Moo-Young Seok

Strain-dependent transition of time-dependent deformation mechanism in single-crystal ZnO evaluated by spherical nanoindentation

We have systematically explored the time-dependent plasticity of (0 0 0 1)-oriented ZnO single crystal through spherical indentation tests. Constant load indentation creep tests were performed in elastic strain and elastic–plastic strain regimes. In both regimes, creep indeed occurred at ambient temperature. With the observed quasi-steady-static strain rate, the stress exponent was found as ~1.361 for elastic contact regime and ~3.077 for elastic–plastic regime, suggesting predominant creep mechanism of diffusion and dislocation, respectively. This strain-dependent transition of creep mechanism is discussed in terms of possible factors affecting the creep in each regime.

Hydrogen-induced softening in nanocrystalline Ni investigated by nanoindentation

Abstract The influence of hydrogen on the plastic deformation of nanocrystalline nickel was analysed by recourse to nanoindentation on the uncharged and hydrogen-charged samples. It was revealed that, in nanocrystalline Ni, hydrogen significantly decreases hardness but does not alter the strain rate sensitivity. Through thermal desorption spectroscopy measurement, charged hydrogen was expected to reside in face-centred cubic lattice, grain boundaries (GBs) and vacancies rather than dislocations. The hydrogen-induced softening behaviour is discussed in terms of the possible roles of hydrogen in GB-assisted dislocation flow mechanism.