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論文

Quantitative analysis of microstructure evolution, stress partitioning and thermodynamics in the dynamic transformation of Fe-14Ni alloy

Li, L.*; 宮本 吾郎*; Zhang, Y.*; Li, M.*; 諸岡 聡; 及川 勝成*; 友田 陽*; 古原 忠*

Journal of Materials Science & Technology, 184, p.221 - 234, 2024/06

Dynamic transformation (DT) of austenite ($$gamma$$) to ferrite ($$alpha$$) in the hot deformation of various carbon steels was widely investigated. However, the nature of DT remains unclear due to the lack of quantitative analysis of stress partitioning between two phases and the uncertainty of local distribution of substitutional elements at the interface in multi-component carbon steels used in the previous studies. Therefore, in the present study, a binary Fe-Ni alloy with $$alpha$$+$$gamma$$ duplex microstructure in equilibrium was prepared and isothermally compressed in $$alpha$$+$$gamma$$ two-phase region to achieve quantitative analysis of microstructure evolution, stress partitioning and thermodynamics during DT. $$gamma$$ to $$alpha$$ DT during isothermal compression and $$alpha$$ to $$gamma$$ reverse transformation on isothermal annealing under unloaded condition after deformation were accompanied by Ni partitioning. The lattice strains during thermomechanical processing were obtained via in-situ neutron diffraction measurement, based on which the stress partitioning behavior between $$gamma$$ and $$alpha$$ was discussed by using the generalized Hooke's law. A thermodynamic framework for the isothermal deformation in solids was established based on the basic laws of thermodynamics, and it was shown that the total Helmholtz free energy change in the deformable material during the isothermal process should be smaller than the work done to the deformable material. Under the present thermodynamic framework, the microstructure evolution in the isothermal compression of Fe-14Ni alloy was well explained by considering the changes in chemical free energy, plastic and elastic energies and the work done to the material. In addition, the stabilization of the soft $$alpha$$ phase in Fe-14Ni alloy by deformation was rationalized since the $$gamma$$ to $$alpha$$ transformation decreased the total Helmholtz free energy by decreasing the elastic and dislocation energies.

論文

Metalloid substitution elevates simultaneously the strength and ductility of face-centered-cubic high-entropy alloys

Wei, D.*; Wang, L.*; Zhang, Y.*; Gong, W.; 都留 智仁; Lobzenko, I.; Jiang, J.*; Harjo, S.; 川崎 卓郎; Bae, J. W.*; et al.

Acta Materialia, 225, p.117571_1 - 117571_16, 2022/02

 被引用回数:46 パーセンタイル:99.77(Materials Science, Multidisciplinary)

Recently-developed high-entropy alloys (HEAs) containing multiple principal metallic elements have ex-tended the compositional space of solid solutions and the range of their mechanical properties. Here we show that the realm of possibilities can be further expanded through substituting the constituent metals with metalloids, which are desirable for tailoring strength/ductility because they have chemical interactions and atomic sizes distinctly different from the host metallic elements. Specifically, the metalloid substitution increases local lattice distortion and short-range chemical inhomogeneities to elevate strength, and in the meantime reduces the stacking fault energy to discourage dynamic recovery and encourage defect accumulation via partial-dislocation-mediated activities. These impart potent dislocation storage to improve the strain hardening capability, which is essential for sustaining large tensile elongation. As such, metalloid substitution into HEAs evades the normally expected strength-ductility trade-off, enabling an unusual synergy of high tensile strength and extraordinary ductility for these single-phase solid solutions.

口頭

(ベイナイト+$$gamma$$)組織中の不均一な炭素分布と$$gamma$$の熱的安定性の関係

渡邊 未来*; 宮本 吾郎*; 諸岡 聡; 古原 忠*

no journal, , 

TRIP鋼では、オーステンパー処理中のベイナイト変態を活用して未変態オーステナイトに炭素を濃化させて安定化し、室温でオーステナイトを残留させている。残留オーステナイトの安定性はその粒径,形態および組成に依存するが、その定量的な理解は進んでいない。そこで、本研究では中性子回折を用いたベイナイト変態中の炭素濃化挙動のその場観察および、FE-EPMAとEBSDの同視野観察により、炭素濃度分布および粒径の不均一性がオーステナイトの熱的安定性に及ぼす関係を調査した。中性子回折の結果として、673K保持におけるオーステナイトの回折プロファイルの変化から、変態後期では、低炭素のオーステナイトが優先的にベイナイト変態することが分かる。一方で、微視組織観察の結果として、粒径の小さいオーステナイトほど炭素濃縮しやすいことがわかる。また、同じ炭素濃度でも粒径が小さいほどオーステナイトとして残留していることから、粒径微細化によるMs点の低下が現れたことが示唆される。

口頭

Fe-C-Mn合金のパーライト変態におけるエネルギー散逸

梅田 岳昌*; Zhang, Y.*; 宮本 吾郎*; 古原 忠*; 諸岡 聡

no journal, , 

パーライトは、鋼の共析変態によって得られ、冷却によって生じた駆動力は炭素の拡散や、フェライト/セメンタイトラメラの形成、パーライト/オーステナイト界面の移動に消費されると考えられてきた。実際には、パーライト変態時に生じるひずみや、成長先端における合金元素の分配や界面偏析によっても駆動力が消費され、より複雑なエネルギー散逸が起こっていると推測される。エネルギー散逸によって界面の移動に寄与できる駆動力が減少し、成長速度も遅くなるが、その定量的な理解がパーライト鋼の組織制御において重要である。しかし変態kineticsに影響を及ぼすエネルギー散逸因子の検討はパーライト変態の場合にはあまり行われていない。そこで本研究では、Fe-C-Mn合金をモデル系として選択し、パーライトの成長界面の移動時におけるエネルギー散逸について定量的評価することを目的とした。

口頭

Fe-C-Mn合金のパーライト変態におけるエネルギー散逸

梅田 岳昌*; Zhang, Y.*; 宮本 吾郎*; 古原 忠*; 諸岡 聡

no journal, , 

共析変態によって得られるパーライト鋼は、フェライトとセメンタイトが層状に積層した微細なラメラ組織を有し、鉄道用レールやピアノ線、橋梁用ワイヤなどに幅広く用いられる重要な鉄鋼材料である。強度特性と大きく関係するラメラ間隔は、パーライト変態の駆動力および成長速度に依存することから、その制御にはパーライト変態の熱力学的な理解が重要である。パーライト変態の駆動力は炭素拡散、界面移動に伴う摩擦、ラメラ界面形成の3つの素過程に消費されると考えられてきた。しかし近年、フェライト/セメンタイト間の格子ミスフィットによって変態時に弾性ひずみエネルギーが蓄積することで、駆動力が消費されることが報告されている。また合金元素を添加すると、界面近傍における拡散や界面偏析が起こることで実質的な変態の駆動力もさらに変化すると考えられる。変態挙動に影響を与えるこれらの現象には未だ不明点が多く、特に成長界面における合金元素の分布を直接観察した例が少なく、さらに上記因子の効果を定量評価した例もほとんどない。よって本研究では、Fe-C-Mn合金のパーライト成長時の駆動力消費を様々な組織解析技術を用いて解明することを目的とした。その結果、Mn添加材では、$$alpha$$/$$gamma$$界面でのMn偏析によるソリュートドラッグ効果が最も駆動力を消費する大きな要因であることが示唆され、界面移動に伴う摩擦によるエネルギー散逸や、その場中性子回折実験より評価した変態ひずみの効果など他因子も少なからず影響していることが判明した。

口頭

Pearlite growth kinetics in Fe-C-Mn eutectoid steels

Zhang, Y.*; 梅田 岳昌*; 諸岡 聡; 宮本 吾郎*; 古原 忠*

no journal, , 

Essential understanding of the pearlite growth kinetics is of great significance to predict the lamellar spacing and the resultant mechanical properties of pearlitic steels. In this study, a series of eutectoid steels with Mn addition up to 2mass% were isothermally transformed at a temperature range from 873K to 973K to investigate the growth kinetics and the underlying thermodynamics at the migrating interface during pearlite transformation. The microscopic observation revealed that the pearlite growth rate in each alloy becomes increased while the lamellar spacing becomes decreased by lowering the transformation temperature. Mn addition decelerates the growth rate, accompanied by a relatively wider lamellar spacing at each temperature. After analyzing the element distribution in the vicinity of migrating austenite/pearlite interface via three-dimensional atom probe, Mn was found to be enriched at the austenite/pearlitic ferrite interface, whereas the Mn partitioning among the three phases is negligibly small in the 2mass% Mn added alloy isothermally transformed at 873K. Based on the estimation of energy dissipated by various factors, the driving force for pearlite transformation in the Mn-free alloy was found to be consumed by interface friction, carbon partitioning and ferrite/cementite interfacial energy, whereas neutron diffraction analysis indicated that the influence of transformation strain is relatively small. The retardation effects of pearlite growth kinetics in the Mn-added alloy, which is partly due to the reduced driving force for pearlite transformation, can be well explained by further considering the energy dissipation caused by solute drag effects of Mn.

口頭

Pearlite growth kinetics in Fe-C-Mn eutectoid steels

Zhang, Y.*; 梅田 岳昌*; 宮本 吾郎*; 古原 忠*; 諸岡 聡

no journal, , 

Essential understanding of the pearlite growth kinetics is important to predict the lamellar spacing and the resultant mechanical properties of pearlitic steels. In this study, through quantitatively analyzing the microstructural features in the vicinity of pearlite growth interface, the influence of these factors and the underlying thermodynamics of pearlite growth kinetics were clarified. The pearlite growth rate and lamellar spacing were measured based on the microstructural observation via optical microscopy and scanning electron microscopy, respectively. Three-dimensional atom probe (3DAP) was used to analyze the elemental distribution in the vicinity of pearlite growth front, whereas in-situ neutron diffraction at elevated temperatures was performed at J-PARC, BL19 (TAKUMI) to quantify the elastic strain generated during pearlite transformation. Based on the proposed thermodynamic model, the influence of various factors on the pearlite growth kinetics is estimated using the experimental results obtained in this study. It was found that in most transformation conditions, solute drag effects caused by Mn interfacial segregation have the largest contribution in retarding the pearlite growth rate. In contrast, the magnitude of elastic strain in pearlite measured by neutron diffraction is quite small, which marginally affects the pearlite growth kinetics.

口頭

高炭素マルテンサイトの低温焼戻し挙動におよぼす合金元素添加の影響

丸澤 賢人*; Zhang, Y.*; 宮本 吾郎*; 古原 忠*; 諸岡 聡

no journal, , 

マルテンサイトの焼もどし過程は反応熱と熱膨張の物性測定結果よると、合金元素添加により反応が遅延される傾向があると報告されているが、ミクロ組織変化との対応には不明な点が多く残されている。本研究では種々の組織解析手法を組み合わせることで、低温焼もどし時の組織変化に及ぼす合金元素添加の影響を解明することを目的とした。測定試料に対して、室温から600$$^{circ}$$Cまで10$$^{circ}$$C/minの連続加熱中および90$$^{circ}$$Cでの等温保持中のその場中性子回折測定を行った。また種々の温度と時間で焼もどし後のX線回折測定およびミクロ組織観察を行った。中性子回折の結果から、焼入れままの状態では残留オーステナイトのピークが確認され、また強制固溶された炭素により正方晶性(軸比)を有するためマルテンサイトの002回折ピークには明瞭なピーク分離が生じている。一方で、昇温に伴って第2段階の終了温度である300$$^{circ}$$C付近で残留オーステナイトのピークは消失し、またピーク分離は徐々に解消され正方晶性が低下した。この傾向は他の合金材についても同様であった。次に、軸比の変化に着目すると、焼入れままではAl添加材はその他の合金材と比べ、最も高い軸比を有しており、その後遅延効果により第1段階後期まで正方晶性が維持され、200$$^{circ}$$CでFe-C2元合金と一致した。一方でFe-C2元合金と比べ、Mn, Cr添加材の軸比変化はほとんど同様で、Si添加材は第0段階で軸比低下がわずかに促進されるという結果が得られた。

口頭

Multi-aspect characterization of low-temperature tempering behaviors in high-carbon martensite

Zhang, Y.*; 丸澤 賢人*; 工藤 航平*; 諸岡 聡; 宮本 吾郎*; 古原 忠*

no journal, , 

As-quenched martensite in carbon steels needs to be tempered to restore its ductility and toughness for practical applications. During tempering, a series of reactions relevant to carbon diffusion are known to occur sequentially, causing changes in microstructure in tempered martensite. In this study, multi-aspect characterization using various advanced characterization techniques were performed, with an aim to clarify the low-temperature tempering behaviors of high-carbon martensitic steels. An Fe-0.8 mass% binary alloy was mainly used in this study, and 4 ternary alloys with further 2 at% addition of Mn, Si, Cr or Al, were also investigated for comparison. All the alloys were water quenched after austenitization to obtain the as-quenched martensite as the starting microstructure. Tempering processes were performed either by continuous heating or isothermal holding under various conditions. Afterwards, the changes in physical properties of tempered martensite were analyzed via calorimetry, dilatometry, and resistometry, whereas the microstructural evolutions were characterized via transmission electron microscopy, in-situ neutron diffraction, and three-dimensional atom probe tomography. The experimental results revealed the continuous occurrence of different tempering stages, including carbon clustering, precipitation of metastable iron carbide, decomposition of retained austenite, and precipitation of cementite. In addition, the tetragonality of martensite became continuously lowered due to the reduction in solute carbon content by tempering. Among all the investigated elements, the addition of Al was found to have the largest retardation effects on the tempering kinetics, which was caused by its suppression effect on carbon diffusivity.

口頭

高炭素マルテンサイトにおける低温焼戻し挙動の多面的解析

Zhang, Y.*; 宮本 吾郎*; 古原 忠*; 丸澤 賢人*; 工藤 航平*; 諸岡 聡

no journal, , 

In this study, following our previous work on low-temperature tempering kinetics, multi-aspect characterization (in-situ neutron diffraction, TEM and 3DAP) via combining various traditional and advanced experimental approaches was performed, with an aim to thoroughly elucidate the low-temperature tempering behaviors and microstructural evolutions in martensite. As the result, the lattice parameters of a- and c-axes of martensite became increased and decreased, respectively, during continuous heating, resulting in continuous weakening of its tetragonality especially in the temperature range of the 1st stage of $$varepsilon$$/$$eta$$-carbide precipitation (340 K $$sim$$ 500 K). On the other hand, the lattice parameter of austenite stopped increasing at $$sim$$ 450 K much earlier before the onset of the 2nd stage, indicated the occurrence of carbon depletion.

口頭

高炭素マルテンサイトの低温焼戻し挙動に及ぼす合金元素の影響

丸澤 賢人*; 工藤 航平*; Zhang, Y.*; 宮本 吾郎*; 古原 忠*; 諸岡 聡

no journal, , 

The objective of this study is to clarify microstructure evolution during low-temperature tempering of high carbon martensite with addition of various third alloying elements, by particularly focusing the early-stage reactions. Fe-0.8mass%C (Base alloy) and Fe-0.8mass%C-2at%M (M alloy: M = Al, Si, Mn, Cr) were used. The as-quenched specimens were either continuously heated to 873 K at various rates of 5-20 K/min, or isothermally held at various temperatures of 333-453 K, whose kinetics were evaluated by calorimetric, dilatometric and resistometric analyses, respectively. Furthermore, in-situ neutron diffraction experiment and microstructure observation using TEM and 3DAP were conducted. As the result, the tetragonality is found to be gradually decreased during heating, especially in the temperature range of the 1st stage of tempering. Its decreasing rate is strongly retarded by Al addition, whereas the effects of other alloying elements are relatively small. Such effects were also recognized by the calorimetric and dilatometric analyses. In these specimens, heterogeneity in carbon distribution is induced by precipitation of metastable iron carbide. When the $$mu$$ parameter (deviation from the binomial distribution) is used to evaluate the degree of heterogeneity, a much lower value in the Al alloy than the other alloys indicates retarded carbide precipitation by Al addition. This is also consistent with the smaller reduction in solute carbon content in the Al alloy. By assuming the linear relationship between tetragonality and solute carbon content, this observation corresponds to the retardation effect on decreasing rate of tetragonality.

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