Deformation Mechanics and Microstructure Evolution of Material Removal at Small Length-Scale

Deformation mechanics and microstructure evolution of material removal at the micrometer length-scale were studied via performing plane strain machining inside a scanning electron microscope. In Cu, deformation mechanics was characterized by performing digital image correlation on secondary electron images of deformation zone. Corresponding microstructure evolutions were examined by orientation imaging microscopy using electron backscattered diffraction. While the deformation geometry and rate are identical, resultant microstructure in deformed chips spans the entire gamut from conventional ultrafine-grained structure to complete lack of refinement among different crystal orientations due to variation in dislocation evolution, i.e. orientation-dependent deformation anisotropy. Subsequent examination of machining surface revealed topographical defects along some grain boundaries. It is hypothesized that rampant ductile fracture occurs due to elevated dislocation densities in the vicinity of grain boundary, which is imprinted on the freshly generated surface as revealed by in situ experiments. This phenomenon essentially limits the precision of diamond turning-based processes in nanometrically-smooth roughness profiles. In Mg AZ31, we demonstrated that refining one characteristic dimension of the deformation geometry to the 100nm-scale triggers a brittle-to-ductile transition at ambient temperatures. The other two dimensions can be substantially larger or even macroscopic. The ability to accommodate shear strains greater than 200% in this configuration is independent of the orientation of the crystals with respect to the loading direction. Refining the characteristic dimension enables accommodation of large plastic strains via multi-slip by reducing the mismatch in critical resolved shear stress of available slip systems, while engendering the characteristically dramatic microstructure refinement. In pure magnesium, the deformed chip features much larger grain sizes compared with AZ31 due to rampant dynamic recrystallization in the absence of solute dragging effect. These observations imply that achieving the combination of low-density, high strength and formability in bulk Mg (alloys) can be accomplished by designing microstructure and composites in fewer than 3 dimensions without requiring precise control over crystal orientations