Activation Volume Determination in FCC Nanopillars

Abstract

Activation volume (V*) is a fundamental parameter describing thermally activated flow processes in crystalline materials. Its determination is fundamentally defined by the stress response to an instantaneous change in the inelastic strain rate. V* values were measured for polycrystalline nickel (Ni) and copper (Cu) nano-pillars, fabricated in-house by electron beam lithography and electroplating processes, using a new strain rate change test procedure developed for FemtoTool’s FT-NMT03, an in-situ nanomechanical testing apparatus. Pillars were compressed in a scanning electron microscope at temperatures of 300 ± 1 K operating the FT-NMT03 in displacement control at a base nominal strain rate of 0.004 s-1 with rate change factors of 1/4 or 1/10. The instrument response replicates the step-ramp rate change method developed previously at Queen’s for large-scale specimen testing. Continuous rate tests, which are typical in the literature, were also made to compare to rate change results. V* values for ¼ rate change tests on Cu were found to be 35.4 ± 2.4 b3 and 34.4 ± 1.3 b3, and for 1/10 rate change tests, found to be 17.3 ± 2.0 b3 and 29.9± 2.0 b3 with or without step-ramp compensation, respectively. V* for factor of four continuous rate tests on Ni were 2.4 ± 0.1 b3 on average. V* values for 1/4 change tests on Ni were 28.3±2.6 b3, and 29.4±3.0 b3, and for 1/10 change tests on Ni were 27.6 ± 1.3 b3 and 21.0 ± 2.5 b3 with and without steps, respectively. It was determined that step compensation did not significantly impact measurements with the FT-NMT03 compared to those made without compensation for low-factor rate changes, but for rate change factors of 10 or greater, tests without compensation underestimated V* enough to alter conclusions about rate-controlling mechanisms. This work validates the step-ramp method for V* determination and the uncertainty in data previously reported in the nano-pillar literature.