Faculty Profiles - OKAYASU Mitsuhiro

Papers - OKAYASU Mitsuhiro

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Mechanical Properties of Epoxy- and Dicyclopentadiene-Based Carbon-Fiber-Reinforced Plastics at Low and Room Temperatures

Takashi Fujiwara, Masaki Takeuchi, Che Daoyuan, Yao Liang, Naoki Nishioka, Mitsuhiro Okayasu

Applied Composite Materials 2025

Publishing type：Research paper (scientific journal)

In this study, the material properties of carbon-fiber-reinforced polymers (CFRPs) were investigated at room temperature (20 °C) and cryogenic temperature (–196 °C). Two types of CFRPs incorporating thermoset resins, namely epoxy (E) and dicyclopentadiene (D), were employed to evaluate their mechanical properties and failure behaviors. For low-temperature testing, an originally designed apparatus was utilized, in which mechanical properties were evaluated using a polystyrene container filled with liquid nitrogen (LN). The LN level was maintained automatically via a capacitance sensor-based monitoring system that regulated its replenishment as necessary. The results revealed that the bending and fatigue strengths of both CFRP types increased by approximately 30% at –196 °C compared to those at 20 °C. This enhancement may be attributed to various reasons, including increased internal strain induced by the disparity in thermal expansion coefficients between the carbon fiber and the resin. CFRP exhibited relatively higher creep resistance at –196 °C, compared to that at 20 °C. Under static bending load, E-CFRP displayed a sudden stress drop at approximately 2% bending strain, likely due to delamination between the epoxy resin and carbon fibers. In contrast, D-CFRP demonstrated enhanced strain tolerance, sustaining strains exceeding 5% without severe delamination, which can be attributed to the superior wettability of dicyclopentadiene with carbon. Details of the wettability and the above material properties of both CFRPs were analyzed in this paper.

DOI： 10.1007/s10443-024-10302-4

Hydrogen Embrittlement Characteristics of a Die-Casting Die after a High-Pressure Die-Casting Operation

Mitsuhiro Okayasu, Katsunori Shigematsu

International Journal of Metalcasting 18 ( 1 ) 810 - 820 2024.1

Language：English Publishing type：Research paper (scientific journal) Publisher：SPRINGER INT PUBL AG

In this work, the failure characteristics of a die-casting die manufactured from conventional hot-worked steel (SKD61) were investigated experimentally. A die with heat-checking failure on its cavity due to high-pressure die casting was employed. Apparently, the microstructural characteristics of the die around the regions exhibiting heat checking were unaltered. However, the hardness values decreased slightly because of the die heating via the injection of molten aluminum alloy, which is the tempering-like effect. Hydrogen was detected in the die around the regions with heat checking, which can slightly reduce the tensile fracture strain, i.e., hydrogen embrittlement (HE). The extent of HE was sensitive to the loading speed, particularly to a low loading speed. When heated to 450 degrees C, hydrogen gas was detected in a commercial die lubricant, which may lead to hydrogen infiltration in the die. This paper discusses the details pertaining to hydrogen penetration and die failure mechanisms.

DOI： 10.1007/s40962-023-01081-y

Effects of cutting conditions on materials properties of austenitic stainless steels

Ryutaro Kondo, Mitsuhiro Okayasu, Hao Wen, Jin Kawasaki

Journal of Engineering Research 2024

Publishing type：Research paper (scientific journal)

Austenitic stainless steel SUS304 is widely used in various engineering applications, including semiconductor manufacturing equipment and food processing machinery, owing to its exceptional material characteristics such as non-magnetic properties, high corrosion resistance, strength, and ductility. However, concerns have arisen in recent years regarding potential alterations in the material properties of austenitic stainless steel after machining processes. As the cutting process was conducted widely to make various engineering applications with different machining conditions, an examination of the effect of machining conditions on the material properties is important. In the present study, the influence of cutting conditions on the microstructure and mechanical of SUS304 was experimentally and numerically investigated, where three types of ball-end mills with rake angles (- 20°, − 3°, and 5°) and different cutting speeds (50 m･min−1 and 100 m･min−1) were conducted with plane and side machining to investigate the cutting properties. Material properties of SUS304 can be changed by the cutting resistance of the above cutting process, in particular the low rake angles under plane machining, where the high cutting resistance makes high dislocation density and strain-induced martensite formation. This occurrence made the low corrosion resistance as well as low ductility. On the contrary, no significant effect of the cutting speeds on the cutting resistance was detected compared to the condition of the rake angle on the plane machining. The reasons behind this will be discussed in detail systematically in this paper. From this approach, it could be clarified to maintain the high material properties of SUS304 even after the cutting process, which could be useful to make design of the engineering applications.

DOI： 10.1016/j.jer.2024.04.014

Hydrogen Embrittlement of Austenitic Stainless Steels with Different Surface Treatments

Mitsuhiro Okayasu, Hao Wen, Ryutaro Kondo

International Journal of Steel Structures 24 ( 3 ) 477 - 487 2024

Publishing type：Research paper (scientific journal)

This study evaluated the effect of surface conditions on the hydrogen embrittlement (HE) characteristics of SUS304 austenitic stainless steel, as SUS304 is one of the candidate structural materials for hydrogen energy systems. Various machining processes, including milling (ML), shot peening (SP), and cold rolling (CR), were employed to modify the surface roughness, internal strain, and microstructural characteristics of the test sample. Namely, this approach was conducted as the surface-absorbed hydrogen is related to effective hydrogen: a high internal strain was obtained in the entire CR area along with a rough surface, while SP and ML samples displayed high strain levels near the surface. The strain value was reflected in the hardness level due to their work hardening and strain-induced martensite formation. Concerning this, the hardness values of CR and SP samples were higher than 6 and 2 times the as-received samples. The hydrogen content charged to the samples was contingent upon the strain level: higher strain corresponded to elevated hydrogen content, particularly in CR samples. Despite the notable high hydrogen content in CR samples, HE was not detected in the tensile test. Conversely, even with a low hydrogen content, severe HE was observed in all samples during the fatigue test. The susceptibility of stainless steel to HE proved sensitive to cyclic loading, wherein surface-absorbed hydrogen migrated to the crack tip during cyclic loading. Detailed discussions on the reasons for these observations are provided based on the experimental results.

DOI： 10.1007/s13296-024-00829-4

Hydrogen Embrittlement Characteristics of Low Carbon Steels After Heat and Surface Treatments

Mitsuhiro Okayasu, Takehiro Kokado

Transactions of the Indian Institute of Metals 77 ( 8 ) 2127 - 2137 2024

Publishing type：Research paper (scientific journal)

The hydrogen embrittlement (HE) characteristics of the low carbon steels: Fe-xC-1.5Mn-0.03Si (0.05C, 0.1C, and 0.15C) with ultimate tensile strength (UTS) of ~ 1200 MPa, were examined experimentally after heating and surface treatment processes. For this investigation, the low carbon steels with different microstructural characteristics, including phase structures, precipitates, and internal strain, were employed. The high internal strain was seen in the Fe-0.15C steels after cold rolling (CR), water quenching (WQ), and bake hardening (BH) process, while the low strain was obtained in the steel samples after the annealing process. The high strain for CR and WQ (and BH) is caused by severe plastic deformation and the phase transformation of the martensite structure, respectively. Carbide particles were precipitated in the WQ and BH samples, and the amount of carbide was correlated with their carbon content. HE occurred only in the CR-0.15C and WQ-0.15C samples although the low UTS of ~ 1200 MPa was used, where the fracture strain decreased significantly. HE was attributed to the high internal strain and high carbon content which led to the hydrogen trapping site. A multiple regression analysis was also carried out to estimate the extent of HE of the low carbon steels.

DOI： 10.1007/s12666-024-03288-x

Formability Improvement for Bending Magnesium Alloy AZ31 Sheet via Microstructural Control

Mitsuhiro Okayasu, Takumi Ogawara, Minoru Matsushima

Journal of Materials Engineering and Performance 2024

Publishing type：Research paper (scientific journal)

To investigate the formability of the AZ31 magnesium alloy, U-bent samples were fabricated through the cold- and hot-bending processes using the 1-mm thick AZ31 sheet. The hot-bending (HB) process was conducted at various temperatures (200-400 °C). On the other hand, the cold-bending (CB) process was carried out using the AZ31 sheet with and without tensile pre-strain (0, 5, and 10%) before heating to various temperatures: 200-450 °C. The AZ31 sheet made via the HB process at above 300 °C exhibited a higher formability with a lower springback of ϕ = 5° (0%-300 °C-HB) compared with that for the CB-AZ31 (0%-25 °C-CB), with ϕ = 36°. To improve the CB formability of the AZ31 alloy, the microstructure was controlled via the pre-strain and heating processes. In this instance, the grain growth occurred significantly in the AZ31 alloy with 5% pre-strain before heating to more than 250 °C (5%-T > 250 °C-CB). The large grains led to low yield stress, low hardness, and severe plastic deformation (twin deformation and grain rotation), resulting in high CB formability. The mechanical properties and deformation characteristics of the CB- and HB-AZ31 samples were investigated via several experimental approaches to interpret the change in the formability of the AZ31 alloy.

DOI： 10.1007/s11665-024-09822-8

Hydrogen embrittlement properties of several stainless steels

Mitsuhiro Okayasu, Hisanobu Matsuura

International Journal of Fracture 248 ( 1 ) 201 - 220 2024

Publishing type：Research paper (scientific journal)

The hydrogen embrittlement (HE) characteristics of various stainless steels were investigated. In this study, as-received, heated (1100 °C, 15 h), and cold-rolled (30% strain) γ-austenite (AS), α-ferrite (FS), α′-martensite (MS), and γ–α duplex (DS) stainless steels were employed. For as-received stainless steels, severe HE occurred for DS and MS with static tensile loading, while no clear and weak HE was observed for AS and FS, respectively. This could be attributed to the different extent of hydrogen diffusivity in the stainless steel. A large amount of hydrogen penetrated to (i) lattice vacancy with low atomic density for body-centered cubic FS, DS, and MS, compared to that for face-centered cubic (AS); (ii) the phase boundary between γ-austenite and α-ferrite for DS; and (iii) the boundary between the Cr base precipitate and the martensite matrix for MS. HE also occurred strongly for heated-DS owing to the grain growth, i.e., a high hydrogen concentration in grain and phase boundaries. Although no clear HE was detected in as-received AS with static loading, HE occurred in cold-rolled AS, where hydrogen penetrated lattice vacancies and α′-martensite formed through strain-induced martensite. Owing to strain-induced martensite created during cyclic loading, HE was detected even for as-received AS, which is dissimilar to the result of the tensile test. Details of HE characteristics of the strainless steels were examined using the four stainless steels with different microstructures, diferent strain level and oxide layer. Moreover, those were investigated under different loading conditions, such as constant, static, and cyclic loading.

DOI： 10.1007/s10704-024-00809-z

Microstructural Control and Alloy Design for Improving the Resistance to Delayed Fracture of Ultrahigh-Strength Automotive Steel Sheets

Takehide Senuma, Mitsuhiro Okayasu, Hardy Mohrbacher

Metals 13 ( 8 ) 2023.8

Publishing type：Research paper (scientific journal)

The demand for higher-strength automotive steel sheets has increased significantly for lightweight and safe body concepts. However, the increment of the steel strength is often limited by the potential occurrence of delayed fracture. This paper discusses proper microstructure control and alloy design to improve the resistance against the delayed fracture of ultrahigh-strength automotive steel sheets in order to increase the usable upper limit of their strength and provides basic data serving as a practical guide for solving the problem of delayed fracture in ultrahigh-strength automotive steel sheets. It is confirmed that grain refinement, the appropriate dual-phase structure of martensite with ferrite or retained austenite, and surface decarburization, increase the resistance to delayed fracture. In terms of alloy design, the effects of Nb, Mo, and B on the delayed fracture resistance of hot-stamped steels have been investigated. The results suggest that there are other reasons for Nb to improve delayed fracture resistance in addition to grain refinement and the ability to trap hydrogen by its precipitates, as has been conventionally believed. Regarding Mo, it was clearly demonstrated that the segregation of this element at the grain boundary plays a main role in improving the delayed fracture resistance.

DOI： 10.3390/met13081368

An Edge Device Framework in SEMAR IoT Application Server Platform

Yohanes Yohanie Fridelin Panduman, Nobuo Funabiki, Sho Ito, Radhiatul Husna, Minoru Kuribayashi, Mitsuhiro Okayasu, Junya Shimazu, Sritrusta Sukaridhoto

Information (Switzerland) 14 ( 6 ) 2023.6

Publishing type：Research paper (scientific journal)

Nowadays, the Internet of Things (IoT) has become widely used at various places and for various applications. To facilitate this trend, we have developed the IoT application server platform called SEMAR (Smart Environmental Monitoring and Analytical in Real-Time), which offers standard features for collecting, displaying, and analyzing sensor data. An edge device is usually installed to connect sensors with the server, where the interface configuration, the data processing, the communication protocol, and the transmission interval need to be defined by the user. In this paper, we proposed an edge device framework for SEMAR to remotely optimize the edge device utilization with three phases. In the initialization phase, it automatically downloads the configuration file to the device through HTTP communications. In the service phase, it converts data from various sensors into the standard data format and sends it to the server periodically. In the update phase, it remotely updates the configuration through MQTT communications. For evaluations, we applied the proposal to the fingerprint-based indoor localization system (FILS15.4) and the data logging system. The results confirm the effectiveness in utilizing SEMAR to develop IoT application systems.

DOI： 10.3390/info14060312

Material properties of Japanese and western wooden nails

Mitsuhiro Okayasu, Kohei Uotani, Toshiro Tanaka

MATERIALIA 28 2023.5

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCI LTD

The corrosion characteristics of Japanese (J) and Western (W) nails with and without coating the Fe3O4 layer on the nail surface were examined experimentally to understand their mechanical properties and corrodibility. In this study, (i) one of the historical J-nails made by the forging process and (ii) commonly employed commercial W-nails created by extruding were used. Although the grain size and strain value differed depending on the nail and its location(sharpened tip, body, and flattened head), the microstructure of both nails consisted of ferrite and cementite structures. A high internal strain was created in the W-nail due to the nail creation by the extruding process. In particular, a high strain was obtained in the sharpened tip and flattened head areas of the W-nail compared to its body area, caused by severe plastic deformation. A high strain was also obtained in the sharpened tip area of the J-nail owing to the severe forging process. Such a high strain was related to the high hardness. The high hardness of the sharpened tip area of the J-nail (220 HV) was slightly lower than that of the W-nail's tip area. The hardness of the J -nail decreased non-linearly to its head area, which was affected by a low strain and largely grown grains. The corrosion occurred on the J- and W-nail surfaces, where lepidocrocite (gamma-FeOOH) was detected due to the weak-acidity oxidation, i.e., acid rain. The corrosion extent was different depending on the nail. Although the corrosion was detected for the J-nail after exposure for 3 months, the corrosion occurred on W-nail after exposure to the atmosphere for a week. Different corrosion extents between J- and W-nails were affected by their microstructural characteristics and strain levels, i.e., different surface energy. The corrosion resistance of both nails was improved by coating the Fe3O4 layer on the nail surface, where the corrosion extent for both J- and W-nails was similar due to the corrosion protection by the Fe3O4 layer. The corrosion characteristics directly affected their mechanical properties: tensile strength and ductility. This work systematically analyzed details of the mechanical properties of the nails after corrosion.

DOI： 10.1016/j.mtla.2023.101717

Hydrogen embrittlement characteristics of the hot- and cold-stamped 22MnB5 steel

Mitsuhiro Okayasu, Yuki Tanaka

International Journal of Fracture 240 ( 2 ) 243 - 255 2023.4

Language：English Publishing type：Research paper (scientific journal) Publisher：SPRINGER

The hydrogen embrittlement (HE) characteristics of hot-stamped (HS) 22MnB5 steels were investigated experimentally with and without low-temperature heating. The HS process was carried out on a 22MnB5 plate that was heated to the temperature of its austenite region above the A(3) line before die quenching. Because of the varying degrees of work hardening and quenchability, the material properties of the HS samples differed depending on the area. Due to the variety of hydrogen-trapping sites, HE occurred significantly more with different systems. HE occurred with intergranular failure in the prior austenite grain and lath boundaries of the martensite phase in the HS samples without low-temperature heating. Moreover, the resistance of HE increased because of the creation of the epsilon-carbide trapping site in the HS samples after low-temperature heating. However, Ti-base precipitates were created during the heating process acceleration of HE. Specifically, the trapping sites obtained in this study play different HE roles: epsilon-carbide improves HE resistance, while the grain boundary of prior austenite and Ti-base precipitates led to HE. Furthermore, this study explored the HE mechanisms in detail.

DOI： 10.1007/s10704-022-00684-6

Effects of Artificial Agingon Material Properties of Die-Cast Al-Si-Cu-Mg-xNa Alloy

Naoki Sahara, Reiya Ikeda, Shaohua Wu, Mitsuhiro Okayasu

INTERNATIONAL JOURNAL OF METALCASTING 17 ( 1 ) 515 - 525 2023.1

Language：English Publishing type：Research paper (scientific journal) Publisher：SPRINGER INT PUBL AG

The influence of sodium on the microstructural characteristics and mechanical properties of die-cast Al-Si-Cu-Mg (ADC12) alloy has been examined experimentally. Mechanical properties of the die-cast Al-Si-Cu-Mg alloy have increased with the addition of 0.03% Na, because of modification of the shape of the Si eutectic particles from acicular to fine and fibrous appearing. However, the mechanical properties decreased when 0.04% Na or more was added, due to the formation of shrinkage porosity. The fine and fibrous appearing eutectic Si has been created by the interruption of the nucleation and growth of Si eutectic by Na. The mechanical properties of Na containing die-cast ADC12 alloy were further enhanced by a T5 treating which is a low-temperature aging, giving rise to significant precipitation of fine CuAl2 and Mg2Si. In contrast, the mechanical properties of die-cast ADC12 T6-treated samples were lower due to the formation of blister defects. Thus, the solid solution process in the T6 treatment may not make severe precipitation hardening of die-cast ADC12 due to overaging. Moreover, severe precipitation hardening occurred by the artificial aging after microstructural refinement by Na.

DOI： 10.1007/s40962-022-00793-x

Effect of Artificial Aging on the Mechanical and Fatigue Properties of the Hot-Stamped 6022 Aluminum Alloy

Mitsuhiro Okayasu, Masakazu Okawa

Journal of Materials Engineering and Performance 31 ( 8 ) 6386 - 6394 2022.8

Language：English Publishing type：Research paper (scientific journal) Publisher：SPRINGER

The mechanical and fatigue properties of the 6022 aluminum alloy were investigated experimentally using U-bend samples fabricated through the hot-stamping process. The hot-stamping process was carried out at 550 degrees C before conducting artificial aging under several conditions. The alloy obtained via the hot-stamping process exhibited a lower degree of internal strain and a higher formability with a lower spring back compared with that obtained via the cold-stamping process. As the hot-stamping process was conducted at a temperature close to the solution temperature of the 6022 alloy, precipitation hardening occurred after artificial aging. The hardness of the hot-stamped 6022 alloy increased with increasing the aging temperature, and the high hardness was detected at 170 degrees C for 4 and 12 h due to the different strengthening mechanisms, namely dislocations and Mg2Si base precipitates. Like the results of the hardness, a high ultimate tensile strength and a high fatigue strength were obtained for the hot-stamped 6022 alloy after artificial aging at 170 degrees C for 4 h. The tensile strength is more than 35% higher than that of the cold-stamped sample. Although a high tensile strength was obtained after the aging process, the resistance to crack growth was not very high. This was attributed to the high crack driving force caused by the weak crack closure and low strain energy of the aged 6022 alloy. The failure characteristics of the hot-stamped 6022 samples were investigated via several experimental approaches to understand in detail the material properties of the aged 6022 alloy.

DOI： 10.1007/s11665-022-06716-5

Effect of the microstructural characteristics of die-cast ADC12 alloy controlled by Na and Cu on the mechanical properties of the alloy

Mitsuhiro Okayasu, Naoki Sahara, Norihito Mayama

Materials Science and Engineering A 831 142120 2022.1

Publishing type：Research paper (scientific journal)

The mechanical properties of a die-cast Al–Si–Cu alloy (DC ADC12) were improved using new processing techniques that induce grain refinement and precipitation hardening. Several gravity-cast ADC12 (GC ADC12) samples with different microstructural sizes were also evaluated to better understand the material properties of DC ADC12. The material properties of DC ADC12 indicate that the alloy has both advantages (fine grains) and disadvantages (defects). The mechanical properties of ADC12 can be improved by adding Na before the T5 process. The grain refinement of ADC12 was achieved by introducing Na using benign and inexpensive NaHCO3 rather than metallic Na. The Na elements including sodium oxide retained in the melt cause fine spherical eutectic structures to form in the spherical α-Al matrix by interrupting the Si phase growth. The phenomenon can be detected via a three-dimensional atom probe tomography analysis. Precipitation hardening of the die-cast Al alloy was achieved by subjecting it to T5 treatment and a large amount of Cu, replacing the T6 treatment, for 3 h at 175 °C, because the die casting process can be replaced with the solid solution process in T6 owing to its high cooling rate. Precipitation hardening occurred in the grain-refined DC ADC12-Na sample during the T5 process, which was verified via in situ observation of its microstructure using laser microscopy during the heating and cooling processes. Al2MgCu, CuAl2, and fine Si particles, which were different from the precipitates obtained using the T6 process, were produced in the spherical α-Al phase. The demonstrated techniques resulted in remarkable improvements in the ultimate tensile strength and fracture strain of DC ADC12.

DOI： 10.1016/j.msea.2021.142120

Effect of carbon nanotubes on the mechanical properties of cast Al–Si–Cu alloys

Mitsuhiro Okayasu, Shauha Wu, Indah Uswatun Hasanah

Materials Science and Technology (United Kingdom) 38 ( 4 ) 237 - 245 2022

Publishing type：Research paper (scientific journal)

To enhance the mechanical properties of cast Al–Si–Cu alloys (ADC12), 0.3% of carbon nanotubes (CNTs) was added. The cast ADC12–0.3%CNT samples were synthesised via heated mould continuous casting for the creation of fine grains, and the artificial aging process was conducted at 173°C for 13 h to obtain precipitation hardening. The reinforcement and refinement of eutectic Si and precipitates (Al2Cu and Mg2Si) were achieved by adding CNTs, which leads to a high tensile strength of about 450 MPa and a relatively high ductility of about 10%. The obtained tensile strength and 0.2% proof stress are more than twice as high as those of the conventional cast ADC12 alloy. The refinement of the microstructures could be due to the interruption of their growth caused by the CNTs.

DOI： 10.1080/02670836.2022.2037058

Effect of Cooling Rate on the Precipitation Characteristics of Cast Al-Si-Cu Alloy

M. Okayasu, N. Sahara, M. Touda

Archives of Foundry Engineering 21 ( 4 ) 55 - 60 2021.12

Publishing type：Research paper (scientific journal)

The influence of the cooling rate on the extent of precipitation hardening of cast aluminum alloy (ADC12) was investigated experimentally. This study explored the cooling rate of the solidification of Cu in the α-Al phase to improve the mechanical properties of ADC12 after an aging process (Cu based precipitation hardening). The solid solution of Cu occurred in the α-Al phases during the casting process at cooling rates exceeding 0.03°C/s. This process was replaced with a solid solution process of T6 treatments. The extent of the solid solution varied depending on the cooling rate; with a higher cooling rate, a more extensive solid solution was formed. For the cast ADC12 alloy made at a high cooling rate, high precipitation hardening occurred after low-temperature heating (at 175°C for 20 h), which improved the mechanical properties of the cast Al alloys. However, the low-temperature heating at the higher temperature for a longer time decreased the hardness due to over aging.

DOI： 10.24425/afe.2021.138679

The effects of the microstructural characteristics of Fe–0.33C–1.2Mn–xNb–xMo steels on hydrogen embrittlement fracture

Mitsuhiro Okayasu, Ryo Arai, Takehide Senuma

International Journal of Fracture 231 ( 2 ) 257 - 274 2021.10

Publishing type：Research paper (scientific journal)

The hydrogen embrittlement (HE) characteristics of Fe–0.33C–1.2Mn–xNb–xMo steels were investigated experimentally using various samples with differing microstructural characteristics. HE in steels was affected by hydrogen trapping sites: ε-carbide-based, Nb-based, and Mo-based precipitates, which were effective at enhancing HE resistance. In contrast, the prior austenite (γ) grain boundary within steel could act as hydrogen trapping sites and accelerate HE. In addition, hydrogen trapping occurred around the crack, leading to an acceleration of crack growth rate. There are various trapping sites in the steels with negative and positive effects on HE. The extent of the HE was clarified via tensile strength and resistance of delayed failure. Furthermore, the HE characteristics were analyzed using the samples with different quantity of hydrogen charged with two different methods. Based upon the above work, high HE resistance of the steel was proposed as Fe–0.33C–1.2Mn–0.05Nb–0.5Mo steels after a bake-hardening process at 170 °C for 20 min.

DOI： 10.1007/s10704-021-00586-z

Hydrogen embrittlement characteristics of hot-stamped 22MnB5 steel

Mitsuhiro Okayasu, Takafumi Fujiwara

International Journal of Hydrogen Energy 46 ( 37 ) 19657 - 19669 2021.5

Publishing type：Research paper (scientific journal)

Hydrogen embrittlement (HE) characteristics of 22MnB5 steel (U-bent specimen) manufactured using hot-stamping process at various temperatures were experimentally and numerically investigated. Steel resistance to HE was examined through delayed failure tests under static and cyclic loading during hydrogen charging. First, the low cyclic loading caused severe HE, in which a clear difference in the extent of HE was obtained depending on the hot-stamped sample, which directly affected the microstructural characteristics and stress–strain distribution. The hot-stamped samples with large martensite phase showed low resistance to HE compared with those with small martensite phase because of the high concentration of hydrogen trapped in the phase boundaries. Moreover, the dual phase (ferrite and martensite) of the hot-stamped samples reduced their resistance to HE, which is caused by the hydrogen trapped in the laminar-shaped pearlite phase. The resistance to HE was improved by low-temperature heating at 200 °C for 1 h because of the generation of ε-carbides as trap sites as they render the hydrogen non-diffusible. Furthermore, the internal strain in the U-bent sample could accelerate HE because of the high concentration of hydrogen. These results were verified by experimental and numerical analyses. Thus, the hydrogen trapping mechanism was proposed as a valid mechanism for HE in 22MnB5.

DOI： 10.1016/j.ijhydene.2021.03.092

Effects of microstructural characteristics on the hydrogen embrittlement characteristics of austenitic, ferritic, and γ–α duplex stainless steels

Mitsuhiro Okayasu, Takafumi Fujiwara

Materials Science and Engineering A 807 2021.3

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE SA

Hydrogen embrittlement (HE) characteristics of γ (AS), α (FS), and γ–α duplex (DS) stainless steels were examined experimentally and numerically. Severe HE occurred in the DS sample, whereas weak HE was detected in the AS and FS samples. This was attributed to the high hydrogen concentrations at the DS-trapping sites. Hydrogen trapping occurred in the low atomic density zones in the boundaries between α and γ phases in DS sample. The chemical bonding between atomic-scale phase boundaries was weakened by hydrogen penetration. This resulted in a crack growth along the DS α/γ phase boundaries. The ductility of DS decreased as the hydrogen content increased, especially when it exceeded 15 ppm. In contrast, the weak HE observed among AS and FS samples was attributed to the small hydrogen levels that infiltrated both samples. Finally, HE mechanism was proposed on the basis of these experimental results.

DOI： 10.1016/j.msea.2021.140851

A study of the mechanical properties of a Mg-Al-Zn alloy (AZ91) produced via Thixomolding

Mitsuhiro Okayasu, Takahiro Fukui

International Journal of Material Forming 14 ( 2 ) 271 - 280 2021.3

Language：English Publishing type：Research paper (scientific journal) Publisher：SPRINGER FRANCE

In this work, the mechanical properties and failure characteristics of a Mg-9.0Al-0.8Zn alloy (AZ91) fabricated via the Thixomolding process (TC) were investigated and compared with those of alloys formed via other casting processes, including cold-chamber die-casting (CD) and hot-chamber die-casting (HD). The microstructure of the three cast samples mainly consisted of the α-Mg phase and eutectic Mg17Al12. Fine, uniformly organized spherical α-Mg grains formed in the TC sample, and this morphology can be related to the TC process. A relatively small microstructure, comparable to the TC sample, was also obtained with CD because of the high solidification rate. However, large grains and cast defects were observed in the HD sample. Shrinkage porosity cast defects were dominant with HD, and they were caused by the low casting pressure. The ultimate tensile strength and 0.2% proof stress of the TC sample were 279 MPa and 192 MPa, respectively, which was ~10% higher than for the CD sample. The cast defects and large grains for the HD sample caused a significant reduction in its tensile properties, although its 0.2% proof stress was relatively similar to the CD sample. High fatigue strength and high crack growth resistance were realized with TC. Failure analysis was also carried out to probe the excellent mechanical properties of the TC sample.

DOI： 10.1007/s12289-020-01589-2

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