Deformation twinning in ultra-fine grained Ni at room temperature and low strain rate

► The DTs in Ni with ultra-fine grains were observed at room temperature under a low strain rate. ► The grain rotation and/or grain boundary migration result in a large number of misfit dislocations or partial dislocations at grain boundaries, which mediates nucleation of DTs. ► The deformation of the Ni single wire was dominated by DTs and revealed the fine super-plasticity.