Characterization of the microstructure, grain boundaries and texture of nanostructured electrodeposited CoNi by use of Electron Backscatter Diffraction (EBSD)

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The mechanical behaviour of electrodeposited nanocrystalline metals have been the subject of considerable research in the past decade. This interest is strongly related to the unexpected response of these materials to deformation. However, since the properties of materials are direct related to their microstructure characteristics, it is remarkable how little research on the microstructure and texture of nanocrystalline deposits has been conducted in this time. The need of a systematic study of the microstructural characteristics of these materials has, therefore, motivated this work, which deals with a detailed characterization of nanostructured electrodeposited CoNi. The aim of this work was, however, not only to describe the general microstructure and texture development in CoNi electrodeposited film, but to understand the fundaments of their formation and therefore provide fundamental knowledge for further studies.
In order to relate the microstructure of the studied CoNi films to the texture, the characterization was performed mainly by electron backscatter diffraction (EBSD). This technique allows a microstructure related texture analysis of relative large sample areas with a spatial resolution of 30 to 50 nm. Furthermore, x-ray diffraction, energy dispersive spectroscopy and transmission electron microscopy was applied to study the macrotexture evolution of the film, the alloy composition and microstructure feature smaller than 30 nm, respectively. To achieve an understanding of the microstructure and texture evolution throughout the film, the deposit films were investigated on the substrate interface, on the bath interface and on the cross section. The work presents a detailed description of the microstructure, the crystallographic texture and the grain boundary character of electrodeposited CoNi depending on the additive concentration in the deposit bath and on the film thickness. These three major sample characteristics are presented in separately subchapter of the experimental results first for a CoNi deposit produced in a bath with additive level of 0.02 g/l and later as a comparison between samples from bath with different additive concentration. A further subchapter of the experimental results presents the study of grain boundary plane and it relationship to the grain morphology of the deposit film. This latter study was performed by a new technique, which combines precise material removal with a focused ion beam and orientation microscopy (3D-EBSD) fully automatically, allowing the three dimensional study of samples with spatial resolution of 50 × 50 × 50 nm3.
The microstructure of the CoNi electrodeposited samples has been described in terms of phase concentration and distribution and in terms of the grain size. Whereas the latter was studied as a function of the film thickness, considering only high angle grain boundaries and considering all grain boundaries with misorientation larger than 2°. The grain size distribution was studied and correlated to the number of grain and to the area fraction of these grains. Although a preliminary x-ray diffraction study did not reveals a two phase microstructure, the CoNi deposits consist of fcc and hcp phases, whereas the concentration of the fcc phase is higher at lower film layers and decreases with the film thickness. The grains in the film show a columnar morphology with grains extended in the film growth direction. The average grain diameter (grain size perpendicular to the growth direction) increases with the thickness of the film. The length of these grains in film growth direction depends on the grain orientation and on their orientation gradient (defect density). The higher the defect density, the shorter the grain, because arrangements of dislocation cause the grain to deflect from the preferential growth direction. The columnar grains reveal a strong (0211)//GD texture (GD = growth direction), which becomes sharper with the film thickness indicating a strong growth selection process. Although the majority of the grains show such columnar morphology, several grains are equiaxed. These equiaxed grains are more randomly oriented than the columnar ones and appear as clusters. Their fraction increases slightly with the film thickness. The deposit reveals a high fraction of high angle grain boundaries. The fraction and distribution of phase boundaries, twin boundaries and general grain boundaries were characterized depending on the film thickness and on the additive level in the bath. A topographic analysis of the location of the twin boundaries showed that two 〈1102〉 57° twins occur together with one 63° to 69° boundary in a triple point with a common 〈0211〉 rotation axis. The grain boundary plane of such arrangement was studied and it was found that the twin plane as well as the (0001) basal plane forms low energy grain interfaces, the latter in case of an incoherent twin and a conventional large angle grain boundary. These low energy grain boundaries promote the crystal growth in those sites and facilitate the formation of columnar grains.
The additive concentration in the deposit bath influences the microstructure, texture and grain boundary character of the CoNi films significantly only after a certain concentration (additive level >0.02 g/l). When 0.04 g/l of saccharin is added to the bath the surface morphology, the grain size and the texture is changed considerably, however the
formation of columnar grain is not completely suppressed. At this additive level, columnar grain are formed only in the beginning of the growth, and are significantly smaller in both, growth direction and perpendicular to it. These columnar grains reveal the same crystallographic texture 〈0211〉||ND as the columnar grains of the samples from lower saccharin levels. After several micrometers of film thickness the preferential growth in the 〈0211〉 direction is reduced and the microstructure becomes completely nanocrystalline. These fully nanocrystalline grains reveals a weak (111)fcc preferential orientation.
Most of the studies on the mechanical properties of nanocrystalline electrodeposited metals discuss the microstructure of the studied sample only in terms of the average grain size. The results of this study shows that the microstructure of electrodeposited CoNi is very complex and depends strongly on the film thickness. These results show clearly that further microstructure characterization is needed to achieve a comprehensive understand of the mechanical behavior of nanocrystalline metals.