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MATERIALS TRANSACTIONS Vol. 60 (2019), No. 2

ISIJ International
belloff
オンライン版ISSN: 1347-5320
冊子版ISSN: 1345-9678
発行機関: The Japan Institute of Metals and Materials

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キーワードランキング

21 Nov. (Last 30 Days)

MATERIALS TRANSACTIONS Vol. 60 (2019), No. 2

Multi-Phase-Field Modeling of Transformation Kinetics at Multiple Scales and Its Application to Welding of Steel

Munekazu Ohno, Yasushi Shibuta, Tomohiro Takaki

pp. 170-179

DOI:

10.2320/matertrans.ME201711

抄録

Production processes of structural materials generally involve a variety of microstructural evolutions, spatiotemporal scales of which are different by several orders of magnitude. In this study, a multi-phase-field model for simulating transformation phenomena at multiple scales is developed by considering mesoscopic kinetics of interest based on diffuse interface description without curvature effect. In particular, the present model is developed for simulations of microstructural evolutions during welding of carbon steels. In this model, the motion of dendrite envelope is described to simulate solidification not at a dendritic scale but at a scale of grain structure. Moreover, the pinning effect is described based on a mean-field approximation, which allows for simulations of grain growth with existence of very fine particles. The present model is applied to two-dimensional simulations for welding processes of carbon steel. The microstructural evolution involving the melting, solidification, austenite grain growth and pinning effect due to very fine particles is demonstrated.

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Estimation of Thermodynamic and Interfacial Parameters of Metallic Materials by Molecular Dynamics Simulations

Yasushi Shibuta

pp. 180-188

DOI:

10.2320/matertrans.ME201712

抄録

The range of application of molecular dynamics (MD) simulations is rapidly expanding owing to the recent advance in high-performance computing. Since only the coordinate and velocity of atoms in the system are directly obtained from MD simulations, it is important to correctly understand how the coordinate and velocity of atoms are converted into thermodynamic and interfacial properties. Here, MD-based techniques for estimating the thermodynamic and interfacial properties of metallic materials are assessed by considering practical examples of the melting point of a pure metal, the solidus and liquidus compositions of a binary alloy, the grain boundary energy, the solid-liquid energy, and the kinetic coefficient.

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Prediction of Fatigue Strength in Steels by Linear Regression and Neural Network

Takayuki Shiraiwa, Yuto Miyazawa, Manabu Enoki

pp. 189-198

DOI:

10.2320/matertrans.ME201714

抄録

This paper examines machine learning methods to predict fatigue strength with high accuracy using existing database. The fatigue database was automatically classified by hierarchical clustering method, and a group of carbon steels was selected as a target of machine learning. In linear regression analyses, a model selection was conducted from all possible combinations of explanatory variables based on cross validation technique. The derived linear regression model provided more accurate prediction than existing empirical rules. In neural network models, local and global sensitivity analyses were performed and the results of virtual experiments were consistent with existing knowledge in materials engineering. It demonstrated that the machine learning method provides prediction of fatigue performance with high accuracy and is one of promising method to accelerate material development.

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Modeling and Crystal Plasticity Simulations of Lath Martensitic Steel under Fatigue Loading

Fabien Briffod, Takayuki Shiraiwa, Manabu Enoki

pp. 199-206

DOI:

10.2320/matertrans.ME201713

抄録

A computational study for the modeling of lath martensitic steels, considering morphological and crystallographic features, is presented. A two-dimensional multi-scale tessellation is proposed to generate idealized microstructures with several scales of heterogeneities. The proposed approach is applied to lath martensite where prior austenite grain, packet and block boundaries are explicitly considered as well as their crystallographic relationships. The role of the different sources of heterogeneity on fatigue crack initiation is then investigated by finite element simulations including a single-crystal plasticity model. A fatigue criterion based on the Tanaka-Mura model is evaluated. The results indicate that block morphology and their orientation relationship significantly affects the strain distribution and the predicted location of crack initiation. In this regard, the effective critical size for crack initiation in low-carbon steels appears to be the block size as the Tanaka-Mura model mainly predicts cracks initiation along the block direction.

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Cyclic Yield Characterization for Low-Carbon Steel with HAZ Microstructures

Hide-aki Nishikawa, Yoshiyuki Furuya

pp. 207-212

DOI:

10.2320/matertrans.ME201717

抄録

To characterize the relationship between the cyclic yield and monotonic tensile properties of HAZ microstructures, incremental step tests were carried out on a total of eight microstructures of low-carbon steel that had been subjected to several simulated HAZ heat treatments. The results showed that most of the cyclic stress-strain relationships evaluated using incremental step tests roughly agreed with those from constant-stress amplitude tests. Furthermore, the cyclic yield stresses of simulated HAZ microstructures were proportional to these tensile strengths, similar to ordinary carbon and low-alloy steels. Although cyclic yield coefficients were also proportional to these tensile strengths, the slope of the proportional line is steeper than that of ordinary steels. Approximation formulas using monotonic tensile stress to calculate cyclic hardening coefficients for simulated HAZ were therefore determined as the least-squares line compiled from the experimental data.

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Prediction of Creep Rupture Time Using Constitutive Laws and Damage Rules in 9Cr–1Mo–V–Nb Steel Welds

Kozo Koiwa, Masaaki Tabuchi, Masahiko Demura, Masayoshi Yamazaki, Makoto Watanabe

pp. 213-221

DOI:

10.2320/matertrans.ME201703

抄録

Long-term creep tests for the parent metal, a simulated heat-affected zone (HAZ) and a weld joint were conducted based on 9Cr–1Mo–V–Nb steel. The creep rupture time, tr, for the weld joint was predicted by computationally simulating creep damage based on Norton’s law combined with the time exhaustion rule (TER), where the equivalent stress (σeq), maximum principal stress (σ1), or Huddleston stress (σhud) was used to evaluate the rupture time. Creep damage analysis was also conducted based on the Hayhurst-type damage mechanics rule (HDR), in which creep rupture time was evaluated in relation to the rupture stress, σr. The computed creep rupture times and damage distributions were compared with the experimentally obtained rupture time and void distribution of the actual weld joint, respectively. Furthermore, the effect of the bevel angle of the HAZ was examined. The key findings from this study were as follows: (1) cautious predictions could be obtained for a loading stress of 80 MPa and higher; (2) the computed fracture initiation positions were on the HAZ boundaries, consistent with the type IV fracture observed at the stress of 80 MPa; (3) the magnitudes of the predicted rupture times were tr (σ1) < tr (σhud) ≈ tr (σr) < tr (σeq); (4) the bevel angle dependence previously reported was reasonably reproduced with the TER models that used σeq and σhud and with the HDR model.

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Dominating Driven Factors of Hydrogen Diffusion and Concentration for the Weld Joint–Coupled Analysis of Heat Transfer Induced Thermal Stress Driven Hydrogen Diffusion–

A. Toshimitsu Yokobori, Jr., Go Ozeki, Toshihito Ohmi, Tadashi Kasuya, Nobuyuki Ishikawa, Satoshi Minamoto, Manabu Enoki

pp. 222-229

DOI:

10.2320/matertrans.ME201716

抄録

Hydrogen embrittlement cracking caused at a weld joint is considered to be dominated by hydrogen diffusion and concentration driven by thermal stress induced by heat transfer during cooling process. The gradient of hydrostatic stress component is considered to be a driven force of hydrogen transportation. However, this problem concerns the occurrence phenomenon during cooling process. Therefore, diffusion coefficient, yield stress and Young’s modulus are changed corresponding with temperature change. Especially, diffusion coefficient shows the space gradient corresponding with space gradient of temperature caused by heat transfer. This affects the diffusion equation of hydrogen as a driven force of hydrogen diffusion. Under these backgrounds, to clarify not only the effect of local thermal stress but also that of space gradient of diffusion coefficient on hydrogen release and trap, the hydrogen diffusion analysis based on our proposed α multiplication and FEM-FDM methods was conducted by introducing the terms of gradients of diffusion coefficient and temperature into the diffusion equation. The following results were obtained. The space gradient of diffusion coefficient was found to contribute the release of hydrogen from the site of stress concentration when the gradient of local hydrogen concentration takes the same sign as that of diffusion coefficient. Concerning the prevention of hydrogen embrittlement cracking at weld joint, these results show that not only Pre-Heat Treatment (PHT) which is a mechanical factor, but also the space gradient of diffusion coefficient which is a factor of material science was found to be one of effective factor of release of hydrogen from a site of stress concentration.

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Microstructure and Mechanical Properties of Laser Welded Al–Mg–Si Alloy Joints

Jiaxing Gu, Shanglei Yang, Chenfeng Duan, Qi Xiong, Yuan Wang

pp. 230-236

DOI:

10.2320/matertrans.M2018333

抄録

In this paper, Al–Mg–Si alloy with the thickness of 2.5 mm was overlap welded by fiber laser. Optical microscopy, scanning electron microscopy, energy dispersive spectroscopy, hardness tester, and electro-hydraulic servo testing machine were used for the microstructure observation and mechanical property examination. Specimens extracted from the welded plates were tested at room temperature for the determination of the tensile and fatigue properties of the welded joints. Results showed the columnar grains were formed in the fusion boundary, while equiaxed grains were formed in the fusion zone; the lowest hardness in the fusion zone is 72.0 HV, gradually increasing from the heat-affected zone to the BM. Tensile shear strength of the welded joint is 96.0 Mpa, which is about 25% of the tensile strength of base metal. The fatigue limit at 1 × 106 conditional cycles is 22.5 MPa. The fractography showed intergranular and quasi-cleavage fracture. Bending deformation is related to the magnitude of the force. As the stress lower, the deformation become more severe.

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Key Factor for the Transformation from hcp to 18R-Type Long-Period Stacking Ordered Structure in Mg Alloys

Masafumi Matsushita, Takafumi Nagata, Jozef Bednarcik, Norimasa Nishiyama, Shoya Kawano, Satoshi Iikubo, Yuji Kubota, Ryo Morishita, Tetsuo Irifune, Michiaki Yamasaki, Yoshihito Kawamura, Masanori Enoki, Hiroshi Ohtani

pp. 237-245

DOI:

10.2320/matertrans.M2018213

抄録

Cast Mg85Y9Zn6 has an 18R-type LPSO structure. However, Mg85Y9Zn6 recovered after being subjected to a loading pressure of 7 GPa at 973 K shows a fine dual-phase structure composed of a face-centered cubic (fcc) structure showing a superlattice (D03), as well as a hexagonal close-packed structure (hcp:2H). The D03/hcp structure transformed to 18R-type LPSO during heating at ambient pressure. In this research, the transformation process from the D03/hcp structure to 18R-type LPSO structure was discussed by means of in situ XRD and first-principles calculation. At first, lattice volume of 2H increased with an increase in the temperature, because of the Zn and Y emitted from the D03 phase into the 2H lattice. After the volume expansion of 2H lattice, the structure collapsed due to insert of random stacking faults (SFs). Then, a formation of 18R-type LPSO structure occurred. Based on a first-principles calculation for pure Mg, volume expansion of the 2H lattice causes the transformation to an 18R structure. Furthermore, the results of free energy calculations for the hcp and fcc structures in the Mg–Y–Zn ternary system show that the segregation of Y and Zn atoms on SFs occurs by the Suzuki effect. These segregated Y and Zn atoms in SF layers, which have a local fcc structure, create a synergy between the stacking and chemical modulations. Present result insists that the volume increase of 2H lattice takes place first, and then the transformation from the hcp structure to 18R stacking occurs.

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Microstructural Evolution and Mechanical Properties of a Three-Phase Alloy in the Cr–Mo–Nb System

Li Peng, Ken-ichi Ikeda, Toshiaki Horiuchi, Seiji Miura

pp. 246-253

DOI:

10.2320/matertrans.M2018269

抄録

A new three-phase alloy of 50Cr–30Mo–20Nb (at%) was studied based on the Cr–Mo–Nb ternary phase diagram, and was composed of a Cr-rich BCC1 phase, a Mo-rich BCC2 phase and a NbCr2 Laves phase after heat treatment at around 1523 K or lower. A supersaturated BCC single-phase solid solution alloy obtained by homogenization at 1973 K for 1 h underwent microstructural evolution during heat treatment at 1473 K. Intragranular precipitation of the Cr-rich BCC1 phase occurred, which led to the formation of an alternating BCC1/BCC2 two-phase microstructure through a discontinuous precipitation process, followed by precipitation of the Laves phase at the BCC1/BCC2 interphase boundaries. At a higher temperature of 1523 K, a similar microstructure was observed, with increased BCC decomposition and Laves precipitation rates, while the alloy consisted of BCC and Laves phases at 1773 K. The mechanical properties of alloys heat treated at 1473 K for various periods after the solid-solution treatment were also investigated. A maximum fracture strength of 1493 MPa and a minimum hardness of 773 ± 7 HV were obtained for an alloy aged for 24 h, where the BCC1/BCC2 two-phase microstructure dominated. The Vickers hardness of an alloy aged for 72 h, which had a fine-grained microstructure that included the Laves phase, was 839 ± 8 HV under a load of 0.5 kgf, and no obvious microcracks were observed.

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Mechanical Properties and Nanostructure of Multi-Layered Al–Zn–Mg Alloy with Compositional Gradient

Kazufumi Sato, Katsushi Matsumoto, Hiroshi Okuda

pp. 254-262

DOI:

10.2320/matertrans.L-M2018852

抄録

The mechanical properties and nanostructure of the multi-layered aluminum alloy sheet were investigated by tensile test, hardness test, electron probe microanalysis and micro-small-angle X-ray scattering in scanning mode, focusing on the distributions through the thickness. The multi-layered sheets consisting of highly concentrated Al–Mg and Al–Zn alloys show a remarkable increase in proof stress after interdiffusion and artificial aging. The predominant layers to contribute to the proof stress change from the layers with higher Zn/Mg ratio in T4 temper to the layers with lower Zn/Mg ratio after artificial aging. These age-hardening responses depend on the layers, which are large in the layers with higher Zn/Mg ratio, whereas small in the layer with lower Zn/Mg ratio. These noticeable bulk properties are ascribed to the local change in the types, volumes and morphologies of the G.P. zones and/or metastable phases depending on the concentration profiles through the thickness, which are produced from these unique multi-layered structures.

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Quantitative Analysis on Light Elements Solution Strengthening in Pure Titanium Sintered Materials by Labusch Model Using Experimental Data

Shota Kariya, Mizuki Fukuo, Junko Umeda, Katsuyoshi Kondoh

pp. 263-268

DOI:

10.2320/matertrans.Y-M2018849

抄録

Solid solution strengthening effect by oxygen (O) and nitrogen (N) atoms of α-titanium (Ti) materials was quantitatively evaluated using Labusch model by consideration of the experimental data. When using Labusch model to predict solid solution strengthening improvement, an application of the isotropic strains by solute elements is generally assumed to estimate Fm value. It is, however, difficult to exactly calculate Fm value for α-Ti materials with O and N solute atoms because the anisotropic strains are induced in α-Ti crystal with hcp structure by these elements. In this study, Fm value was experimentally derived from the relationship between 0.2% yield stress and solute elements (O and N atoms) content of powder metallurgy Ti materials. As a result, the strengthening improvement was proportional to c2/3/Sf (c: soluted atom content, Sf: Schmid factor), and its factor of proportionality of Ti–O and Ti–N materials was 4.17 × 103 and 3.29 × 103, respectively. According to this analysis, it was clarified that Fm value of Ti–O and Ti–N materials was 6.22 × 10−10 and 5.21 × 10−10, respectively, and then the estimated strengthening improvement by using these values was significantly agreed with the experimental results of PM Ti materials with O and N solution atoms. This Paper was Originally Published in Japanese in J. Jpn. Soc. Powder Powder Metallurgy 65 (2018) 407–413.

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Effect of Heat Treatment on the Microstructure and Mechanical Properties of High-Strength Ti–6Al–4V–5Fe Alloy

Zhenyu Wang, Libin Liu, Ligang Zhang, Jinwen Sheng, Di Wu, Miwen Yuan

pp. 269-276

DOI:

10.2320/matertrans.M2018267

抄録

The effect of heat treatment on the microstructure characteristics and mechanical properties of the high-strength and low-cost Ti–6Al–4V–5Fe alloy was investigated. Two-phase (α + β) and single-phase (β) solution treatments and aging were applied to determine the relationship between microstructures and properties. The size of the grain after treatment of the solution in the β single phase was only dozens of micron for the primary α(αp), which exhibited an obvious pinning effect on grain growth. The morphology and volume fraction of αp phase were highly sensitive to the heat treatment temperature and remarkably influenced the properties of the alloy. When the solution temperature was 780°C and the aging temperature was 550°C, the largest proportion (40%) of the globular αp phase and small secondary α(αs) phases resulted in the best performance of the alloy, with an ultimate strength of up to 1300 MPa and 9.57% elongation. The fracture surface of tensile specimens was systematically studied, showing a ductile mode of tensile failure after the sample was treated below 800°C and then aged. However, it exhibited a brittle mode when the alloy is treated above 800°C.

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Galvanic Microencapsulation (GME) Using Zero-Valent Aluminum and Zero-Valent Iron to Suppress Pyrite Oxidation

Sophea Seng, Carlito Baltazar Tabelin, Motoya Kojima, Naoki Hiroyoshi, Mayumi Ito

pp. 277-286

DOI:

10.2320/matertrans.M-M2018851

抄録

Pyrite is a common gangue mineral in mine wastes, and its oxidation is the primary cause of acid mine drainage (AMD) formation, which is a very serious environmental problem encountered worldwide. To address this problem, we developed a new technique to suppress pyrite oxidation called galvanic microencapsulation (GME). Galvanic interaction occurs when two conductive or semi-conductive materials with different rest potentials interact with one another. The material with a lower rest potential becomes the anode and is oxidized while the other one with the higher rest potential becomes the cathode and is galvanically protected. In this study, the effects on pyrite oxidation of zero-valent aluminum (ZVAl) or zero-valent iron (ZVI) dosages, leaching time, and pH were elucidated. In addition, the suppression mechanisms involved during GME were investigated by electrochemical measurements and surface-sensitive characterization techniques. The results showed that pyrite oxidation was suppressed in the presence of ZVAl or ZVI. With time, galvanic interaction between pyrite and ZVAl in the first 3 days was negligible, which could be attributed to the Al-oxyhydroxide coating on ZVAl. After 7 days, however, ZVAl exhibited substantial suppressive effects on pyrite oxidation. In comparison, the suppressive effects of ZVI on pyrite oxidation were observed after just 1 day. Cyclic voltammetry and chronoamperometry measurements showed that the suppressive effects of ZVAl and ZVI were predominantly due to galvanic interactions.Although ZVAl and ZVI could limit pyrite oxidation, their suppressive effects were only temporary because the surface of pyrite was not passivated by an unreactive coating. To induce coating formation and prolong the suppression of pyrite oxidation, phosphate was added together with ZVI. Only ZVI was selected for these experiments because of the potential formation of iron phosphate, a very stable material even under acidic conditions. In the presence of phosphate, suppression of pyrite oxidation by ZVI was dramatically improved because of the combined effects of galvanic interactions and coating formation.

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Gold Recovery from Waste Printed Circuit Boards by Advanced Hydrometallurgical Processing

Batnasan Altansukh, Kazutoshi Haga, Hsin-Hsiung Huang, Atsushi Shibayama

pp. 287-296

DOI:

10.2320/matertrans.M2018328

抄録

The scope of this study was to improve the hydrometallurgical processes involving iodine-iodide leaching and precipitation for recovery of gold from waste printed circuit boards. Firstly, the influence of different precipitating agents, namely ascorbic acid, trisodium citrate and sodium hydroxide on the recovery of gold from gold-iodide leach liquor were investigated in order to define the most effective precipitating agent. The leach liquor was prepared by dissolving pure gold chips in 1:6 molar ratio of iodine-iodide solution at 40°C, 550 rpm for 12 h. The variables, which affect the efficiency of gold precipitation from the leach liquor, were the molar ratio of precipitating agents to gold, pH and redox potential of the solutions. The attained high gold precipitation efficiency from the leach liquor was more than 99% under the highly acidic (pH < 1.6) and alkaline conditions (pH > 13) induced by 0.1 M ascorbic acid and 0.1 M sodium hydroxide respectively, but 64.5% of gold at a weak alkaline condition (pH 8) with 0.1 M trisodium citrate. Secondly, physico-chemical properties of resultant colloidal solutions and prepared gold particles were examined. Finally, recycling of waste printed circuit boards (WPCBs) via iodine-iodide leaching followed by the ascorbic acid reduction was discussed. Results indicate that over 95% of gold extracted from WPCBs by two-step iodine-iodide leaching under the defined conditions, while the dissolution efficiencies of other precious metals (Ag, Pd) and metal impurities (Cu, Al, Fe, Ni, Pb and Zn) were less than 1% and 3%, respectively. The vast majority of Au (99.8%), Cu (95.6%) and Ag (76.8%) were precipitated from the pregnant leach solution by ascorbic acid reduction at ambient conditions.

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The Effects of Additives on the Electrodeposition of a Zn–Zr Oxide Composite from Dispersed Particle-Free Solution

Yosuke Hara, Daiki Ueda, Satoshi Oue, Hiroaki Nakano

pp. 297-305

DOI:

10.2320/matertrans.M2018353

抄録

Electrodeposition of a Zn–Zr oxide composite was performed under galvanostatic conditions from an unagitated sulfate solution containing Zn2+ and Zr ions, as well as additives, such as NO3− ions and polyethylene glycol (PEG), at pH 2 and 313 K. The effect of these additives on the codeposition of Zr oxide and its polarization behavior, as well as the microstructure of the deposits, was investigated. The Zr content in the deposits obtained at varying current densities increased significantly with the addition of 2.0 g·dm−3 of NaNO3. Zn–Zr oxide films deposited from the NaNO3-containing solution showed a massive structure composed of fine crystals without crystalline Zn platelets, although large cracks were observed between the large crystals. EDX analysis revealed that Zr codeposited on the massive crystals as a fine concave-convex oxide. The corrosion current density of the Zn–Zr oxide films deposited from the NaNO3-containing solution was almost the same as that of pure Zn deposits, showing that there is no improvement in corrosion resistance when Zn is codeposited with Zr oxide. Moreover, Zr content in the deposits obtained from the PEG-containing solution increased significantly along with increasing current density above 1000 A·m−2. With the addition of 1000 mg·dm−3 of PEG, the crystalline Zn platelets disappeared, and the deposits were instead composed of fine mesh-like crystals with a preferred orientation of the Zn plane, resulting in a smooth surface. The cathodic current density for the reduction of dissolved oxygen on the Zn–Zr oxide films deposited from the PEG-containing solution was smaller than that of the pure Zn deposits, and as a result, the corrosion current density of the Zn–Zr oxide films was smaller than that of the pure Zn deposits. The increase in Zr content in the deposits with NO3− ions and PEG is attributed to the acceleration of the hydrolysis of Zr ions. This Paper was Originally Published in Japanese in J. Japan Inst. Metals 82 (2018) 366–374.

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Assessment of Hydrogen Absorption into Steel during Sacrificial Dissolution of Zinc and Zinc Coatings in Various pH Solutions

Gaku Kitahara, Aya Tsuji, Takashi Asada, Tomohiro Suzuki, Keitaro Horikawa, Hidetoshi Kobayashi

pp. 306-315

DOI:

10.2320/matertrans.M2018343

抄録

Hydrogen absorption into steel during the sacrificial dissolution of zinc and zinc coatings was investigated using the proposed hydrogen permeation method. The amount of absorbed hydrogen was small when the zinc coating was intact, but increased markedly at the onset of sacrificial dissolution of the zinc coating. The amount of absorbed hydrogen following zinc sacrificial dissolution was well-correlated to the zinc dissolution rate and was larger than the amount absorbed by iron corrosion in the entire pH range. Hydrogen evolution and absorption were inhibited by the deposition of zinc compounds such as hydrozincite and simonkolleite. The results herein suggest that zinc coating promotes hydrogen absorption in severe environments that involve frequently exposure to rainwater or immersion in seawater. The method proposed in this study can be used to quantitatively evaluate the change in hydrogen absorption caused by zinc coating.

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Theoretical Analysis of Maximum Pulling Rate in Capillary Shaping of Pure Aluminum

Jun Yaokawa, Yasushi Iwata, Yoshio Sugiyama, Mitsuhiro Kobayashi, Yuta Egawa

pp. 316-321

DOI:

10.2320/matertrans.F-M2018850

抄録

The capillary shaping is one of the most attractive options for the fabrication of aluminum frame components of a light weight car body structure. In this study, the maximum pulling rate of a pure aluminum fabricated by this technique was investigated using heat transfer analysis. Numerical and theoretical methods were applied to consider various cooling conditions. The maximum pulling rate depends on h/b, where b is the thickness of a product and h is the heat transfer coefficient between the coolant air and product. The maximum pulling rate increases with increasing h/b and decreasing melt temperature. Critical cooling length, which contributes to increased pulling rate, is smaller than 100 mm and decreases with increasing h/b. This Paper was Originally Published in Japanese in J. JFS 89 (2017) 631–637.

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Solidification Structure and Secondary Particles in Vertical-Type High-Speed Twin-Roll Cast 3003 Aluminum Alloy Strip

Ram Song, Shinji Kumai

pp. 322-329

DOI:

10.2320/matertrans.F-M2018853

抄録

The effect of cooling rate on microstructure of vertical-type high-speed twin-roll cast 3003 aluminum alloy strip was investigated. The solidification structure was characterized in terms of grain size and kinds, size, morphology and chemical composition of secondary particles. The 3003 aluminum alloy strip consisted of cell structure, dendritic structure, globular grains and eutectic structure along the strip thickness direction. From the relationship between the cooling rates and as-cast grain size, the cooling rate of high-speed twin-roll cast strip surface area was estimated as 3.1 × 103 K/s. Significant differences in the formation of secondary particles were found between direct chill (DC)-casting and the high-speed twin-roll casting as a result of the different cooling rates; Al6(Mn,Fe) and α-Al(Mn,Fe)Si phase were identified in the DC-cast sample, whereas only α-Al(Mn,Fe)Si was predominant in the high-speed twin-roll cast strip. The Al6(Mn,Fe) particles in DC-cast sample were script-like morphology with high aspect-ratio. In contrast, α-Al(Mn,Fe)Si particles in the high-speed twin-roll cast strip was spore-like morphology. Most α-Al(Mn,Fe)Si particles in the strip were considered to be formed as one of the eutectic components from the liquid droplets trapped in the inter-dendrite regions. In particular, Fe-rich α-Al(Mn,Fe)Si phase was formed at strip central area due to the increase in Fe segregation at the growth front.

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Critically Percolated States in High-Entropy Alloys with Exact Equi-Atomicity

Akira Takeuchi, Kunio Yubuta, Takeshi Wada

pp. 330-337

DOI:

10.2320/matertrans.M2018216

抄録

The formation of site-percolated states of exact equiatomic high-entropy alloys (HEAs) with body-centered-cubic (bcc) and face-centered-cubic (fcc) structures was investigated where their critical concentrations (pcsite) are given as 0.245 and 0.198, respectively, from conventional percolation theory. Molecular dynamics simulations were performed for WNbMoTa and WNbMoTaV HEAs with a bcc structure and AuCuNiPt and AuCuNiPdPt HEAs with an fcc structure. The simulation conditions included a generalized embedded atom method potential under NTp ensemble where the number of elements (N), absolute temperature (T), and pressure (p) were maintained constant. N-element alloys (N = 4 and 5) with a fraction of constituent elements (x = 1/N) were initially prepared in 10 × 10 × 10 supercells randomly in terms of chemical species and were simulated under atmospheric pressure at T = 1000 K. The total pair-distribution functions of the alloys revealed that the nearest neighbor distance (dn) for fcc ranged from 0.20 to 0.33 nm, whereas dn and the second neighbor distance (dnn) for bcc ranged from 0.235 to 0.305 nm and 0.305 to 0.370 nm, respectively. A 3-dimensional topological analysis for atomic correlations revealed that the alloys were in percolated and isolated states, respectively, when x ≥ pcsite and x < pcsite and that the values of 1/pcsite correspond to the ideal values of N for exact equi-atomic HEAs. Furthermore, it was observed that exact equi-atomic quaternary alloys (N = 4) with a bcc structure and quinary alloys (N = 5) with an fcc structure are in the critically percolated states.

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On the Solid Solubility Extension by Rapid Quenching and Spinodal Decomposition during Aging in Melt-Spun Cu–Ti Alloys

Shin-ichiro Kondo, Hiromichi Nakashima, Takao Morimura

pp. 338-345

DOI:

10.2320/matertrans.M2018255

抄録

We have examined solid solubility extension for melt-spun Cu–Ti alloys (4, 6, 7, and 8 mass% Ti) in comparison with that for conventional quenched alloys. X-ray diffraction (XRD) measurements revealed that the solid solubility extension for the melt-spun alloys was in the range of 7–8 mass% Ti, whereas that for quenched alloys was less than 6 mass% Ti. After annealing the melt-spun alloys at 673 K, the XRD measurements revealed sidebands with no intermetallic compound peaks, bright-field transmission electron microscopy images showed modulated structures, and selected-area diffraction patterns exhibited satellite structures; taken together, these experimental results confirm the occurrence of spinodal decomposition. In this study, however, distinct superlattice reflections were not observed.

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Effect of Chromium Content on Heat Treatment Behavior of Multi-Alloyed White Cast Iron for Abrasive Wear Resistance

Jatupon Opapaiboon, Mawin Supradist Na Ayudhaya, Prasonk Sricharoenchai, Sudsakorn Inthidech, Yasuhiro Matsubara

pp. 346-354

DOI:

10.2320/matertrans.M2018318

抄録

The effect of chromium (Cr) content on heat treatment behavior of multi-alloyed white cast iron with basic alloy composition of 5 mass% Mo, W and V each and 2 mass%C was investigated. Cast iron with varying Cr content from 3 to 9% was prepared. Specimens were annealed at 1223 K and then hardened using fan air cooling from 1323 and 1373 K austenitizing. Hardened specimens were tempered between 673 and 873 K with 50 K intervals. In the as-cast state, the microstructure of specimens with Cr content less than 5 mass% consisted of primary austenite and eutectic structure of (γ+MC) along with (γ+M2C). The (γ+M7C3) was observed in specimens with Cr content of more than 5 mass%. In as-hardened state, the hardness increased to the highest value at 5 mass%Cr and subsequently decreased with an increase in the Cr content. The volume fraction of retained austenite (Vγ) also behaved in the same way with reference to hardness. In the tempered state, evident secondary hardening was observed in all specimens. Maximum tempered hardness (HTmax) was obtained at 773–798 K tempering. The Vγ values decreased continuously as the tempering temperature increased and they were overall less than 5% at HTmax. The degree of secondary hardening (ΔHs) increased proportionally with a rise of Vγ in as-hardened state. The HTmax increased first and then decreased as the Cr content increased. The highest values of HTmax were obtained in 5 mass%Cr specimen regardless of austenitizing temperature.

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Development of α/γ Transformable FeCrAl-ODS Alloys by Nickel Addition

Tomonori Nishikawa, Shenghua Zhang, Shigeharu Ukai, Naoko Oono, Shigenari Hayashi

pp. 355-363

DOI:

10.2320/matertrans.M2018271

抄録

Utilizing γ/α transformation allows for a wider range of structure control for Al2O3-forming FeCrAl alloys considered as a candidate for the advanced fast reactor fuel cladding. In this research, nickel addition was explored as a method for creating γ-austenite at 1000°C and retaining α-ferrite at room temperature through computing phase diagram by FactSage and implementing HT-XRD & EPMA experiments. In addition, Al2O3 scale formation by oxidation test was implemented at 1000°C. Those results indicated that structure control by the γ/α transformation is realized with appropriate aluminum and nickel addition, keeping continuous Al2O3 scale formation.

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Microstructure and Property of Sn–37Pb Solder Bumps in Ø0.6 mm Ball during Thermal Shock

Guisheng Gan, Daquan Xia, Xin Liu, Cong Liu, Hanlin Cheng, Zhongzhen Ming, Haoyang Gao, Donghua Yang, Yi-ping Wu

pp. 364-368

DOI:

10.2320/matertrans.M2018289

抄録

Sn37Pb solder bumps with Ø0.6 mm ball were used to conduct thermal shock test, microstructure and property of solder bumps were investigated. The experimental results have shown that the shear strength of solder bumps was 62.89 MPa after reflow, then dropped to between 47.31 MPa and 50.86 MPa with increasing of thermal shock cycles, but finally reached to 61.96 MPa at 2000 cycles again. Interfacial IMCs were typical scallop-type and loose in the solder bumps, but were serrated and become smoother and compact after thermal shock with 1500 cycles. The IMCs thickness of solder bumps was about 1.80∼3.05 µm, but the composition of IMCs was Cu6Sn5 whether thermal shock or not. 1/25 of solder bumps at 200∼1000 cycles, 2/25 of solder bumps at 1500 cycles and 3/25 of solder bumps at 2000 cycles were failure respectively. Obvious tearing crack and fiber tissue was observed in the solder bumps of as-received, and then became large after thermal shock, but dimples of solder bumps deepened after thermal shock at 2000 cycles.

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Computational Modeling for Coarsening of (Fe,Cr)2B in Borated Stainless Steel

Chi-Hyoung Won, Jae Hoon Jang, Chang-Hoon Lee, Tae-Ho Lee, Namhyun Kang

pp. 369-372

DOI:

10.2320/matertrans.M2018336

抄録

The microstructure of borated stainless steels subjected to high temperature annealing process near solidus was confirmed using scanning electron microscopy. The spheroidization and coarsening of (Fe,Cr)2B occurred significantly during annealing at 1200°C. The coarsening rate was much faster than that predicted by existing models due to the anisotropy of the precipitate, i.e., its non-spherical shape. We simulated the coarsening behavior in a multi-component diffusional simulation including the anisotropic effect with respect to annealing time. The model illustrated the coarsening behavior of (Fe,Cr)2B well in borated stainless steels, and the interfacial energy between the precipitate and the austenite matrix was estimated to be 1.8 J·m−2.

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