Fabrication, thermal stability and mechanical characterization of electrodeposited nanocrystalline face centered cubic nickel-iron alloys

Various compositions of FCC (face centered cubic) Ni-Fe alloys with a grain size of less than 15 nm were successfully fabricated using the electrodeposition technique. It was found that the grain size, lattice strain, texture, lattice parameter, microhardness as well as the microcracking pattern are all dependent on the iron content of deposits.; Tensile results showed that defect-free nanocrystalline FCC metals are not inherently brittle and exhibit a good combination of super-high strength and a reasonable tensile elongation. Due to the high quality of samples, the tensile ductility obtained in the current study is a noticeable improvement in comparison to the previously reported results. In the case of the Ni-15%Fe alloy with a grain size of below 9 nm, the approximately 6% plastic tensile elongation is the first time to be reported in FCC metals at such grain size level. Based on the fractographic analysis, a model describing the fracture process in nanocrystalline metals was proposed. It was found that the stable propagation of the microcracks preceded the final fracture.; Low temperature annealing had considerable effect on the mechanical behavior of nanocrystalline Ni-Fe alloys and resulted in an increase in the strength and a reduction in the ductility, which is probably due to the grain boundary relaxation. In addition, the as-deposited samples exhibited loading-rate sensitivity and the annealed samples showed otherwise. These findings suggest that the grain boundary does play a significant role in the deformation process for nanocrystalline materials.; Isochronal and isothermal annealings were applied to study the thermal stability and the grain growth kinetics in nanocrystalline metals. A stabilization of nanocrystalline structure was found by the addition of iron to nickel. It is also of interest that two temperature regimes were identified in terms of the grain growth. Based on the calculation of the activation energy, it is suggested that the grain boundary diffusion dominates at low temperatures, whereas the lattice diffusion starts to make a contribution to the grain growth within the high temperature range. An abnormal grain growth was observed and the grain growth can still be described by the generalized parabolic grain growth model.; In conclusion, this dissertation elucidates that the addition of alloying element, grain size and the grain boundary state are three key parameters to be considered when studying the nanostructured materials.