Papers by Author: Uta Klement

Abstract: Axial suspension plasma spraying (ASPS) is a relatively new, innovative spraying technique which has produced thermal barrier coatings (TBCs) with attractive properties such as high durability and low thermal conductivity. Using a suspension, it is possible to spray with finer powder particles resulting in coatings that have a columnar microstructure and contain a wide range of pore sizes, both nm-and μm-sized pores. To optimize the thermal properties and to maintain them during service of the components, it will be important to design TBCs with optimal porosity. Hence, an important part in the assessment of ASPS coatings is therefore the characterization of the microstructure and how it is build up, and the determination of porosity. Both aspects are addressed by performing measurement on splats and ASPS-coating using electron backscatter diffraction (EBSD) technique and by measuring porosity by Mercury Intrusion Porosimetry (MIP).

Abstract: Electrodeposition is an advanced synthesis technique which involves the creation of a coating or free-standing material through an electrolytic process. Organic additives such as saccharin have been frequently used in electroplating operations to moderate deposit growth rates and to control film quality. In the present study, plating of Nickel without additives has resulted in a sub-microcrystalline microstructure and a <110>-fibre texture in growth direction. Structural units in form of groups of grains possessing a common <110>-zone axis in growth direction and low-Σ relationships between them have been found in the microstructure by use of EBSD. Upon annealing, grain growth sets in. However, the structural units and the texture are preserved up to 550°C. This means that the structural units stabilize the microstructure; there is no orientation change when grain growth occurs (e.g. by twinning). The low-Σ boundaries of the structural units are described in detail and texture development upon annealing is discussed in connection with results from previous studies on Ni and Ni-alloys of different initial texture.

Abstract: The microstructure of a Ni–18 wt.% Fe electrodeposits having a banded structure is described in detail. The aim is to investigate the influence of the banded structure on grain growth behaviour and texture and to elucidate if there are other mechanisms operative in the stabilization of nanocrystalline electrodeposits. Spectroscopy techniques have been used to characterize the variations in alloy/impurity concentration perpendicular to the growth direction. The influence of these chemical variations on the microstructural evolution has been monitored by in-situ annealing treatments in the TEM. Local texture of the annealed material has been determined by use of the electron backscatter diffraction (EBSD) technique. SEM and TEM investigations have shown that the banded structure is not related to phase changes and that grain growth is not affected by the banded structure, i.e. there is no preferred growth along bands. The first grown grains have <100>, <112> and <111> orientations with the growth direction and upon further grain growth a <111> fibre texture with respect to the growth direction of the electrodeposits is formed. The banded structure seems not to affect the general behaviour of nanocrystalline electrodeposits.

Abstract: Organic additives such as saccharin have been frequently used in electroplating operations to moderate deposit growth rates and to control film quality. In this study, texture development upon annealing of pulse-electroplated Nickel produced without additives is analyzed by use of electron backscatter diffraction technique. Plating without additives results in a microstructure with slightly elongated grains and a fibre texture in growth direction and this texture is conserved upon annealing up to 600°C. Structural units in form of groups of elongated grains possessing a common zone axis in growth direction and twin relationships between themselves are found in the microstructure. For revealing the influence of additives, the observations are compared with results obtained for Ni and Ni-Fe plated in the presence of additives where during abnormal grain growth the initial fibre texture changes to an energetically more favourable texture by twinning. The lack of additives is assumed to be responsible for the observed differences in texture and microstructure development.

Abstract: Optical microscopy on the etched cross-section of a nanocrystalline Ni-18 wt.% Fe electrodeposit revealed the existence of a banded structure perpendicular to the growth direction. To evaluate if the banded structure is affecting grain growth and texture development, EBSD orientation maps were obtained after annealing for 30 min at 300, 350, and 400°C. Grown grains were found to be random in shape and no preferential sites for grain growth were observed. The texture of the grown grains is changing upon annealing and the final fibre texture parallel to growth direction of the electrodeposit can be obtained from texture components found at lower annealing temperatures when performing one or two consecutive twinning operations.

Abstract: White layers, formed during hard turning and wire electric discharge machining, were characterized by means of X-ray diffraction, scanning electron microscopy and transmission electron microscopy. Different cutting speeds and flank wear were utilized in order to obtain different thermally and/or plastically deformed white layer. Since the white layer after wire electric discharge machining is mainly thermally induced, it was used as a reference structure. In the investigation, both bainitic and martensitic structures were studied. With a constant flank wear of 0.175 mm the thickness of the white layer increased from 1.5 μm to 3 μm as the cutting speed was increased. In both processes the white layer were characterized by nanosized grains and surface tensile residual stresses. M₃C carbides were observed in the hard turned white layer, indicating that the time and temperature needed for completely dissolving the carbides were not reached during cutting. For both materials the white layers formed by wire electric discharge machining consisted of ~ 30 vol. % of retained austenite. Observation regarding the volume fraction of the retained austenite in the white layer formed by hard turning for martensitic material showed an increase in the volume fraction of retained austenite from ~ 2 - 3 vol. % to ~ 6 vol. %, while this observation was not seen in the white layer formed in the bainitic material.

Abstract: Thick (up to 5 mm) Ni electrodeposits were produced by the pulsed electrodeposition (PED) technique. The PED-Ni was investigated in planar and cross-sections using high resolution scanning electron microscopy. Grain size and local texture were studied by electron backscatter diffraction. Thermal stability and grain growth behaviour were investigated using in-situ annealing in the transmission electron microscope. It is observed that columnar grains are present in the material and that the orientation of grains is not uniform. Textures and in-situ annealing behaviour are compared to previous data on nanocrystalline PED-Ni and Ni-Fe, where a subgrain coalescence model adopted from recrystallization is used to describe the occurrence of abnormal grain growth upon annealing and where twinning was found to be responsible for the texture development.