Effects of pre-straining and coating on plastic deformation of aluminum micropillars

The plastic deformation of micropillars is known to be affected by whether dislocations can escape easily from the material volume, and the extent to which the dislocations mutually interact during the deformation. In this work, pre-straining and coating are used to modify the initial dislocation content and the constraints on the escape of dislocations. Aluminum micropillars in the size range from ∼1 to ∼6 μm, with or without thin coating by tungsten deposition and pre-straining by 7%, were compressed using a flat-punch nanoindenter to study their plasticity behavior. The results reveal very different behavior between the size regime of a few microns and that of ∼1 μm. For pillars a few microns large, coating leads to significant strengthening, and pre-straining by 7% also produces a mild strengthening effect. The proof strength also exhibits good correlation with the square root of the residual dislocation density measured by transmission electron microscopy after deformation, indicating that strength in this size regime is controlled by dislocation interactions as in traditional Taylor hardening. Coating evidently helps retain dislocations inside the pillar, and pre-straining increases the initial dislocation content; both effects lead to more severe strain hardening during deformation. For smaller pillars ∼1 μm in size, however, pre-straining results in softening, although coating still leads to strengthening, and the strength exhibits no correlation with the residual dislocation density, which remains close to the initial value even with coating. These suggest dislocation starvation in this small size regime and that strength is controlled by the availability of mobile dislocations. Coating cannot effectively trap dislocations inside the pillar, but can still strengthen the pillar, presumably because dislocation nucleation is more difficult at the coated surface.