Kinetics of length-scale dependent plastic deformation of gold microspheres
- PDF / 550,375 Bytes
- 9 Pages / 584.957 x 782.986 pts Page_size
- 5 Downloads / 227 Views
The size and strain-rate dependence of plastic deformation in Au microspheres of diameter ranging from 0.8 to 6.0 lm was investigated at room-temperature using flat-punch micro-compression testing. The contact yield stress was observed to increase with decreasing microsphere diameter. The apparent activation volume, V*, associated with the rate dependent plastic deformation remained essentially constant between 4 and 6b3 for 0.8 and 1.0 lm spheres over strains up to 20% whereas it increased from 12 to 42b3 for the larger 3.0 and 6.0 lm diameter specimens. The initiation of plastic deformation within the microspheres was also found to be highly dependent upon sphere diameter and strain rate with associated V*, and apparent activation energy, Q*, values of 0.4b3 and 0.02 eV for 0.8 lm diameter spheres increasing to 4.1b3 and 0.16 eV for 6.0 lm diameter spheres. These values indicate that initial plasticity is controlled by heterogeneous nucleation events that are consistent with a surface self-diffusion mechanism.
I. INTRODUCTION
Metal microspheres are used for a variety of applications ranging from biomedical drug delivery systems to large surface area electrode material for energy storage devices. Their usefulness in these applications arises from their high surface to volume ratio and relative ease of fabrication. While it is known that submicrometer size materials display considerably enhanced mechanical strength compared to their bulk counterparts, a complete understanding of the operative plastic deformation mechanisms, particularly their strain rate dependence, is still largely incomplete but is absolutely necessary to realize the full potential of metal microspheres.1–6 A frequently observed characteristic of submicrometer size metal samples is that they display plastic deformation occurring by a stochastic process of discrete displacement jumps corresponding to the motion of single, or groups of, dislocations.7–10 While the resolved shear stress required to initiate plastic deformation in these small samples is very high, it is less than the ideal theoretical shear strength of the bulk crystal.11–13 This suggests that, dislocations are being nucleated at inhomogeneities within, or from the surface of, the sample.14 The high strength and interrupted nature of the ductility of these small samples is therefore likely to be related to the conditions for nucleation of dislocations from these sites rather than the conditions necessary for Contributing Editor: Mathias Göken a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2017.223
the motion of existing dislocations past obstacles within the sample. This is the basic tenant behind the dislocation starvation model for plastic deformation of submicrometer sized metal samples.7 To date, few studies of microspheres,15–18 nanodots,9 nanoposts19 and nanowires20–22 exist that investigate the kinetics of this deformation process and most of the studies in this area have used a computational/numerical modeling approach. Experimental i
Data Loading...
