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Superplastic Deformation of Defect-Free Au Nanowires via Coherent Twin Propagation
Since 2005, researchers have known via molecular dynamics simulations that ultra-small (i.e. < 5 nm diameter) FCC metal nanowires can exhibit unique shape memory and pseudoelastic behavior driven by surface stress effects resulting from their very small cross sectional dimensions - see (http://prl.aps.org/abstract/PRL/v95/i25/e255504) and (http://pubs.acs.org/doi/full/10.1021/nl0515910). The process involves tensile loading of a rhombic <110> nanowire with {111} transverse surfaces that, after about 40% tensile strain, can reorient to a <100> nanowire with {100} transverse surfaces, and a square cross section. The shape memory effect or pseudoelasticity is determined by whether the nanowire is stable in the new <100> orientation, or whether additional thermal energy is required to cause the surface-stress-driven reorientation back to <110>/{111}.
Experimental studies of these deformation paths have not existed until recently, when in the attached paper recently published in Nano Letters (http://pubs.acs.org/doi/abs/10.1021/nl2022306), we found that <110> gold nanowires with {111} transverse surfaces can indeed reorient under tensile loading to <100> wires with {100} surfaces. The reorientation occurs through ~40% tensile strain, is driven by coherent and long range (micrometer scale) propagation of twin boundaries, and occurs for the range of nanowire diameters tested (40-150 nm). The results are novel and demonstrate the gold nanowires to uniquely be both superstrong and superplastic.
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Few Observations.......
Dear Prof Park,
Nice to see your work on Nano Letter on Super plastic Deformation in Au nanowire. I am having few observations:
1. It is well known that <110> orientaion is energetically more stable as compared to <100> oriented FCC metallic nanowire, based on first principle and MD simulations. A large no. of shape memory and pseudoelasticity is reported in the literature. It was nice to see those behaviors via experimentally.
As we know that the stress-strain behavior reported in region II is basically the phase transformation/reorientaion of an initial <110> oriented phase to <100> phase, which is recoverable. How this region could be considered as plastic deformation?
2. As literature says, for ceramics, failure strain of ~100% and for metals ~400-500% are considered as superplasticity. The failure strain reported in the paper is order of ~50% in metallic nanowire, should be considered as superplastic.
Or, it should have been superelastic behavior, as that region II is recoverable (as per atomistic simulations results)?
Anyway I have not gone through the paper in details. I ll read this intersting article. Thanx for sharing the manuscript.
Vijay Kumar Sutrakar
answers
Dear Vijay:
Thanks for your comments and for your interest. To answer your questions:
1. The region II is considered "plastic" in this case because there is no recovery back to the initial <110> orientation. For these wires, they are actually synthesized chemically as the lower energy <110>/{111} wire with rhombic cross section - there is no surface-stress-induced reorientation from <100> to <110>.
2. Again, for the sizes considered experimentally (50-150 nm), there is no recovery due to surface stress since the cross section is too large. I suspect that as the synthesis approach improves and smaller cross section nanowires can be made, then the surface-stress-induced reorientation may occur, but we will have to wait to test that.