  In recent years, twinned nanopillars have attracted tremendous attention for
research due to their superior mechanical properties. However, most of the
studies were focused on nanopillars with twin boundaries (TBs) perpendicular to
loading direction. Nanopillars with TBs parallel to loading direction have
received minimal interest. In this backdrop, the present study is aimed at
understanding the role of axial TBs on strength and deformation behaviour of Cu
nanopillars using atomistic simulations. Tensile and compression tests have
been performed on $<$112$>$ nanopillars with and without TBs. Twinned
nanopillars with twin boundary spacing in the range 1.6-5 nm were considered.
The results indicate that, under both tension and compression, yield strength
increases with decreasing twin boundary spacing and is always higher than that
of perfect nanopillars. Under compression, the deformation in $<$112$>$ perfect
as well as twinned nanopillars proceeds by the slip of extended dislocations.
In twinned nanopillars, an extensive cross-slip by way of Friedel-Escaig and
Fleischer mechanisms has been observed in compression. On the other hand, under
tensile loading, the deformation in perfect nanopillars occurs by partial
slip/twinning, while in twinned nanopillars, it proceeds by the slip of
extended dislocations. This extended dislocation activity is facilitated by
stair-rod formation and its dissociation on the twin boundary. Similar to
compressive loading, the extended dislocations under tensile loading also
exhibit cross-slip activity in twinned nanopillars. However, this cross-slip
activity occurs only through Fleischer mechanism and no Friedel-Escaig
mechanism of cross-slip has been observed under tensile loading.
