Solid State Phenomena Vols. 172-174

Abstract: The study presents the latest developments in terms of speed and integration of the electron backscatter diffraction (EBSD) and energy dispersive spectroscopy (EDS) techniques. The microstructural features and texture of a commercially available tool steel have been analyzed by simultaneous EBSD/EDS measurements. The EDS data was used for confirming/correcting the EBSD results as well as for detecting the presence of ultrafine carbide precipitates. The results indicate the formation of two different types of carbides inside a ferritic matrix. Most of the matrix was found to be composed of fully recrystallized grains with average diameters around 10 microns. Zones characterized by finer submicron scale grains could also be identified locally as well as grains containing networks of subgrain boundaries. This study demonstrates that the combination of the two techniques, i.e. EBSD and EDS, results in a powerful tool for a fast, reliable and complete characterization of multiphase materials.

Abstract: From neutron diffraction it is known that the BaCeO₃ perovskite undergoes a sequence of phase transformations from high temperature cubic C to rhombohedral R, to orthorhombic O1 (Imma) and to orthorhombic O2 (Pnma). Doping Y³⁺ on the Ce⁴⁺ site introduces charge compensating O vacancies (V_O) that may be partially filled with OH complexes with exposition to H₂O, so making the material an ionic conductor. Anelastic relaxation experiments have been carried out on samples doped with 2%Y and 10%Y; the real part s’(T) of the complex elastic compliance presents softenings at the transitions, and the loss s’’/s’ curves allow the content of V_O and H to be monitored. Doping has a strong effect on the temperature of the Pnma/R transition: with 10%Y in the fully hydrated state T_O1-R increases up to 750 K while after full outgassing falls to 500 K, meaning that the introduction of ~5% V_O shifts the transition of 250 K. While the effect of cation substitution on the transitions temperature is easily explained in terms of simple arguments usually valid for perovskites based on bond length considerations, the remarkable stabilization of the R phase by V_O requires to take into account the anomalous sequence of phase transitions of undoped BaCeO₃, where the R structure transforms into orthorhombic Pnma on cooling with the loss of an octahedral tilt system.

Abstract: Magnetic structure in a layered perovskite manganite, La_2-2xSr_1+2xMn₂O₇ (x = 0.307) has been investigated by neutron diffraction measurements. We found that the ground state (at 4 K) has a ferromagnetic structure with magnetic moment of Mn ions being aligned in a direction inclined by 10 degree from the c-axis. The magnetic structure changes to a canted antiferromagnetic one (CAFM-I) at about 20 K and then to another canted antiferromagnetic one (CAFM-II) at about 80 K. Here the magnetic moment lies in the ab-plane in CAFM-II but not in CAFM-I. The magnetic structure then changes to an antiferromagnetic one with magnetic moment lies in the ab-plane at about 90 K, and then to a paramagnetic one at about 100 K.

Abstract: Low dimensional nanostructures, e.g. nanowires, self-assembled through heteroepitaxy, present a variety of crystallographic features that do not always follow conventional V-W or S-K growth mode. Applying Δg parallelism rules and edge-to-edge matching (E2EM) model in β-DySi₂/Si and CoSi₂/Si systems provides a better understanding of the natural preference of the interface orientation and the orientation relationship (OR) during heteroepitaxial growth. This may help improving the quality of nanowires through optimizing the substrate orientation.

Abstract: Segregation of solute atoms and vacancies to migrating D03–type antiphase boundaries (APBs) in Fe3Al of the stoichirometric and an Al-rich compositions has been investigated in detail by a phase-field method developed in the previous study [Koizumi et al. Acta Mater 2009;57:3039] focusing on the quantitative relationship among the segregation, APB energy, driving-force, drag-force and APB mobility. It has been revealed that the drag-force by solute segregation depends considerably on the alloy composition and is responsible for the significant composition dependence of the migration kinetics of D03-APB whereas the composition dependences of driving-force and mobility are negligibly small.