High strain rate testing of 3D printed copper micropillars
The field of micromechanics involve the mechanical metrology of materials at small length scales using different experiments including micropillar compression, microtensile testing and microcantilever bending. Currently, this field almost exclusively relies on focused ion beam (FIB) based milling to obtain the micron scale samples with the required specific geometries for mechanical testing. This serial sample preparation method using FIB milling is unfortunately both tedious and time consuming. Consequently, majority of the previous micromechanical studies on different materials including metals, ceramics and glass are limited to a few data points and typically, statistically relevant testing is not possible. Further, despite many efforts to expand the range of micro and nanomechanical testing in terms of forces, temperatures and loading conditions, the achievable strain rates are still limited to ~0.1/s.
In this presentation, for the first time, the fabrication of 3D printed copper micropillars and a thorough investigation of their microstructural and mechanical properties under application relevant extreme strain rates will be described. First, a localized electroplating technique with force-feedback control used to create a copper micropillar array with 100 self-similar micropillars will be explained. The microstructural investigation conducted using FIB and electron backscatter diffraction (EBSD) combination to identify the grain size and the sigma three coherent twin boundaries present in the micropillars will be subsequently shown. The rate-dependent compressive properties of the copper micropillars across five orders of magnitude in strain rate from ~0.001/s to ~500/s, achieved using a piezobased in situ mechanical tester, will then be presented as a function of grain size from nanocrystalline to oligocrystalline. Remarkably, a yield stress saturation beyond 0.1/s strain rate was identified in nanocrystalline copper micropillars. The presentation will also report a combined HRSEM, thermal activation analysis and EBSD/FIB based characterization conducted on the deformed copper micropillars to identify the deformation mechanisms behind the stress-strain signatures. Finally, to show that this manufacturing technique can be used to fabricate complex mesoscale metal architectures, a case study on copper microsprings and their mechanical characterization under monotonic and fatigue-like cyclic loading conditions will be shown.