Nobuo Asahi

Microscopic Observation of Undercooled Silver-Gold Alloy Droplets

The solidification behaviour of undercooled Ag–Au alloy droplets with about 0.05 mm diameter has been observed using a high-temperature microscope. In this alloy system, the ratio of the maximum amount of undercooling to the liquidus temperature of each alloy composition ranged from 0.170 to 0.180 and a growth of a dendritic structure was almost always observed in the cross section of each droplet after solidification. It was found from the results that the solidification of undercooled Ag–Au alloy droplets behaved in the same way as that of undercooled pure silver or pure gold.

The Transformation Kinetics of Amorphous Fe80B20 and Fe78B20Mo2 Alloys Using Electrical Resistivity Measurement

The kinetics of the transformation of roller-quenched amorphous Fe80B20 and Fe78B20Mo2 alloys were investigated by electrical resistivity measurement, and the crystallization of these alloys was analyzed using the Jounson-Mehl-Avrami equation. From the isothermal annealing, the activation energies for crystallization were determined as 2.5 eV for Fe80B20 and 3.9 eV for Fe78B20Mo2.

Crystallization of amorphous Zr-20 at %Ni alloy

The crystallization process of an amorphous Zr80Ni20 alloy was investigated by X-ray diffractometry, electrical resistivity measurement and differential scanning calorimetry (DSC). The resistivity (at 300 K) of amorphous ribbon used in this experiment was 1.7 ×102 µΩcm. The temperature coefficient of resistivity was -1.04 ×10-4/K, in the region between 300 and 370 K. The crystallization temperature determined by resistivity measurement was 615 K. The activation energies for the crystallization were found to be 2.4 eV from the resistivity data and also 2.5 eV from DSC data. A time-temperature-transformation (T-T-T) diagram consisting of two straight lines was obtained.

Dendritic Structures Developed on Fe–FeSi Eutectic Thin Films below the Eutectic Point

When Fe-FeSi eutectic thin films are maintained at temperatures below the eutectic point for various periods of time, and then cooled at suitable rates, dendritic structures appear on the specimen surfaces. According to electron microscopic observation, the dendritic structures obtained in such a way are quite similar to those obtained at the eutectic point. Considering that dendritic growth usually takes place when a melt is supercooled, it is concluded from the above fact that the eutectic point of Fe-FeSi eutectic thin film is lower than that of the bulk eutectic alloy. The nature of this phenomenon was studied to see whether it could be explained by existing theory.

Lamella to Dendrite Transition in Solidifying Hypereutectic Fe–FeSi Alloy Films

The critical condition for the transition from lammellar to dendritic structures in hypereutictic Fe–FeSi alloy films, ranging in composition from 35 to 40 at%Si, has been determined by means of unidirectional solidification. The critical condition is expressed by the following relation; G/R=1.6×107C0+4.7×106 where G is temperature gradient in a melt at the liquid-solid interface, R is the solidification rate and C0 is the initial composition of Si in the melt. The liquid diffusion coefficient obtained using the above result was 4.1×10-5 cm2/s.

The Transition from Dendritic to Lamellar Structures in Hypoeutectic Fe-FeSi Alloy Films

Hypoeutectic Fe-FeSi alloy films, in the composition range from 29 to 33 at%Si, were unidirectionally solidified under temperature gradients in a melt at the liquid-solid interface from 20 to 90 K/cm, at solidification rates from 7.2×10-5 to 2.5×10-3 cm/s. The solidification structures are classfied into two types, lamellar and dendritic structures. The critical condition for the transition from dendritic to lamellar structures is expressed in the following relation; G/R=-2.7×105C0+9.0×106 where G is the temperature gradient in the melt at the liquid-solid interface, R is the solidification rate and C0 is the initial composition of Si of the melt.

On Dendritic Structures Caused by Contact between Iron and Silicon below Their Eutectic Temperatures

When iron and silicon were brought into contact by the method of vacuum evaporation and annealed at temperatures below their lowest eutectic points for various time periods, many indications of dendritic crystailization were observed on the specimen surfaces. The dendritic structures obtained in such a way were compared with those obtained at temperatures higher than the eutectic point. Both structures were so much alike that it was almost impossible to distinguish one from the other. Based on the well-known fact that dendritic growth takes place when the melt is supercooled, it follows that fusion or fusion-like phenomenon has occurred at a temperature below the lowest eutectic point. The cause of such anomaly was studied and explained by means of a metastable quasi-equilibrium state.

Behaviors of Eutectic Crystals below Their Eutectic Points (II)

It is found that the crystal growth of silicon in contact with iron is possible below the eutectic points of the iron-silicon binary system. It is also noteworthy that the behavior of crystal growth in the case above mentioned is quite similar to the crystallization of silicon in the gold-silicon system at a temperature much higher than the gold-silicon eutectic point. Such findings will be explained by considering the similarity between the gold-silicon equilibrium phase diagram and a meta-stable iron-silicon phase diagram proposed by the present authors.