Abstract
The elastic-plastic transition or yield behaviour was observed during compression of nanocrystalline nickel pillars fabricated by electroplating into electron beam lithography (EBL) forms or by focused ion beam (FIB) milling of the same electroplated structure. The experimental methodology allowed structures with different surface defect source/obstacles and similar bulk defects to be compared. Pillars with 1 to 2 um diameter cross-sections were made with height to diameter aspect ratios from 2:1 to 6:1, grain size of 91 ± 23 nm, and <110>, <111>, <100> textures in the growth direction. The pillars were compressed under a flat punch at constant loading rates from 1.5 to 30 μΝ/s using an instrumented indenter. EBL pillars generally showed a gradual elastic-plastic transition similar to yielding in bulk nickel followed by strain bursts. In contrast, FIB-milled pillars consistently showed stochastic yielding. The Young’s modulus was 55 to 350 GPa for all pillars, agreeing with values in the literature. The yield stress at 0.2% offset strain for EBL pillars ranged from 480 to 1800 MPa, and 320 to 700 MPa for FIB-milled pillars that was narrowed to 550 to 650 MPa after subsequent annealing, with initial deformation localized near the tops of FIB-milled pillars due to geometrical tapering. EBL pillar shear yield strengths exceed the literature reported for Ni and appear to lose their size dependence.
Published Version
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