Tadami Taoka

Metallurgical Investigations with a 500 kV Electron Microscope

Metallurgical investigations such as recrystallization, martensitic transformation and dynamic behaviors of dislocations have been carried out with a 500 kV electron microscope. The critical foil thickness sufficient to observe the same dynamic behaviors of these phenomena as occurred in bulk specimens is about 1 µ for recrystallization in aluminum and iron alloys, and 3 µ for cell formation in aluminum. On the other hand, foil thickness which allows observation with the 500 kV electron microscope is practically above 8 µ for annealed aluminum and above 2 µ for annealed iron and copper, and it is about one half in order to allow continuous observation of the dynamic behaviors of phenomena. It is concluded that the 500 kV electron microscope has great advantages for the dynamic investigations of metals, especially of materials with the atomic number smaller than 15 such as aluminum and many of ionic crystals.

Orientation dependence of yield stress in 4.4% silicon iron single crystals

Abstract Single crystals of a 4.4%Si-Fe with systematically selected orientations were deformed in tension or compression at room temperature. Slip bands were observed to be nearly parallel to traces of either {110} or {112} plane in most specimens. Two anomalous results were obtained on orientation dependence of yield stress. First, the Schmid law did not hold for the {110}〈111〉 slip system, that is, the critical resolved shear stress increased as the plane of maximum Schmid factor deviated from the {110} plane towards the neighbouring {112} cross slip plane. This is discussed in relation to differences in the dragging stress for the motion of screw dislocations due to differences in the density of jogs which were produced by cross slip. The difference in density of dragging points among specimens with different stress axes was, in fact, observed by electron microscopy using thin foils parallel to slip planes. Secondly, the critical resolved shear stress for a {112}〈111〉 slip in the twinning shear direction was about 13% lower than that in the opposite direction. This fact is tentatively attributed to different Peierls -Nabarro stress between the two opposite senses of motion of a dislocation on a {112} plane.

The Nature of Stacking Faults and Partial Dislocations in Deformed Ni3Ga Single Crystal

The nature of stacking faults and partial dislocations in deformed Ni3Ga single crystal having the L12 structure has been analysed by transmission electron microscopy on the basis of the diffraction theory of the electron microscope images. The results showed that stacking faults are of the superlattice intrinsic type and that the majority of stacking fault ribbons are bounded by two superlattice Shockley partials with parallel and opposite a/3 Burgers vectors. This latter fact is incompatible with any formation mechanisms so far proposed. Most partials lay parallel to directions 30deg apart from the Burgers vector due probably to the sessile dissociation mechansim proposed by Giamei et al. The formation mechanism of stacking faults is discussed.

Magnetic-Field-Free Objective Lens around a Specimen for Observing Fine Structure of Ferromagnetic Materials in a Transmission Electron Microscope

In order to observe the fine structures of ferromagnetic materials or to observe their magnetic domain structures, special magnetic-field-free lenses in which specimens are set in the upper pole-piece have been developed. In these objective lenses, the spherical aberration depends largely on the shape of the lens, especially, (1) the distance between the positions of the specimen and the peak of the lens field, (2) the diameters of the pole-piece bores and the top face of the upper pole-piece, and (3) the gap length. In a lens with a pole-piece of optimum dimensions, the resulting resolution reached 0.7 nm and the leakage field strength was 0.35 mT. Complicated domain structures of ferromagnetic materials could be clearly observed.