  The slip strength of individual slip systems at different strain rates will
control the mechanical response and strongly influence the anisotropy of
plastic deformation. In this work, the slip activity and strain rate
sensitivity of the <a> basal, <a> prismatic, and <c+a> pyramidal slip systems
are explored by testing at variable strain rates (from 10E-4 s^-1 to 125 s^-1)
using single crystal micropillar compression tests. These systematic
experiments enable the direct fitting of the strain rate sensitivities of the
different slips using a simple analytical model and this model reveals that
deformation in polycrystals will be accommodated using different slip systems
depending on the strain rate of deformation in addition to the stress state
(i.e. Schmid's law). It was found that the engineering yield stress increases
with strain rate, and this varied by slip systems. Activation of the prismatic
slip system results in a high density of parallel, clearly discrete slip
planes, while the activation of the <c+a> pyramidal slip leads to the plastic
collapse of the pillar, leading to a 'mushroom' morphology of the deformed
pillar. This characterization and model provide insight that helps inform metal
forming and understanding of the mechanical performance of these engineering
alloys in the extremes of service conditions.
