In this work the grain size effects on the mechanical behaviour of polycrstalline nickel from micro to nanoscale were studied. Pulse-electrodeposited nanocrystalline nickel was heat-treated to produce different grain sizes. The interaction between dislocations nucleated under the indenter tip and individual grain boundaries was examined by performing nanoindents always in the center of the largest grains in a given metallographic section with a nanoindenting atomic force microscope. The results show that hardness not only depends on the grain size, but also on the ratio of the indent size to the grain size. With increasing indentation depth, hardness increasesor decreases, depending on the relationship between the grain size and the plastic zone size. Direct dislocation-boundary-interaction was observed in the grain size range of 300 nm to 900 nm, where the pop-in width increases with increasing grain size. Later pop-ins occur at lower loads with increasing pop-in widths and could be regarded as the sign of activation of new dislocation sources in the adjacent grains. The result is in agreement with the theoreticalprediction of the classical Hall-Petch model. Strain rate-controlled tensile and nanoindentation experiments are performed to reveal the grain size effects on deformation mechanisms. Results show that with decreasing grain size, the strain rate sensitivity increases and the activation volume decreases. Quantitative analyses of the activation volume show different dislocation sources in coarse-grained nickel and nanocrystalline nickel. Room temperature creep behaviour was observed in nanocrystalline nickel. Insitu bending experiments on bulk nanonickel in an atomic force microscope, for the first time, show that grain boundary sliding at room temperature plays an important role during bending and fatigue tests, and the fatigue crack is intergranular. These results demonstrated that in nanocrystalline nickel the grain boundary mediated deformation processes play a significant role.
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