UC San Diego

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. Unique mechanical response, such as spallation failure and shear localization, manifests 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 transformation, which override thermal softening. In recent studies, FeNiCoAlTaB (NCATB) and VNbMoTa have achieved some outstanding quasi-static mechanical properties. However, their mechanical properties at high strain rate conditions are not so well documented. In our research, we will study their dynamic properties and correlate them with fundamental microstructural mechanisms. This will be conducted by developing appropriate heat treatments that maximize the performance of these alloys. The dynamic properties are primarily studied using a split-Hopkinson pressure bar. By designing specimens with distinctive geometry, we are able to operate uni-axial compressive tests and dynamic shearing tests with conventional cylindrical specimens and forced hat-shape specimens, respectively. The NCATB HEA exhibits high strength at both loading conditions, revealing its potential as a candidate of structural materials. The incredible resistance toward shear localization with strong strain hardening ability indicates novel deformation mechanisms contrary to the well-studied martensitic transformation and microbands. Surprisingly, besides the well-studied martensitic transformation and dislocation mechanisms, the newly discovered 9R represents a novel pathway to mediate plastic deformation. The results of this dissertation research shed light on the deformation mechanisms of HEAs of medium/high stacking fault energy while experiencing extreme dynamic shearing loading.