Probing Microplasticity in Small-Scale FCC Crystals via Dynamic Mechanical Analysis
In small-scale metallic systems, collective dislocation activity has been correlated with size effects in strength and with a steplike plastic response under uniaxial compression and tension. Yielding and plastic flow in these samples is often accompanied by the emergence of multiple dislocation avalanches. Dislocations might be active preyield, but their activity typically cannot be discerned because of the inherent instrumental noise in detecting equipment. We apply alternate current load perturbations via dynamic mechanical analysis during quasistatic uniaxial compression experiments on single crystalline Cu nanopillars with diameters of 500 nm and compute dynamic moduli at frequencies 0.1, 0.3, 1, and 10 Hz under progressively higher static loads until yielding. By tracking the collective aspects of the oscillatory stress-strain-time series in multiple samples, we observe an evolving dissipative component of the dislocation network response that signifies the transition from elastic behavior to dislocation avalanches in the globally preyield regime. We postulate that microplasticity, which is associated with the combination of dislocation avalanches and slow viscoplastic relaxations, is the cause of the dependency of dynamic modulus on the driving rate and the quasistatic stress. We construct a continuum mesoscopic dislocation dynamics model to compute the frequency response of stress over strain and obtain a consistent agreement with experimental observations. The results of our experiments and simulations present a pathway to discern and quantify correlated dislocation activity in the preyield regime of deforming crystals.
© 2017 American Physical Society. Received 23 December 2016; published 14 April 2017. We would like to thank Eric K. Gustafson, Koji Arai, Eric Quintero, and the members of the Seismic Isolation and Suspension Working Group in the LIGO Scientific Collaboration, as well as Peter Ispanovity and Erik Van der Giessen for helpful discussions. We thank Ottman Turtuliano for assistance with fused silica sample preparation. We gratefully acknowledge the financial support from NSF (DMR-1204864, JRG) and the Department of Energy, Basic Energy Sciences [Early Career Faculty grant (JRG) and DE-SC0014109 (SP)]. LIGO was constructed by the California Institute of Technology and Massachusetts Institute of Technology using funding from the National Science Foundation, and operates under Cooperative Agreement No. PHY-0757058. Advanced LIGO was built under Award No. PHY-0823459. This paper carries LIGO Document No. LIGO-P1600316.
Submitted - 1612.09011.pdf
Published - PhysRevLett.118.155501.pdf
Supplemental Material - Supplemental__1_.pdf