  Polycrystalline hcp metals - an important class of engineering materials -
typically exhibit complex plasticity because of a limited number of slip
systems. Among these metals, deformation is even more complicated in cobalt as
it commonly contains residual fcc phase due to the incomplete martensitic
fcc$\rightarrow$hcp transformation upon cooling. In this work, we employ a
combination of in-situ (acoustic emission, AE) and ex-situ (scanning electron
microscopy, SEM) techniques in order to examine deformation dynamics in pure
polycrystalline cobalt varying in grain size and the content of residual fcc
phase prepared using systematic thermal treatment and cycling. We reveal that
the presence of the fcc phase and special ~71{\deg} grain boundaries between
different hcp martensite variants brings about higher deformability and
strength. The fcc phase provides additional slip systems and also accommodates
deformation via the stress-induced fcc$\rightarrow$hcp transformation during
loading. On the other hand, special boundaries enhance structural integrity and
suppress the formation of critical defects. Both these non-trivial effects can
dominate over the influence of grain size, being a traditional microstructural
variable. The ex-situ SEM experiments further reveal that the stress-induced
fcc$\rightarrow$hcp transformation is sluggish and only partial even at high
strains, and it does not give rise to detectable AE signals, unlike in other
materials exhibiting martensitic transformation. In turn, these insights into
cobalt plasticity provide new avenues for the microstructure and performance
optimization towards the desired applications through the modern concept of
grain boundary engineering.
