Kojiro F. Kobayashi

Formation and crystallization of Al-Fe-Si amorphous alloys

Rapid solidification experiments of Al-based Al-Fe-Si ternary alloys have been performed both by the gun and single roller methods. Glass forming tendency is found to be largest near theβ-phase (Al9Fe2Si2) composition, where the amorphous state is obtained both by the gun and single roller methods. Crystallization behaviour of amorphous AlFe13.0Si17.4 alloy, nearβ-phase composition, followed the course of (i) the appearance of fineα-Al particles and (ii) the transformation of the matrix intoβ-phase. The ratio of crystallization temperature and the melting point,Tx/Tm, for AlFe13.0Si217.4 amorphous alloy is smaller than 0.5 indicating the difficulty of glass formation of these alloys.

The solidification process of highly undercooled bulk Cu-O melts

The effect of undercooling on grain structure was investigated in pure copper and alloys up to Cu ∼ 0.39wt% 0 (eutectic composition), in which grain refinement does not occur at any degree of bath undercooled when the oxygen content is less than 300 p.p.m. Grain refinement occurs in these alloys when the oxygen content exceeds about 300 p.p.m. and the undercooling prior to nucleation exceeds 100 K without quenching. Fragmentation affects primary, secondary and tertiary dendrite arms during and after recalescence. Quenching after recalescence at various solidification times retains transient grain structures. When the sample, which should have achieved complete grain refinement by furnace cooling, is quenched immediately after nucleation, the structure shows a trace of radiating fan-shaped grains originating from a single point of nucleation.

Structural changes during mechanical alloying of elemental aluminium and molybdenum powders

Mechanical alloying of elemental aluminium and molybdenum powders was performed by conventional low-energy ball milling. Seven compositions (Al-3, 10, 17, 20, 27, 50 and 75 at% Mo), as well as elemental aluminium and molybdenum powders, were milled up to 1000 h. The structural changes during milling were followed by X-ray diffraction. In all cases milling produced refinement of the microstructure, and prepared powders were microcrystalline with grain sizes of the order of nanometres. A supersaturated solid solution of molybdenum in aluminium was formed. Only for the Al-75 at% Mo powders was a solution of aluminium in molybdenum observed.

Crystal growth of the primary silicon in an Al-16 wt % Si alloy

Studies on the crystallographic growth habit of primary silicon crystals in an Al-16 wt% Si alloy were carried out by X-ray micro focus Laue analysis and ECP (electron channelling pattern) analysis.The plate-like primary silicon crystals grow by the same mechanism as that for germanium dendrites, i.e. the TPRE (twin plane re-entrant edges) mechanism.The spherical primary silicon crystal in sodium treated melts is composed of several pyramidal grains with tops at the centre of the sphere. Many of these grains have a twin relation to each other. The sodium enriched regions are found at the boundaries of these pyramidal silicon grains.The external surfaces of the spherical primary crystals exhibit regular crystal facets. The surface facets are most frequently parallel to {111} plane but there are also some facets parallel to other less densely packed planes such as {100}, {211} and so on.

Continuous heating of rapidly solidified Al-Cr ribbons

Rapidly solidified Al-Cr alloys have been prepared by a single roller method. Microstructures and the subsequent structural changes were examined as a function of alloy content and heat treatment. The phase transformation process of the ribbons at the continuous heating stage (0.33 K sec−1 ) was determined by differential scanning calorimetry (DSC), X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The precipitation temperature ofθ in the supersaturated solid solution decreased with increasing degree of supersaturation, but that at the grain boundaries did not so greatly depend on the supersaturation.

Growth of quasi-crystals from the supersaturated solid solution

Abstract Al-6.0 at % Mn alloy was rapidly quenched from the melt and the supersaturated solid was obtained. Upon annealing, a precipitate with icosahedral symmetry was formed. The external morphology of the precipitate has 20 extruding arms, the directions of which match with the 20 three fold symmetry axes of a regular dodecahedron. All the 20 arms of a precipitate particle activate the same electron diffraction spot.

Growth rate of crystals of pure metals nucleated from the undercooled melt

Abstract Crystal growth rates ( R ) of pure metals have been measured as a function of the bath undercooling, ΔT . The obtained relations are R = 2.3 ( ΔT ) 1.65 cm / s for lead , R = 0.94( ΔT ) 1.43 cm / s for antimony , R = 0.22( ΔT ) 1.81 cm / s for tin , R = 0.0025( ΔT ) 2.87 cm / s for tellurium and R = 0.022( ΔT ) 1.67 cm / s for bismuth .

New metastable phases in the Al―Mg system

Nouvelles phases metastables detectees dans les alliages Al-∼34,3 at.% Mg par microscopie electronique et calorimetrie

Crystallographic orientation relationships between primary silicon and aluminium crystals

Crystallographic orientation relationships between the primary silicon crystal and the aluminium crystal heterogeneously nucleated on the silicon surface in hypereutectic Al-Si alloys, were studied by the micro-focus X-ray diffraction analysis. The apparently random orientation relationships obtained by X-ray analysis have been classified into simple relationships by taking the twinning in the primary silicon crystals into consideration. The epitaxial relationships between silicon and aluminium crystals in untreated alloys, and that in sodium-treated alloys, are found to be distinctly different.