Quasi-static and shock-wave loading of ultrafine-grained aluminum: effect of microstructural characteristics

The interrelation between microstructural characteristics and mechanical properties under quasi-static and shock-wave (dynamic) loading was investigated in ultrafine-grained aluminum processed by accumulative roll bonding (ARB) for 4, 7, 10 and 14 cycles. The microstructural parameters such as the size of the elements of grain–subgrain structure, grain size and fraction of high-angle grain boundaries were obtained using transmission electron microscopy (TEM) and electron back scatter diffraction (EBSD). Indentation and tensile tests at the strain rate of 1 × 10⁻⁴ s⁻¹ were applied as the quasi-static loading, the impact by aluminum flyer-plates with the impact velocity of 620 ± 30 m s⁻¹ was the shock-wave loading. The strain rate in the rarefaction wave before spall fracture was 2 × 10⁵–7 × 10⁵ s⁻¹ in the latter case. It is shown that the dislocation substructures and low-angle subboundaries significantly affect the strength properties under quasi-static conditions, while the grain size (the areas bounded by only high-angle boundaries) and fraction of high-angle grain boundaries mainly define the dynamic strength properties. The different influence of the microstructural characteristics on the quasi-static and dynamic mechanical properties is related to the easier dislocation cross-slip under high strain rates.