Bringing casting to microfabrication: processing and deformation of microcastings

This work extends the range of pathways for the production of metallic microcomponents by downscaling metal casting. This is accomplished by using either of two different molding techniques, namely femtosecond laser micromachining or lithographic silicon microfabrication, in combination with metal pressure infiltration followed by solidification and mold leaching. Structures produced in this work, of size ranging from a few tens of micrometer to a few millimeters and with features of size down to around 2 µm, are cast out of engineering metals (silver, copper, gold, and their alloys). Femtosecond laser machining, on one hand, enables the fabrication of freeform structures that are difficult or impossible to produce by any other means. On the other hand, combining silicon microfabrication with metal pressure casting allows the production of 2D and 2.5 D microcomponents with massive parallelization. Microcasting is shown with both molds to feature excellent dimensional control, high reproducibility, the potential for full density, the possibility to alloy with great freedom the infiltrant alloy composition, and a capacity to replicate essentially any continuous shaped hollow with micrometric precision. Microcasting, in both forms developed here, provides a new way to efficiently produce micron-scale single-crystalline specimens that are amenable to tensile testing. By conducting in-situ displacement-controlled tensile tests under the scanning electron microscope at room- and elevated temperatures (200 °C and 400 °C), it is demonstrated that the mechanical response of microcast silver or copper, either bare or coated with ceramic, shows attractive strength and ductility values, indicating in turn the high microstructural quality of the material produced while providing insights into small-scale plasticity. The latter are achieved by investigating the influence of size, crystal orientation, and temperature on the deformation, yield, strain burst statistics, and strain hardening of these micrometric dense metal samples and comparing data with bulk counterparts that are cast to have a diameter near 1 mm. Results show clear evidence of small-scale plasticity. The yield stress of microcastings is affected by both size and temperature and shows evidence of the presence of a scale-dependent density of geometrically necessary dislocations in the cast metal, while no effect of the crystal orientation is measured. The decrease in yield stress at elevated temperatures can fully be attributed to the concomitant decrease in shear modulus, suggesting that the mechanisms that govern the initiation of plastic flow in microcast silver are likely the same from 20 °C up to 400 °C. The work hardening rate also decreases with increasing temperature; however, it does so faster than do elastic properties. The complementary cumulative distributions of strain burst amplitudes agree with an exponentially truncated power-law distribution that has the expected power-law exponent. There is, within error, no measured dependence of the cutoff intensity on sample diameter or test temperature.