杉山 明

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.

© 2018 Trans Tech Publications, Switzerland. Time-resolved and in-situ observations using synchrotron radiation X-rays were performed to observe solidification of cast iron (CE=4.5, 0.02mass%Mg). Morphology of graphite particles was influenced by specimen holder material. In the Al2O3 holder, graphite particles were spheroidal at the beginning and then deviated from the spheroidal shape. In addition, the coupled eutectic solidification of austenite and graphite occurred at the final stage. In contrast, the divorced eutectic solidification, in which graphite particles and austenite dendrites independently grew, was selected until the end of solidification in MgO holder. Spheroidal graphite particles were engulfed by austenite. Consequently, typical microstructure observed in ductile cast iron was reproduced in the in-situ observation. The results suggested that oxygen potential, which was determined by Al2O3 or MgO (specimen holder) in the observations, could be an important factor for the selection of eutectic growth mode and graphite morphology.

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.

Synchrotron X-ray radiography at 125 frames per second was used to study deformation mechanisms in semi-solid Fe-C alloys at high solid fraction and shear strain rates of 10(-1)/s. An image correlation approach was also used to quantify the shear strain fields and study shear-induced dilation and the origin of shear cracking. It was shown that, at high solid fraction (90 to 93 pct solid), rearrangement including rotation and translation of solid particles became restricted and shear strain localized into narrow liquid-filled channels/fissures. Shear cracking was shown to originate from inadequate liquid flow into the expanding spaces between solid particles caused by shear-induced dilation. At lower solid fraction (similar to 85 pct solid), solid particles rearranged with a significantly higher component of rotation leading to more shear-induced dilation and a wider shear band.

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．

In-situ observation of deformation at the grain scale in semi-solid Al-Cu alloys has been carried out using synchrotron X-ray radiography. The deformed microstructure was characterized by the quantitative analysis of solid motion from the image sequences during shear in semi-solid Al-Cu alloys. The influence of solid particle shape on shear deformation was also examined by comparing with in-situ observation of deformation in water-polystyrene particle mixtures where the polystyrene particles are spherical with a diameter of 500 μm. In both semi-solid Al-Cu alloys and water-particle mixtures, solid particles located close to the shear plane developed an increasing component perpendicular to the shear plane, and a relatively high shear strain rate and decreased solid fraction was localized in a shear domain. The shear band width in semi-solid Al-Cu alloys (~10 mean particles diameter) was approximately two times wider than that formed in water-particle mixtures due to the difference in the solid particle shape. © 2013 ISIJ.

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 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 delta phase to gamma phase was observed. At lower cooling rates, gamma phase was produced in semisolid state of delta phase and liquid, indicating the peritectic reaction occurred during solidification. In contrast, delta phase transformed into gamma 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.

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.

金属合金の凝固過程の時間分解イメージング(凝固その場観察)が,SPring-8などの第3世代放射光施設で可能になりつつある.凝固その場観察は,凝固組織の形成過程や偏析などの欠陥の形成過程を直接知ることができる.さらに,デンドライト成長モデルとその場観察を組み合わせて,いくつかの熱物性値(拡散係数,界面エネルギー)を評価できる可能性がある.

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．

This paper presents the microsegregation between the secondary dendrite arms and the macrosegregation between the grains for the Fe-Si-Al alloys. The influence of the solute enrichment around the dendrite envelope on the columnar grain of the Sn-Bi alloys is also presented. The segregation was observed by the time-resolved X-ray. Although the segregation between the arms was observed, the segregation was eliminated by the diffusion in the solid phase after the local solidification finished. On the other hand, the segregation between the grains was approximately 4 times higher than that between the secondary arms. The observation suggested that the solute transport though the dendrite arms to the liquid region significantly enhanced the segregation at the grain boundary. The grain selection during the unidirectional solidification of the Sn-Bi alloys was also influenced by the solute enrichment. around the dendrite envelope. The solute transport from the mushy region has to be considered to improve the accuracy in the prediction of the segregation and the grain structure.

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。

ステンレス代替合金として注目されているFe-Crは、熱膨張率の小さなインバー合金としても知られている。素粉末からミリング合成したFe-Cr合金を固化成形し、ミリング条件、焼結条件や組成がインバー特性に及ぼす影響を調査した。

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.