Online

Picard, Ève-Audrey

Mechanical Engineering

2021

M.S.

Nanocrystalline metals and alloys have been proven to possess unprecedentedly higher tensile strength than coarse-grained conventional metals. The extreme grain refinement in nanocrystalline metals, however, negatively affects these materials by reducing their ductility through grain-boundary embrittlement and shear localization mechanisms that are promoted by segregation of solute atoms to the interfaces. Different segregation behaviors described in the literature can be divided into either heterogeneous or homogeneous types. Yet current understanding of the impact of solute atom arrangements within grain boundary networks on mechanical properties of cubic and hexagonal nanocrystals remains limited. In this thesis, hybrid Monte-Carlo and molecular dynamics simulations were used to study the segregation behavior of Ni solute atoms in polycrystals made of FCC Ag-Ni, FCC Al-Ni, BCC Nb-Ni, and HCP Zr-Ni alloys. Solute segregation in 4 binary alloys with a constant solute content of 4 at.% Ni was simulated and quantified at the same homologous temperature and at their respective maximum solubility temperature. A spectrum of segregation configurations varying from fully heterogeneous to fully homogeneous was found: Pure heterogeneous segregation (Ag96Ni4 500 K), homogeneous segregation with second-phase precipitates (Al96Ni4 378 K, Nb96Ni4 1110 K), homogeneous segregation with small-scale Ni clusters (Nb96Ni4 1564 K, Zr96Ni4 464 K), and pure homogeneous segregation with amorphous intergranular films (Al96Ni4 913 K, Zr96Ni4 1118 K). These differences in segregation behavior are shown to lead to significant variations in stress-strain response for each alloy. It is found that segregation involving the presence of grain-boundary precipitates with homogeneous segregation behavior promoted the most significant shear localization.