Microstructural Evolution of a Nanocrystalline Ni-Co Alloy and Ni Introduced by Impact Deformation

Abstract

Nanocrystalline (nc) materials exhibit superior mechanical properties compared with coarse-grained materials. Like other structural materials, in applications, nc materials are subject to tensile, shear and/or compressive stresses. While the mechanical behaviours and microstructural evolution of nc materials subject to tension, shearing or compression have been widely investigated, little has been conducted on structural behaviours under high strain rate impact. The aim of this research project is to explore how the microstructures of nc materials (nc Ni-32.8 at%Co alloy and nc pure Ni fabricated by electrodeposition) evolve under high strain rate impact deformation. High strain rate compressive deformation was conducted using split Hopkinson pressure bars at room temperature. The subsequent structural characterisation was conducted using X-ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Very interesting results were revealed for the two nc materials; in particular, significant 〈110〉 texture formation and grain growth occurred in nc Ni-Co, but not in nc pure Ni. The underlying mechanisms can be attributed to grain rotation, and the difference between Ni-Co and Ni to the differing stacking fault energy. This thesis is organised as follows. Chapter 1 provides a brief introduction to the research background, including synthesis methods, mechanical properties and deformation mechanisms of nc materials, texture evolution of deformed materials and the grain growth mechanism. The fundamental working principles of the research tools including SEM, TEM and the experimental procedures including transmission Kikuchi diffraction (TKD)/TEM specimen preparation are presented in Chapter 2. TKD and TEM characterisation results on the morphology and crystal structure before and after deformation are described in Chapter 3. Analyses of the TKD and TEM data and deductions regarding the underlying mechanism are discussed in Chapter 4. Chapter 5 offers conclusions and avenues for possible further work.