In situ TEM investigation of deformation and fracture mechanism in nanocrystalline nickel

The strength of a material is known to increase with the decreasing grain size and will reach its peak strength at certain critical grain size. It was proposed and has been widely accepted that this results from the deformation mechanism crossover, i.e. a continuous transition from dislocation nucleation and motion to grain boundary mediated plasticity. Evidence for this has been sought for many years, however, to date, direct experimental confirmation remains elusive.; By solving the challenging problems encountered in previous studies, in situ dynamic dark field transmission electron microscope (TEM) investigations combined with in situ high resolution TEM observations have been performed successfully on high purity nanocrystalline nickel samples with an average grain size about 10nm, which show: (1) grain agglomerates formed very frequently and rapidly in many locations apparently independently of one another under influence of the applied stress, (2) both inter- and intra-grain agglomerate fractures are observed in response to the deformation, (3) trapped dislocations are frequently observed in grains which may be still in a strained state and no deformation twinning was detected, (4) trapped lattice dislocations were observed to move and annihilate during the stress relaxation. These TEM observations (i) for the first time provide conclusive experimental evidence that grain boundary mediated plasticity, such as grain boundary sliding and grain rotation, has become a prominent deformation mode for as deposited Ni. Theoretical analysis suggested that the deformation mechanism crossover resulted from the competition between the deformation controlled by nucleation and motion of dislocations and the deformation controlled by grain boundary related deformation accommodated mainly by grain boundary diffusion with decreasing grain size, (ii) confirmed the speculation that dislocations are most probably observed in stressed grains, (iii) suggested that the dimpled fracture surface of nanocrystalline materials may result from those newly formed grain agglomerates. Additionally, direct measurement of lattice distortions during straining revealed that grain interiors may experience ultra-high elastic distortions during tensile deformation.