The evolution of defects in Mo alloy nanofibers with initial dislocation densities ranging from 0 to 1.6 1014 m2 were studied using
an in situ “push-to-pull” device in conjunction with a nanoindenter in a transmission electron microscope. Digital image correlation was
used to determine stress and strain in local areas of deformation. When they had no initial dislocations the Mo alloy nanofibers suffered
sudden catastrophic elongation following elastic deformation to ultrahigh stresses. At the other extreme fibers with a high dislocation
density underwent sustained homogeneous deformation after yielding at much lower stresses. Between these two extremes nanofibers
with intermediate dislocation densities demonstrated a clear exhaustion hardening behavior, where the progressive exhaustion of dislocations
and dislocation sources increases the stress required to drive plasticity. This is consistent with the idea that mechanical size effects
(“smaller is stronger”) are due to the fact that nanostructures usually have fewer defects that can operate at lower stresses. By monitoring
the evolution of stress locally we find that exhaustion hardening causes the stress in the nanofibers to surpass the critical stress predicted
for self-multiplication, supporting a plasticity mechanism that has been hypothesized to account for the rapid strain softening observed
in nanoscale bcc materials at high stresses.
Published by Elsevier Ltd. on behalf of Acta Materialia Inc.