Gregory Sparks1 Robert Maass1

1, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States

Reducing the sample size has become increasingly popular for quantifications of various mechanical size-effects, one of which is that virtually all stress-strain curves from metallic single crystals become intermittent, underlining that plastic flow is not a smooth process. Previous investigations on the intermittent stress-strain behavior of small-scale crystals have focused on the scale-freeness of displacement jump-size distributions, reporting pure or truncated power-law behavior that, via similar scaling exponents, suggest a universal aspect of plasticity, which is shared by vastly different microstructures and materials. However, investigating the time-resolved spatiotemporal avalanche dynamics of the same intermittent flow, revealed that there are indications for a materials-specific response, as expressed by different velocity relaxation behavior and average avalanche shape-functions in fcc and bcc crystals [1].
One fundamental aspect of our recent efforts in tracing nano-scale slip dynamics in single crystals is that there is a materials-dependent and test-system dependent range of finite slip velocities [2, 3]. Whilst the scatter of the recorded velocities is large and exhibits some agreement with recently predicted scaling laws, each so far investigated material has a recorded maximum slip velocity. The fact that finite values are seen, suggests that macroscopically smooth flow may be a manifestation of a time-scale mismatch between the internal processes, that is nano-scale slip (dislocation avalanches), and the applied rate. Here we investigate this proposition experimentally, by tracing the time-resolved nano-scale slip dynamics of Nb microcrystals across more than five orders of magnitude in displacement rate.

[1] G. Sparks, J. Sickle, K.A. Dahmen, R. Maass. Shapes and velocity relaxation of dislocation avalanches in fcc and bcc crystals, (2017).
[2] R. Maass, P.M. Derlet, J.R. Greer. Independence of Slip Velocities on Applied Stress in Small Crystals, Small 11 (2015) 341-351.
[3] G. Sparks, P.S. Phani, U. Hangen, R. Maass. Spatiotemporal slip dynamics during deformation of gold micro-crystals, Acta Materialia 122 (2017) 109-119.