Deformation behaviour and microstructure of nanocrystalline electrodeposited and high pressure torsioned nickel

Microstructural analysis and mechanical testing have been performed on two types of nanocrystalline (NC) Ni samples and compared to coarse-grained (CG) Ni. The NC electrodeposited (ED) Ni is characterized by a mean grain size of 21 nm, a narrow grain size distribution and the absence of dislocations observed in undeformed conditions. NC high pressure torsioned (HPT) Ni shows a mean grain size of 105 nm with a broad grain size distribution. Grains observed in transmission electron microscopy exhibit dislocations and dislocation cells. The mechanical properties deduced from tensile and indentation tests of NC ED Ni show a significantly higher strength than the one of NC HPT Ni. Both materials show a limited total plastic deformation of less than 5% and a quasi-linear decrease in the work hardening rate with increasing stress. Strain rate jump tests have shown for both materials similar strain rate sensitivities, which are one order of magnitude higher than in CG Ni. A significantly smaller apparent activation volume Vap has been measured for NC Ni as compared to CG Ni. NC ED Ni shows a decrease in Vap with increasing stress, while an increase of Vap was measured for NC HPT Ni. Using the repeated stress relaxation technique, an effective activation volume and associated parameters describing the change in the mobile dislocation density have been estimated for NC ED Ni and CG Ni and are discussed in terms of the possible deformation kinetics. Results have shown that the apparent activation volume of NC ED Ni deduced from relaxation tests is more than three times higher than the one deduced from jump tests, while in CG Ni both methods give consistently similar values. This can be taken as an indication for a breakdown in the application of such analysis to 20 nm grain sized metals.