Study of Deformation Behavior of Nanocrystalline Nickel using Nanoindentation Techniques

Nanocrystalline materials with grain size less than 100 nm have been receiving much attention because of their unparallel properties compared with their microcrystalline counterparts. Because of its high hardness, nanocrystalline nickel has been used for MEMS. Long term thermomechnical properties and deformation mechanism at both ambient and elevated temperatures need to be evaluated which is vital for reliability of its applications as structural material.

In this thesis, nanoindentation creep of nanocrystalline nickel with an as-deposited grain size of 14 nm was characterized at elevated temperatures. The nanoindentation creep rate was observed to scale with temperature and applied load (or stress), and could be expressed by an empirical power-law equation for describing conventional crystalline solids. Creep activation energy was found to be close to that for grain boundary self-diffusion in nickel. The activation volume was also evaluated using a stress relaxation technique. The creep results were compared with those for fine-grained nickel in the literature. Possible mechanisms were discussed in light of the creep rate and temperature ranges.

To provide a direct comparison, uniaxial creep tests were conducted on nanocrystalline nickel with an as-deposited grain size of 14 nm at 398 K. It was found that stress exponents under the two test conditions are almost the same, indicating a similar creep mechanism. However, the strain rate measured by nanoindentation creep was about 100 times faster than that by uniaxial creep. The rate difference was discussed in terms of stress states and the appropriate selection of Tabor factor.

To further explore the time-dependent plastic behavior, multiple unload-reload tests were conducted on electrodeposited nanocrystalline nickel in both compression and tension. A hysteresis was observed during each unload-reload cycle, indicating irreversible energy dissipation. The dissipated energy was evaluated and the energy dissipation rate was found to increase with the flow stress to the third power and sensitive to the stress state (tension or compression). A mechanistic model based on grain boundary sliding was proposed to describe the unload-reload behavior. Experimental results were found to be in good agreement with the model predictions, suggesting the observed hysteresis was indeed caused by grain boundary sliding.