Room temperature stress relaxation in nanocrystalline Ni measured by micropillar compression and miniature tension

In this study, we report a micropillar stress relaxation technique employing a stable displacement-controlled, in-situ scanning electron microscope indenter, and unusually large micropillars to precisely measure stress relaxation in electroplated nanocrystalline Ni thin films. The observed stress relaxation is significant under constant displacement: even well below the 0.2% offset yield strength, the stresses relax by ∼4% within a minute; in the work hardening regime, stress relaxes by ∼9% in 1 min. A logarithmic fit of the relaxation curves is consistent with an Arrhenius thermal activation of plasticity and suggests an activation volume in the vicinity of ∼10b3. The apparent and effective activation volumes diverge at lower strains, particularly in the “elastic” regime. These measurements are compared to similar measurements performed on free-standing thin film tensile coupons. Both methods yield similar results, thereby validating the applicability of pillar compression to capture time-dependent plasticity. To our knowledge, these are the first micropillar stress relaxation experiments on metals ever reported.