been performed on nanopillar containing one and two
TBs and their position is varied within nanopillar from the center to the
surface. The results indicate that in nanopillar containing a single TB, the
strength is maximum when the TB is located at the center of the nanopillar and
it decreases as the TB is shifted towards the nanopillar surface. On the other
hand in nanopillar containing two TBs, the maximum strength is observed when
the twin boundaries are placed at distance of one fourth or one fifth the
pillar size from the surfaces and it decreases when TBs are moved on either
side. The present study demonstrates that the mechanical properties of the
twinned nanopillars can be controlled by carefully tailoring the position of
the TBs within the nanopillars.
