Time- and size-dependent nanomechanical behavior of nanocrystalline metal pillars

Abstract

Due to mechanical robustness, high productivity, and conductivity, metallic materials still have a great importance in application for small-scale device/component as well as traditional structural materials. In this context, there have been intensive efforts to characterize and understand the mechanical properties of small-volume structures. At early stage, most research efforts on small-scale mechanical behavior have focused on single crystalline pillars. However, it is noted that they are far from representing real materials operating in practical applications, whose microstructure is often complex, containing boundaries and interfaces. Accordingly, much attention has been focused on the effects of multiple grain boundaries (especially, nanocrystalline) within small deformed volume. Although a number of experimental findings on mechanical behavior of small-volume structures with nano-sized grains are available, there are still issues remaining unsolved. In this thesis, I attempt to shed a light on the issues and thus to extend our current knowledge on the combined effect both limited internal and external size on time-dependent plastic deformation. As the first step, nanocrystalline pillar are prepared through e-beam lithography and electroplating, which technique was selected as the most suitable fabrication method. This process has important merits; free from surface damage generated from focused ion beam milling and high throughput with hundreds of uniform pillars at once. Then, time-dependent deformation behavior under various loading conditions was systematically investigated through micro-compression tests on the prepared nanocrystalline metal pillars. First, creep deformation of nanocrystalline nickel at room temperature was critically explored through a series of creep and quasi-static compression experiments on pillar with same outer diameter but different surface-to-volume ratio; rod and tube. Enhanced creep rates in tubes as compared to rods, establishes the facilitating role played by the free surface in time-dependent deformation. Creep stress exponent, n, and strain-rate sensitivity, m, were compared to examine connections between creep and the rate-dependent plasticity, if any. Second, influence of cyclic load, with the maximum stress well within the elastic regime, on the strength of nanocrystalline nickel was investigated through quasi-static compression experiments on cyclic-loaded sub-μm-sized pillars, fabricated through electron beam lithography and electroplating. Results show that prior-cycling enhances both yield and flow strengths of nanocrystalline Ni without significant loss in plasticity. Changes in strain-rate sensitivity and activation volume for deformation upon cycling were measured and utilized to discuss possible mechanisms responsible for the observed strengthening. Final topic is the rate-sensitive plastic flow and its dependence on sample dimension. The effects of specimen size and strain rate on the plastic deformation response of sub-mm-sized nanocrystalline Cu pillars were examined through a series of micro-compression experiments, with particular emphasis on the stochastic nature of the measured responses. A large number of micropillars with two different diameters, both having an average grain size of 6 nm, were prepared by employing the single batch process of e-beam lithography and electroplating and tested. By recourse to statistical analysis, it was recognized the yield strength and flow stress increase with pillar size and strain rate. Further, the rate sensitivity in smaller pillars was more pronounced, implying synergetic interactions between the deformed volume and the strain rate imposed. The coupling influence of size and rate on yield was analyzed by estimating the parameters in a statistical distribution having Weibull-like formula, revealing that the enhanced role of free surface in smaller pillar may make it easy to trigger yielding. The size dependence of rate-sensitive plastic flow was also statistically examined in detail and discussed in terms of strain-rate sensitivity, activation volume, and the combined roles of free surfaces and grain boundaries.