Atomic scale analysis of nanocrystalline materials by advanced microscopy

Abstract

In this thesis, relatively new and advanced microscopy techniques are used to overcome many challenging problems in characterisation of nanocrystalline (nc-) materials. They include grain boundaries’ orientation and misorientation evaluation by transmission Kikuchi diffraction (TKD) and atomic scale analysis of segregation at grain boundaries by atom probe microscopy (APM). For each of these techniques, systematic investigations were carried out to optimise specimen preparation methods, data acquisition and analysis parameters. This resulted in fundamental knowledge that could be adopted to study a wide range of nc- materials. Two different sets of nc- materials are considered in this work: (i) bulk engineering materials (duplex stainless steels (DSS)) processed by high pressure torsion (HPT); and (ii) novel binary thin film systems including Al-O, Ni-P and Cu synthesized by direct current (DC) - magnetron sputtering and electron beam evaporation methods. In DSS samples, TKD work with the support of high resolution transmission electron microscopy has shown deformation twinning in body centred cubic ferrite phase, which is a significant experimental finding in contrast to the abundant theoretical modelling work in literature. Further, ferrite-ferrite grain boundaries and ferrite-austenite interphase boundaries are captured in APM experiments for studying segregation of alloying elements. This provided a way to calculate interfacial excess of segregating elements such as Mo, P, B, and W. Even in Al-O thin films, solute excess of O atoms and O rich clusters at grain boundaries is directly evidenced by APM and quantified by advanced computational methods. APM data also showed that O and P additions in Al-O and Ni-P led to a reduction in grain size of the as-deposited films. AP results facilitated and led to the studies of grain boundary pinning effect on stress coupled grain boundary mediated deformation mechanisms in these films.