Strain-rate effects and dynamic behavior of high entropy alloys

The novel class of multicomponent alloys, also known as high-entropy alloys (HEAs) exhibits excellent properties under low strain-rate conditions. These are especially revealed in the high strength of nanocrystalline CoCrFeMnNi and AlNbTiV alloys, and in the high fracture toughness of AlCoCrCuFeNi and NbMoTaW alloys. Nevertheless, up to now, the dynamic behavior of these high-entropy alloys has not been investigated to the same extent as the quasi-static response. A significantly different mechanical response, such as spallation failure and shear localization, manifests itself when materials are subjected to dynamic loading. Shear localization is an essential precursor to shear failure; studies addressing retardation of its onset are important because of their relevance to applications such as armor for military use. The resistance to shear localization is associated with the extensive work hardening ability enabled by dislocation slip, twinning, and phase transformations which override thermal softening. Apart from shear localization, in contrast to the conventional fracture failure dictated by fracture toughness, spallation resulting from tensile pulses and involving propagating micro-cracks and/or micro-voids also plays an important role in dynamic performance. Although distinctive behaviors have also been reported for other conventional metallic materials under dynamic loading, the unique characteristics of HEAs warrant this review.