Abstract Traditional metallography relies on the imaging of individual section planes. However, conclusions as to spatial shapes and microstructural arrangements can only be drawn to a limited extent. The idea to reconstruct three-dimensional microstructures from metallographic serial sections is therefore obvious and not at all new. However, the manual process of preparing a great number of individual sections and assembling them into image stacks is time-consuming and laborious and therefore constitutes an obstacle to frequent use. This is why the Federal Institute for Materials Research and Testing, or BAM for short (Bundesanstalt für Materialforschung und -prüfung), is developing a robot-assisted 3D metallography system performing the tasks of preparation and image acquisition on a metallographic section fully automatically and repeatedly. Preparation includes grinding, polishing and optional etching of the section surface. Image acquisition is performed using a light optical microscope with autofocus at several magnification levels. The obtained image stack is then pre-processed, segmented and converted to a 3D model resembling a microtomographic image, but with a higher lateral resolution at large volumes. As opposed to tomographic techniques, it is possible to perform traditional chemical etching for contrasting. The integration of a scanning electron microscope is in the planning stages. Studies conducted so far have demonstrated the possibility of visualizing hot gas corrosion layers, gray cast irons and ceramic-based microelectronic structures (vias).

The steam side oxidation of ferritic–martensitic VM12-SHC steel was investigated under thermo-cyclic conditions in water steam at 620/320 °C and 30 bar with a focus on assessing the influence of pre-oxidation time, specimen geometry and surface finish. The specimens were pre-oxidized under isothermal conditions in water steam at 620 °C and 30 bar for 500 h or 1500 h. After pre-oxidation treatment, all specimens were subjected up to 258 thermal cycles. Three different geometries—rectangular coupons, U-shaped ring segments and ring samples—were investigated to evaluate the influence of open/closed shape, and flat/curved surface on corrosion rate. At the same time, two types of surface finish were considered: “as received” and “ground.” The formation of a protective scale by pre-oxidation was investigated. EBSD and ESMA analyses revealed that the Cr-content of the alloy appeared to be insufficient for obtaining a protective oxide scale under studied conditions, at the same time the anayses confirmed that initial oxidation depends on presence of minor alloying elements as Si and Mn, strong oxide formers which can alter the kinetics and morphology of the corrosion reaction. Moreover, rectangular coupons with small wall thickness and flat surface exhibited the highest corrosion rate, while “ground” curved samples showed only local oxidation. This indicates that for same pre-oxidation time, oxidation kinetics is controlled by curvature.

Processes of interdiffusion have been studied in the Ni-Re system. The method of electron-microprobe analysis was used to measure the interdiffusion coefficients D̃ in Ni4.4Re/Ni, Ni7Re/Ni Ni9Re/Ni, and Ni9Re/Ni4.4Re (wt %) diffusion pairs in the temperature range of 1050–1350°C. The averaged values of the preexponential factor and the activation energy for interdiffusion were found to be D 0 = 1.16 × 10−4 m2/s and Q = 317 kJ/mol, respectively. The value D 0 ∼ 10−4 m2/s is typical of substitutional alloying elements in fcc metals; the value of Q is determined to be close to that for W and is significantly higher than those that correspond to other alloying elements in nickel superalloys. No clearly pronounced concentration dependence of the coefficients of interdiffusion has been revealed at a rhenium concentration of 4.4–9 wt %.

типично для легирующих элементов замещения в ГЦК-металлах, а Q близко к таковому для W и значительно выше, чем для других легирующих элементов в никелевых жаропрочных сплавах. Явной концентрационной зависимости коэффициент взаимной диффузии в интервале исследованных концентраций Re 4.4–9 мас. % не обнаружено. в диффузионных парах Ni4.4Re/Ni; Ni7Re/Ni; Ni9Re/Ni и Ni9Re/Ni4.4Re (мас. %) в интервале температур 1050–1350°С. Усредненные значения предэкспоненциального множителя и энергии активации взаимной диффузии соответственно составляют D0 = 1.16 ? 10-4 м2 и Q = 317 кДж/моль. Данное значение D0 10-4 м2 типично для легирующих элементов замещения в ГЦК-металлах, а Q близко к таковому для W и значительно выше, чем для других легирующих элементов в никелевых жаропрочных сплавах. Явной концентрационной зависимости коэффициент взаимной диффузии в интервале исследованных концентраций Re 4.4–9 мас. % не обнаружено.

The materials in oxyfuel power plant will be subjected to CO2− and SO2−rich gases on the fireside. The oxidation behaviour of two 9–12 % Cr steels T92 and VM12 was studied under dry oxyfuel environment in the temperature range of 580–650 °C for up to 1,000 h. The oxide structure and morphology were analyzed using various experimental techniques. A complex temperature dependence of oxidation rate is observed for both T92 and VM12 whereby the oxidation rate decreased with increasing temperature. This is attributed to increased Cr-enrichment in the inner scale with increasing temperature. T92 and VM12 alloys are also susceptible to carburization in an oxyfuel environment.

Optical and scanning electron microscopy, as well as electron microprobe analysis and electron backscatter diffraction, have been used to study diffusion processes that occur in a diffusion pair that consistsof a single-crystal CMSX-10 nickel-base superalloy and polycrystalline nickel, at temperatures of 1050–1250°C. It has been found that, in this system, the distributions of γ-stabilizing elements (Cr, Co, W, and Re) are described by the Boltzmann solution for diffusion between two semiinfinite plates of a binary alloy. The processing of these distributions has shown that the diffusion coefficients of Cr, Co, W, and Re in the multicomponent system are close to those in binary alloys of these elements with Ni. The diffusion redistribution of the elements leads to the dissolution of the γ′ phase in the nickel-base superalloy, growth of nickel grains toward the superalloy constituent of the diffusion pair, and the formation of porosity on both sides of the migrating interface, which is determined from a crystal misorientation of the alloy single crystal and nickel grains.

Методами оптической и растровой электронной микроскопии, а также микрорентгеноспектрального анализа и дифракции обратноотраженных электронов исследованы диффузионные процессы, происходящие в диффузионной паре монокристалл никелевого жаропрочного сплава CMSX-10 – поликристаллический Ni при температурах в интервале 1050–1250°C. Установлено, что в данной системе распределения -стабилизирующих элементов Cr, Co, W и Re описываются решением Больцмана для диффузии между двумя полубесконечными пластинами бинарного сплава. Обработка этих распределений показала, что коэффициенты диффузии Cr, Co, W и Re в многокомпонентоной системе близки к коэффицентам диффузии в бинарных сплавах этих элементов с Ni. В результате диффузионного перераспределения элементов происходит растворение -фазы в никелевом жаропрочном сплаве, рост никелевых зерен в сторону сплава и образование пористости по обе стороны мигрирующей поверхности раздела, определяемой по кристаллографической разориентации монокристалла сплава и никелевых зерен.

Basic research on the corrosive effect of flue gases has been performed at the BAM Federal Institute for Materials Research and Testing (Germany). Conditions at both high and low temperatures were simulated in specially designed experiments. Carburization occured in flue gases with high CO2 content and temperatures higher than 500°C. In SO2 containing flue gases sulphur was detected in the oxide scale. At lower temperatures no corrosion was observed when gases with low humidity were investigated. Humidity higher than 1500ppm was corrosive and all steels with Cr contents lower than 12% revealed corroded surfaces. At low temperatures below 10°C a mixture of sulphuric and nitric acid condensed on metal surfaces. Acid condensation caused severe corrosion. Humidity, CO2, O2, and SO2 contents are the important factors determining corrosion. Below 300°C acid condensation is the primary reason for corrosion. Low humidity and low temperatures are conditions which can be expected in the CO2 separation and treatment process. This work includes major conditions of the flue gas and CO2 stream in CCS plants and CCS technology.