Noriaki Nakatsuka

In situ observation of solidification phenomena in Al–Cu and Fe–Si–Al alloys

Abstract Synchrotron radiation enables the observation of solidification in metallic alloys. In situ observations of solidification for Al–Cu alloys (5, 10 and 15 wt-%Cu) are reported. Nucleation and fragmentation of dendrite arms were often observed in the 15 and 10%Cu alloys when unidirectional solidification was performed from the planar interface. In contrast, nucleation and fragmentation were rarely observed in the 5%Cu alloys. The nucleation ahead of the solidifying front and the fragmentation in the mushy region strongly depended on alloy composition. This paper also presents in situ observation of solidification of Fe–10Si–0·5Al (at-%) alloys. The dendritic growth of δ-Fe was clearly observed using this technique. The development of X-ray imaging techniques enables the solidification of various conventional cast alloys such as Al, Ni and Fe alloys to be observed and will be increasingly used to investigate solidification phenomena.

In situ observation of nucleation, fragmentation and microstructure evolution in Sn–Bi and Al–Cu alloys

Abstract This paper presents recent progress of in situ observation for the microstructure evolution during solidification. Nucleation and fragmentation of dendrite arms are important issues for controlling microstructure during solidification. However, there are few studies on in situ observation of nucleation and fragmentation in metallic alloys. Time resolved X-ray imaging technique has been developed to observe solidification of metallic alloy systems in situ. Fragmentation of dendrite arms often occurred at the root after growth velocity was reduced for the Sn–13 at.-%Bi alloys and the Al–15 mass%Cu alloys. In the Al–15 mass%Cu alloys, both of nucleation and fragmentation contribute to formation of grain structure. The result suggested that fragmentation should be considered for controlling grain structure.

In-situ observation of peritectic solidification in Sn-Cd and Fe-C alloys

Time-resolved absorption imaging using synchrotron radiation X-rays allows us to observe solidification of metallic alloys of interest. This paper presents peritectic solidification in Sn-Cd alloy and Fe-C alloys. In unidirectional solidification of Sn-Cd alloy, the formation of a banded structure, in which two phases were alternatively piled up in the growth direction, was clearly observed. Sequence of nucleation or fluctuation of triple junction (primary phase / secondary phase / liquid) resulted in the banded structure. Ease of nucleation for both phases contributed to the banded structure formation. In carbon steel (Fe-0.45mass%C), the transformation from δ phase to γ phase was observed. At lower cooling rates, γ phase was produced in semisolid state of δ phase and liquid, indicating the peritectic reaction occurred during solidification. In contrast, δ phase transformed into γ phase when solidification nearly completed at temperatures 100K below the liquidus temperature. Namely, the transformation seemed to be massive. The observation showed that two different transformation modes operated in Fe-C alloy.

Massive transformation from ? phase to ? phase in Fe?C alloys and strain induced in solidifying shell

Time-resolved in situ observations using synchrotron radiation X-rays showed that γ nucleation difficulties resulted in a massive transformation from the δ phase to the γ phase. The massive transformation occurred even in high-carbon steel (0.45 mass% C) at a cooling rate of 0.33 K/s or more. A crystallographic relationship in which the close-packed (111) plane of the γ phase tended to coincide with the close-packed (110) plane of the δ phase suggested that the γ phase nucleated in a favorable plane in the δ phase. Observations of the sequence in the initial stage of solidification showed that the induced strain and strain rate in the massive transformation were larger than those expected in peritectic solidification. These observations will help us to understand deformation of solidifying shells and to build a physical model.

Three-dimensional alignment of FeSi2 with orthorhombic symmetry by an anisotropic magnetic field

This paper presents the three-dimensional alignment of β-FeSi2 particles in a resin under an anisotropic magnetic field. The alignments obtained under a static magnetic field and a rotating magnetic field proved that the magnetic susceptibilities of β-FeSi2 are χc>χb>χa. The magnetic anisotropy was sufficiently large under a magnetic field of 10T for the crystallographic alignment. Under an anisotropic magnetic field (oscillating magnetic field), a-, b- and c-axes of β-FeSi2 particles suspended in a resin were aligned each other. A pseudo single crystal β-FeSi2 with the orthorhombic structure was fabricated by the oscillating magnetic field.

Synchrotron radiography of direct-shear in semi-solid alloys

Understanding phenomena occurring at the scale of the crystals during the deformation of semi-solid alloys is important for the development of physically-based rheological models. A range of deformation mechanisms have been proposed including agglomeration and disagglomeration, viscoplastic deformation of the solid skeleton, and granular phenomena such as jamming and dilatancy. This paper overviews in-situ experiments that directly image crystal-scale deformation mechanisms in equiaxed Al alloys at solid fractions shortly after the crystals have impinged to form a loose crystal network. Direct evidence is presented for granular deformation mechanisms including shear-induced dilation in both equiaxed-dendritic and globular microstructures. Modelling approaches suitable for capturing this behaviour are then discussed.

Time evolution of the high temperature region formed by laser induced breakdown and of the development of the flame kernel in the constant volume combustion vessel

The lean combustion is one of the key techniques for the advanced internal combustion systems due to the requirement of the higher thermal efficiency. Since the successful ignition must be guaranteed even in the lean combustion, advanced ignition systems have been developed in this decade. Laser ignition is one of the advanced ignition systems which have the profits of the flexibility in the position and the timing of ignition. To develop this ignition system for the actual combustion system, it is required to reveal the underlying physics of the laser ignition. Particularly, the time evolution of high temperature region formed by laser induced breakdown should be discussed. In this study, therefore, the time evolution of the high temperature region formed by the laser induced breakdown and the development of flame kernel were observed by using high-speed imaging. The ignition trials of methane/air lean premixed mixture were carried out in the constant volume combustion vessel to obtain minimum laser pulse energy for ignition (MPE). Results showed that the light emission from plasma formed by laser induced breakdown remained at least in several tens nano-seconds. In addition, there were large differences between the breakdown threshold and the MPE, which meant that the breakdown threshold did not determine the minimum pulse energy for ignition.

A Study of Fine Particle Spheroidization Process by Elevated-Pressure Pure Oxygen Flame

Particle spheroidization by using the flame method is an effective process to produce spheroidized particles, which have high fluidity and filling fraction, from mechanically-pulverized materials. Although it has been reported that ultra fine particles attached on the surface of spheroidized particles after the spheroidization process, the physics underlying the formation of the ultra fine particles in the flame have not been clarified. In consequence, the techniques to control the amount of the attached ultra fine particles on spheroidized particles have not been established yet. The purpose of the present research is, therefore, to control the amount of the ultra fine particles attached on spheroidized particles after the spheroidization process and to clarify the effect of pressure of the combustion field on the amount of the attached ultra fine particles. In order to clarify the effect of pressure of the combustion field, the SEM photographs, particle size distribution and the specific surface area with BET method are measured. These results clearly show that the ultra fine particles attached on the spheroidized particles were reduced with the increase in pressure of the combustion field. This can be derived from inhibited vaporization of the particles because of the shortened flame length and the subsequent reduced heat quantity added to particles whereas the flame temperature rises with the increase in pressure of the combustion field.Copyright

Decomposition of Toluene as a Biomass Tar Through Partial Combustion

For the electric power generation by the woody biomass gasification, tar is incidentally formed at the same time. Tar means a compound of many kinds of aromatic hydrocarbons and causes some troubles, for example, clogging pipes when it is cooled and condensed before being supplied to the gas engine for electric power generation. One way for reducing tar is oxidative and thermal cracking by partial combustion of the producer gas in the gas reformer that is a stage subsequent to the biomass gasifier. During the partial combustion process of the producer gas, inverse diffusion flame is formed when oxidizer is supplied to producer gas. Cracking and polymerization of tar occur simultaneously at the proximity of the inverse diffusion flame. This polymerization of tar into soot is, however, a significant problem in the gas reformer. Experimental study was performed to clarify the effect of hydrogen concentration in the combustion region on soot formation and the growth of polycyclic aromatic hydrocarbons (PAHs) that is precursor of soot. In the present study, hydrogen concentration at the proximity of the inverse diffusion flame was controlled by the small amount of hydrogen addition to the oxidizer. The main results were as follows. Soot formation was suppressed by the small amount of hydrogen addition (approximately 0.5% to the total enthalpy of the producer gas). The suppression of soot formation was caused by higher concentration of hydrogen at the proximity of the combustion region since the aromatic radicals were neutralized before they could combine together or with acetylene. Carbon yield was increased with the increase in the amount of hydrogen added to the oxidizer as carbon content in the undetectable components by the integrated gas chromatograph such as the soot was decreased. In addition, the increase of carbon yield resulted mainly from the increase in carbon monoxide stemmed from reforming of high-boiling components such as soot.Copyright