Akira Sugiyama

The microstructure evolution in ductile cast iron with magnesium addition was observed in situ by using X-ray radiography (two-dimensional observation) and time-resolved tomography (three-dimensional observation) in the BL20XU of a synchrotron radiation facility, SPring-8 (Hyogo, Japan). In the two-dimensional observation, graphite nodules nucleated in the melt and floated up immediately after nucleation. The floating was terminated by engulfment of graphite nodules into austenite dendrites. The radiography indicated that the average floating distance was shorter than the dendrite arm spacings in the 100-μm-thick specimen. Because the short distance could be influenced by the sample confinement, time-resolved tomography was performed by using a pink X-ray beam in the BL28B2 of SPring-8. Graphite nodules that nucleated in the melt (probably on magnesium–oxygen–sulfur inclusions in the melt) floated and were engulfed by austenite dendrites within several seconds, even in the bulk specimen. Although the average distance in the bulk specimen was approximately twice as large as that in the 100-μm-thick specimen, floating after nucleation and engulfment into austenite dendrites within a short duration were observed commonly from both techniques. The sequence of nucleation and engulfment had a critical effect on the number and size of the graphite nodules.

Synchrotron X-ray radiography was used to study tensile and compressive deformations of semi solid Al-Cu and/or Fe-C alloys. In the case of tensile deformation of globular Al-Cu sample at similar to 60% solid, relatively high strain regions were formed even at mean strain of 0.005. The normal strain rate at the regions was 10 times as high as mean normal strain rate (3.45x10(-3) s(-1)). At mean strain of 0.04, tensile deformation was localized in the high strain region, resulting in the formation of internal cracking in the plane normal to the tensile axis. On the other hand, in the case of compressive deformations of globular Al-Cu sample at similar to 55% solid and polygonal Fe-C sample at similar to 73% solid, shear bands with decreased solid fraction were formed at the domains tilted by approximately 45 degrees with respect to compressive plane. Rearrangement of solid particles including translation and rotation caused the shear induced dilation at the shear domains. Shear strain was localized at the shear domain with decreased solid fraction. Deformation of the polygonal solid particle of Fe-C sample caused a force to transmit over a longer distance than for the globular Al-Cu sample. Shear fracture finally occurred due to inadequate liquid flow into the expanding spaces between solid particles caused by shear-induced dilation. The solid/solid interaction including impingement between solid particles and rearrangement has significant role in the compressive deformation. These observations demonstrated that the mechanism of cracking formations induced by compressive deformation was totally different from that in the tensile deformation.

Radiography using a synchrotron radiation X-ray source was performed to examine solidification and melting behaviors in hypereutectic cast iron specimens containing 0.002 and 0.05mass pctMg. The solidification sequence in the alloy containing 0.002mass pctMg was (1) nucleation and growth of graphite particles of which transformed to a flake-like shape, (2) growth of gamma-Fe dendrites, (3) nucleation of graphite particles ahead of the interface just prior to the eutectic solidification, and (4) the eutectic solidification. In contrast, (1) and (2) occurred nearly at the same time in the specimen containing 0.05 mass pct Mg. The addition of 0.05mass pctMg significantly reduced the temperature range in which the graphite particles grew as the primary phase. Image-based analysis of melting behavior showed that even 0.05 mass pct addition was sufficient to modify the phase equilibrium of the liquid, gamma-Fe, and graphite phases. Thus, the observed influence of Mg on the solidification sequence was attributed to the modification of the phase equilibrium. The influence was consistently explained by considering the shift of the eutectic composition to the carbon side in the pseudo-ternary system. It was also suggested that supersaturation of carbon in the melt increased as the temperature decreased even though the primary graphite particles existed. The supersaturation may cause the nucleation of the graphite particles just before the eutectic solidification. (C) The Minerals, Metals & Materials Society and ASM International 2015

In situ observation of deformation in globular Al-Cu samples at a solid fraction of similar to 50% and a global shear strain rate of 10(-1) s(-1) was performed using time-resolved X-ray imaging. The solid particle motion during shear was quantitatively analysed. The force was transmitted though the contacts between solid particles over a long distance parallel to the shear plane (18 mean grain size, d) after only a 1d increment of the Al2O3 push-plate motion. On the other hand, the distance of transmitted force in the perpendicular direction to the shear plane was restricted to approximately 11d even for a high displacement of the Al2O3 push-plate. A relatively high shear strain rate became localized at the shear domain after a small amount of deformation (a 1d increment). The solid fraction decreased in the region of localized shear strain rate. The shear band width, where the shear strain was localized and the solid fraction decreased, remained mostly unchanged over a 4 d increment of Al2O3 push-plate motion.

Al-Cu合金の固液共存状態における変形挙動を，放射光を用いて直接観察した．また水-粒子混合体の変形挙動と比較し，変形の組織に及ぼす影響を調べた．

透過X線の輝度と線吸収係数の関係から，凝固過程における溶質分布の時間分解計測を試みた．

Time-resolved in situ observations using synchrotron radiation X-rays showed that gamma nucleation difficulties resulted in a massive transformation from the delta phase to the gamma 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 delta phase tended to coincide with the close-packed (110) plane of the delta phase suggested that the gamma phase nucleated in a favorable plane in the delta 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.

半溶融状態の炭素鋼に変形を与えた場合の結晶粒の挙動について直接観察を行った．

鋳鉄の凝固過程の直接観察結果を報告した．亜共晶鋳鉄において，γデンドライトに続き，共晶黒鉛の成長などが観察された．

Time-resolved X-ray imaging of dendritic solidification for pure Fe and carbon steels with sufficient spatial and time resolutions has been developed for the first time by overcoming essential problems in low contrast between solid and liquid phases and in high melting temperatures. Static observation showed that the solid/liquid interface in pure Fe specimen was determined by the absorption contrast at photon energy ranging from 16 to 30 keV. In addition, the phase contrast was also observed in the vicinity of the interface. Dynamic observation showed that cellular growth in pure Fe specimen was observed at a growth velocity up to 400 mu m/s. Feasibility observation was also performed for two different carbon steels (0.0025 mass% C and 0.45 mass% C). Growing dendrites were observed in-situ at a growth velocity up to 500 mu m/s. This study proves that the developed imaging enabled to observe solidification phenomena in-situ for various kinds of steels.

X-ray imaging using synchrotron radiation has enabled to observe solidification of metallic alloys in-situ. The observation provides direct information on evolution of microstructure and some defects. In addition, the precise measurement of dendrite tip shape can be used to evaluate physical properties (diffusivity and interfacial energy) with the dendrite growth models.

Al-In過偏晶合金の凝固過程のその場観察を行った．ロッド状の規則構造の形成について冷却条件等の影響を明らかにした．

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. © 2009 W. S. Maney &amp Son Ltd.

凝固過程における組織形成をその場観察した．核生成やデンドライトの分断が組織形成に重要である．

This paper presents some results of direct observation of mold filling in a specially designed die-casting by X-ray diffraction, including comparison with numerical simulation. Based on such work the authors discuss how to prevent gas entrapment and propose new methods.

数値シミュレーションでは結果の妥当性を検証する必要がある。透過X線による溶湯の充てん過程を直接観察し、計算結果と比較することで、表面張力の重要性が明らかとなった。

Ti、C、Ni素粉末を用いてﾒｶﾆｶﾙｱﾛｲﾝｸﾞで合成することによって微細TiCが分散したサーメットの作製を目指す。MA時にTiとCの割合を化学量論組成から明示的にずらし（Ti75C25）、成形時に不足のCを添加することで成形中にTiCが合成され、ナノサイズの微細粒子とすることができた。

Al基アモルファス合金を結晶化温度以下で成形し、バルク状のアモルファス合金の作成を試みた。低温で形成するために超硬合金製ダイとパンチを用い、超高圧低温成形を行うことでアモルファス状態を維持したままバルク状の成形体が形成できることが明らかとなった。

Ti-Al合金は、高融点の金属間化合物であり。抗耐食性構造材料として期待されている。TiとAlは溶解法では作成困難であるがメカニカルアロイング法によって容易にTi-Al合金を作成できた。

FeとCrの素形材粉末からの合成の可能性について調査した。Crの配合割合を変化させて合成を行い、最適な組成と合成条件、成形条件について明らかにした。

ステンレス合金として知られているFe-18Cr-8Ni合金を、メカニカルアロイング法によって合成した。この粉末を固化した試料の組織は微細となり、強度が増加した。また、γ相安定組成であるにもかかわらずα相合金となった。

サーメットを焼結法でつくる場合の燃焼合成反応による合成熱を利用してZrO2とNi合金の接合を試みた。サーメットはNi系であるので、Ni合金との接合性はよかった。また、ZrO2も高い反応熱によって局部的に強固に接合しており、固化成形に成功した。

Fe52Cr48+5mass% X(X=Si, C) were synthesized by mechanical alloying (MA) of the elemental powders of Fe, Cr and Si/C using a planetary ball milling for 180ks under 1.33kPa argon gas atomosphere. These MA powders were consolidated by spark plasma sintering (SPS) process. The X-ray diffraction patterns of mechanical alloyed Fe52Cr48+5mass%Si and Fe52Cr48-+-5mass%C powders were almost similar with that of mechanical alloyed Fe52Cr48 powder. The σ-phase was obtained by only SPS of Fe52Cr48+5mass%Si. In this MA-SPS process, Fe52Cr48 alloy without σ-phase was synthesized and formed because of the few Si contamination in this process. The sintered Fe52Cr48 alloy without σ-phase had a coefficient of thermal expansion lower than that of SUS304 stainless steel.

In order to clarify the formation mechanism of V-type segregation, flow phenomena was examined by using two kinds of samples : solid particle-water mixture and aqueous solution of NH_4Cl. The former was used to simulate the slipping phenomena of equiaxed crystals in the mushy zone and the latter to simulate the remelting of mushy zone of ingot. These samples were sucked down through the outlet located at the center of the bottom of a cavity with constant velocities, which is called artificial suction. Most particles or equiaxed crystals and liquid in the upper part of the cavity moved down vertically. The V-type movement or slipping of particles were not observed. In the case of aqueous solution of NH_4Cl, equiaxed crystals moved down vertically during the artificial suction and the V-type movement or slipping were not observed. However, the V-type segregation were formed in the mushy zone. It is concluded that the V-type segregation were not formed by the slipping phenomena but by the remelting.

固相反応であるメカニカルアロイング法を用いて、FeAlの合成を試みた。Al-richな状態でミリング合成した粉末に不足分のFeを添加して成形することでFeAl金属化合物を成形できた。

  The flow behavior of solid-liquid two phase flow during mold filling effects greatly on the properties of castings such as semi-solid metal products and particle-reinforced metal matrix composites. In this study, the flow behavior of solid-liquid mixture flowing into a rectangular cavity with a core was investigated using a water-polystyrene particle mixture. The shape of the free surface of the mixture in the cavity with a core was more stable than without. Even a small core of 2mm×2mm was effective. The main reason for the phenomena is the change in the flow path of the mixture due to the core. Although gas was entrapped near the core, the amount of the gas was less for the core with the same width as the gate than the cavity without the core. The low-concentration region of solid particles appeared at the V-shaped flow boundary, which existed upward from the bottom of the core. The concentration of particles decreased at the center of the rotation of flow, which existed under, at the side of, and above the core.

The solid-liquid two phase flow influences on the properties of materials such as the semi-solid metals or the particulate-reinforced metal matrix composites. Therefore the flow phenomena of water-polystyrene mixture with high fraction solid in a suddenly enlarged cavity were examined. The mixture in an airtight vessel was ejected vertically into the cavity by increasing the internal pressure of the vessel. The shape of free surface of the mixture became more stable with increasing fraction solid. The main reason of suppressing the disturbance of the free surface is not the increase of the mean relative viscosity but the increase of fraction solid of the region near the free surface. The concentration of particles decreased at the center of rotary motion in the early stages of filling and at the boundary between the rising particles and the piled-up particles, which existed above filling ingate. It was the inertial force of particles that held the low-concentration region at the center of rotation. The low-concentration region above the ingate is caused by the collision of particles.