UBC Theses and Dissertations
Dynamic recrystallization of nano/micro particles under high velocity impact Funes Rojas, David
This thesis studies the microstructural evolution of single-crystal face-centred-cubic nano and micro size particles impacted at high velocity, mainly using molecular dynamics simulations. The first chapter focuses on Silver single-crystal nanocubes impacted along different orientations, combined with different crystallographic orientations. Upon impact, some preferential lattice orientations can develop more or less intricate microstructures with grains of different sizes. These cases correspond with crystallographic orientations where at least eight or more slip systems are simultaneously activated, which leads to an avalanche of dislocations. The interaction of dislocations and the severe plastic work leads to dynamic recrystallization that creates a microstructure with grain size varying from fine grains near the impacted area, and coarse ones on the opposite end. On the other hand, dynamic recrystallization was not observed for the orientations with asynchronously activated slip systems besides considerable shock-wave pressures, plastic deformation, and large dislocation densities. These results are in good agreement with experimental works carried out in Silver microcubes. This study is then extended to a representative sample of face-centred cubic metals. It is found that there is a critical velocity for all studied materials after which dislocation density and grain refinement are maximized. These findings are validated on a sprayed novel CoCrFe0.75NiMo0.3Nb0.125 high-entropy alloy. This sprayed alloy exhibited extraordinary mechanical properties reaching a yield strength of ∼ 1745 MPa, an ultimate strength of ∼ 2622 MPa with a maximum strain at failure of ∼ 9%, outperforming all other CoCrFeNi-based alloys. These outstanding mechanical properties are partially the result of grain refinement and dislocation density developed during the impact of the micro-particles during cold spray. It is shown via molecular dynamics simulations that the recrystallization of high-entropy alloys via dislocation emission plays a major role in the outstanding mechanical properties of the sprayed alloy. Overall, this thesis sheds light on the factors that promote dynamic recrystallization, providing insights to create a pathway to control the microstructure and atomic structure in face-centred metallic materials.
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