On the Hardness and Strain Rate Sensitivity of Electrodeposited Nanocrystalline Ni-18 wt% Co Alloy Studied by Nanoindentation

Ni-18wt% Co foils made by electrodeposition possesses an average grain size, computed using diffraction contrast in TEM, of about 30 nm. Vickers microindentation and depth-sensing nanoindentation have been adapted to assess the deformation parameters such as hardness, strain rate sensitivity (SRS) and activation volume. These foils with single-phase fcc-structured solid solution exhibit the hardness values of 4.5 +/- 0.1 GPa and 6.2 +/- 0.2 GPa measured by microindentation and nanoindentation, respectively. The dependence between hardness and applied load in the present solid solution cannot be attributed to the indentation size effect, but to the changes in internal friction and/or reduction in stacking fault energy that may have resulted from the Co additions. An elastic modulus of 193 +/- 3 has been realized in these foils. Performing nanoindentation at various loading rates and subsequent analysis has resulted in a SRS of 0.017 and an activation volume of 7.6 b(3). These values suggest that in these nanocrystalline Ni-18Co foils made by electrodeposition, interfaces such as grain boundaries, triple junctions and quadruple junctions play a governing role in dictating the deformation kinetics. A model reported in the literature has been successfully modified to reasonably explain the dependence of SRS on grain size for various Ni-based alloys including the one reported in the present study. However, the model fails to follow the established dependence for the materials with grain size below 10 nm as the deformation mechanisms at these extremely finer length scales (below 10 nm) are expected to be totally different from considerations applicable for the present alloy with a grain size of 30 nm.