Faculty Profiles - OKAYASU Mitsuhiro

Papers - OKAYASU Mitsuhiro

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Piezoelectric properties of lead zirconate titanate ceramics at low and high temperatures

Mitsuhiro Okayasu, Masakazu Okawa

Advances in Applied Ceramics 120 ( 3 ) 127 - 133 2021

Language：English Publishing type：Research paper (scientific journal) Publisher：TAYLOR & FRANCIS LTD

The material properties and damage characteristics of lead zirconate titanate(PZT) ceramics were investigated at various temperatures. A positive voltage was obtained when the sample was cooled from 20°C to −190°C, while a negative voltage was obtained when the sample was heated from −190°C to 180°C. The difference between the positive and negative values depended on the thermal stress. Compressive stress generated a more positive voltage in the cooling process, while tensile stress led to a more negative voltage in the heating process. The voltage values also depended on the cooling (or heating) rate of the sample, e.g. the greater the cooling (or heating) rate, the greater the voltage. When cyclic loading was conducted mechanically at −190°C, the voltage reduced, but it was recovered after heating to 20°C. Damage of the PZT ceramic (90° domain switching) was detected when the sample was cooled to −190°C due to the high thermal stress.

DOI： 10.1080/17436753.2021.1904765

Microstructural Characteristics of Unidirectionally Solidified Cast Al–Si–Cu Alloy

Shaohua Wu, Mitsuhiro Okayasu, Tatsuya Kuwada

International Journal of Metalcasting 15 ( 3 ) 1073 - 1083 2021

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

The properties of a conventional Al–Si–Cu alloy (ADC12) material, fabricated using a unidirectional casting process and slowly solidified in the range 0.02–0.14 °C s−1, were determined. The microstructural characteristics of the cast sample were dependent on sample area and resulted from changes in the amount of Si present during solidification. In the lower and middle regions, where the α-Al phase formed relatively organized crystal structures of different patterns, e.g., 〈101〉, columnar grain growth of α-Al dendrites with a low eutectic Si content was observed. Although columnar grain growth was also found in the upper region, it was randomly formed and the area was narrower. Random crystal orientation (i.e., weak control of unidirectional solidification) was created by interrupting columnar α-Al dendrite growth, which resulted from changes in the dynamics of the alloyed Si atoms. Eutectic Si is considered the only Si precipitate in ADC12; however, primary Si was also formed in the middle and upper regions, which was attributed to high Si concentrations resulting from Si migration to the upper region. Fine and coarse microstructures were observed in the lower and upper regions, respectively, with the middle region acting as a transition zone in which the amount of Si rapidly increased following transport between the lower and upper regions. A high amount of hard Si precipitate in the upper region of the sample resulted in high hardness values. In contrast, due to its fine microstructure with unidirectional crystal formation, the lower region exhibited high tensile strength and high ductility.

DOI： 10.1007/s40962-020-00542-y

Decarburizing behavior and its effect on mechanical properties of ultrahigh strength steel sheets

Kuwata Kentaro, Takemoto Yoshito, Okayasu Mitsuhiro, Bian Jian, Senuma Takehide

ISIJ International 61 ( 4 ) 1300 - 1308 2021

Publishing type：Research paper (scientific journal)

In this study, the decarburization behavior, and its effect on mechanical properties of 2 000 MPa class martensitic steel were investigated with the purpose of collecting the basic data to realize the further strengthening of structural members for automobiles. Concerning the decarburization behavior, the maximum of the decarburization rate of 2 000 MPa class martensitic steel with a basic chemical composition of 0.35%C-1.2%Mn was found around 750°C and the thickness of the decarburized zone grew proportional to the square root of time. The addition of Nb reduced the decarburization rate. Concerning the effect of decarburization on mechanical properties, the decarburization significantly improved bendability even though the thickness of the decarburized layer is relatively thin. An observation of the crack behavior revealed that the initiation and propagation of the crack were suppressed by the decarburization layer. The resistance to delayed fracture was improved by decarburization. This improvement is presumed to be based on a similar mechanism for the improvement of bendability.

DOI： 10.2355/isijinternational.ISIJINT-2020-416

Examination of Hydrogen Diffusivity in Carbon Steels Using a Newly Developed Hydrogen Permeation System

M. Okayasu, M. Sato

Experimental Mechanics 2021

Publishing type：Research paper (scientific journal)

Background: Hydrogen energy has received increased attention because of environmental needs and because it is an attractive replacement for fossil fuels. However, the presence of hydrogen in steels is known to be prejudicial to their global performance due to reduced ductility and unpredictable failures. Objective: To understand the diffusivity of hydrogen in carbon steels, a new hydrogen permeation system was developed to reveal hydrogen diffusion characteristics in carbon steels. Methods: Hydrogen gas was applied directly to one of the chambers to determine if hydrogen would permeate through the steel plate and into the other chamber. The hydrogen gas charged into the chamber at a pressure of 0.7 MPa after air removal using a vacuum pump, and fresh air was charged into the other chamber at atmospheric pressure before the measurement of the penetrated hydrogen. Results: Hydrogen did not diffuse substantially through a steel plate heated to less than 50 °C, but it did diffuse effectively through a carbon steel plate heated to more than 100 °C. The amount of hydrogen that diffused through the steel plate increased nonlinearly with increasing plate temperature and charging time. However, the diffusion amount saturated at 100 °C even after more than 2 h of charging, and hydrogen penetration through the steel plate stopped. In particular, the hydrogen atoms trapped in the steel plate interrupted the penetration of newly charged hydrogen atoms. Conclusion: By using newly proposed system, it is now possible to accurately quantify the diffusion amount of the hydrogen in the carbon steels, and hydrogen charging of the carbon steels was found to be irreversible. Based upon the results, hydrogen trapping system and hydrogen embrittlement characteristics were proposed.

DOI： 10.1007/s11340-021-00753-2

Failure Characteristics of PZT Ceramic During Cyclic Loading

Mitsuhiro Okayasu, Tsukasa Ogawa

Journal of Electronic Materials 49 ( 9 ) 5534 - 5541 2020.9

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

Failure characteristics of PbZrTiO3 (PZT) ceramic plates are investigated under cyclic loading with rods of different diameters, i.e., different contact areas (0-20 mm). The voltage generated under loading by the rod with the smallest diameter (contact area) is higher than those for the larger contact areas. This is due to the high strain induced in the PZT ceramic. However, the opposite trend is seen when the loading exceeds 60 N, i.e., the voltage obtained for the smallest contact area is lower. This is caused by failure of the PZT ceramic. The voltage generated under cyclic loading by the 5-mm, 10-mm, 15-mm, and 20-mm rods drops by about 10% in the early cyclic loading stage, but then remains constant until 10,000 cycles. The reduction in voltage is influenced mainly by 90 degrees domain switching. In this case, many grains (about 15% of the total) are switched: a random domain orientation is switched to the < 100 > direction perpendicular to the ceramic plate, i.e., a crystalline texture is formed. In contrast, there is significant reduction in voltage under loading by the 0-mm rod (point contact). As the extent of domain switching for the 0-mm rod is similar to that for the other rods, the reduction in electrical generation can be attributed to crack generation resulting from the high deformation.

DOI： 10.1007/s11664-020-08303-7

The effect of precipitations (NbC and carbide) in Fe-C-Mn-xNb steels on hydrogen embrittlement characteristics

Mitsuhiro Okayasu, Masaya Sato, Daiki Ishida, Takehide Senuma

Materials Science and Engineering A 791 2020.7

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

Hydrogen embrittlement (HE) characteristics in Fe-C-Mn-xNb steels were examined via various analyses, including electron backscatter diffraction analysis, scanning transmission electron microscopy and three-dimensional atom-probe tomography. For the investigation, the steel samples were prepared with varying Nb contents and heat treatment processes. The material properties of steel samples that were subjected to: (i) water quenching and (ii) quenching and tempering at 170 degrees C for 20 min, were determined to be nearly similar, although different degrees of HE were detected. After the tempering process, epsilon-carbide precipitated clearly in the matrix, which could act as a trapping site for hydrogen atoms and lead to improved HE resistance. Moreover, with addition of Nb, niobium base precipitates (e.g., NbC) with a diameter of a few nanometers were obtained in the martensite matrix, which could also function as hydrogen trapping sites. There was slight improvement in the HE resistance with NbC. Hydrogen-assisted failure mechanisms under both static and cyclic loading were observed with intergranular brittle cracking for the water quenched sample, even though the brittle and ductile mix failure mode was detected for the sample after the quenching and tempering process.

DOI： 10.1016/j.msea.2020.139598

Microstructure-dependent hydrogen diffusion and trapping in high-tensile steel

Mitsuhiro Okayasu, Jun Motojima

Materials Science and Engineering A 790 2020.7

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

In this work, the hydrogen embrittlement (HE) characteristics of high-tensile steel sheets with different microstructural characteristics were investigated. The sheets were fabricated via cold rolling (CR), water quenching (WQ), baking hardening (BH), and low-temperature annealing (LT), and their HE characteristics were clarified by examining the relationships between the microstructural characteristics and the severity of HE. Severe HE occurred in the WQ sample with hydrogen trapping at the boundaries of the retained austenite phases, resulting in intergranular and cleavage-like brittle failure. A reduction in HE was realized after the BH and LT processes. In these cases, hydrogen trapping was divided between the epsilon-carbide in the lattice spacings and at the boundaries of retained austenite, resulting in a mixed ductile/brittle failure mode. The extent of HE in the CR sample was similar to those in the BH and LT samples. However, the trapping sites were different; hydrogen trapping in the CR sample occurred in the slip band and around dislocations, resulting in delamination-like brittle failure on the slip planes. The extent of HE was also affected by the strain rate. More severe HE occurred in both the WQ and BH samples loaded slowly at 0.01 mm min(-1) compared to the samples loaded 1.0 mm min(-1) (i.e., intergranular failure). In this case, HE was affected by the large amount of hydrogen atoms trapped at the boundaries of the retained austenite phases. The hydrogen atoms in the lattice structure and epsilon-carbide migrated to the boundaries via dislocation movement. The extent of deterioration in tensile strength was two times higher in the samples loaded at the higher speed of 1.0 mm min(-1) compared to those loaded at 0.01 mm min(-1). Finally, the hydrogen trapping and failure mechanisms on the nano and atomic scales were discussed based on the results of the microstructural analyses.

DOI： 10.1016/j.msea.2020.139418

Hot-stamping technology for carbon fiber reinforced thermoplastic plates based on electrical resistance heating

Mitsuhiro Okayasu, Masaya Sato

Journal of Composite Materials 54 ( 10 ) 1353 - 1361 2020.5

Language：English Publishing type：Research paper (scientific journal) Publisher：SAGE PUBLICATIONS LTD

In the present work, a hot-stamping system for carbon fiber reinforced thermoplastic (CFRTP) plates based on electrical resistance heating was developed, where CFRTP consisted of polyphenylene and polyacrylonitrile. With the hot-stamping process, a simple hat-shaped sample was made. The heating rate and maximum sample temperature varied depending on the electrical resistance of the CFRTP plate. Moreover, the contact conditions between the electrodes and the CFRTP plate also affected the sample temperature owing to their influence on the electrical resistance, which was determined by the amount of exposed carbon fiber (CF) on the sample surface. Temperature measurements performed using samples with various amounts of exposed CF (20%-95% CF) revealed that approximately 65% CF afforded the highest sample temperature and fastest heating rate. The CFRTP plate underwent non-uniform heating, especially during the early stages, e.g. less than 10 s. Sample heating to 150celcius resulted in permanent deformation of the hat-shaped CFRTP samples with less springback, whereas heating to higher temperatures above the melting point led to meandering of the samples. In contrast, CFRTP samples subjected to hot-stamping at lower temperatures, such as 110celcius, exhibited rough surfaces. In addition to the sample temperature, the formability of CFRTP during hot-stamping was affected by the holding time. When hot-stamping was performed without a holding time, even at high temperatures of 150celcius and above, low-quality samples with dented surfaces and irregular sample thickness were obtained. The results of this study indicate that a temperature of 150celcius and a holding time of 10 s are optimal for fabricating high-quality hot-stamped CFRTP with smooth surfaces and uniform thickness.

DOI： 10.1177/0021998319877559

An Electrical Resistance Joining Technology for Carbon Fiber-Reinforced Polyphenylene Sulfide Composites

Mitsuhiro Okayasu, Tomohumi Kubota

Journal of Materials Engineering and Performance 29 ( 4 ) 2134 - 2143 2020.4

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

This paper proposes a fusion bonding technology with electrical resistance heating for carbon fiber-reinforced plastic using polyphenylene sulfide thermoplastic resin with 60% carbon fiber (CFRTP), and suitable bonding conditions are investigated experimentally. The concept of the fusion bonding system is that the thermoplastic resin in the CFRTP sheet is heated to near its melting point of 280 degrees C via electric charging after clamping two CFRTP sheets at high pressures, 310 kPa, using electrodes made from aluminum rods. The heating temperature was controlled using the electrical resistances (5-20 O) between the two CFRTP sheets which were controlled using the exposed carbon fiber (CF) on the CFRTP surface: a lower exposure rate gives a higher electrical resistance, e.g., 15 O for CF20% and 5 O for CF80%. In addition, the sample temperature was dependent on the electric current and the electric charging time. Two CFRTP sheets were successfully bonded using the fusion bonding system with the maximum strength of more than 30 MPa, where an electric current of 4.0 A was applied for 10 s. The bonding strength was attributed to changes in the material properties, especially the bonding resin. The hardened bonding resin was created from heat processing at 250 degrees C, which led to a high bonding strength.

DOI： 10.1007/s11665-020-04764-3

Effects of loading contact on electric-power generation of lead zirconate titanate piezoelectric ceramic plate

Mitsuhiro Okayasu, Tsukasa Ogawa

Journal of Advanced Ceramics 8 ( 4 ) 509 - 518 2019.12

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

To better understand the generation of electric power for piezoelectric PbZrTiO3 (PZT) ceramic plate (phi 25 mm), an attempt was made to investigate experimentally and numerically electric-power generation characteristics during cyclic bending under various loading fixtures (phi 0-phi 20 mm), i.e., different contact areas. Increasing the load-contact area on the PZT ceramic leads to a nonlinear decrease in the generated voltage. Decreasing contact area basically enhances the generated voltage, although the voltage saturates during loading when the contact area is less than phi 5 mm. A similar voltage is generated for phi 0 and phi 5 mm, which is attributed to strain status (ratio of compressive and tensile strain) and material failure due to different stress distribution in the PZT ceramic. On the basis of the obtained electric generation voltage, suitable loading conditions are clarified by loading with the phi 5 mm fixture, which generates a higher voltage and a longer lifetime of the PZT ceramic. From this approach, it is appeared that the area contact with the area ratio of 0.04 (phi 5 mm/phi 20 mm) is suitable to obtain the high efficiency of the electric voltage.

DOI： 10.1007/s40145-019-0331-7

Mechanical and fatigue properties of long carbon fiber reinforced plastics at low temperature

Mitsuhiro Okayasu, Yuki Tsuchiya

Journal of Science-Advanced Materials and Devices 4 ( 4 ) 577 - 583 2019.12

Language：English Publishing type：Research paper (scientific journal) Publisher：VIETNAM NATL UNIV

The mechanical properties of long unidirectional (UD) and crossply (CR) carbon fiber reinforced plastics (CFRPs) were investigated at a low temperature (-196 degrees C). The CFRPs were fabricated from 60 vol.% carbon fiber and epoxy resin. The bending strength of the UD-CFRP was approximately twice that of the CR-CFRP. The high strength of the UD-CFRP was directly attributed to the amount of carbon fiber oriented along the loading direction: 60% for UD-CFRP compared with 30% for CR-CFRP. The low-temperature (-196 degrees C) tensile and fatigue strengths of the UD-CFRP were over 1.5 times greater than those at room temperature (20 degrees C). This was attributed to the increased epoxy strength at low temperatures along with the internal compressive stress arising from the different thermal expansion coefficients of the carbon fiber and epoxy. Both the epoxy strength and internal compressive strength were employed as factors in a compound law to numerically estimate the low-temperature tensile strength. This work presents a systematic analysis for changes in the CFRP material properties at low temperatures. (C) 2019 Publishing services by Elsevier B.V. on behalf of Vietnam National University. Hanoi.

DOI： 10.1016/j.jsamd.2019.10.002

Effect of Microstructural Characteristics on Mechanical Properties of Ferritic Stainless Steel

Mitsuhiro Okayasu, Tomoki Shigeoka

Journal of Materials Engineering and Performance 28 ( 11 ) 6771 - 6778 2019.11

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

To improve the mechanical properties of hot-rolled ferritic stainless steel (SUS430), the microstructural characteristics of SUS430 were changed using a heating process under various conditions. The hardness of SUS430 decreased upon the increase in the heating temperature to 900 degrees C, and the hardness increased when the sample was heated to temperatures greater than 900 degrees C. The high hardness of the sample heated at 1000 degrees C (H-1000 degrees C) is attributed to the heating time: A high hardness was obtained for a H-1000 degrees C sample that was heated for 1 h (H1000 degrees C-1h), but this decreased when the heating time was increased to more than 1 h. The high hardness of H1000 degrees C-1h is caused by the fine Cr23C6 precipitates that are distributed in the sample around the grain boundaries. On the other hand, the large precipitates of Cr23C6 in H1000 degrees C-12h decrease the hardness. The hardness value of SUS430 is directly attributed to the mechanical properties and the ultimate tensile strength. The tensile strength of H1000 degrees C-1h was found to be about 200% and 20% higher than the as-received and H1000 degrees C-12h samples, respectively. Despite the increase in the tensile strength of the H1000 degrees C-1h sample, the ductility was not found to decrease significantly, for example, the fracture strain was approximately 25%. This occurrence is affected by a severe slip in the ferrite base grain, and the high strength of H1000 degrees C-1h is influenced by the interruption of the slip by the Cr23C6 precipitates. Unlike the tensile strength, similar fatigue properties were observed for both H1000 degrees C-1h and H1000 degrees C-12h, which is associated with the low crack driving force of H1000 degrees C-12h, caused by the roughness-induced crack closure arising from the large Cr23C6 precipitates.

DOI： 10.1007/s11665-019-04426-z

Fatigue Failure Characteristics of Single-Crystal Fe-3%Al Alloy

Mitsuhiro Okayasu, Keisuke Noda

Journal of Materials Engineering and Performance 28 ( 10 ) 5978 - 5987 2019.10

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

To obtain a better understanding of the mechanical properties and failure characteristics of the single-crystal Fe-3%Al alloy (SC), tensile and fatigue properties of the SC and polycrystalline Fe-3%Al alloy (PC) have been investigated. The tensile strength and fracture strain for the SC samples were different depending on the loading direction, i.e., < 111 >, < 110 > and < 100 >, which could be explained by the Schmid factor. Fatigue strength for the SC samples was basically correlated with their tensile strength. Different fatigue properties were obtained for the PC sample, e.g., the high fatigue strength was obtained in the early fatigue stage, and significant reduction in the fatigue strength was appeared in the later fatigue stage, e.g., low endurance limit at 10(7) cycles. The reason for this was caused by the material brittleness due to the accumulated dislocation at the grain boundaries. Because the high ductile SC samples made severe plastic deformation ahead of the crack tip, the crack growth rate did not enhance even if the high stress intensity factor range was applied in Region II (da/dN versus Delta K). The crack growth rate dropped just before the final fracture. Effective stress intensity factor range was estimated using the crack closure model, but the crack growth rate is found to be affected strongly by the slip system of SC sample.

DOI： 10.1007/s11665-019-04331-5

Structural Health Monitoring System for Remote Inspection of Material Failure

Mitsuhiro Okayasu, Toshiki Yamasaki

Journal of Nondestructive Evaluation 38 ( 2 ) 2019.6

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

A new structural health monitoring (SHM) system is proposed for inspecting the failure characteristics of engineering materials, which are monitored remotely. This system is controlled using electrical devices with wireless networking system. A PZT ceramic transducer, an accelerometer and a strain gauge are used to understand the failure characteristics. These sensors are attached to the test sample and reveal its static and dynamic strain behaviour, i.e., information on the material failure (crack generation) and related strains. It is also designed that a dummy plate with a PZT material is attached to the test sample. The dummy plate is designed to fracture earlier than the test sample during the loading process, whereupon the SHM system is triggered via the piezoelectric effect. A compact prototype SHM system is created to evaluate the effectiveness of our system. The failure characteristics are monitored well using the prototype SHM system, which might be able to predict the lifetime of the test sample, i.e., the SHM system reveals the critical strain value and crack growth rate just before the final failure of the test samples.

DOI： 10.1007/s10921-019-0592-7

Effect of Microstructural Characteristics on Mechanical Properties of Austenitic, Ferritic, and - Duplex Stainless Steels

Mitsuhiro Okayasu, Daiki Ishida

Metallurgical and Materials Transactions A 50A ( 3 ) 1380 - 1388 2019.3

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

Materials properties of (austenite), (ferrite), and - duplex stainless steels were experimentally examined using samples with different grain sizes (8 to 1000 mu m) and different ratios of the to phase ( proportion: 35 to 78pct). The mechanical properties (hardness and tensile strength) of the duplex stainless steel were about 1.5 times higher than those of the austenitic and ferritic stainless steels. Two main reasons for the high strength of duplex stainless steel were identified as follows: (i) severe interruption of slip deformation in the phase on the phase; (ii) a high misorientation angle around phase boundaries between the and phases, caused by bonding of the different lattice structures: -fcc and -bcc. The ultimate tensile strength of duplex stainless steel increased with increasing proportion of the phase to 50pct, but decreased with a further increase in the amount of phase. The mechanical properties improved with decreasing grain size of the stainless steels, which follows the Hall-Petch relationship; however, the reverse relationship was obtained for ferritic stainless steel, especially with large grain sizes (100 to 1000 mu m), in which the size of hard Cr23C6 precipitates increased with increasing grain size.

DOI： 10.1007/s11661-018-5083-4

Influence of microstructure on the mechanical properties and hydrogen embrittlement characteristics of 1800MPa grade hot-stamped 22MnB5 steel

Mitsuhiro Okayasu, Lele Yang

Journal of Materials Science 54 ( 6 ) 5061 - 5073 2019.3

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

The influence of the microstructural characteristics on the mechanical and hydrogen embrittlement properties of 1800MPa grade hot-stamped 22MnB5 steel was experimentally investigated using samples processed under various hot-stamping conditions, i.e., different heating temperatures and strain levels. The tensile strength increased with increasing hot-stamping temperature up to approximately 920 degrees C and subsequently decreased owing to the increasing sizes of the lath martensite and prior austenite phases. Some degree of internal strain was introduced into the 22MnB5 steel specimen during hot stamping at 920 degrees C, which led to a slightly higher hardness although no clear microstructural change was observed. The severity of hydrogen embrittlement of the hot-stamped 22MnB5 steel samples was investigated after immersion in a NH4SCN solution, and the degree of hydrogen embrittlement was found to be directly associated with the amount of hydrogen that penetrated into the grain boundary and lath martensite boundary. The high-strength 22MnB5 steel with a very small lath martensite phase exhibited severe hydrogen embrittlement due to the large amount of hydrogen in the sample, and the high internal strain (or high dislocation density) could lead to accelerated hydrogen embrittlement. Severe hydrogen embrittlement occurred upon charging with more than approximately 0.8ppm hydrogen. Based on the obtained results, models are proposed for the hydrogen embrittlement characteristics of hot-stamped 22MnB5 steel.

DOI： 10.1007/s10853-018-3175-6

Fatigue properties of a nanocrystalline titanium based bulk metallic glassy alloy

Mitsuhiro Okayasu, Tomoki Shigeoka

Journal of Science-Advanced Materials and Devices 3 ( 4 ) 478 - 484 2018.12

Language：English Publishing type：Research paper (scientific journal) Publisher：VIETNAM NATL UNIV

To obtain a better understanding of the fatigue properties and crack growth characteristics of a nanocrystalline titanium based bulk metal glasses (Ti-BMG) made by vacuumed casting process, the fatigue failure mechanisms of Ti-BMG have been investigated via S - N and da/dN - Delta K tests. For comparison, the crystalline Ti alloy Ti-Al6V4 was also employed. The fatigue strength in the early fatigue stage was high for Ti-BMG due to the high tensile strength. However, the fatigue strength decreased significantly in the late fatigue stage. The higher slope of S - N relation was detected for Ti-BMG, which crossed that for the Ti-Al6V4 sample around 5 x 10(3) cycles. In the higher Region II, the fatigue crack growth rate was of similar level for both Ti-BMG and Ti-Al6V4 due to their similar strain energy. In the lower Region II, however, the lower crack growth resistance was obtained for Ti-BMG, as compared to Ti-Al6V4. This was attributed to the high crack driving force for Ti-BMG, caused by the weak roughness-induced crack closure. Such crack closing characteristics of Ti-BMG were systematically investigated by various experimental techniques. (c) 2018 The Authors. Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi.

DOI： 10.1016/j.jsamd.2018.10.001

Method for reducing aluminum alloy heat loss in high pressure die casting shot sleeves

M. Okayasu, S. Wu, T. Hirayama

International Journal of Cast Metals Research 31 ( 5 ) 308 - 316 2018.9

Publishing type：Research paper (scientific journal)

To reduce the amount of casting defects in diecast components, a new shot-sleeve is proposed. Cold flakes are generated by solidification of the casting material in the shot-sleeve before the injection process. To solve this problem, a thermal insulation system is introduced on the inner shot-sleeve surface, with tiny grooves being machined to create an air gap between the melt and the sleeve surface. Various casting materials with different material properties are used to examine the insulating effect of the shot-sleeve. The degree of thermal insulation is found to be influenced by the material properties, with the surface tension being significant factors. This thermal insulation effect allows the production of high-quality samples and therefore with good mechanical properties. When casting materials with low surface tension are used, the melt is able to penetrate deeply into the grooves of the shot-sleeve, lessening the insulation effect and leading to the defect.

DOI： 10.1080/13640461.2018.1470370

Tensile Properties of Unsaturated Polyester and Epoxy Resin Reinforced with Recycled Carbon-Fiber-Reinforced Plastic

Mitsuhiro Okayasu, Yuta Kondo

Applied Composite Materials 25 ( 3 ) 561 - 568 2018.6

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

To better understand the mechanical properties of recycled carbon-fiber-reinforced plastic (rCFRP), CFRP crushed into small pieces was mixed randomly in different proportions (0-30 wt%) with two different resins: unsaturated polyester and epoxy resin. Two different sizes of crushed CFRP were used: 0.1 mm x 0.007 mm (milled CFRP) and 30 mm x 2 mm (chopped CFRP). The tensile strength of rCFRP was found to depend on both the proportion and the size of the CFRP pieces. It increased with increasing proportion of chopped CFRP, but decreased with increasing proportion of milled CFRP. There was no clear dependence of the tensile strength on the resin that was used. A low fracture strain was found for rCFRP samples made with chopped CFRP, in contrast to those made with milled CFRP. The fracture strain was found to increase with increasing content of milled CFRP up to 20 wt%, at which point, coalescence of existing microvoids occurred. However, there was a reduction in fracture strain for rCFRP with 30 wt% of milled CFRP, owing to the formation of defects (blow holes). Overall, the fracture strain was higher for rCFRPs based on epoxy resin than for those based on unsaturated polyester with the same CFRP content, because of the high ductility of the epoxy resin. The different tensile properties reflected different failure characteristics, with the use of chopped CFRP leading to a complicated rough fracture surface and with milled CFRP causing ductile failure through the presence of tiny dimple-like fractures. However, for a high content of milled CFRP (30 wt%), large blow holes were observed, leading to low ductility.

DOI： 10.1007/s10443-017-9635-3

MECHANICAL PROPERTIES OF CAST Al-Mg-5 ALLOY PRODUCED BY HEATED MOLD CONTINUOUS CASTING

Mitsuhiro Okayasu, Shuhei Takeuchi

International Journal of Metalcasting 12 ( 2 ) 298 - 306 2018.4

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

To make the high strength and high ductility of the cast aluminum alloys, the Al alloy samples are created by the heated mold continuous casting (HMC) process. Because of the rapid and unidirectional solidification in the HMC process, tiny grains with uniformly orientated crystals are created. In this case, influence of the solidification speed of HMC on the mechanical properties of the Al-Mg-based alloy is studied. The microstructures are altered with the casting speed because of the different solidification rates. The mean secondary dendrite arm spacing (SDAS) for the HMC-Al-Mg alloy made at the high casting speed of 2.7 mm/s is about one-fifth of that for the conventional gravity cast Al-Mg alloy. Nonlinear relationship between the tensile properties (strength and ductility) and the cooling rate are obtained with sigma(UTS) and sigma(0.2) increasing and epsilon(f) decreasing with increasing the cooling rate. The high strength of the HMC samples at the high cooling rate results from the existence of tiny microstructures. The high ductility of the HMC sample at the low cooling rate is affected by the uniform crystal orientation, where shear slips are facilitated by the uniform crystal orientation.

DOI： 10.1007/s40962-017-0163-6

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