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論文

${it In situ}$ neutron diffraction revealing the achievement of excellent combination of strength and ductility in metastable austenitic steel by grain refinement

Mao, W.; Gong, W.; Harjo, S.; 諸岡 聡; Gao, S.*; 川崎 卓郎; 辻 伸泰*

Journal of Materials Science & Technology, 176, p.69 - 82, 2024/03

 被引用回数:0 パーセンタイル:0(Materials Science, Multidisciplinary)

Fe-24Ni-0.3C(wt.%)準安定オーステナイト鋼の降伏応力は、平均結晶粒径が35$$mu$$m(粗粒[CG])から0.5$$mu$$m(超微細粒[UFG])に減少すると3.5倍(158$$rightarrow$$551MPa)に増加したが、引張伸びは大きく維持された(0.87$$rightarrow$$0.82)。結晶粒径が力学特性と変形機構に及ぼす影響を定量的に明らかにするため、室温での引張変形中にCGとUFG Fe-24Ni-0.3C鋼のその場中性子回折測定を行った。CGとUFG試料における塑性変形の初期段階は転位すべりによって支配され、変形後期には変形誘起マルテンサイト変態(DIMT)も生じた。結晶粒の微細化により、DIMTの開始応力が大きく増加し、ひずみに関するDIMTの速度が抑制されることがわかった。結果として、(i)結晶粒微細化によりオー ステナイトが安定化し、DIMTに対して最も安定な結晶粒である$$<$$111$$>$$//LD(LD:負荷方向)オーステナイト粒でのDIMTの開始が大幅に遅れた。その結果、UFG試験片の$$<$$111$$>$$//LDオーステナイト粒のほとんどはマルテンサイトに変態しなかった。(ii)結晶粒の微細化は、マルテンサイト変態の自己促進効果も抑制した。それにもかかわらず、UFG試験片の変態速度が低いDIMTは、CG試験片のより応力を増加させるのに効率がよく、変形中に均一な変形を維持するのに適していた。以上の現象は、UFG準安定オーステナイト鋼の優れた強度と延性の両立に相互に寄与している。

論文

Cryogenic deformation behavior of a dual-phase Mg-Li alloy investigated by in-situ neutron diffraction

Gong, W.; Gholizadeh, R.*; 川崎 卓郎; 相澤 一也; Harjo, S.

Magnesium Technology 2024, p.89 - 90, 2024/03

Mg and its alloys generally exist as a single hcp crystal structure at room temperature. It is widely recognized that the application of Mg alloys is hindered by their limited formability at room temperature, primarily attributable to the scarcity of readily activated deformation modes within the hcp phase. The addition of Li in Mg alloy can stable the bcc phase at room temperature, and these dual-phase (hcp+bcc) Mg-Li alloys exhibit excellent formability. The Li-enriched bcc phase has been frequently considered as the origin for improving formability. However, these Mg-Li alloys show poor work-hardening ability and the resultant low strength at room temperature. Considering that the dislocation recovery can be suppressed by decreasing the deformation temperature and the activity of deformation mode may be changed with temperature, we investigated the deformation behavior of a commercial LZ91 magnesium alloy at cryogenic temperatures using in-situ neutron diffraction.

論文

Fe-5Mn-0.1C中Mn鋼におけるリューダース帯伝播中の階層的不均一変形; その場走査型電子顕微鏡観察

小山 元道*; 山下 享介*; 諸岡 聡; Yang, Z.*; Varanasi, R. S.*; 北條 智彦*; 川崎 卓郎; Harjo, S.

鉄と鋼, 110(3), p.205 - 216, 2024/02

${it In situ}$ deformation experiments with cold-rolled and intercritically annealed Fe-5Mn-0.1C steel were carried out at ambient temperature to characterize the deformation heterogeneity during L$"u$ders band propagation. Deformation band formation, which is a precursor phenomenon of L$"u$ders band propagation, occurred even in the macroscopically elastic deformation stage. The deformation bands in the L$"u$ders front grew from both the side edges to the center of the specimen. After macroscopic yielding, the thin deformation bands grew via band branching, thickening, multiple band initiation, and their coalescence, the behavior of which was heterogeneous. Thick deformation bands formed irregularly in front of the region where the thin deformation bands were densified. The thin deformation bands were not further densified when the spacing of the bands was below $$sim$$ 10 $$mu$$m. Instead, the regions between the deformation bands showed a homogeneous plasticity evolution. The growth of the thin deformation bands was discontinuous, which may be due to the presence of ferrite groups in the propagation path of the deformation bands. Based on these observations, a model for discontinuous L$"u$ders band propagation has been proposed.

論文

Martensitic transformation behavior of Fe-Ni-C alloys monitored by ${it in-situ}$ neutron diffraction during cryogenic cooling

山下 享介*; Harjo, S.; 川崎 卓郎; 諸岡 聡; Gong, W.; 藤井 英俊*; 友田 陽*

ISIJ International, 64(2), p.192 - 201, 2024/01

${it In situ}$ neutron diffraction measurements were performed on Fe-33Ni-0.004C alloy (33Ni alloy) and Fe-27Ni-0.5C alloy (27Ni-0.5C alloy) during cooling from room temperature to the cryogenic temperature (4 K) to evaluate changes in the lattice constants of austenite and martensite, and changes in the tetragonality of martensite due to thermally induced martensitic transformation. As the martensitic transformation progressed, the lattice constants of austenite in both alloys deviated to smaller values than those predicted considering the thermal shrinkage, accompanied by an increase in the full width at half maximum of austenite. The fresh martensite formed in both alloys had a body-centered tetragonal (BCT) structure, regardless of the carbon content. The tetragonality of martensite decreased with progressive martensitic transformation during cooling in the 33Ni alloy, but was almost constant in the 27Ni-0.5C alloy. This suggests that carbon is necessary to maintain the tetragonality of martensite during cooling. The tetragonality of martensite in the 27Ni-0.5C alloy decreased during room temperature aging because of carbon mobility.

論文

Probing deformation behavior of a refractory high-entropy alloy using ${it in situ}$ neutron diffraction

Zhou, Y.*; Song, W.*; Zhang, F.*; Wu, Y.*; Lei, Z.*; Jiao, M.*; Zhang, X.*; Dong, J.*; Zhang, Y.*; Yang, M.*; et al.

Journal of Alloys and Compounds, 971, p.172635_1 - 172635_7, 2024/01

 被引用回数:0 パーセンタイル:0(Chemistry, Physical)

The grain orientation-dependent lattice strain evolution of a (TiZrHfNb)$$_{98}$$$$N_2$$ refractory high-entropy alloy (HEA) during tensile loading has been investigated using ${it in situ}$ neutron diffraction. The equivalent strain-hardening rate of each of the primary $$<hkl>$$-oriented grain families was found to be relatively low, manifesting the macroscopically weak work-hardening ability of such a body-centered cubic (BCC)-structured HEA. This finding is indicative of a dislocation planar slip mode that is confined in a few single-slip planes and leads to in-plane softening by high pile-up stresses.

論文

Principal preferred orientation evaluation of steel materials using time-of-flight neutron diffraction

徐 平光; Zhang, S.-Y.*; Harjo, S.; Vogel, S. C.*; 友田 陽*

Quantum Beam Science (Internet), 8(1), p.7_1 - 7_13, 2024/01

Comprehensive information about the ${it in situ}$ microstructure and crystal structure evolution during the preparation/production processes of various materials is in great demand in order to precisely control the microstructure morphology and the preferred orientation characteristics for the excellent strength-ductility-toughness balance of advanced engineering materials. ${it In situ}$ isothermal annealing experiments of cold rolled 17Ni-0.2C (mass %) martensitic steel sheets were carried out by using the TAKUMI and ENGIN-X time-of-flight neutron diffractometers, respectively. The inverse pole figures based on full-profile refinement were extracted to roughly evaluate the preferred orientation features along three principal sample directions of investigated steel sheets using the General Structure Analysis System (GSAS) software with built-in generalized spherical harmonic functions. The consistent rolling direction (RD) inverse pole figures from TAKUMI and ENGIN-X have confirmed that the time-of-flight neutron diffraction has high repeatability and statistical reliability, revealing that the principal preferred orientation evaluation of steel materials is available through 90$$^{circ}$$ TD $$rightarrow$$ ND (transverse direction $$rightarrow$$ normal direction) rotation of the investigated specimen on the sample stage during two neutron diffraction experiments. Moreover, these RD, TD and ND inverse pole figures before and after in situ experiments were compared with the corresponding inverse pole figures recalculated respectively from the MUSASI-L complete pole figure measurement and the HIPPO in situ microstructure evaluation. The similar orientation distribution characteristics suggested that the principal preferred orientation evaluation method can be applied to in situ microstructure evolution of bulk orthorhombic materials and spatially resolved principal preferred orientation mappings of large engineering structure parts.

論文

Recrystallization of bulk nanostructured magnesium alloy AZ31 after severe plastic deformation; An in situ diffraction study

Liss, K.-D.*; Han, J.-K.*; Blankenburg, M.*; Lienert, U.*; Harjo, S.; 川崎 卓郎; 徐 平光; 行武 栄太郎*; Kawasaki, M.*

Journal of Materials Science, 23 Pages, 2024/00

 被引用回数:0 パーセンタイル:0.01(Materials Science, Multidisciplinary)

The magnesium alloy AZ31, which has undergone high-pressure torsion processing, was subjected to in situ annealing micro-beam synchrotron high-energy X-ray diffraction and compared to the as-received rolled sheet material that was investigated through in situ neutron diffraction. While the latter only exhibits thermal expansion and minor recovery, the nanostructured specimen displays a complex evolution, including recovery, strong recrystallization, phase transformations, and various regimes of grain growth. Nanometer-scale grain sizes, determined using Williamson-Hall analysis, exhibit seamless growth, aligning with the transition to larger grains, as assessed through the occupancy of single grain reflections on the diffraction rings. The study uncovers strain anomalies resulting from thermal expansion, segregation of Al atoms, and the kinetics of vacancy creation and annihilation. Notably, a substantial number of excess vacancies were generated through high-pressure torsion, and maintained for driving the recrystallization and forming highly activated volumes for diffusion and phase precipitation during heating. The unsystematic scatter observed in the Williamson-Hall plot indicates high dislocation densities following severe plastic deformation, which significantly decreases during recrystallization. Subsequently, dislocations reappear during grain growth, likely in response to torque gradients in larger grains. It is worth noting that the characteristics of unsystematic scatter differ for dislocations created at high and low temperatures, underscoring the strong temperature dependence of slip system activation.

論文

Stress evaluation method by neutron diffraction for HCP-structured magnesium alloy

Harjo, S.; Gong, W.; 川崎 卓郎

Quantum Beam Science (Internet), 7(4), p.32_1 - 32_13, 2023/12

Tensile deformation in situ neutron diffraction of an extruded AZ31 alloy was performed to validate conventional procedures and to develop new procedures for stress evaluation from lattice strains by diffraction measurements of HCP-structured magnesium alloys. Increases in the lattice strains with respect to the applied true stress after yielding largely vary among [${it hk.l}$] grains. The newly proposed procedure of stress evaluation from the lattice strains shows very high accuracy and reliability by weighting the volume fraction of [${it hk.l}$] grains and evaluating them in many [${it hk.l}$] orientations in addition to multiplication by the diffraction elastic constant. When multiple ${it hk.l}$ peaks cannot be obtained simultaneously, we recommend to use the 12.1 peak for stress evaluation.

論文

Effects of dislocation arrangement and character on the work hardening of lath martensitic steels

段野下 宙志*; 長谷川 寛*; 樋口 翔*; 松田 広志*; Gong, W.; 川崎 卓郎; Harjo, S.; 梅澤 修*

Scripta Materialia, 236, p.115648_1 - 115648_5, 2023/11

 被引用回数:0 パーセンタイル:0(Nanoscience & Nanotechnology)

Work-hardening behavior of a lath martensitic Fe-18Ni alloy during tensile deformation is discussed based on the Taylor's equation. The dislocation characteristics are monitored using in situ neutron diffraction. In the specimens of as-quenched (AQ) and tempered at 573 K (T573), the dislocations are extremely dense and randomly arranged. The dislocations in AQ and T573 form dislocation cells as deformation progresses. Consequently, a composite condition comprising cell walls and cell interiors is formed, and the coefficient $$alpha$$ in the Taylor's equation increases. Cells are already present in the specimen tempered at 773 K (T773), which has a low dislocation density and a large fraction of edge-type dislocations. As deformation continues, the dislocation density of T773 increases, its cell size decreases, and its composite condition become stronger. Simultaneously, the edge-type dislocation fraction decreases, keeping $$alpha$$ unchanged. Thus, both the dislocation arrangement and character affected $$alpha$$, thereby affecting the work-hardening behavior.

論文

Measurement of mechanical behavior of $$^{11}$$B-enriched MgB$$_{2}$$ wire using a pulsed neutron source

町屋 修太郎*; 長村 光造*; 菱沼 良光*; 谷口 博康*; Harjo, S.; 川崎 卓郎

Quantum Beam Science (Internet), 7(4), p.34_1 - 34_17, 2023/10

Measuring the actual strain on the MgB$$_2$$ filaments are of paramount importance, since tensile stress and strain diminish the critical current. In this study, the strain measurement using neutron diffraction during tensile loading was established. We fabricated a MgB$$_2$$ wire enriched with boron-11, an isotope having a smaller neutron absorption cross- section than natural boron, and succeeded in obtaining changes in the lattice constant under tensile loading through Rietveld analysis.

論文

Quantitatively evaluating the huge L$"u$ders band deformation in an ultrafine grain stainless steel by combining ${it in situ}$ neutron diffraction and digital image correlation analysis

Mao, W.; Gao, S.*; Gong, W.; Harjo, S.; 川崎 卓郎; 辻 伸泰*

Scripta Materialia, 235, p.115642_1 - 115642_6, 2023/10

 被引用回数:1 パーセンタイル:0(Nanoscience & Nanotechnology)

本研究では、巨大なリューダースバンド変形を示す超微細粒(UFG)ステンレス鋼の引張その場中性子回折とデジタル画像相関測定を行い、バンド伝播中の歪硬化に対するオーステナイト母相と変形誘起マルテンサイトの個々の寄与を評価した。定量的解析の結果、UFG構造によって流動応力が大幅に増加した場合、オーステナイト母相のひずみ硬化は均一な変形を維持するには不十分であることがわかった。リューダースバンドの伝播に必要なひずみ硬化は、マルテンサイトの形成とその内部の高い内部応力によってもたらされた。

論文

Quantitatively evaluating respective contribution of austenite and deformation-induced martensite to flow stress, plastic strain, and strain hardening rate in tensile deformed TRIP steel

Mao, W.; Gao, S.*; Gong, W.; Bai, Y.*; Harjo, S.; Park, M.-H.*; 柴田 曉伸*; 辻 伸泰*

Acta Materialia, 256, p.119139_1 - 119139_16, 2023/09

 被引用回数:5 パーセンタイル:90.35(Materials Science, Multidisciplinary)

変形誘起塑性(TRIP)鋼は、変形誘起マルテンサイト変態(DIMT)に伴う加工硬化率の向上により、強度と延性の優れた組み合わせを示す。TRIP鋼や合金の加工硬化挙動におけるDIMTの役割を定量的に評価することは、強度と延性の両立を可能にする先進材料を設計するための指針を与えるが、変形中に相組成が変化し続け、応力と塑性ひずみの両方が構成相間で動的に分配されるため、その評価は困難である。本研究では、Fe-24Ni-0.3C(wt.%)TRIPオーステナイト鋼の引張変形とその場中性子回折測定を行った。中性子回折測定による応力分割と相分割に基づく解析手法を提案し、試験片の引張流動応力と加工硬化率を、オーステナイト母相,変形誘起マルテンサイト、DIMT変態速度に関連する因子に分解し、試料の加工硬化挙動における各因子の役割を考察した。さらに、回折プロファイル解析により測定した転位密度を用いてオーステナイトとマルテンサイト間の塑性ひずみ分配を間接的に推定し、材料中のオーステナイトとマルテンサイト間の応力・ひずみ分配の全体像を構築した。その結果、変形誘起マルテンサイト変態速度とマルテンサイトが負担する相応力の両方が、材料の全体的な引張特性に重要な役割を果たしていることが示唆された。提案した分解解析法は、TRIP現象を示す多相合金の機械的挙動を調べるために広く適用できる可能性がある。

論文

Strengthening of $$alpha$$Mg and long-period stacking ordered phases in a Mg-Zn-Y alloy by hot-extrusion with low extrusion ratio

Harjo, S.; Gong, W.; 相澤 一也; 川崎 卓郎; 山崎 倫昭*

Acta Materialia, 255, p.119029_1 - 119029_12, 2023/08

 被引用回数:6 パーセンタイル:95.52(Materials Science, Multidisciplinary)

An as-cast sample and two hot-extruded samples with different extrusion ratios (R) of Mg$$_{97}$$Zn$$_{1}$$Y$$_{2}$$ alloy containing the HCP $$alpha$$ matrix ($$alpha$$Mg) and the long-period stacking ordered phase (LPSO) of about 25-vol%, were used in tensile deformation in situ neutron diffraction experiments, to elucidate the effects of uniquely different microstructural evolutions in $$alpha$$Mg and LPSO with varying the R value to the mechanical properties. $$alpha$$Mg behaved as the soft phase and LPSO as the hard phase, and hot-extrusion improved the strength of both. At the R value of 5.0, a bimodal microstructure was created in $$alpha$$Mg, increasing largely the yield strength of $$alpha$$Mg. With increasing the R value to 12.5, the bimodal microstructure of $$alpha$$Mg collapsed and the yield strength of $$alpha$$Mg decreased. However, the strength of LPSO increased monotonously with increasing the R value due to the developments of kink bands and texture.

論文

中性子回折によるハイエントロピー合金の変形挙動のその場観察

Harjo, S.

日本結晶学会誌, 65(3), p.178 - 182, 2023/08

Observations of deformation behavior of high entropy alloys using ${it in situ}$ neutron diffraction measurements during deformation at various temperatures are reviewed. Neutrons are used to investigate stresses and crystallographic microstructures inside engineering materials, taking advantage of their large penetrating power and the ability to see the arrangement of atoms by diffraction methods. The important structural details of high entropy alloys such as internal stresses, phase conditions, dislocations, texture etc. are discussed in relation to the deformation conditions. Some highlights are introduced: (a) Cooperative deformation in CrMnFeCoNi alloy at ultralow temperatures, (b) Stacking fault energies in CrFeCoNi and CrCoNi alloys, and (c) Load redistribution in eutectic high entropy alloy AlCoCrFeNi$$_{2.1}$$ during high temperature deformation.

論文

${it In situ}$ neutron diffraction study on the deformation behavior of the plastic inorganic semiconductor Ag$$_{2}$$S

Wang, Y.*; Gong, W.; 川崎 卓郎; Harjo, S.; Zhang, K.*; Zhang, Z. D.*; Li, B.*

Applied Physics Letters, 123(1), p.011903_1 - 011903_6, 2023/07

 被引用回数:1 パーセンタイル:54.89(Physics, Applied)

Bulk Ag$$_{2}$$S is a plastic inorganic semiconductor at room temperature. It exhibits a compressive strain greater than 50%, which is highly different from brittle conventional counterparts, such as silicon. Here, we present the experimental investigation of the deformation behavior in a plastic inorganic semiconductor Ag$$_{2}$$S using ${it in situ}$ neutron diffraction during compressive deformation at room and elevated temperatures. At room temperature, the lattice strain partitioning among $$hkl$$-orientated grain families could be responsible for the significant work-hardening behavior in the bulk Ag$$_{2}$$S with a monoclinic structure. The rapid accumulation of lattice defects and remarkable development of the deformation texture suggest that dislocation slip promotes plasticity. At 453 K, a monoclinic phase transforms into a body-centered cubic phase. A stress plateau appears at $$sim$$-4.8 MPa, followed by a rehardening state. The deformation mode of bulk Ag$$_{2}$$S at the initial stage is likely attributable to the migration of silver ions, and as strain increases, it is closer to that of room temperature, leading to rehardening.

論文

Lattice parameters of austenite and martensite during transformation for Fe-18Ni alloy investigated through ${it in situ}$ neutron diffraction

Gong, W.; Harjo, S.; 友田 陽*; 諸岡 聡; 川崎 卓郎; 柴田 曉伸*; 辻 伸泰*

Acta Materialia, 250, p.118860_1 - 118860_16, 2023/05

 被引用回数:2 パーセンタイル:74.65(Materials Science, Multidisciplinary)

Martensitic transformation is accompanied by the generation of microscale and macroscale internal stresses during cooling below the martensitic transformation start temperature. These internal stresses have been determined through X-ray or neutron diffraction, but the reported results are not consistent, probably because the measured lattice parameter is influenced not only by the internal stress but also by several factors, including solute elements and crystal defects. Therefore, ${it in situ}$ neutron diffraction combined with dilatometry measurements during martensitic transformation and subsequent cyclic tempering were performed for an Fe-18Ni alloy. The phase strains calculated by lattice parameter variations show that a hydrostatic compressive strain in austenite and a tensile strain in martensite arose as the martensitic transformation progressed during continuous cooling or isothermal holding. However, the phase stresses of austenite and martensite estimated from these strains failed to hold stress balance law when dense crystal defects involved in the processes. After these crystal defects were removed by appropriate tempering, the stress balance law held well. Meanwhile, the phase stresses of austenite and martensite were changed to opposite, revealing their true identity. Various crystal defects in austenite and martensite, introduced by plastic accommodation, were suggested to affect their lattice parameters and then their phase stresses.

論文

High-density nanoprecipitates and phase reversion via maraging enable ultrastrong yet strain-hardenable medium-entropy alloy

Kwon, H.*; Sathiyamoorthi, P.*; Gangaraju, M. K.*; Zargaran, A.*; Wang, J.*; Heo, Y.-U.*; Harjo, S.; Gong, W.; Lee, B.-J.*; Kim, H. S.*

Acta Materialia, 248, p.118810_1 - 118810_12, 2023/04

 被引用回数:13 パーセンタイル:99.28(Materials Science, Multidisciplinary)

Maraging steels, known for ultrahigh strength and good fracture toughness, derive their superior properties from lath martensite structure with high-density nanoprecipitates. In this work, we designed a novel Fe-based medium-entropy alloy with a chemical composition of Fe$$_{60}$$Co$$_{25}$$Ni$$_{10}$$Mo$$_5$$ in atomic% by utilizing the characteristics of the maraging steels. By a single-step aging of only 10 min at 650 $$^{circ}$$C, the alloy showed microstructures consisting of a very high number density of (Fe, Co, Ni)$$_7$$Mo$$_6$$-type nanoprecipitates in lath martensite structure and reverted FCC phase, which led to ultrahigh yield strength higher than 2 GPa. This work demonstrates a novel direction to produce strong and ductile materials by expanding the horizons of material design with the aid of high-entropy concept and overcoming the limits of conventional materials.

論文

Competitive strengthening between dislocation slip and twinning in cast-wrought and additively manufactured CrCoNi medium entropy alloys

Woo, W.*; Kim, Y. S.*; Chae, H. B.*; Lee, S. Y.*; Jeong, J. S.*; Lee, C. M.*; Won, J. W.*; Na, Y. S.*; 川崎 卓郎; Harjo, S.; et al.

Acta Materialia, 246, p.118699_1 - 118699_13, 2023/03

 被引用回数:11 パーセンタイル:98.19(Materials Science, Multidisciplinary)

In situ neutron diffraction experiments have been performed under loading in cast-wrought (CW) and additively manufactured (AM) equiatomic CoCrNi medium-entropy alloys. The diffraction line profile analysis correlated the faulting-embedded crystal structure to the dislocation density, stacking/twin fault probability, and stacking fault energy as a function of strain. The results showed the initial dislocation density of 1.8$$times$$10$$^{13}$$ m$$^{-2}$$ in CW and 1.3$$times$$10$$^{14}$$ m$$^{-2}$$ in AM. It significantly increased up to 1.3$$times$$10$$^{15}$$ m-$$^{-2}$$ in CW and 1.7$$times$$10$$^{15}$$ m$$^{-2}$$ in AM near fracture. The dislocation density contributed to the flow stress of 470 MPa in CW and 600 MPa in AM, respectively. Meanwhile, the twin fault probability of CW (2.7%) was about two times higher than AM (1.3%) and the stacking fault probability showed the similar tendency. The twinning provided strengthening of 360 MPa in CW and 180 MPa in AM. Such a favorable strengthening via deformation twinning in CW and dislocation slip in AM was attributed to the stacking fault energy. It was estimated as 18.6 mJ/m$$^{2}$$ in CW and 37.5 mJ/m$$^{2}$$ in AM by the strain field of dislocations incorporated model. Dense dislocations, deformation twinning, and atomic-scale stacking structure were examined by using electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM).

論文

Evolution of austenite lattice parameter during isothermal transformation in a 0.4 C low alloyed steel

Wang, Y.*; 友田 陽*; 大村 孝仁*; Gong, W.; Harjo, S.

Materialia, 27, p.101685_1 - 101685_9, 2023/03

The evolution of internal stress during displacive transformation is a topic of continuous debate. Neutron diffraction was used to study the isothermal bainite transformation in a 0.4 C low alloyed steel from 773 to 623 K to provide a clearer basis for discussion regarding the change in the austenite lattice parameter. According to diffraction profile analysis, fresh bainite possesses a body-centered tetragonal structure, and its c/a ratio decreases rapidly over time. The austenite lattice parameter increases or decreases depending on whether the transformation temperature is above or below the nose of the Time-Temperature-Transformation (TTT) curve. This isothermal transformation behavior can be divided into two categories: above and below the nose of TTT curve, which correspond to the upper and lower bainites, respectively. The internal stresses caused by the transformation strains are relaxed by dislocation motion and vacancy formation. The yielded dislocations and vacancies not only affect the broadening of both austenite and bainitic ferrite diffraction peaks but also the lattice parameter.

論文

Neutron stress measurement of W/Ti composite in cryogenic temperatures using time-of-flight method

西田 真之*; Harjo, S.; 川崎 卓郎; 山下 享介*; Gong, W.

Quantum Beam Science (Internet), 7(1), p.8_1 - 8_15, 2023/03

In this study, the thermal stress alterations generated in a tungsten fiber reinforced titanium composite (W/Ti composite) were evaluated by the neutron stress measurement method at cryogenic temperatures. The W/Ti composite thermal loads were repeated from room temperature to the cryogenic temperature (10 K), and alterations in thermal residual stress were evaluated using the neutron in situ stress measurement method. In this measurement, the stress alterations in the titanium matrix and the tungsten fibers were measured. This measurement was carried out by TAKUMI (MLF-BL19) of J-PARC, a neutron research facility in the Japan Atomic Agency. The measurement method of TAKUMI is the time-of-flight (TOF) method. Owing to this measurement method, the measurement time was significantly shortened compared to the angle-dispersion type measurement by a diffractometer. As a result of the measurement, large compressive stresses of about 1 GPa were generated in the tungsten fibers, and tensile stresses of about 100 MPa existed in the titanium matrix. The thermal stresses due to the temperature change between room temperature and cryogenic temperature is caused by the difference of thermal expansions between the tungsten fibers and the titanium matrix, and these stress values can be approximated by a simple elastic theory equation.

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