Delayed plasticity in nanoindentation of annealed crystals

Abstract

Continuum constitutive relations used in the design of macro-sized components assume that the elastic limit of a crystalline solid is time independent. Recent experiments using the nanoindentation technique, however, reveal that the elastic limit of submicron-sized metallic volumes decreases as time under load increases. A submicron metallic volume can sustain a static load in the elastic regime initially, but transition to plastic deformation may occur after some waiting time. In this paper, the characteristics of this type of delayed plasticity are reviewed. The available experimental data suggest that homogeneous nucleation of the plasticity events, which was frequently discussed in the recent literature, occurs only at sufficiently high loads within a narrow range. In a lower and broader load range, the nucleation of the plasticity events occurs at a history dependent rate, thus via a damage-accumulation mechanism not compatible with the homogeneous nucleation theory. A model based on the diffusion-controlled, subcritical growth of a Frank loop just underneath the indenter is proposed in this work to explain the history dependent nucleation of instability observed at lower loads. By fitting to the available nanoindentation data in Ni₃Al, it is apparent that self-diffusion along the indenter-sample interface, rather than through the bulk, is likely to be the controlling factor for the growth of the Frank loop to a critical size to yield a dislocation avalanche.

Acknowledgements

The work described in this paper was supported by a grant from the Research Grants Council of the Hong Kong Special Administration Region, P.R. China (Project No. HKU7194/04E). Helpful discussion with Prof. I.P. Jones is gratefully acknowledged.