1 Department of Mechanical Engineering, Technical University of Denmark2 Center for Electron Nanoscopy, Technical University of Denmark3 Centre for oil and gas – DTU, Center, Technical University of Denmark
Grain boundary engineering aims for a deliberate manipulation of the grain boundary characteristics to improve the properties of polycrystalline materials. Despite the emergence of some successful industrial applications, the mechanism(s) by which the boundary specific properties can be improved is not yet well-understood. This, at least partly, owes to the lack of robust characterization methods for analyzing the nature of grain boundaries including the grain boundary plane characteristics, until recently. In the past decade, significant improvements in the 2-dimensional and 3-dimensional analysis of the grain boundaries have happened. These improvements, for example by high-resolution imaging techniques and orientation imaging microscopy for additional crystallographic information, provide the possibilities for thorough characterization of the grain boundaries and based on that, it is possible to engineer new materials. In this study, one of the most widely used electrolytes for electrodeposition is chosen for the synthesis of nickel films and based on thorough characterization of the boundaries the potentials in grain boundary engineering are outlined. The internal structure of the nickel films both in the as-deposited state and after thermal annealing is investigated and experimental methods for grain boundary characterization are accordingly applied to essentially different microstructures. Supplementary characterizations with X-ray diffraction, orientation imaging microscopy, and focused ion beam microscopy were applied. Using additive freeWatts electrolyte, coarse columnar microstructures with <211>, <100>, and <210> texture and fairly high fraction of twin boundaries are synthesized. In <210> textured nickel film, multiple twinning occurs which brings about an arrangement of the favorable boundaries that break the network of general grain boundaries. Successful dedicated synthesis of a <210> textured nickel film fulfilling the requirements of grain boundary engineered materials, suggests improved boundary specific properties. However, the <210> textured nickel film shows fairly low thermal stability and growth twins annihilate by thermal treatment at 600 degree C. In contrast, for <211> oriented grains, growth nano-twins which are enveloped within columnar grains show a high thermal stability even after thermal treatment at 600 degree C. In order to exploit the high thermal stability of nano-twins, the effect of different electrodeposition conditions and alloying cobalt on the strength of <211> texture and twin formation are studied. Using the Watts electrolyte with a common sulfur-free additive, nano-crystalline nickel films with different characteristics in as-deposited state are synthesized as a function of the deposition conditions. The microstructure of thermally treated nano-crystalline nickel films, show low fraction of favorable boundaries. The grain size and texture development due to thermal treatment is studied too and it is argued that prior to the major grain growth strain energy minimization plays the major role in the microstructure evolution while after major grain growth interface energy minimization has the major role. Differences in as-deposited microstructural characteristics, brings about differences in grain size and grain boundary characteristics after thermal treatment. It is suggested that triple lines, at least partly, contribute to the observed differences and potentials for \Grain Boundary Junction Engineering" are outlined.