Sample size effect on creep in bending: An interplay between strain gradient and surface proximity effects

Miniature cantilevers are associated with amplification in strain gradient and dislocation recovery. These contrasting phenomena interplay to impart strengthening via the injection of geometrically necessary dislocations because of strain gradients and softening due to the surface proximity effect. In this work, creep studies on Al cantilevers of thicknesses ranging from 0.070 to 7 mm demonstrate this interplay. The 2D strain field and dislocation substructure were examined to understand the steady-state creep response in miniaturized cantilevers. The average creep response of the cantilever strengthens with decreasing cantilever thickness; however, regimes of hardening, softening, and bulk-like behavior were observed while traveling along the length of the cantilever. Consistently, refinement in the steady-state dislocation substructure was observed in the regions of large strain gradients. The locations in a miniaturized cantilever with insignificant strain gradients registered surface proximity effect-induced enhanced creep rates. An analytical model has been developed to describe the strain gradient-surface proximity interplay. Graphical abstract