Abstract: Although the rate-dependence of metals has been widely researched, the deformation mechanism under small-scale impact conditions lacked exploration and in-depth understanding. Using quasi-static nanoindentation (strain rate, SR, < 1 s^-1) and high strain-rate nano-impact (SR > 10³ s^-1) with a pyramidal Berkovich tip, this study investigates the influence of SR on the deformation response of an aluminium single crystal (110). The underlying microstructural variance was analyzed using on-axis TKD and TEM. The results show that the impact deformation involves great elastic recovery and different substructural characteristics. In contrast to the uniform sub-grain substructure with medium and high-angle grain boundaries formed during quasi-static indentation, the substructure formed under impact has a more heterogeneous nature including microbands near the surface and sub-grains underneath with dominant low-angle grain boundaries. The significant change in substructure for the impact deformation comes from suppressed thermally activated dislocation motion, leading to the conversion of dislocation glide from wave-like (quasi-static) to planar regime (impacting), and the insufficient rearrangement of geometrically necessary dislocations (GNDs). The heterogeneous microstructure develops due to the competition between high strain-rate-induced planar-slip and strain gradient-induced GND rearrangement, as well as the uneven distribution of SR and strain gradient. Moreover, the underlying incipient mechanism of the microband is proposed, in which successive primary dislocations are nucleated at the surface and glide perpendicular to flanks to pile up. Finally, the influence of SR on indentation size effects is discussed.

Key words: Nanoindentation, Nano-impact, Microstructure, Aluminum single crystal, Microbands, Dislocations