MATERIALS TRANSACTIONS
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ONLINE ISSN: 1347-5320
PRINT ISSN: 1345-9678

MATERIALS TRANSACTIONS Vol. 60 (2019), No. 9

  • Radiation Tolerance of Ultrafine-Grained Materials Fabricated by Severe Plastic Deformation

    pp. 1723-1731

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    DOI:10.2320/matertrans.MF201931

    This paper presents a historical overview of studies on the irradiation behaviour of ultrafine-grained materials produced by severe plastic deformation (SPD), which allows fabricating bulk materials with submicron grain size and nanostructural features, essentially enhancing their strength and functional properties. The dramatically increased interface fraction also provides substantially improved microstructure tolerance and ultrafine-grained material properties for electron, proton, ion or neutron irradiation. Issues related to considering SPD metals and alloys as advanced radiation-resistant materials are discussed.
  • PREFACE

    pp. 1732-1732

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    DOI:10.2320/matertrans.MPR2019906

  • Effect of Forging Temperature on Microstructure Evolution and Tensile Properties of Ti-17 Alloys

    pp. 1733-1739

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    DOI:10.2320/matertrans.ME201911

    In this study, the microstructure, tensile strength, elongation, and reduction of area of near-β Ti alloys (Ti-17) were investigated after being subjected to solution and aging treatments. Ti-17 was forged at temperatures between 700 and 850°C followed by air cooling. Then, the forged Ti-17 was subjected to solution treatment at 800°C for 4 h followed by water quenching and aging treatment at 620°C for 8 h followed by air cooling. Tensile tests were performed at room temperature, 450°C, and 600°C. The change in microstructure at different forging temperatures was exhibited by only the volume fraction and morphology of the grain boundary (GB) α phase. That is, a granular GB α phase was formed in the samples forged at 700 and 750°C. Moreover, a film-like GB α phase was formed in the samples forged at 800 and 850°C. The tensile strength was the same for all the tested samples, indicating that the microstructure has little effect on the tensile strength. The elongation and reduction of area increased with decreasing volume fraction in the GB α phase. It is considered that the film-like morphology slightly improves ductility.
  • Quantitative and Qualitative Relationship between Microstructural Factors and Fatigue Lives under Load- and Strain-Controlled Conditions of Ti–5Al–2Sn–2Zr–4Cr–4Mo (Ti-17) Fabricated Using a 1500-ton Forging Simulator

    pp. 1740-1748

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    DOI:10.2320/matertrans.ME201904

    The fatigue lives of forged Ti-17 using a 1500-ton forging simulator subjected to different solution treatments and a common aging treatment were evaluated under both load- and strain-controlled conditions: high and low cycle fatigue lives, respectively. Then, the tensile properties and microstructures were also examined. Finally, the relationships among fatigue lives and the microstructural factors and tensile properties were examined.The microstructure after solution treatment at 1203 K, which is more than the β transus temperature, and aging treatment exhibits equiaxed prior β grains composed of fine acicular α. On the other hand, the microstructures after solution treatment at temperatures of 1063, 1123, and 1143 K, which are less than the β transus temperature, and aging treatment exhibit elongated prior β grains composed of two different microstructural feature regions, which are acicular α and fine spheroidal α phase regions. The 0.2% proof stress, σ0.2, and tensile strength, σB, increase with increasing solution treatment temperature up to 1143 K within the (α + β) region, but decrease with further increasing solution treatment temperature to 1203 K within the β region. The elongation (EL) and reduction of area (RA) decrease with increasing solution treatment temperature, and it becomes nearly 0% corresponding to a solution treatment temperature of 1203 K. The high cycle fatigue limit increases with increasing solution treatment temperature up to 1143 K, corresponding to the (α + β) region. However, it decreases with further increase in the solution treatment temperature to 1203 K in the β region. The fatigue ratio in high cycle fatigue life region is increasing with decreasing solution treatment temperature, namely increasing the volume fraction of the primary α phase, and it relates well qualitatively with the volume fraction of the primary α phase when the solution treatment temperature is less than the β transus temperature. The low cycle fatigue life increases with decreasing solution treatment temperature, namely increasing the volume fraction of the primary α phase. The low cycle fatigue life relates well quantitatively with the tensile true strain at breaking of the specimen and the volume fraction of the primary α phase for each total strain range of low cycle fatigue testing.
  • Suppression of Grain Boundary α Formation by Addition of Silicon in a Near-β Titanium Alloy

    pp. 1749-1754

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    DOI:10.2320/matertrans.ME201920

    The effect of Si addition on the microstructure and mechanical properties of a near-β titanium alloy Ti-17 with fully lamella microstructure was investigated. It was found that the microstructure of Ti-17 with silicon exhibited the absence of a continuously thick α layer along prior-β grain boundaries (grain boundary α), while the grain boundary α was distinctively formed in Ti-17 without Si. The formation of (Ti, Zr) silicide particles at the prior-β grain interior and its grain boundaries were observed, and the presence of these particles were related to the disappearance of grain boundary α similar to the oxygen scavenging effect of boride particles reported previously. With regard to mechanical properties, Ti-17 with silicon exhibits higher strength and lower ductility compared with Ti-17 without Si. Ductile transgranular fracture morphology was observed even on the fracture surface of Ti-17 with Si after tensile test.
  • Low Springback and Low Young’s Modulus in Ti–29Nb–13Ta–4.6Zr Alloy Modified by Mo Addition

    pp. 1755-1762

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    DOI:10.2320/matertrans.ME201912

    Deformation-induced higher Young’s modulus can satisfy the contradictory requirements of Ti alloys for spinal-fixation applications, which demand a high Young’s modulus to reduce springback during operations and a low Young’s modulus to prevent stress shielding effect for patients after surgeries. In this study, TNTZ–(1, 3, 5)Mo are designed by adding Mo and Ti to Ti–29Nb–13Ta–4.6Zr (TNTZ) in order to maintain low initial Young’s modulus and achieve low springback. All the solutionized alloys show single β phase with increasing the β stability by Mo addition. They show low Young’s moduli less than 65 GPa, similar ultimate tensile strength of 650 MPa and elongation around 20%. The springback of TNTZ–3Mo and TNTZ–5Mo is lower than that of TNTZ and TNTZ–1Mo owing to their more stable β phase. After cold rolling, TNTZ–3Mo shows the largest increasing ratio of 25% in Young’s modulus and the highest ultimate tensile strength owning to the appearance of deformation-induced ω phase. With the low initial Young’s modulus of 59 GPa, TNTZ–3Mo is a potential candidate to make the spinal rods in spinal fixation devices.
  • Effects of Fe on Microstructures and Mechanical Properties of Ti–15Nb–25Zr–(0, 2, 4, 8)Fe Alloys Prepared by Spark Plasma Sintering

    pp. 1763-1768

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    DOI:10.2320/matertrans.ME201913

    Biomedical Ti–15Nb–25Zr–(0, 2, 4, 8)Fe (mol%) alloys are prepared by mixing pure element powders and spark plasma sintering (SPS). Specimens with diameters of 20 mm and thicknesses of 3 mm are obtained by sintering at 1000°C for 10 min followed by cooling in the furnace. Some of the specimens are then heat-treated at 900°C for 1 h followed by water quenching. Zr and Fe are dissolved in Ti; however, segregation of Nb is observed in all of the alloys. The β and α′′ phases are observed in the as-sintered and heat-treated specimens owing to the insufficient diffusion of the alloying elements. Fe stabilizes the β phase and provides a solution-strengthening effect. With the increase in the Fe content in the as-sintered specimen, the compressive strength and micro-Vickers hardness are improved in the Ti–15Nb–25Zr–(0, 2, 4)Fe alloys and slightly decreased in Ti–15Nb–25Zr–8Fe. The as-sintered Ti–15Nb–25Zr–4Fe alloy exhibits the maximum compressive strength of 1740 MPa. Although the plasticity is decreased by the Fe addition, a fracture strain of approximately 17% is obtained for Ti–15Nb–25Zr–4Fe, indicating a good plasticity. The heat treatment cannot eliminate the segregation of Nb, but can improve the plasticity and slightly increase the strengths of Ti–15Nb–25Zr–(0, 2, 4)Fe. Moreover, the heat-treated Ti–15Nb–25Zr–8Fe exhibits a high strength of approximately 1780 MPa and fracture strain of approximately 19%. Therefore, good comprehensive mechanical properties, including high strengths, high hardnesses, and good plasticities, can be obtained in Fe-added β-Ti alloys prepared by SPS and subsequent optional short heat treatment.
  • Development of Low-Cost Manufacturing Process and Effects of Adding Small Amounts of Ta, O, and N on the Physical and Mechanical Properties of Highly Biocompatible Ti Alloys

    pp. 1769-1778

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    DOI:10.2320/matertrans.ME201917

    Zirconium (Zr), niobium (Nb), and tantalum (Ta) are important alloying elements of titanium (Ti) alloys for attaining superior biocompatibility. To develop low-cost manufacturing processes, we examined the effects of melting methods, Ta addition using a Ti–Ta mother alloy, and the addition of small amounts of oxygen (O) and nitrogen (N) on the continuous-hot-rolling, physical, and mechanical properties of biocompatible Ti alloys as well as their microstructure. Seven Ti–Zr-based alloys containing Zr, Nb, and Ta were subjected to vacuum arc melting, induction skull melting (ISM), and levitation induction melting. The use of Ti–30 mass% Ta mother alloy (hereafter, % represents mass%) was effective for melting the biocompatible Ti alloys. ISM was found to be a promising advanced method for biocompatible Ti alloys. A fine granular α-phase structure (approximately 1 µm), high strength, and excellent ductility were obtained in the continuous hot-rolling process. The Ti–Zr-based alloys were strengthened by adding small amounts of N and O. The observed mechanical properties were superior to those of artificial hip stems made of Ti–6Al–4V alloy. The Ti–Zr-based alloys started to melt at approximately 1620°C. The β-transus temperature (Tβ) was in the range from 805 to 850°C. The microstructure was predicted using the temperature difference (ΔT) from Tβ. The specific heat constant (Cp) and thermal conductivity (W) increased with increasing temperature up to Tβ then decreased above Tβ. The machinability of Ti–Zr-based alloy was similar to that of Ti–6Al–4V alloy.
  • Preparation of Titanium Alloy/Bioactive Glass Composite for Biomedical Applications via Selective Laser Melting

    pp. 1779-1784

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    DOI:10.2320/matertrans.ME201914

    Selective laser melting (SLM) is widely used for the additive manufacturing (AM) of metal components, which can produce net shape complex geometries. Novel titanium alloy/bioactive glass composites were successfully fabricated via SLM method for biomedical applications. The fabricated composites contained a Ti5Si3 phase, which resulted from the reaction between the titanium alloy and the bioactive glass. The phase can improve the bonding strength between composite and bone. Additionally, the remained amorphous bioactive glass phase could improve bioactivity. The present study opens a new avenue for developing new titanium alloy/bioactive glass composites with optimal bioactivity and bonding strength with the bone.
  • Production of Superplastic Ti–6Al–7Nb Alloy Using High-Pressure Sliding Process

    pp. 1785-1791

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    DOI:10.2320/matertrans.ME201924

    A Ti–6Al–7Nb alloy was processed by severe plastic deformation through high-pressure sliding (HPS) at room temperature for grain refinement. The microstructure consists of grains with sizes of 200–300 nm in diameter having high and low angles boundaries. Superplasticity appeared with the total elongation of more than 400% and this was more likely when the tensile specimen is deformed in the direction parallel than perpendicular to the sliding direction. However, the superplastic elongation is almost the same irrespective of whether the sliding was made in the single direction or in the reversible directions as far as the total sliding distance is the same. The total elongation is invariably higher for the tensile testing at 1123 K than at the other temperatures, reaching the highest elongation of 790% at the initial strain rate of 1 × 10−3 s−1. The strain rate sensitivity and the activation energy for the deformation were determined to be more than ∼0.3 and 199 kJ/mol, respectively. The microstructural observation reveals that the α phase region covers more than 85% of the tensile specimens after deformation and their grains are equiaxed in shape. It is concluded that the superplastic deformation is mainly controlled by grain boundary sliding through thermally activation by lattice diffusion. This Paper was Originally Published in Japanese in J. JILM 68 (2018) 9–15.
  • Mechanical Properties of Additively Manufactured Porous Titanium with Sub-Millimetre Structural Units

    pp. 1792-1798

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    DOI:10.2320/matertrans.ME201916

    The mechanical properties of metallic materials can be controlled by both alloy design and the construction of an appropriate structure. Porous materials are a promising candidate for bone-related devices because they have a low Young’s modulus and allow bone ingrowth. Recently, great advancements have been made in additive manufacturing technology, also known as three-dimensional printing. This technology enables the arbitrary and independent control of the Young’s modulus, strength of the material, the shape and volume fraction of the pores. Porous titanium samples composed of rhombicuboctahedron-derived units with sub-millimetre dimensions were fabricated by laser additive manufacturing, and the dependence of their mechanical properties on the structural parameters was investigated. Porous Ti samples with five different sets of dimensions were accurately fabricated as designed. The solid Ti parts of the samples were confirmed to contain no remarkable solidification voids by Archimedes’ principle. The porosity can be easily controlled in the present structural design; the measured and designed porosities showed a good linear relationship, although the measured porosity was slightly larger than the designed porosity. The gradient of stress-strain curve in elastic region can be also arbitrarily controlled in porous Ti samples with the present structure; it increased as the minimum cross-sectional area ratio in the plane perpendicular to the loading axis increased. This means that Young’s modulus can be arbitrarily controlled by the present structural control. However, it should be noted that the local mechanical properties such as Young’s modulus of the fabricated samples are not uniform in the overall structures; to improve this, the effect of the structure on the retention and flow of heat must be considered.
  • Enhanced Cell Proliferation on Biomedical Titanium Surfaces by Laser Ablation-Induced Micro- and Nanoscale Hybrid Structures

    pp. 1799-1806

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    DOI:10.2320/matertrans.ME201909

    The cell proliferation performance of pure titanium substrates was enhanced by modifying the surface morphology using an ultraviolet laser with a wavelength of 355 nm and travel speeds ranging from 10∼300 mm/sec. Rat calvarial osteoblast cells were cultured on the sample surfaces for 1∼7 days. The cell proliferation was investigated via 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assays. Scanning electron microscopy observations showed that the laser ablation (LA) surfaces had a hybrid micro- and nanoscale structure consisting of microscale grooves with nanoscale agglomerations on their surface. For a low laser travel speed of 10 mm/sec, the grooves had a width of approximately 5.44∼10.03 µm. For the maximum travel speed of 300 mm/sec, the grooves reduced in height, but increased in width to around 10.97∼20.06 µm. The agglomerations on the grooves had a size of around 30∼100 nm; with larger agglomerations being formed at a lower laser travel speed. The XRD analysis results revealed the presence of titanium compounds (TiO and TiN0.3) on the LA surfaces ablated at lower travel speeds of 10 mm/sec and 50 mm/sec, respectively. The MTT measurements showed that the LA samples yielded a better cell proliferation rate than a sandblasted acid-etched titanium sample or a machined titanium sample. Furthermore, the cell proliferation rate increased with a decreasing laser travel speed. In general, the present results confirm the feasibility of laser ablation surface modification as a means of promoting the cell proliferation rate on titanium bioimplants.
  • Fabrication of the Silicate Containing CaTiO3 Film with Hydrophilic and Smooth Surface on Titanium to Improve Osteoconductivity

    pp. 1807-1813

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    DOI:10.2320/matertrans.ME201905

    It has been reported that hydrophilicity and hydrophobicity of implants influenced the bioactivity. However, it is hard to maintain the hydrophilicity in case of being stored in air. So it is critical to find a way to maintain implants’ hydrophilicity. In general, silicate has been known to contribute the hydrophilicity. In this study, the silicate containing CaTiO3 films have been prepared on Ti substrates by two-step treatment for biomaterial applications. The hydrophilicity, osteoconductivity and protein adsorption of treated specimens have been investigated. The 1st step treatment for Ti is to form TiO2 as precursors, either by anodizing in sulfuric acid solution at 298 K, liquid phase oxidation in nitric acid solution with hydrogen peroxide at 353 K, or thermal oxidation at 673 K in air. Hydrothermal treatment in silicate containing alkaline solution is the 2nd step to convert TiO2 to silicate containing CaTiO3 films. The SEM, XRD, XPS, WCA (water contact angle) investigations and protein adsorption measurements have been carried out to characterize the surface properties. This surface maintained 10 deg. in WCA after 7 d exposure in air, while the specimen without silicate has WCA of more than 40 deg. The osteoconductivity is evaluated based on the contact ratio of formed hard tissue on the implanted specimens after 14 d implantation in rats’ tibia at in vivo test. The as-prepared film not only has exhibited smooth and superhydrophilic surface, but also has achieved high osteoconductivity and great protein adsorption capacity.
  • Formation of Photocatalytically Active TiO2 Layers on Ti–Nb Alloys by Two-Step Thermal Oxidation

    pp. 1814-1820

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    DOI:10.2320/matertrans.ME201901

    A two-step thermal oxidation process was applied to Ti–xNb binary alloys (x = 0, 1, 10, 15, and 30 at%) to prepare anatase-containing TiO2 layers, and their photocatalytic activities were evaluated by measuring the water contact angle and decomposition of methylene blue (MB) under UV irradiation. The condition of the first-step treatment was fixed as heating in Ar–1%CO atmosphere at 1073 K for 3.6 ks, and the subsequent second-step treatment was conducted in air at 673–1073 K for 10.8 ks. The reaction layer formed after the two-step thermal oxidation consisted of TiO2. The anatase fraction of the TiO2 layers increased with decreasing second-step temperature and increasing Nb content of the Ti–Nb alloys. In addition, Nb and carbon were introduced into the TiO2 layers. A water contact angle of around 5° was observed on the TiO2 layers formed at the second-step temperatures of 673–973 K. The rate constant of MB decomposition showed a maximum for an anatase fraction of 0.6–0.8 at which the recombination of exited electrons and holes are suppressed. The TiO2 layer formed on the Ti–10 at%Nb alloy exhibited a higher rate constant of MB decomposition compared with Ti–30 at%Nb, in which the TiNb2O7 phase formed. These results indicate that Nb is an effective alloying element for producing a photocatalytically active TiO2 layer on Ti by the two-step thermal oxidation process. Nevertheless, the presence of an anatase-rich TiO2 layer and an appropriate Nb content in TiO2 are required for achieving high photocatalytic activities.
  • Photocatalytic Performance of an Anodic TiO2 Layer Fabricated in a NH4NO3/Ethylene Glycol Electrolyte with Various Crystallographic Phases

    pp. 1821-1827

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    DOI:10.2320/matertrans.ME201915

    Anodizing Ti substrates in an ammonium nitrate/ethylene glycol electrolyte is an innovative process capable of fabricating nitrogen-doped photocatalytic titanium oxide (TiO2) layer in one step. This fabricated layer comprises both rutile and anatase TiO2 phases; however, a major phase contributing to its excellent visible-light responsive photocatalytic performance is still unknown. In the present work, the crystallographic phase of an anodic layer was controlled by exploiting a post-thermal treatment and relationship between the phase variation and photocatalytic performance was then investigated to determine the major phase contributing to this performance. Post-thermal treatment to the anodic TiO2 layer had little influence on the surface morphology and nitrogen doping, but the crystallographic phase, more specifically the ratio of anatase to rutile phases, changed with the heating temperature. The photocatalytic activity, evaluated by methylene blue decolorization and ethylene decomposition, increased with an increase in the ratio of anatase phase, while the correlation with the rutile phase was not observed. X-ray diffraction (XRD) analysis using a grazing incidence geometry showed that the anatase phase was concentrated in the topmost surface region when compared with the rutile phase. In conclusion, the variation of the photocatalytic performance was related to the growth of the anatase TiO2 phase in the layer, with the treatment temperature of 723 K showing the highest photocatalytic activity.
  • Temperature Dependence of Activation Enthalpy for Yielding in Bimodal Ti–6Al–4V

    pp. 1828-1832

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    DOI:10.2320/matertrans.ME201902

    Mechanical properties relating to the thermally activated process of yielding was investigated in Ti–6Al–4V with a bimodal microstructure, consisting of both primary alpha grains and lamellar colonies of secondary alpha/beta lamellae. The temperature dependence of yield stress, effective stress, activation volume, and activation enthalpy were investigated between 77 K and 650 K. The yield stress and effective stress decreased with increasing temperature. It was found that the temperature dependence of activation enthalpy for yielding shows values between those obtained from basal slips and prismatic slips investigated in single crystalline α-titanium. This suggests that the thermally activated processes which control the yielding of bimodal Ti–6Al–4V is the combination of basal and prismatic slips.
  • High Temperature Tensile Deformation Mode and Microstructural Conversion of Ti–6Al–4V Alloy with the (α + α′) Duplex Starting Microstructure

    pp. 1833-1841

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    DOI:10.2320/matertrans.ME201907

    Herein this work quantitatively clarifies the effects of the α′ martensite phase and its fraction in the (α + α′) duplex microstructure on high temperature deformation mode of the Ti–6Al–4V alloy. The change in the fraction of α′ martensite region results in the change in deformation behavior complicatedly: cooperative occurrences of continuous dynamic recrystallization from the equiaxed α region and discontinuous dynamic recrystallization from the α′ martensite region affect the deformation mode. This work reveals that discontinuous dynamic recrystallization from the α′ martensite acts as an additional accommodation mechanism, resulting in higher ductility associated with enhanced grain boundary sliding. Specifically it is remarkable at lower strain rates. In addition, the Ti–6Al–4V alloy with an (α + α′) duplex microstructure exhibits lower flow stress value and slight higher ductility than that with an equilibrium (α + β) microstructure, implying that the accommodation mechanism for deformation is effectively activated in the α′ martensite microstructure. This work also clarifies the active deformation modes of the grain matrix deformation associated with dislocation glide and the grain boundary sliding quantitatively, revealing the enhanced GBS with increasing strain for the Ti–6Al–4V alloy having higher fraction of α′ martensite region.
  • Effect of Oxygen Addition on the Formation of α′′ Martensite and Athermal ω in Ti–Nb Alloys

    pp. 1842-1849

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    DOI:10.2320/matertrans.ME201908

    The effect of the addition of oxygen on the formation of the microstructure quenched from the β phase of Ti–Nb alloy was investigated based on the solid solution treatment (SST) temperature in the β phase. The alloy ingots of Ti–(12, 14, and 18)-at% Nb–(0, 1, and 3)-at% O were arc-melted. They were homogenized at 1200°C for 3.6 ks and then hot-rolled at 850°C into 1.5-mm thick sheets. The specimens were solution-treated at 1050 to 1200°C for 0.6 ks and then quenched in iced brine. The microstructure of the Ti–(12, 14, and 18)-at% Nb alloys exhibited an α′′ phase regardless of the SST temperature. The addition of oxygen in the Ti–(14 and 18)-at% Nb alloys suppressed the β → α′′ martensitic transformation; therefore, the quenched structure became the β + ωa phase. The further addition of oxygen suppressed the β → ωa transformation during quenching. The effect of oxygen addition on the phase transformations of β → α′′ and β → ωa during quenching from the β phase was weakened with an increasing SST temperature.
  • Effects of {332}〈113〉 Deformation Twinning on Fatigue Behavior of Ti–Mn System Alloys

    pp. 1850-1856

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    DOI:10.2320/matertrans.ME201919

    The effects of {332}〈113〉 deformation twinning, one of the unique deformation modes for metastable β-type Ti alloys, on the fatigue behavior of Ti–Mn system alloys were investigated focusing on fatigue strength, fatigue crack initiation and propagation. Ti–7Mn and Ti–5Mn–3Mo (mass%) alloys which are primarily deformed by dislocation slips and {332}〈113〉 deformation twins, respectively, were subjected to fatigue tests conducted in tensile-tensile mode at room temperature, followed by fracture surface and deformation microstructure analyses. We found for the first time the Ti–5Mn–3Mo alloy has higher fatigue strength as compared to the Ti–7Mn alloy owing to the formation of the {332}〈113〉 deformation twins. The {332}〈113〉 deformation twins are to some extent responsible for the plastic strain accumulation in place of the dislocations during cyclic deformation. Thus, {332}〈113〉 deformation twinning prevents the accumulation of dislocations during cyclic deformation, thereby suppressing fatigue crack initiation. Moreover, formation of the {332}〈113〉 deformation twins around crack tip decreases stress concentration at the crack tip and changes the crack propagation direction, as a result, crack propagation speed is decreased. These results indicate that the {332}〈113〉 deformation twining is crucial for improving the fatigue properties of metastable β-type Ti alloys.
  • α-Phase Precipitation Behavior in β-Type Ti–15V–3Cr–3Sn–3Al Alloys Aged by Different Aging Processes, Investigated Using Transmission Electron Microscopy

    pp. 1857-1863

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    DOI:10.2320/matertrans.ME201910

    Two-step aging method using high temperature to low temperature sequence has been developed to improve the mechanical properties of Ti–15V–3Cr–3Sn–3Al alloys. α-precipitation behavior due to the 1st-step and the 2nd-step aging on this alloy has been investigated using electron microscopy. Due to the 1st-step aging, grain-boundary α-precipitates and extreme fine α-precipitates were observed. Due to the 2nd-step aging, needle-shaped α-precipitates in high density were observed. To clarify roles of the 1st-step aging process, precipitation behavior of a simple-aged specimen at the same condition as the 2nd-step aging was investigated. It is concluded that the principal factor to grow needle-shaped α-precipitates in high density was considered as extreme fine α-phase precipitates due to the 1st-step aging.
  • Effect of C and Al Elements on High Specific Resistance and High Rigidity of Ultra-High Strength TiC(1−X)/Ti Metal Matrix Composites Fabricated by Blended Elemental Reactive Sintering

    pp. 1864-1872

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    DOI:10.2320/matertrans.ME201925

    The effects of C and Al concentrations on the specific resistance and rigidity of newly developed ultra-high strength titanium-based metal matrix composites (Ti-MMCs), fabricated using blended elemental reactive sintering (BERS), were investigated. Both TiC(1−X)/Ti–6Al–4V and Ti–8.6Al–5.7V composites were compared with TiB/, SiC/ and AlN/Ti-MMCs. TiC was found to react with Ti powder and to transform to TiC(0.50–0.62) during sintering. The resulting TiC(0.50–0.62)/Ti–8.6Al–5.7V exhibited a specific resistance of 2.33 µΩm, a Young’s modulus of 135 GPa and a tensile strength of 1.25 GPa, with a substantial elongation of approximately 2.5%. In contrast, TiB/Ti–6Al–4V showed excellent mechanical properties but an extremely low electrical resistance because the conducting TiB particles had a specific resistance of only 0.07 µΩm. Both SiC and AlN also reacted with Ti powder during sintering to form a brittle phase at the interfaces between the particles and the Ti matrix. As a result, Ti–6Al–4V MMCs suitable for use as structural materials could not be fabricated using BERS with SiC or AlN. The high specific resistance of the TiC(0.50–0.62)/Ti–8.6Al–5.7V is partly attributed to the C deficiency of the TiC(0.50–0.62) particles, which results in a specific resistance of approximately 1.7 µΩm. This value is approximately three times higher than the value of 0.52 µΩm for stoichiometric TiC particles. The solubility of excess C and Al in the Ti matrix also increases the specific resistance of the material. This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 83 (2019) 97–106. The fifth author name was revised from Kiyoji Nakamura to Kiyoharu Nakamura. The bibliography information of an article for correction was listed in J. Japan Inst. Met. Mater. 83 (2019) 256.
  • Tunability of Mg2Si Bandgap by Formation of Mg2(Si, C) with an Anti-Fluorite Structure Examined by First-Principles Calculations

    pp. 1873-1880

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    DOI:10.2320/matertrans.M2019082

    We used first-principles calculations to investigate the effects of replacing Si atoms in Mg2Si with C atoms to tune its bandgap and enhance its thermoelectric performance. First-principles calculations suggest that the substitution of Si by C atoms in the Mg2Si lattice forms Mg8Si4−zCz (z = 1, 2, and 3) with a sustained anti-fluorite structure, which results in an unexpected bandgap contraction. However, the bandgap of Mg8Si4(1−x)C4x composed of a supercell structure of a rhombohedral Mg2Si primitive cell and Si-substitution with C has a wide bandgap when the C/Si ratio is sufficiently high. The formation enthalpies from Mg, Si, and C (diamond) are negative under pressures greater than ca. 15 GPa.
  • Strengthening Mechanisms of Powder Metallurgy Extruded CP Titanium Materials with Zirconium and Oxygen Solid Solution via Decomposition of ZrO2 Additives in Sintering

    pp. 1881-1889

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    DOI:10.2320/matertrans.Y-M2019833

    One of the representative high-strength titanium (Ti) alloys used as biomaterials is a commercial Ti–6Al–4V (Ti-64). It has, however, serious problems because Ti-64 contains vanadium, one of highly toxic elements, as the necessary additive to improve the mechanical strength. In this study, in order to develop a high-strength and biocompatible Ti alloy for application to biomaterials, powder metallurgy (PM) α-Ti material with zirconium (Zr) and oxygen (O) solid solution (Ti(Zr,O) alloy) was fabricated from the elemental mixture of CP Ti and ZrO2 powders. During solid-state sintering process, the additive ZrO2 particles were decomposed by reaction with CP Ti powder, and then Zr and O atoms were dissolved in the α-Ti crystals as substitutional and interstitial elements, respectively. These solution elements caused a remarkable increment of the lattice constant of α-Ti (hcp) crystal, and resulted in the significant improvement of tensile strength of Ti alloys. For example, Ti(Zr,O) alloy showed 0.2% yield stress of 1153 MPa when using CP Ti powder mixed with 3 wt.% ZrO2 particles, which was greatly high compared to PM CP Ti material with 0.2% YS of 463 MPa. In addition, the solid solution strengthening mechanism of this alloy was investigated in detail. This Paper was Originally Published in Japanese in J. Jpn. Soc. Powder Powder Metallurgy 65 (2018) 746–755.
  • Goss Orientation Evolution in Ti–5.5Mo–8Al–6Zr Shape Memory Alloy upon Heat Treatment

    pp. 1890-1897

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    DOI:10.2320/matertrans.M2019039

    The effect of heat treatment temperature on texture formation in Ti–5.5Mo–8Al–6Zr (mol%) alloy sheets was systematically investigated in this study. α′′ martensite was induced by cold-rolling. Although the deformation texture of α′′ martensite could not be detected, the formation of {130}α′′⟨310⟩α′′ and (100)α′′[010]α′′ textures was proposed. The specimen heat-treated at 1073 K or higher consisted of a single β phase, and Goss, {110}β⟨113⟩β, and Brass orientations were formed as the recrystallization textures. The Goss orientation, which is irregular for β-Ti alloys, developed with increasing heat treatment temperature, and became the dominant component in the specimens heat-treated at 1173 K. The specimen heat-treated at 973 K consisted of the β + α phase. {113}β⟨110⟩β, {113}β⟨141⟩β, {223}β⟨252⟩β, {223}β⟨692⟩β, and {332}β⟨113⟩β were formed in the β phase, whereas {1120}α⟨1100⟩α–{1122}α⟨1100⟩α was formed in the α phase. Transmission electron microscopy observations revealed that the specimen heat-treated at 973 K was not completely recrystallized. This microstructural difference led to a difference in texture components between the specimens heat-treated at 973 K and at 1073 K or higher.
  • Solidification of Aluminum Plate and Cylinder Induced by Heat Transfer

    pp. 1898-1901

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    DOI:10.2320/matertrans.M2019071

    Solidification of aluminum plate and cylinder was analyzed by tracing the flow of latent heat of solidification from liquid-solid interface to surface and therefrom to outside by heat transfer. Temperature drop at surface below melting point during the solidification was found to be related closely to the thickness of solid layer for each mode of solidification. Temperature gradient set by the above relation enabled to estimate the growing rate of solid layer. Repetition of above calculations from the beginning of solidification revealed the times for solid layer to attain varied thicknesses. Thus the solidification times of plate and cylinder with varied size and varied coefficient of heat transfer were calculated. The results of calculation were shown in numerical tables for each mode of solidification. Solidification of the superheated aluminum plate was analyzed by use of parabolic distribution of temperature in liquid before and during the solidification. This Paper was Originally Published in Japanese in Japan Inst. Met. Mater. 83 (2019) 124–127.
  • Influence of Gamma Irradiation on Properties of Ferroelectric Composite from Cellulose Nanoparticles and Triglycine Sulfate

    pp. 1902-1907

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    DOI:10.2320/matertrans.MT-M2019139

    The present work is devoted to comparative study on electrophysical properties of non-irradiated and gamma-irradiated ferroelectric composites synthesized from triglycine sulfate and cellulose nanoparticles. The investigation of phase transition indicated that depending on composition weight ratio, the phase transition temperature of non-irradiated samples increased by 3 to a few tens of celcius degrees higher than those for single crystal TGS (+49°C), while the characteristic relaxation frequencies were lower. Besides, two areas of linear dispersion and Debye-like relaxation were detected in the studied frequency range. Under the influence of gamma irradiation, the phase transition temperature and characteristic relaxation frequencies decreased as compared to those of the non-irradiated samples. The stated anomalies can be explained by the interaction between cellulose nanoparticles and triglycine sulfate inclusion through hydrogen bonds, and by the defects created by gamma irradiation.
  • Enhanced Property of W–Cu Composites by Minor Addition of Ag

    pp. 1908-1913

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    DOI:10.2320/matertrans.MT-M2019109

    Nano-scale W–20Cu (mass%) and W–18Cu–2Ag (mass%) composite powders were obtained by mechano-thermochemical process, followed by liquid phase sintering process from 1200 to 1300°C. The results indicate that the relative density and electrical conductivity of W–18Cu–2Ag composite were much superior to the W–20Cu composite at all temperatures. Microstructural investigation reveals that minor addition of Ag is beneficial to the densification of W–Cu composite. This can be ascribed to the stronger the wettability and capillary force of liquid Cu–Ag to W compared to that of pure Cu. Furthermore, during cooling Ag can precipitate into the voids between the W particles in W-rich area where is hard to be infiltrated by liquid Cu at high temperature, which is also beneficial to electrical conductivity.
  • Effects of Microstructure on Mechanical Properties of Harmonic Structure Designed Pure Ni

    pp. 1914-1920

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    DOI:10.2320/matertrans.MT-M2019145

    This study aimed at investigating the influence of microstructure on mechanical properties of Harmonic Structured (HS) pure-Ni compacts. The harmonic structure is a heterogeneous microstructure with a spatial distribution of fine grains (FG) and coarse grains (CG), that is, the CG areas (‘Core’) embedded in the matrix of three-dimensionally continuously connected network of FG areas (‘Shell’). The HS pure-Ni samples were fabricated by powder metallurgy route consisting of mechanical milling (MM) of plasma rotated electrode processed pure-Ni powder and subsequent sintering by Spark Plasma Sintering. The plastic deformation at powder particle surface increases with increasing MM time. As a result, after sintering, shell fraction also increases in the HS pure-Ni samples. It was found that the fraction of a “shell” area is an important parameter controlling the balance of the mechanical properties of the HS pure-Ni compacts. The HS pure-Ni with a higher fraction of “shell” area demonstrated higher strength and approximately similar elongation as compared to the homo Ni samples and HS pure-Ni samples containing low shell fraction. Moreover, the effects of strain hardening rates and strain hardening exponents on deformation behaviour of HS pure-Ni samples were also discussed. This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 83 (2019) 231–237.
  • Enhanced Dendrite Coarsening and Microsegregation in Al–Cu Alloy under a Steady Magnetic Field

    pp. 1921-1927

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    DOI:10.2320/matertrans.M2019057

    The final mechanical properties of alloys are significantly influenced by the secondary dendrite arm spacing (SDAS). The application of a steady magnetic field (SMF) during solidification is a novel method to control the SDAS, however, the nature of the change in SDAS under an SMF is still an open question. In this work, dendrite coarsening in the Al–4.5 mass%Cu alloy in an SMF and its effect on microsegregation were investigated experimentally by the quenching technique. The coarsening experiments showed that the SDAS increased in an SMF, which was mainly attributed to the thermoelectric magnetic convection (TEMC) while the change in solid/liquid interfacial tension in the SMF played an adverse role. Further, the variation of the microsegregation level in the SMF was examined by composition measurements. It was shown that the segregation ratio increased in the SMF, which could be ascribed to the reduction of diffusivity in the solid phase and the enlargement of SDAS in the SMF. Using a modified analytical model developed by Voller, the microsegregation levels with and without an SMF were predicted, which was in agreement with the experimental results.
  • Investigation on Simplified Equation for Estimating J-Integral of Adhesive Joint and Its Application to Galvannealed Steel Plate

    pp. 1928-1935

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    DOI:10.2320/matertrans.Z-M2019831

    Structural adhesives have been used to reduce weight and increase rigidity in recent automotive developments. This study focuses on the fracture behaviors of the cohesive zone under mixed-mode conditions, and a method for evaluation of the J-integral parameters using the displacement of the crack tip opening is proposed. The method is an application of the Dugdale model, which is known as an ideal model to work with a localized plasticity ahead of the crack tip. The validity of the method is verified through finite element analysis. Furthermore, the method is applied to adhesive joints on galvannealed steel, and its fracture mechanism is investigated. This Paper was Originally Published in Japanese in J. Soc. Mat. Sci., Japan 67 (2018) 1042–1049.
  • Mechanism behind the Crack Formation in Hydrogen Doping Cz-Si Crystal Growth

    pp. 1936-1942

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    DOI:10.2320/matertrans.M2019046

    In this study, {100} cracks were found in Czochralski (Cz) silicon wafers grown in the atmosphere including hydrogen under the condition of a low Vc/Gc (Vc, growth rate; and Gc, temperature gradient) although the {100} plane is not a cleavage plane of silicon crystals. It was also found that dislocation clusters were associated with the as-grown defects. To reveal the mechanism behind the crack formation, the process of introducing interstitial and hydrogen atoms into a Cz-Si crystal upon solidification was imitated by applying ion irradiation into Cz silicon wafers under three different conditions: silicon ions and hydrogen ions, silicon ions only, and hydrogen ions only. In this case, {100} cracks were only found in the wafer irradiated with both silicon and hydrogen ions. This suggests that the existence of dislocations in silicon is necessary for crack formation. Density functional theory calculations showed that the cleavage energy was decreased by the arrangement of hydrogen atoms on a {100} plane of a silicon crystal, which can explain the formation of {100} cracks during solidification.
  • Investigation of Sintering Behavior in Low-alloyed Steel Powder Containing TiB2 Particles

    pp. 1943-1948

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    DOI:10.2320/matertrans.Y-M2019817

    In our previous study, TiB2-reinforced sintered steel with a high Young’s modulus was obtained for lightweighting of automobile parts. The high Young’s modulus was attributed to the combination of suitable reinforcing particles for the steel and the high relative sintered density provided by hot-working of the sintered billet.In this study, how and why the relative sintered density is increased were investigated via single-pressing and single-sintering, by which near net shape parts were simply and easily manufactured.In steel powder and TiB2 powder compact, the liquid phase of the Fe–B system formed above 1448 K, enhanced the densification. For 2–15 vol% TiB2 addition, the effect of this liquid phase on the densification was appreciable and high relative sintered densities of >98% were achieved by vacuum sintering at 1523 K. This high relative sintered density realized a high Young’s modulus comparable to the calculated value for each TiB2 contents. This Paper was Originally Published in Japanese in J. Jpn. Soc. Powder Powder Metallurgy 65 (2018) 227–232.
  • Anisotropic Electric Conductivity and Battery Performance in C-axis Oriented Lanthanum Silicate Oxyapatite Prepared by Slip Casting in a Strong Magnetic Field

    pp. 1949-1953

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    DOI:10.2320/matertrans.Y-M2019832

    Lanthanum silicate oxyapatite (LSO) has superior properties as a solid electrolyte for solid oxide fuel cell (SOFC) applications. This is because LSO has a higher oxide-ion conductivity compared to yttria-stabilized zirconia at temperatures below 600°C. Textured LSO bulk ceramics were fabricated based on a magnetic field-assisted colloidal processing technique. The c-axis of the LSO was aligned parallel to the applied magnetic field during the consolidation by slip casting. The anisotropic electric conductivity of the textured bulk ceramics was evaluated by the impedance spectroscopy method. It was confirmed that very high conductivity was obtained along the c-axis. Higher fuel cell performance was demonstrated in the textured LSO compared to the random LSO. This Paper was Originally Published in Japanese in J. Jpn. Soc. Powder Powder Metallurgy 65 (2018) 121–126.
  • Influence of Zn Content on Grain Boundary Precipitates on Stress Corrosion Cracking of Al–Zn–Mg Alloys under Environments Containing Chloride Solutions

    pp. 1954-1963

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    DOI:10.2320/matertrans.MT-M2019119

    A commercial AA7075 (Zn: 5.4 mass%) and experimental Al–Zn–Mg alloys with 8.5 or 10.5 mass% of Zn were used to investigate the influence of Zn in the alloys on the composition of grain boundary precipitates and the susceptibility to the stress corrosion cracking (SCC) of the alloys in NaCl solutions. The stress-strain curves of the alloys under the slow strain rate test (SSRT) demonstrated that the susceptibility of the Al–Zn–Mg alloys to SCC increased as the Zn content increased from 5.4 to 8.5 mass% in the alloys. The susceptibility of the Al–10.5Zn–Mg alloy to SCC was not evaluated because of its brittleness. A coarse mainly Zn-containing precipitate was observed at the grain boundary only in the Al–8.5Zn–Mg alloy by STEM/EDS analysis. Calculated Al–Zn–Mg–Cu–Cr phase diagram predicts that MgZn2 is appeared in the Al–8.5Zn–Mg alloy but not in the Al–5.4Zn–Mg alloy. Anodic polarization measurements of the alloys demonstrated that the dissolution current density increased and the pitting corrosion potential decreased as the Zn content in the alloys increased. The generation of active grain boundary precipitates, such as the mainly Zn-containing precipitates, appears to lead to a higher susceptibility to SCC in Al–Zn–Mg alloys.
  • Preparation of Nanoporous CeO2 Catalyst Supports by Chemical Treatment of Amorphous Alloys and Investigation of Ni/CeO2 Catalytic Activity

    pp. 1964-1967

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    DOI:10.2320/matertrans.M2019068

    Nanoporous CeO2 was prepared implementing the dealloying method on precursors consisting of Ce–Al alloys characterized by different atomic arrangements. In fact, the atomic arrangement of the precursor alloy strongly influence the surface area of CeO2 in the final product. Nanoporous CeO2 with quite high surface area were formed when an amorphous Ce–Al alloy was used as the precursor. The catalytic performance of the catalyst that Ni was supported on CeO2 prepared from amorphous alloy were evaluated based on the reaction whereby molecular hydrogen is released from ammonia borane. A high level of catalytic activity was observed due to that fine Ni particles were dispersed on CeO2 prepared from amorphous alloy with quite high surface area.
  • Effect of Titanium on Oxidation Behavior of High-Purity Ferritic Stainless Steel

    pp. 1968-1976

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    DOI:10.2320/matertrans.MT-M2019123

    The effect of Ti addition on the oxidation resistance of high-purity 19%Cr ferritic stainless steels has been investigated during isothermal heat treatment at temperatures between 1073 and 1273 K in air. The microstructures of the scale and scale/metal interface were investigated in detail using scanning electron microscopy-electron backscatter diffraction, field emission-transmission electron microscopy, together with micro-energy dispersive X-ray spectroscopy.Ti addition increased the oxidation mass gains and simultaneously improved the oxidation resistance limit temperature. The formed scale consisted mainly of Cr2O3 regardless of the addition of Ti, but the addition of Ti increased the thickness of the Cr2O3 layer. Moreover, the addition of Ti considerably reduced the grain size of Cr2O3, and this was inferred to increase the oxidation mass gain owing to the easy diffusion of metal ion through grain boundaries. Furthermore, Ti was oxidized in the region underneath the scale/metal interface and formed internal complex oxides, such as Al2TiO5 and Al2Ti7O15 owing to the presence of small amounts of Al in the used steels. θ-Al2O3 also formed in the somewhat deeper region from the interface, where Ti could not be oxidized. The oxygen-getter effect of Ti atoms was postulated to be responsible for improving the oxidation resistance of the alloys. This Paper was Originally Published in Japanese in J. Japan. Inst. Met. Mater. 82 (2018) 130–139. In order to clearly explain, the captions of figures and tables were changed. The references were also changed.
  • Evaluation Method of Formability through Development onto Tangential Planes: Geometrical Evaluation for Forming of Closed Section Parts from Sheet

    pp. 1977-1982

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    DOI:10.2320/matertrans.P-M2019834

    New formability evaluation methods are investigated for closed section parts manufactured from sheet blanks. New methods are based on an analytical approach and geometrical information. The authors proposed developmental evaluation method in which in-plane strain is calculated by the development of product shapes. Two stage development using principal curvatures and a series of tangential planes enables application of developmental evaluation method to shapes with large curvature. In this paper, the results of calculation by the proposed method are compared with the results of FEM simulation regarding on sheet forming into horn tubes. The horn tube, which consists of circular, conical and transient portions, is one of the typical shapes of automotive parts. The conclusion is that the two sets of calculation results are in good qualitative agreement. For product shapes with widely spread, relatively small strain introduced in forming processes, the geometrical condition is dominant for the strain distribution and the prediction accuracy of the developmental evaluation method is improved. This Paper was Originally Published in Japanese in J. JSTP 59 (2018) 241–246.
  • Influence of Various Elements on Primary Crystal Temperature and Carbon Equivalent in Hypo-Eutectic Cast Iron

    pp. 1983-1988

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    DOI:10.2320/matertrans.F-M2019829

    Generally, carbon equivalent is calculated with the following equation; CE = [%C] + (1/3) [%Si]. However, the value calculated with chemical analysis method such as emission spectrochemical analysis is different from that calculated with the primary crystal temperature (Hereafter TL) of the CE meter. In this study, the influence of elements on primary crystal temperature and carbon equivalent in cast iron was examined, and a more accurate equation for calculating carbon equivalent was suggested.The relationship between the various element content and TL from hypo-eutectic to eutectic composition is as follows; TL (°C) = 1625 − 110 [%C] − 25 [%Si] + 3 [%Mn] − 35 [%P] − 71 [%S] − 2 [%Ni] − 7 [%Cr] Dividing this equation with carbon coefficient, a carbon equivalent equation from hypo to eutectic composition is obtained, as follow; CEL = [%C] + 0.23 [%Si] − 0.03 [%Mn] + 0.32 [%P] + 0.64 [%S] + 0.02 [%Ni] + 0.06 [%Cr]. This is calculated from the drop in the solidification temperature and is different from the generally used CE = [%C] + (1/3) [%Si]. We investigated which causes the difference and which is more correct from hypo to eutectic composition.From the review of references, it is assumed that CE = [%C] + (1/3) [%Si] is calculated from the carbon solubility in hyper-eutectic composition. Compared to this, as for the references in which carbon equivalent are calculated from the solidification temperature from hypo to eutectic composition, Si coefficient is not (1/3) but 0.22 to 0.25.From all the following viewpoints, it can be said that CEL = [%C] + 0.23 [%Si] is more correct than CE = [%C] + (1/3) [%Si]: (a) experimental result, (b) cooling curve of CE meter, (c) carbon floatation result, (d) TL and chemical analysis, (e) internal shrinkage test result, and (f) thermodynamic simulation with JMatPro. The silicon coefficient (α) is constant as 0.23 until 3.65% silicon, but it increases linearly if the silicon content exceeds 3.65%. This Paper was Originally Published in Japanese in J. JFS 91 (2019) 87–93. Figure 12 was slightly modified.
  • Effect of Alloying Element on Si Phase Precipitation from Si-Supersaturated α-Al Solid Solution in JIS ADC12 Alloy Die Castings during Heat Treatment

    pp. 1989-1995

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    DOI:10.2320/matertrans.F-M2019830

    The growth of aluminum alloy die castings can be eliminated with a suitable heat treatment. To elucidate the parameters affecting the heat treatment conditions, we investigated the effects of alloying elements on the precipitation behavior of supersaturated silicon in die castings during heat treatment using JIS ADC12 alloy (hereafter, referred to as ADC12 alloy) and Al–11 mass%Si alloy, considering that the former contains several alloying elements while the latter no other elements except for silicon.Most of the supersaturated silicon in ADC12 alloy die castings precipitated with a short time heat treatment, resulting in a large number of fine silicon precipitates dispersed in the primary aluminum phase. However, the supersaturated silicon in Al–11Si alloy die castings needed a long time of heat treatment to precipitate and resulted in fewer and larger silicon precipitates in the primary aluminum phase than that of ADC12 alloy die castings. The concentration of magnesium, copper and silicon etc. was analyzed in the silicon particle near the interface with aluminum matrix in ADC12 alloy die castings using a three dimensional atom probe. These alloying elements of magnesium, copper, etc. are considered to have formed clusters suitable as the nucleuses of silicon precipitation during die casting or at the early stage of heat treatment thus promoting the precipitation of the supersaturated silicon during heat treatment.To confirm this assumption, we examined the growth behavior of the die castings of Al–11Si alloy with an addition of magnesium. The growth of the die castings with magnesium addition was faster than that of Al–11Si alloy die castings during heat treatment, and the promoting effect of magnesium was verified on the precipitation of the supersaturated silicon in ADC12 alloy die castings. Therefore, the presence and the content of alloying elements such as magnesium, copper, etc. should be considered when deciding the heat treatment conditions for removing the growth of the die castings of Al–Si system alloys. This Paper was Originally Published in Japanese in J. JFS 90 (2018) 697–702.
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  • Numerical Simulation of Cut Surface Shape and Residual Stress Distribution in Shearing Process

    pp. 1996-2002

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    DOI:10.2320/matertrans.P-M2019835

    Many analyses relating to shearing of sheet products have already been reported, and an increasing number of elasto-plastic analyses evaluating the residual stress inside sheets have also appeared in recent years. However, because only a few papers have presented measured residual stress distributions, very few in-depth comparisons between calculated stress levels and experimental results are available. In this study, two simulations of the shearing process were carried out, considering the ductile fracture conditions of thin steel sheets. The simulation results of the cut surface shape and residual stress distribution in the vicinity of the cut surface were in good agreement with the experimental results. As another example, after punching a round hole to about half the sheet thickness, i.e., extruding a cylindrical protrusion, a round interlock was formed by subsequently stacking another sheet on the previous sheets. When compared with the X-ray diffraction data, it was found that a simple four-layer interlock simulation model could predict the residual stress distributions with accuracy close to the measured results. The abovementioned simulations are expected to be promising tools benefitting the performance of steel products by reducing residual stress levels. This Paper was Originally Published in Japanese in J. JSTP 59(688) (2018) 65–70.
  • Microscopic Observation of Precipitation Behavior at Friction Stirring Zone of Super Duplex Stainless Steel

    pp. 2003-2007

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    DOI:10.2320/matertrans.M2019069

    To investigate the precipitation behavior of super duplex stainless steel in its weld by friction stir welding (FSW) at a low welding speed, we carried out microstructural observation and analysis. An intermetallic compound phase, σ, was observed in the heat affected zone (HAZ). The σ phase precipitated at the interfaces between δ-Fe (ferrite) and γ-Fe (austenite) grains 1–2 mm away from the stir zone (SZ)/HAZ boundary. On the other hand, the Cr2N was observed together with the belt-like γ-Fe grain aggregates in the vicinity of the advancing side (AS) of the SZ. The other intermetallic phase, χ, was also observed at a triple junction of γ-Fe grains. This demonstrates that the precipitation of the Cr2N and χ phases correlates with the transformation of δ-Fe to γ′-Fe (secondary austenite).
  • Evaluation of Mass Transfer in an Aluminum Melting Furnace Stirred Mechanically during Flux Treatment

    pp. 2008-2015

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    DOI:10.2320/matertrans.M2019055

    The main purpose of the present study is to evaluate mass transfer in an aluminum melting furnace stirred mechanically during flux treatment through water model experiment. Instead of the flux particles, model perlite particles were utilized, and the mass transfer between the pre-coated model particles and water bath was evaluated. Numerical simulation was also carried out to understand the flow pattern and mass transfer mechanisms. The fast impeller rotation speed enhanced the mass transfer. Besides, counter clockwise (CCW) rotation of impeller yielded larger mass transfer as compared to that of the clockwise impeller rotation (CW). The area of higher kinetic energy expanded with increase of impeller rotation speed, and the turbulent energy for the case of CCW rotation was higher compared to the CW case. The mechanism of the mass transfer enhancement under the CCW rotation conditions can be understood in terms of the turbulent energy at the free surface. The averaged downward flow was insufficient to cause the particle entrainment. On the other hand, the turbulent fluctuations were strong enough to cause the particle entrainment.
  • Electrical Property Improvement of Copper Filler Conductive Adhesive with Low-Melting Point Metal Bridge

    pp. 2016-2021

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    DOI:10.2320/matertrans.MH201810

    Conductive adhesives are expected to be the solder alternative bonding materials in low temperature joining. However, the thermal resistivity and electrical resistivity are higher than those of solders. The contact resistances between the metal fillers and resin intrusion are considered to cause the high resistivity. To solve these problems, metallic cross-links are generated between the copper fillers in the conductive adhesives. Low melting point metal (SnBi) fillers which would be molten under curing temperature of the resin are mixed with copper fillers in the resin. In the curing process, the molten SnBi form the metallic bond cross-link between them. In this paper, the electrical resistivity as well as the thermal resistivity depending on the mixture ratio, size of fillers and the SnBi cross-link are investigated through the experiments.
  • A Low Temperature Fabrication and Photoactivity of Al2TiO5 in Cinnamic Acid Degradation

    pp. 2022-2027

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    DOI:10.2320/matertrans.M2019076

    Aluminum titanate was fabricated by sol-gel method using aluminium nitrate, titanium isopropoxide as precursors and citric acid as a complexing reagent. Effects of calcination temperature, citric acid content and calcination duration on the formation of Al2TiO5 (ATO) phase were investigated by X-ray diffraction. The results showed that the ATO-rich mixture with traces of anatase TiO2 was obtained at low calcination temperature (700°C). The properties of obtained ATO were characterized by various methods such as FTIR, N2 isotherm adsorption, DRS, SEM, TEM, and TGA. The point of zero charge of sample was also determined by salt addition method. Owning band gap energy of 3.42 eV the nanostructured ATO could be activated by UV light to become photocatalyst. Indeed, at favorable reaction conditions, detected from the experiment, the obtained ATO gave the photodegradation efficiency of cinnamic acid (CA) approximately 90% after 6 h. ATO also showed high stability and easily separated from solution, consequently after 9 times of reusing the CA degradation extent lightly decreased about 20%.
  • Enhancement of Perpendicular Exchange Bias by Introducing Twin Boundary in Pt/Co/α-Cr2O3/α-V2O3 Epitaxial Film

    pp. 2028-2032

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    DOI:10.2320/matertrans.MT-M2019102

    Perpendicular exchange anisotropy at the Co/α-Cr2O3 interface was investigated using the two types of films: the film with the single crystalline α-Cr2O3 and that with the twinned α-Cr2O3. Exchange anisotropy energy density JK of the film with the single crystalline α-Cr2O3 was ∼0.09 erg/cm2 whereas JK of the film with the twinned α-Cr2O3 was ∼0.43 erg/cm2, more than 4-times enhancement. We discussed the mechanism of the enhancement of JK based on the exchange coupling at the twin boundary and that the spin frustration at the twin boundary can be the origin of the enhancement of JK.
  • TiO2@SiO2 Nanoparticles Functionalized with Para-Aminobenzoic Acid (PABA) by Fisher Esterification Reaction

    pp. 2033-2040

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    DOI:10.2320/matertrans.M2019085

    In this work, hybrid TiO2 nanoparticles embedded in SiO2 were obtained by means of sol-gel process, testing different proportions of the silicon and titanium oxides to obtain TiO2 particles with a high surface content of hydroxyl groups. Anatase crystalline form of TiO2 is a widely used material in active-substance release studies, due to its optimal properties for the transport, distribute and release of different molecules into biological systems. According to the X-ray diffraction analysis, all the proportions of evaluated hybrid systems presented the anatase phase, however, the molar proportion (20:80, TiO2:SiO2) contains a high quantity of hydroxyl groups according to infrared spectroscopy. The spherical morphology of the particles were observed by scanning electron microscopy forming agglomerates. The functionalization of these surfaces was carried out using para-aminobenzoic acid as a drug-binding model that generated a covalent bond between the TiO2@SiO2 system, likewise, thermogravimetric analysis identified the content of functionalized PABA. Dynamic light scattering showed particles and agglomerates around 25–609 nm and negative value of zeta (ζ)-potential in the molar ratio TiO2:SiO2 20:80-PABA. Finally, we demonstrated that PABA hydrolyzes in the presence of human plasma, recovering the nanoparticularized systems with the hydroxyl groups. The hydroxyl groups on the particle surface promote the incorporation of organic molecules that contain carboxyl groups in their structure.
  • The Flow Softening Behavior and Deformation Mechanism of AA7050 Aluminum Alloy

    pp. 2041-2047

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    DOI:10.2320/matertrans.MT-M2019114

    The flow softening behavior and deformation mechanism of AA7050 aluminium alloy are investigated though hot compressive tests using a Gleebe-1500 thermal simulator at strain rates of 0.01∼10 s−1 and temperatures of 300∼450°C. The results show that the maximal relative softening transforms from 350°C to 450°C with increasing strain rate. Based on the processing maps and microstructure characterizations, dynamic recovery, dynamic precipitation and coarsening may be responsible for the softening behavior of AA7050 aluminium alloy at 350°C and 0.01 s−1. With increasing strain rate and temperature, high level of the flow softening is associated with deformation heating, dynamic recovery and dynamic recrystallization. The superimposed processing map with various strains is established, indicating that the optimum hot workability domain is the temperature range of 390∼450°C and strain rate range of 0.1∼0.01 s−1.
  • Three-Dimensional Morphology of C15–Al2Ca Precipitates in a Mg–Al–Ca Alloy

    pp. 2048-2052

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    DOI:10.2320/matertrans.MT-M2019149

    The three-dimensional morphology and thickness of an Al2Ca Laves phase with a C15 crystal structure, which precipitated within the primary α-Mg grain of a Mg–5Al–1.5Ca alloy that had been over-aged at 523 K for 100 h, were investigated using high-resolution transmission electron microscopy. The C15–Al2Ca precipitate exhibits a hexagonal plate-like morphology, with a planar surface parallel to the (0001)α basal plane and the sides of the hexagonal plate parallel to the {1120} second columnar plane of the α matrix. A typical coffee bean contrast was clearly visible around the precipitate, which is indicative of the coherent precipitation of the C15–Al2Ca phase with respect to the α-Mg matrix. The thickness of the Al2Ca precipitate, which corresponds to six layers of the (111)C15 plane composed of Ca atoms, was evaluated as approximately 1.5 nm. This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 83 (2019) 193–197.
  • Positive Effects of the Ultrasound on Biosynthesis, Characteristics and Antibacterial Activity of Silver Nanoparticles Using Fortunella Japonica

    pp. 2053-2058

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    DOI:10.2320/matertrans.M2019065

    Silver nanoparticles (AgNPs) were biosynthesized using Fortunella Japonica extract as an reducing agent through a rapidly and ecofriendly assisted ultrasound method. Effect of ultrasound assistant on synthesis duration, properties and antibacterial activity of AgNPs against Escherichia coli, Bacillus subtilis and Bacillus cereus were reported. The suitable duration for AgNPs assisted ultrasound biosynthesis was 90 minutes that is 60 minutes shorter than stirring-assisted method. The obtained AgNPs with a face centred cubic structure were nearly spherical in shape and uniform in size distribution with the average nanosize of 11.6 nm, being smaller than the size of AgNPs prepared by stirring-assisted method. AgNPs sample exhibited effective antibacterial activity against all three bacteria with average diameter of inhibition zones of over 15 mm and minimum inhibitory concentration of 4.13 µg/mL. The results showed that the ultrasound assistant has positive effects such as shortening synthesis duration, reducing average particle size and increasing antibacterial activity of AgNPs.
  • Direct Deoxidation of Ti by Mg in MgCl2–HoCl3 Flux

    pp. 2059-2068

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    DOI:10.2320/matertrans.MT-M2019135

    A new method for removing the dissolved oxygen (O) in titanium (Ti) is developed, wherein magnesium chloride–holmium chloride (MgCl2–HoCl3) and Mg are used as a flux and a reducing agent, respectively. Through the thermodynamic assessment using a [ This part of text is omitted ] diagram as well as the experimental results, the deoxidation of Ti to a level below 1000 mass ppm O (and even 500 mass ppm O) via the formation reaction of holmium oxychloride (HoOCl), O (in Ti) + Mg + HoCl3 → HoOCl + MgCl2, was confirmed. The deoxidation limit decreases with the increase of the activity of HoCl3 in the MgCl2–HoCl3 flux. One advantage of this method is that the activity of the deoxidized product, aMgO, in the system can be effectively maintained at a low level by the formation of HoOCl. The E–pO2− diagram of the M–O–Cl system (M = Ho, Mg) constructed in this study indicates that the electrochemical deoxidation of Ti scraps in MgCl2–HoCl3 system will be more effective because the aMgO can be further decreased via the formation of HoOCl, and/or the electrochemical oxidation of oxide ions on the carbon anode. This new deoxidation technique using rare-earth-containing MgCl2 flux can be applied to the recycling of Ti scraps in the future.
  • Erratum

    pp. 2069-2069

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    DOI:10.2320/matertrans.ERM201901

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