UC Irvine

Grain boundaries are important planar defects which influence a variety of bulk properties, while the structures and behaviors of grain boundaries can be altered by solute atoms to enable the design of beneficial material performance. Different interactions between solutes and grain boundaries exist. One example is that under certain conditions, solutes travel to grain boundaries and change grain boundary chemistry and structure, to vary the so-called “grain boundary complexion” state. Current studies on grain boundary complexions have been primarily limited to binary alloys. While in ternary or quaternary systems, multi-element segregation might be utilized to create thicker amorphous intergranular films (AIFs), a structural feature that helps toughen nanocrystalline alloys. Another example is the migration of grain boundaries at the presence of solutes, a situation in which there are both segregated dopants on boundaries and dopants as solid solution additions in the surrounding crystal. A promising dopant concentration might be found at which grain boundary motion in alloys can be changed and therefore leads to optimal microstructure beneficial for material performance. To provide guidance to experiments as well as explaining the physical mechanisms responsible for the experimental observations, atomistic modeling is chosen to address the challenges mentioned above. In this thesis, certain criteria for selecting appropriate interatomic potentials for modeling are first established. Then, the interfacial segregation and structural transition behavior in ternary alloys have been investigated and the possibility of forming thicker AIFs in certain ternary alloys by controlling the ratio of different dopant elements has been demonstrated, which could open up the opportunity of tuning the structure and properties of materials by co-doping. In the second part of the thesis, the migration of twin boundaries, basal-prismatic interfaces and conjugate twin boundaries (also referred to as twin tips in some cases) in pure Mg and Mg alloys has been studied. By understanding the effect of segregated dopants as well as randomly distributed dopants in the matrix on the motion of a group of special boundaries, it is able to alter the microstructure evolution by alloying and further tune the mechanical properties of Mg alloys.