Search

Analysis of presolar silicate grains provides new knowledge on interstellar and circumstellar environments and can be used to test models of the Galactic chemical evolution. However, structural information of these grains is rare because sample preparation for transmission electron microscopy is very difficult due to the small dimensions of these grains (<0.5 μm). With the use of the focused ion beam technique thin foils from these grains for transmission electron microscopy analysis can be prepared. Nevertheless, reaching the required precision of some tens of nanometres for the preparation of the transmission electron microscopy foil in the place of interest is not trivial. Furthermore, in the current samples, the grain of interest can only be identified by its different isotopic composition; i.e. there is no contrast difference in scanning electron microscopy or transmission electron microscopy images which allow the identification of the grain. Therefore, the grain has to be marked in some way before preparing the transmission electron microscopy foil. In the present paper, a method for transmission electron microscopy foil preparation of grains about 200 to 400 nm in diameter is presented. The method utilizes marking of the grain by Pt deposition and milling of holes to aid in the exact orientation of the transmission electron microscopy foil with respect to the grain. The proposed method will be explained in detail by using an example grain. [ABSTRACT FROM AUTHOR]

Abstract: Three-dimensional tomography was performed on hydrothermal degraded zirconia by sequential focussed ion beam (FIB) sectioning and field emission scanning electron microscopy (FE-SEM) observation. By means of image analysis the distribution of microcracks produced under the surface was reconstructed and characterized. Results show a microcrack network preferentially oriented parallel to the surface, and a gradient in microcrack density from the surface, which is coherent with other measurements that reveal a decrease in monoclinic phase. The elastic properties of the reconstructed volume are simulated, showing that the deterioration of mechanical properties of degraded zirconia can be mainly attributed to microcracking, with a minor contribution of the phase transformation. [Copyright &y& Elsevier]

The modified ASTM A297 Gr. HH austenitic stainless steels are a group of refractory alloys designed for high temperature applications that required elevated mechanical strength, such as those found in petrochemical furnace structures and in the steel industry. This study examines the overall microstructural features and cyclic oxidation behaviour of modified HH steels with 0.09%Nb and 0.77%Nb additions. The investigations involved thermodynamic simulations and microstructure analyses conducted by optical and scanning electron microscopy of the substrate materials in the as-cast and aged conditions. The cyclic oxidation tests were performed at 900 °C in atmospheric conditions with oxidation times up to 960h. The resulting oxide scales were analyzed by X-ray diffraction and scanning electron microscopy. The thermodynamic simulations and microstructure analyses of the as-cast steels revealed larger amounts of niobium carbides in the 0.77%Nb steel than in the 0.09%Nb steel, in connection with larger amounts of chromium in solution. Despite this precipitation difference, no significant changes were noticed in the long-term oxidation behaviour of the tested materials. • The influence of Nb addition on refractory steel microstructure was analyzed. • Thermodynamic analyses of precipitations were performed. • In the as-cast condition, Nb addition led to Cr enrichment of the austenitic matrix. • No noticeable improvement in oxidation resistance was observed with Nb addition. [ABSTRACT FROM AUTHOR]