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MATERIALS TRANSACTIONS Vol. 46 (2005), No. 4

ISIJ International
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ONLINE ISSN: 1347-5320
PRINT ISSN: 1345-9678
Publisher: The Japan Institute of Metals and Materials

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MATERIALS TRANSACTIONS Vol. 46 (2005), No. 4

Recent Advances in Domain Analysis

Yasukazu Murakami, Daisuke Shindo

pp. 743-755

DOI:

10.2320/matertrans.46.743

Abstract

Recent progress in domain analysis, which has been attained by the notable advancement of transmission electron microscopy (TEM), is reviewed by selecting certain topics related to displacive and magnetic phase transformations. Energy-filtered TEM, which separates the elastic scattering from the unwanted inelastic one, enabled us to explore the complex phenomena in the precursor state of martensitic transformations. In particular, the discovery of the peculiar nano-scaled domains in the parent phase has offered a novel picture on the lattice distortion in the precursor state and the subsequent nucleation process of a martensite. With respect to the analysis of magnetic domains, the authors have devised a technique that precisely evaluates the magnetization distribution based on electron holography. The method was applied to the complicated magnetic microstructure as observed near the Curie temperature and/or Néel temperature in hole-doped manganites. In addition to these topics, similarities of crystallographic domains between martensites and charge-ordered manganites, correlations between crystallographic microstructures (e.g., antiphase boundaries) and magnetic domains, and future challenging issues regarding domain analysis will also be mentioned.

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Recent Advances in Domain Analysis

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Hydrogen-Promoted Grain Boundary Embrittlement and Vacancy Activity in Metals: Insights from Ab Initio Total Energy Calculatons

Wen-Tong Geng, Arthur J. Freeman, Gregory B. Olson, Yoshitaka Tateyama, Takahisa Ohno

pp. 756-760

DOI:

10.2320/matertrans.46.756

Abstract

The rapid diffusion of H in metals permits an easy segregation to the grain boundary and an easy trapping to the vacancy. H-induced intergranular embrittlement in metals such as Fe and Ni is generally a result of coalition of segregated H and other embrittling impurities at the grain boundary. Ab initio total energy calculations based on the density functional theory have shown that H alone can also weaken the cohesion across the grain boundary. The stronger binding of H with a free surface than with a grain boundary, which results in grain boundary embrittlement according to the Rice–Wang theory, can be ascribed to its monovalency. New tensile experiments point to a H-enhanced vacancy contribution to the increased susceptibility of steel to H embrittlement. Ab initio density functional calculations on the energetics of interstitial H, vacancy, and H-monovacancy complexes (VacHn) in bcc Fe have shown that the predominant complex under ambient condition of H pressure is VacH2, not VacH6 as previously suggested by effective-medium theory calculations. The linear structure of VacH2 clusters, a consequence of repulsion between negatively charged H atoms, facilitates the formation of linear and tabular vacancy clusters and such anisotropic clusters may lead to void or crack nucleation on the cleavage planes. On the other hand, the H-induced increase of vacancy cluster formation energy is a support of the experimentally observed enhancement of dislocation mobility in the presence of H, which, through the mechanism of H-enhanced localized plasticity, makes fracture easier.

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Hydrogen-Promoted Grain Boundary Embrittlement and Vacancy Activity in Metals: Insights from Ab Initio Total Energy Calculatons

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Composition-Dependent Thermoelectric Properties of (PbTe)100−x(Bi2Te3-Sb2Te3)x (0.1≤x≤5)

Pinwen Zhu, Yoshio Imai, Yukihiro Isoda, Yoshikazi Shinohara, Xiaopeng Jia, Guangtian Zou

pp. 761-764

DOI:

10.2320/matertrans.46.761

Abstract

We report room-temperature thermoelectric properties of polycrystalline (PbTe)100−x(Bi2Te3-Sb2Te3)x (0.1≤x≤5). At x≤0.25, with a decrease of x, the electrical resistivity increases dramatically whereas the Seebeck coefficient and the thermal conductivity nearly keep constant. At x≥0.5, the electrical conductivity, the thermal conductivity and the Seebeck coefficient in absolute value slightly decrease with increasing x. It is found that, the thermal conductivity of the PbTe sample with Bi2Te3 and Sb2Te3 as the sources of dopants is much smaller than that of doping with other dopants at the same carrier concentration. With an increase of x, the figure of merit, Z, which is calculated from the measured quantities, increases at first and then decreases and shows a maximum value of 8.97×10−4 K−1 at x=0.2. This value is several times higher than that of small grain-size PbTe containing other dopants.

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Composition-Dependent Thermoelectric Properties of (PbTe)100−x(Bi2Te3-Sb2Te3)x (0.1≤x≤5)

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Polymeric Co-C60 Compound Phase Evolved in Atomistically Mixed Thin Films

Seiji Sakai, Hiroshi Naramoto, Vasily Lavrentiev, Kazumasa Narumi, Masaki Maekawa, Atsuo Kawasuso, Tsuyoshi Yaita, Yuji Baba

pp. 765-768

DOI:

10.2320/matertrans.46.765

Abstract

A systematic study of the atom-level mixtures between Co and C60 has made possible to find a new C60-based compound phase, CoxC60 (x≤5). It is confirmed with the EXAFS analysis and the positron annihilation study that the polymeric structure is developed three-dimensionally by bridging two C60 molecules with the covalently bonded Co atom.

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Polymeric Co-C60 Compound Phase Evolved in Atomistically Mixed Thin Films

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Precipitation of Copper Sulfide in Ultra Low Carbon Steel Containing Residual Level of Copper

Yasuhide Ishiguro, Kaoru Sato, Takashi Murayama

pp. 769-778

DOI:

10.2320/matertrans.46.769

Abstract

The influence of residual copper (\\fallingdotseq0.01 mass%) on sulfide precipitation in ultra low carbon steel was studied with microanalysis using Field Emission-Transmission Electron Microscopy (FE-TEM), X-Ray Diffraction (XRD) of the extracted precipitates and quantitative chemical analysis of extraction residue.
A small amount of copper (\\fallingdotseq0.01 mass%) plays an important role in sulfide formation in steel, as does manganese. Two types of sulfides were found in ultra low carbon steel with residual copper: (1) MnS covered with copper sulfide (Cu–S) and (2) “free-standing” Cu–S, with sizes of φ100∼300 nm and less than about φ40 nm, respectively. The atomic ratio of Cu/S was determined to be 1.8±0.3 by Energy Dispersive X-ray spectroscopy (EDX), and the phase was determined to be Cu8S5 from XRD of the extracted precipitates.
Quantitative chemical analysis revealed that almost all sulfur (71 mass ppm) was precipitated as sulfides in steel containing 73 ppm of sulfur. The concentration of sulfur as Cu8S5 was 24 ppm, which accounts for about 35% of the total quantity of sulfur. This indicates that the formation of copper sulfide has to be taken into account in precise analysis of sulfides in low Mn and S systems (Mn\\fallingdotseq0.20%, S\\fallingdotseq0.007%).
The precipitation of Cu–S in water-quenched (WQ) samples and the morphologies of MnS-cored BN & Cu–S-covered MnS in hot-rolled and WQ samples suggest that Cu–S is formed at temperatures between “below 750°C” and 620°C and is also precipitated even during a short time in water-quenching.

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Precipitation of Copper Sulfide in Ultra Low Carbon Steel Containing Residual Level of Copper

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Microstructural Changes at the Initial Stage of Precipitation in an Aluminum–Silicon Alloy

Keiyu Nakagawa, Teruto Kanadani, Laurence Anthony, Hatsujiro Hashimoto

pp. 779-783

DOI:

10.2320/matertrans.46.779

Abstract

In this study, we use a transmission electron microscope (TEM) to observe the microstructure of an Al–Si alloy from immediately after quenching to the initial stage of precipitation. Results reveal the presence of patches immediately after quenching that are thought to be Si clusters, and these are coherent with the {111}matrix. At the initial stage of aging, Si crystals with a plate-like structure are observed, contradicting the round shape previously described. We propose that the Si clusters that appear immediately after quenching act as nuclei for the plate-like Si crystals. As the aging time increases, these increase in length and thickness, becoming thick plate-like Si phase precipitates.

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Microstructural Changes at the Initial Stage of Precipitation in an Aluminum–Silicon Alloy

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Precipitation of Carbonitrides and Their Strengthening upon Non-quench Aging for Micro-alloyed Acicular Ferrite Pipeline Steels

Ming-Chun Zhao, Toshihiro Hanamura, Hai Qiu, Ke Yang

pp. 784-789

DOI:

10.2320/matertrans.46.784

Abstract

A non-quench aging (reheating hot-rolled plates to 600°C for isothermal aging), different from the conventional quench aging (quench followed by temper), was carried out to improve the strength level for acicular ferrite (AF) pipeline steels, while their impact energies and ductilities did not decrease obviously, and the Charpy impact fracture appearances were similar between the as-rolled and the aged samples, showing the ductile dimple rupture over the whole fracture surface. The strengthening was ascribed to the further additional precipitation of microalloyed carbonitrides, and the little degradation of ductile and toughness is believed to relate to both the tempering of martensite in martensite/austenite (M/A) islands and the improvement of microstructural uniformity upon the non-quench aging. The nucleation and strengthening of the carbonitrides in the current non-quench aging were hypothetically calculated with the present mathematical modes and hypothesis based on the thermodynamic consideration and Orowan looping mechanism, which was somewhat consistent with the present experiment, indicating that the strengthening mechanism of the present AF pipeline steels by the current non-quench aging evidently resulted from the additional precipitation of carbonitrides in ferrite region of approximately 600°C.

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Precipitation of Carbonitrides and Their Strengthening upon Non-quench Aging for Micro-alloyed Acicular Ferrite Pipeline Steels

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Stress-induced Martensitic Transformation and Superelasticity of Alloys: Experiment and Theory

Victor A. L’vov, Alexei A. Rudenko, Volodymyr A. Chernenko, Eduard Cesari, Jaume Pons, Takeshi Kanomata

pp. 790-797

DOI:

10.2320/matertrans.46.790

Abstract

The superelasticity of alloys undergoing a stress-induced martensitic transformation is studied in both experimental and theoretical way. Experimental stress–strain dependencies illustrating the different types of superelastic behavior are taken from Ni–Mn–Ga alloys typifying thermoelastic martensites and then modeled theoretically using a statistical approach to describe the growth of stress-induced martensite in the matrix of parent (austenitic) phase. A good agreement between the experimental and theoretical results is achieved whereby a physical interpretation of the different types of stress–strain dependencies is made and a quantitative evaluation of the main parameters controlling transformational behavior of the alloys is carried out.

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Stress-induced Martensitic Transformation and Superelasticity of Alloys: Experiment and Theory

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Retraction:Nanoindentation Characteristics of In-Situ Formed Cu–Hf–Ti–Ag–Ta Bulk Metallic Glass Composites

Zan Bian, Hidemi Kato, Akihisa Inoue

pp. 798-804

DOI:

10.2320/matertrans.46.798

Abstract

This article was retracted. See the Notification.

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Retraction:Nanoindentation Characteristics of In-Situ Formed Cu–Hf–Ti–Ag–Ta Bulk Metallic Glass Composites

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The Effect of Strain Rate on the Impact Behaviour of Fe–2 mass% Ni Sintered Alloy

Woei-Shyan Lee, Jan-Kung Chou

pp. 805-811

DOI:

10.2320/matertrans.46.805

Abstract

The impact behaviour of Fe–2 mass% Ni sintered alloy as a function of strain rate has been studied using a material testing system at strain rate of 10−3, 10−2 and 10−1 s−1 and a split Hopkinson bar at strain rates ranging from 2.5×103 s−1 to 6.8×103 s−1. The mechanical properties of the sintered alloy, including its impact strength, rate of work hardening and strain rate sensitivity are found to be significantly influenced by the strain rate at which deformation takes place. A constitutive law based on the Khan–Huang–Liang model is applied to predict the rate-dependent plastic flow behaviour of the sintered alloy. The model predictions are found to be in good agreement with the observed experimental response. Even under heavy deformation conditions, none of the specimens were observed to fracture. Microstructural observations reveal that the grain size of the deformed specimen decreases as the strain rate increases. The reduction of the grain size can lead to an increase of the flow stress due to the enhancement of the grain boundary area.

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The Effect of Strain Rate on the Impact Behaviour of Fe–2 mass% Ni Sintered Alloy

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High Temperature Phase Relations in FeOX(X=1 and 1.33)–CaO–SiO2 Systems under Various Oxygen Partial Pressure

Hector M. Henao, Florian Kongoli, Kimio Itagaki

pp. 812-819

DOI:

10.2320/matertrans.46.812

Abstract

As a fundamental study of slags in metallurgical processes, liquidus lines in the FeO(wüstite)–CaO–SiO2 and Fe3O4(spinel)–CaO–SiO2 systems were determined at temperatures between 1573 and 1673 K and partial pressures of oxygen between 10−4 and 36 Pa using a quenching method with a crucible of platinum foil. It was found that the two phases (solid wüstite and liquid) region in the FeO–CaO–SiO2 system (1573 K) increased remarkably by changing the partial pressure of oxygen from 10−4 to 10−3 Pa. It was also found that the two phases (solid spinel and liquid) region in the Fe3O4–CaO–SiO2 system (1573 K) at PO2 of 10−2 Pa was a little larger than that in the FeO–CaO–SiO2 system at PO2 of 10−3 Pa but it was extended considerably when the oxygen pressure was increased to 10−1 Pa. However, the two liquids region did no change too much in the range of oxygen pressure above 10−1 Pa even though the liquidus line became almost parallel to the Fe3O4–CaO axis in the ternary diagram when the oxygen pressure was increased. Furthermore, it was found that the two phase region in the Fe3O4–CaO–SiO2 system at PO2 of 10−1 Pa decreased remarkably with increasing temperature. On the basis of the obtained and reported data, the homogeneous liquid regions in the FeOX–CaO–SiO2 systems (1573 K) at PO2 of 10−3 Pa, 10−1 Pa and in air were compared.

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High Temperature Phase Relations in FeOX(X=1 and 1.33)–CaO–SiO2 Systems under Various Oxygen Partial Pressure

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Surface Modification of Gamma-Titanium Aluminides with L12-Type Tri-Aluminides by Pack Cementation

Takashi Kimura, Tohru Awane, Ke Wai Gao, Lijie Qiao, Kenki Hashimoto

pp. 820-826

DOI:

10.2320/matertrans.46.820

Abstract

The surface of an L10-type TiAl base alloy has been modified by coating it with an L12-phase (AlMn)3Ti(V) using a pack-cementation method in a temperature range of 1273–1473 K and with a treatment time of 18–72 ks. The structural morphology and composition distribution in the coat and substrate were analyzed by EPMA and XRD to examine the factors that control the thickness of the coat. Two-phase equilibrium was confirmed between the L10-phase substrate and the L12-phase coat at each temperature tested. Aluminum particulates, 1–2 μm in diameter, were observed inside the coat and were confirmed to be related to the experimental condition and to the increase in coat thickness. The coat is 35 μm thick at temperature range of 1373–1473 K, while at the temperature range of 1273–1323 K the thickness is 11–12 μm. The temperature dependence of coating layer is different from that was reported previously. The results are then discussed from facts basically revealed in this experiment and/or based on information about the phase equilibrium between L10 and L12.

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Surface Modification of Gamma-Titanium Aluminides with L12-Type Tri-Aluminides by Pack Cementation

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Formation of SiO and Related Si-Based Materials Through Carbothermic Reduction of Silica-Containing Slag

S. V. Komarov, D. V. Kuznetsov, V. V. Levina, M. Hirasawa

pp. 827-834

DOI:

10.2320/matertrans.46.827

Abstract

Silicon based nanoparticulates of various composition and morphology have been produced by smelting reduction method which includes a carbothermic reduction of SiO2–Al2O3–CaO starting materials to SiO vapor at 2073 K and transfer of the vapor with a carrier-gas to cooler surfaces inside the experimental reactor where the nanoparticulates were deposited. The chemical composition of the starting materials was matched to the basic composition of silica-rich coal ash which is considered as a potential source of Si in this study. Special emphasis was placed upon examining the degree of SiO2 reduction from starting materials, purity and morphology of the as-obtained nanomaterials. It is shown that up to 20% of Si can be converted from the silica-based melt into rounded nanoparticles, nanoparticle chains and nanowires containing Si,O and C in variable proportions depending on deposition temperature and gas flow conditions. The diameter of nanoparticulates was estimated to be in the range of 20∼100 nm. The nanoparticles and chains were found to be deposited at lower temperature locations (293∼1320 K) while the nanowires were obtained at higher temperatures (1320∼1570 K). There was a tendency for an increase in Si concentration in order of nanowires, nanoparticles chains and nanoparticles. The carbon concentration, on the contrary, was much higher in nanowires as compared to that in nanoparticles. Although the degree of SiO2 reduction from silica-containing melt to SiO vapor is limited because a part of SiO2 reacts with C producing SiC, its good controllability, high productivity and possibility for processing cheap starting materials make the smelting reduction a very attractive technique for production of silicon based nanostructured materials.

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Formation of SiO and Related Si-Based Materials Through Carbothermic Reduction of Silica-Containing Slag

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Lap Joint of A5083 Aluminum Alloy and SS400 Steel by Friction Stir Welding

Kittipong Kimapong, Takehiko Watanabe

pp. 835-841

DOI:

10.2320/matertrans.46.835

Abstract

A lap joint of A5083 aluminum alloy and SS400 steel was produced by Friction Stir Welding (FSW) using the various process parameters such as rotational speed, traverse speed and pin depth. The variations of welding parameters produced various characteristic interfaces and had conspicuous influences on the joint properties. Increasing the rotational speed decreased the shear load of the joint due to producing a thick FeAl3 intermetallic compound (IMC) at the interface. When traverse speed increased, the shear load increased because IMC thickness at the interface decreased, however, when the speed was so high, an incomplete interface was formed. Increasing the pin depth produced a thick FeAl3 IMC phase and an incomplete interface that directly deteriorated the shear load of the joint.

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Lap Joint of A5083 Aluminum Alloy and SS400 Steel by Friction Stir Welding

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Growth Rate of Reaction Layer between SiO2 and Molten Al above 1473 K

Noboru Yoshikawa, Akira Hattori, Shoji Taniguchi

pp. 842-845

DOI:

10.2320/matertrans.46.842

Abstract

Growth rate of reaction layer between molten Al and SiO2 was measured in a temperature range between 1473 K and 1723 K, and was compared with the reported rates between molten Al and mullite. A maximum growth rate was obtained at 1623 K, above which the rate decreased and the reaction halted with formation of an initial thin layer. The same phenomenon had been reported for the reaction between Al and mullite, however, temperatures of the maximum reaction rate were lower than the present (SiO2) case.
According to the observation of microstructures in the reacted layer, gradient of Si concentration became larger as an increase of the reaction temperature up to 1573 K, and closure of the Al phase occurred at 1723 K. Growth rate of the reaction layer was large at 1573 K, however, the residual Si concentration in the reacted layer was also large, indicating that the growth rate is not determined by the rate of Si removal.
Relationship between the microstructure and the growth rate was discussed in comparison with the mullite reaction cases, not only considering the competition between the rates of the reaction and the mass transfer of Al and Si in the reacted layer, but also considering the Al2O3 network structure change.

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Growth Rate of Reaction Layer between SiO2 and Molten Al above 1473 K

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The Effect of Long-Term Isothermal Aging on Dynamic Fracture Toughness of Type 17-4 PH SS at 350°C

Wang Jun, Zou Hong, Wu Xiao-yong, Li Cong, Qiu Shao-yu, Shen Bao-luo

pp. 846-851

DOI:

10.2320/matertrans.46.846

Abstract

The change of impact performance with the extension of aging time on the type 17-4 PH stainless steel at service temperature (350°C) is studied in this paper, the alternation of dynamic fracture toughness and fractography of the type stainless steel for various holding time at this temperature are also researched by instrumental impact test and scanning electron microscope. All results indicate that the plastic deformation energy (EPL), tearing energy (ETE), absorbed-in-fracture energy (Et) and dynamic fracture toughness (KId) of this type alloy are all monotonically decreased with the continuation of time at 350°C,the characteristic of the decline is the KId fall is fast at the forepart of aging test and then descend is slowness, it says the toughness of the alloy is degraded and the degradation was mainly taken place at the early stage of aging test. The fracture mode of this alloy changes from dimple fracture into cleavage fracture and intergranular rapture with the extension of aging time. Those illuminate that the fracture toughness of this alloy has degraded with the extension of aging time at 350°C.

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The Effect of Long-Term Isothermal Aging on Dynamic Fracture Toughness of Type 17-4 PH SS at 350°C

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Shape Memory Behavior of Ti–22Nb–(0.5–2.0)O(at%) Biomedical Alloys

Jae Il Kim, Hee Young Kim, Hideki Hosoda, Shuichi Miyazaki

pp. 852-857

DOI:

10.2320/matertrans.46.852

Abstract

In order to develop new biomedical shape memory alloys, mechanical properties and shape memory behavior of Ti–22Nb–(0.5–2.0)O(at%) alloys were investigated by tensile tests at various temperatures and X-ray diffraction measurement. The Ti–Nb–O alloys were fabricated by an arc melting method and then homogenized at 1273 K for 7.2 ks. The ingots were cold-rolled with a reduction up to 98.5% in thickness and solution treated at 1173 K for 1.8 ks. The fracture stress of both as-rolled specimen and solution treated specimen increased with increasing oxygen content. A fracture stress of 1.37 GPa was obtained in the as-rolled Ti–22Nb–2O alloy. The martensitic transformation temperature decreased by 160 K per 1 at% increase of oxygen content. Superior shape memory effect and superelastic behavior were observed at room temperature in the Ti–22Nb–(0.5–1.5)O alloys. The addition of oxygen stabilized superelastic behavior of Ti–Nb alloys by increasing the critical stress for permanent deformation. The maximum shape recovery strain of 4.0% and the critical stress of 900 MPa for permanent deformation were obtained in the Ti–22Nb–0.5O and Ti–22Nb–1.5O alloys, respectively.

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Shape Memory Behavior of Ti–22Nb–(0.5–2.0)O(at%) Biomedical Alloys

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Effect of Tantalum on Corrosion Resistance of Ni–Nb(–Ta)–Ti–Zr Glassy Alloys at High Temperature

Chunling Qin, Wei Zhang, Hiromichi Nakata, Hisamichi Kimura, Katsuhiko Asami, Akihisa Inoue

pp. 858-862

DOI:

10.2320/matertrans.46.858

Abstract

Ni60Nb20−xTaxTi15Zr5 (x=0, 5, 15 and 20 at%) glassy alloys with high thermal stability and high corrosion resistance have been synthesized by the melt spinning technique. The substitution of tantalum for a portion of niobium in the Ni60Nb20Ti15Zr5 glassy alloy causes a significant increase in the supercooled liquid region (ΔTx=Tx−Tg) from 52 K at 0 at% Ta to 67 K at 20 at% Ta accompanied by an increase in Tg and Tx. The glassy alloys are spontaneously passivated in a wide passive region and at low passive current densities of the order of 10−2 A·m−2 in pH 2 H2SO4 and pH 2 H2SO4 + 500 ppm NaCl solutions at a high temperature of 353 K. On the other hand, in pH 2 H2SO4 + 500 ppm NaF solution at 353 K, the alloys without Ta and with 5 at% Ta suffer general corrosion, while the passive current densities decrease by about two orders of magnitude with the addition of 15–20 at% Ta to the Ni–Nb–Ti–Zr glassy alloy. It indicates that the effect of the addition of tantalum is more pronounced for improving the corrosion resistance in fluoride containing solutions than the other two types of solutions.

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Effect of Tantalum on Corrosion Resistance of Ni–Nb(–Ta)–Ti–Zr Glassy Alloys at High Temperature

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Surface Modification of SUS304 Stainless Steel Using Carbon Push-Ahead Effect by Low Temperature Plasma Nitriding

Masato Tsujikawa, Daisuke Yoshida, Naohiko Yamauchi, Nobuhiro Ueda, Takumi Sone

pp. 863-868

DOI:

10.2320/matertrans.46.863

Abstract

Low temperature nitriding can harden the steel surface. However, the consequent thickness is limited because it depends on the formation of non-equilibrium phase at low temperature diffusion treatment. The hardness drops abruptly at the interface between the treated layer and the substrate as a result of nitriding. In this paper, duplex surface layers produced by combinations of carburizing and nitriding were developed using the carbon push-ahead effect of nitriding using relatively low-temperature plasma treatment. The location of the low-temperature carburized layer was pushed ahead following nitriding to form a support layer for a hard nitrided surface layer. The treatment combination was optimized for a thicker hard layer with good corrosion resistance.

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Surface Modification of SUS304 Stainless Steel Using Carbon Push-Ahead Effect by Low Temperature Plasma Nitriding

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Fatigue Strength of Alloy 718 at High-Temperatures

Kazuo Kobayashi, Koji Yamaguchi

pp. 869-871

DOI:

10.2320/matertrans.46.869

Abstract

The high-temperature fatigue properties of an Alloy 718 were investigated up to 107 cycles. Combined with the data for base metals and welded joints, the relationship between stress amplitude and number of cycles to failure (fatigue life) was characterized by two steps in the S–N curve. The fracture mechanisms were different between high and low stress levels. At high stress levels, fatigue fractures initiated from the specimen surface, whereas at low stress levels, the initiation was inside the surface. The internal fracture initiation sites were crystallographic flat facets of austenitic grains.

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Fatigue Strength of Alloy 718 at High-Temperatures

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Effect of Film Thickness on Structural and Electrical Properties of Sputter-Deposited Nickel Oxide Films

Hao-Long Chen, Yang-Ming Lu, Weng-Sing Hwang

pp. 872-879

DOI:

10.2320/matertrans.46.872

Abstract

This work studies dependences of resistivity, carrier concentration, mobility and structural properties on the thickness of nickel oxide (NiO) films deposited onto glass substrates by RF magnetron sputtering in a pure oxygen atmosphere at an RF power 200 W. The electrical properties were measured by Hall Effect measurements. The X-ray diffraction (XRD) and transmission electron microscope (TEM) analyses of nickel oxide films indicates that these films are polycrystalline when the samples are prepared with an unheated substrate (Ts=303 K) and using a substrate at a higher substrate temperature (Ts=673 K). The thickness of the films varied in the range from 50 to 300 nm. The variations of the microstructural parameters, such as crystallite size (L), dislocation density (δ), stacking fault probability (α), strain (ε) and density (D), with film thickness and substrate temperature were investigated. The results show the crystallite sizes increaser as the thickness of the film increases. The variation of the dislocation density and the stacking fault probabilities and strain decrease as the thickness increases. The resistivity of NiO film is increased with an increase in film thickness, which is related to the decrease of carrier concentration with film thickness. The NiO film with a thickness of 200 nm has a minimum resistivity of 0.69×10−2 Ωm when deposited at substrate temperature of 303 K.

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Effect of Film Thickness on Structural and Electrical Properties of Sputter-Deposited Nickel Oxide Films

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Influence of Excess Si on the Morphology and Thermal Stability of Metastable Precipitates Formed in an Al–Mg–Si Alloy

Koichiro Fukui, Mahoto Takeda, Takao Endo

pp. 880-884

DOI:

10.2320/matertrans.46.880

Abstract

The influence of excess Si on the precipitation behaviour in an Al–Mg–Si alloy was studied by means of Vickers microhardness tests, high-resolution transmission electron microscopy (HRTEM) and differential scanning calorimetry (DSC). The metastable-phase precipitates formed during isothermal ageing up to the time to peak hardness were investigated from the viewpoints of the morphology, the density of precipitates and the change of the exothermic heat caused by the formation of precipitates in the DSC measurements. The changes in the density of precipitates and the exothermic heat with excess Si at peak ageing time showed a similar tendency to that of the peak hardness curves. Four exothermic peaks of meatastable-phase precipitates appeared in the DSC curves of the alloy specimens isothermally aged for a short time. It was revealed that the precipitates named type-1, -2 and -3 in this work, correspond to the exothermic peaks (1), (2) and (3), respectively.

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Influence of Excess Si on the Morphology and Thermal Stability of Metastable Precipitates Formed in an Al–Mg–Si Alloy

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Transmission Electron Microscopic Study of Tetracalcium Phosphate Surface-Treated in Diammonium Hydrogen Phosphate Solution

I-Chang Wang, Jiin-Huey Chern Lin, Chien-Ping Ju

pp. 885-890

DOI:

10.2320/matertrans.46.885

Abstract

The present study investigates the changes in microstructure and microchemistry (particularly Ca/P ratio) during whisker formation on the surface of a monolithic TTCP powder in a basic phosphoric acid solution. The XRD results indicate that when TTCP powder was treated for 10 min or less in (NH4)2HPO4 solution, the monolithic TTCP phase remains unchanged. When treated for 30 min, apatite whiskers appear, which continue to grow with treating time. When treated for 24 h, the apatite phase becomes dominant. TEM results show that on the surface of 1 min-treated TTCP are observed a large amount of globular-shaped fine particles which are substantially amorphous with an average Ca/P ratio of about 1.2. When treated for 10 min, whiskers primarily having a crystal structure of TTCP with an average Ca/P ratio of about 2.1 are observed to grow radially from TTCP surface. With time the whiskers continue to grow in length and width. The whiskers on 24 h-treated surface become Ca-rich apatite with an average Ca/P ratio of about 1.8. Both TTCP and apatite whiskers are essentially non-stoichiometric in chemistry.

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Transmission Electron Microscopic Study of Tetracalcium Phosphate Surface-Treated in Diammonium Hydrogen Phosphate Solution

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Formation, Crystallized Structure and Magnetic Properties of Fe–Pt–B Amorphous Alloys

Akihisa Inoue, Wei Zhang, Takako Tsurui, Dmitri V. Louzguine

pp. 891-894

DOI:

10.2320/matertrans.46.891

Abstract

An amorphous phase was formed for (Fe0.75Pt0.25)100−xBx (x=25 and 30) alloys by melt-spinning. Their crystallization temperature were 671 and 667 K, respectively. The amorphous alloys exhibited good soft magnetic properties. The crystallized structure consisted of γ1-FePt (fct), Fe2B and γ-FePt (fcc) phases, and their average grain sizes were about 15 nm after annealing at 785 K for 900 s. The remanence (Br), reduced remanence (Mr⁄Ms), coercivity (iHc) and maximum energy product (BH)max were 0.96 T, 0.83, 340 kA/m and 102.2 kJ/m3, respectively, for the 25 at%B alloy, and 0.91 T, 0.82, 368 kA/m and 96.5 kJ/m3, respectively, for the 30 at%B alloy. The reversible demagnetization loop behavior was also confirmed. Consequently, the good hard magnetic properties are interpreted as resulting from exchange magnetic coupling among nanoscale hard γ1-FePt and soft Fe2B and γ-FePt magnetic phases.

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Formation, Crystallized Structure and Magnetic Properties of Fe–Pt–B Amorphous Alloys

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