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Volume 64, Issue 11
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Kouichi Maruyama, Takashi Matsuo, Masaharu Yamaguchi2000 Volume 64 Issue 11 Pages 957
Published: 2000
Released on J-STAGE: April 24, 2008
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Akito Nitta2000 Volume 64 Issue 11 Pages 958-964
Published: 2000
Released on J-STAGE: April 24, 2008
JOURNAL FREE ACCESSIncreased turbine inlet temperature (TIT) leads to improvement in the thermal efficiency of gas turbines. At present, Japanese national projects for the development of high-temperature gas turbines with a TIT of 1700°C are being pursued from the perspective of global environmental preservation, so as to reduce the emission of greenhouse gases such as carbon dioxide. One of the most important aspects of these projects is the development of materials for application in very high temperature components such as blades and vanes in gas turbines. This paper describes the current situation and future plans with respect to development of such materials by means of a review of the status of Japanese national projects.View full abstractDownload PDF (944K) -
Masaharu Yamaguchi2000 Volume 64 Issue 11 Pages 965-970
Published: 2000
Released on J-STAGE: April 24, 2008
JOURNAL FREE ACCESSA great deal of fundamental and developmental research has been made on high-temperature structural intermetallics such as aluminides formed with either titanium, nickel or iron. Of these high-temperature intermetallics, TiAl-base alloys with great potential in both aerospace and automotive applications have been attracting particular attention. Recently TiAl turbocharger wheels have finally started being used for turbochargers for commercial passenger cars of a special type. This paper summarizes the current status of the research and development of these high-temperature intermetallics and provides a perspective on what directions we should go for the future research and development of high-temperature intermetallics.View full abstractDownload PDF (610K) -
Toshimitsu Tetsui2000 Volume 64 Issue 11 Pages 971-976
Published: 2000
Released on J-STAGE: April 24, 2008
JOURNAL FREE ACCESSThis report begins by indicating the position of TiAl as a high-temperature structural material in comparison with metallic materials and ceramics. The primary advantage of TiAl is that it exhibits excellent high-temperature strength for a lightweight material, while its chief disadvantage is brittleness. However, its endurance temperature is lower than for ceramics or for single crystal superalloys, and currently only small components can be readily fabricated. Accordingly, likely suitable applications for TiAl are those that presently employ conventionally cast or forged superalloys, and for which specific strength becomes applied stress. In cases where weight reduction directly improved equipment performance, the disadvantage of TiAl brittleness will be outweighed. Given the foregoing properties, research on practical applications for TiAl has centered on turbochargers for passenger vehicles, exhaust valves for passenger vehicle engines, turbine blades for jet engines, and structural component for aircraft. With respect to the former two applications that in which efforts at practical development have been most active, this report summarizes research results in terms of materials development, manufacturing technology development, and empirical testing. Finally, a development example undertaken by the authors with respect to passenger vehicle turbochargers is also presented. This project involved the development of high-performance TiAl alloy and the requisite manufacturing technology, and concluded in successful practical application following numerous empirical tests. The resulting high-response TiAl turbochargers are used in Mitsubishi Motors Corp. Lancer Evolution Series automobiles, marketed since January 1999.View full abstractDownload PDF (1135K) -
Yoshiharu Waku2000 Volume 64 Issue 11 Pages 977-984
Published: 2000
Released on J-STAGE: April 24, 2008
JOURNAL FREE ACCESSMGCs(Melt Growth Composites) with a new microstructure such as Al2O3/Y3Al5O12(YAG) system, Al2O3/Er3Al5O12(EAG) system and Al2O3/GdAlO3(GAP) system have been fabricated by unidirectional solidification. The MGCs have a microstructure, in which single crystal Al2O3 and single crystal complex oxide compound (YAG or EAG or GAP) are continuously connected and finely entangled in three-dimensions without grain boundaries. The MGCs are thermally stable and have the following properties: (1) The Al2O3/GAP MGC shows substantial plastic deformation at 1873 K with a flexural yield stress of about 690 MPa, where the plastic deformation occured by dislocation motion in each phase. (2) In the case of Al2O3/YAG MGCs, the flexural strength at room temperature can be maintained almost up to the melring point, (3) the compressive creep strain of Al2O3/YAG MGCs at 1873 K and a creep stress of 200 MPa is around 1/900 times lower than that of sintered composites of the same composition, and (4) Al2O3/YAG MGCs and Al2O3/EAG MGCs show neither weigth gain nor grain growth, even upon heating at 1973 K in an air atmosphere for 1000 hours. Consequently, these MGCs can be considered for several useful application such as new aero gas turbines and power generation systems with non-cooled turbine blabes at very high temperatures.View full abstractDownload PDF (2115K) -
Tamaki Shibayama, Heishichiro Takahashi2000 Volume 64 Issue 11 Pages 985-991
Published: 2000
Released on J-STAGE: April 24, 2008
JOURNAL FREE ACCESSSiC fiber-reinforced SiC matrix composites (SiC/SiC composites) are attractive structural materials for next-generation high-temperature applications, because the density of such a composite is only one third of the conventional heat-resistance one, and it has superior oxidation resistance due to the formation of SiO2 on the surface in the air at elevated temperatures. A research and development activity to realize high efficiency gas turbine materials and aerospace materials replaced C/C composites has been initiated. Continuous fiber-reinforced ceramic composites (CFCC), including SiC/SiC composites, should be showed to prevent crack propagation due to deflection and scattering of micro cracks initiated in the matrix around the interphase. As the crack-opening displacement (COD) is increased, the reinforced fibers should bridge the matrix and they will break after pull out. The apparent fracture resistance can be increased. As a result of this sequence of phenomena, the CFCC shows the better mechanical properties than that of the monolithic ceramics. Therefore, interpretation of the fracture mechanism at the interface between the fibers and the matrix is essential to improve mechanical properties. Currently, many efforts are being made to develop the advanced SiC/SiC composites from “state-of-the-art” materials by a number of research organizations, including universities, national institutes and industries in the world. In this paper, the current status of research and development of SiC/SiC composites is described together with our results of microstructure analysis using advanced microscopy, and future prospects are also discussed.View full abstractDownload PDF (1355K) -
Sadao Nishikiori, Shoju Masaki2000 Volume 64 Issue 11 Pages 992-998
Published: 2000
Released on J-STAGE: April 24, 2008
JOURNAL FREE ACCESSAdvanced materials are very important to improve the performance of aircraft engines. Superalloys and titanium alloys have been developed to raise the operating temperature and reduce the weight of engine components. However, engines in the 21st century will require advanced materials with higher heat-resistance, superior mechanical properties and reduced weight. Intermetallic compounds and composites have been considered as alternate materials, replacing the existing Ni-based superalloys. In particular, Gamma-TiAl and Ceramic Matrix Composite are expected to be revolutionary materials. In order for both advanced materials, TiAl and CMC, to be widely employed in structural use, it is important to understand the effects of metallurgical features on various properties. We have focused on the development of these advanced materials. In addition, with an industrial view to applications, problems are discussed through our experience in engine tests.View full abstractDownload PDF (1476K) -
Hirofumi Inoue, Takeshi Takahashi, Takayuki Takasugi2000 Volume 64 Issue 11 Pages 999-1006
Published: 2000
Released on J-STAGE: April 24, 2008
JOURNAL FREE ACCESSTensile properties of hot rolled binary TiAl-based alloys were investigated in conjunction with microstructures and textures. The textures formed by hot rolling were analyzed mainly in the γ-phase with a large volume fraction. Tensile tests at room temperature and 1073 K were carried out using specimens with angles of 0° , 45° and 90° to the rolling direction. Microstructures and textures after hot rolling significantly depended on the alloy composition. Hot rolled Ti-48 mol%Al alloys consisting of equiaxed γ and lamellar grains had a strong {010)⟨100] orientation similar to the cube orientation in fcc metals and a near-β fiber texture similar to rolling texture in fcc. Yield stress at 1073 K showed higher values at the 90° direction than at the 0° direction, while yield stress at room temperature exhibited an inverse anisotropy. Such anisotropy can be explained qualitatively by the Taylor factor calculated from the γ-phase texture. On the other hand, microstructures of hot rolled Ti-46 mol%Al alloys were composed of elongated coarse lamellar grains accompanied with fine equiaxed γ grains. Most of the γ⁄α2 lamellar boundaries were inclined in an angle range of 0° to 30° from the rolling plane. The γ-phase basically had the preferred orientations on {320) and {032) fibers, probably corresponding to lamellar orientations. Yield stress at room temperature showed the lowest values at the 45° direction and the highest values at the 90° direction. This anisotropic strength can be explained based on the average tilt angles of lamellae to the tensile axis.View full abstractDownload PDF (1067K) -
Keizo Hashimoto, Yukio Shimada, Naofumi Akikawa2000 Volume 64 Issue 11 Pages 1007-1011
Published: 2000
Released on J-STAGE: April 24, 2008
JOURNAL FREE ACCESSTensile fatigue tests at both ambient and high temperatures have been conducted using Nb doped titanium aluminides, and the results have been compared with that of IN718. Fatigue behavior at ambient temperature shows no clear limit stress of fatigue life, which varies from short to more than 105 cycles within the very narrow range of stress amplitude. TEM observations reveal that dislocation structures of fatigued specimens show an inhomogeneous dislocation density at the adjacent grains. In the case of high temperature fatigue tests, the fatigue life increase with decreasing the stress amplitude, the limit stress amplitude of the fatigue life has been determined to be 400 MPa. After the high temperature fatigue tests, TEM observations reveal that specimens show relatively stable dislocation structures which are composed of the dislocation wall, di-pole and polygonization inside the grains.View full abstractDownload PDF (824K) -
Yoshinobu Motohashi, Youji Murata, Toshio Kaminaga, Takaaki Sakuma, To ...2000 Volume 64 Issue 11 Pages 1012-1019
Published: 2000
Released on J-STAGE: April 24, 2008
JOURNAL FREE ACCESSIntermetalic compound like TiAl having characteristics between metals and ceramics generally shows lower room temperature ductility than metallic materials. We therefore expect that surface processing to make the Ti-Al specimens has a great influence on the ductility. In this study, we have used Ti-48 at%Al alloy. To prepare different surface conditions of the specimens, the following surface processings were made: electrolytic polishing, buff polishing, grinding, and machining with slow or rapid sending speed. For these specimens, bending tests were carried out at room temperature and results that the ductility is greatly influenced by differences in the surface conditions were obtained. The effect of surface conditions on bending properties has been examined under an assumption that the degradation of room temperature ductility may be caused by surface roughness, microcrack, residual stress layer and/or work hardening layer produced in the course of the surface processing. We have concluded that the most important factor is the work hardening layer from the fact that the elongation decreases as surface hardenss increases.View full abstractDownload PDF (1631K) -
Michiko Yoshihara, Young-Won Kim2000 Volume 64 Issue 11 Pages 1020-1026
Published: 2000
Released on J-STAGE: April 24, 2008
JOURNAL FREE ACCESSThe oxidation behavior of nine titanium aluminide alloys has been studied in air, with isothermal conditions at 870°C up to 200 h and 2 h-cyclic conditions both at 760°C and 870°C up to 1000 h. The isothermal oxidation curves for the alloys consist of three stages, that is, the first stage with a linear mass gain, the second with a parabolic increase and the third with a linear mass gain. The alloys with higher Nb contents show smaller mass gains and smaller parabolic rate constants in the second stage. The oxide scales formed on the alloys are basically similar to those on the binary TiAl, however, the high Nb alloys tend to have more continuous and more compact Al2O3 layers in the scales. The enrichment of nitrogen as well as Nb at scale/matrix interface was observed by EPMA analysis. The nitride such as TiN seems to be continuous on the high Nb alloys. The improved oxidation resistance of the Nb-containing alloys is attributable to suppression of TiO2 growth by doping effect and probably also to the nitride layer as a diffusion barrier. However, even 8-10 at%Nb contents are not sufficient for long term cyclic oxidation at 870°C, resulting in a layered scale structure. Small additions of C, B, Si and Hf do not appear to influence the oxidation behavior. The addition of 1 at%Mo did not show any improvement in oxidation resistance under both isothermal nor cyclic oxidation conditions. The addition of W may stabilize the oxidation behavior to be protective.View full abstractDownload PDF (892K) -
Hiroyuki Kawaura, Hiroshi Kawahara, Kazuaki Nishino, Takashi Saito2000 Volume 64 Issue 11 Pages 1027-1032
Published: 2000
Released on J-STAGE: April 24, 2008
JOURNAL FREE ACCESSA new surface treatment by shot blasts using WO3 powder has been developed. This has been most effective in improving the cyclic oxidation resistance of TiAl-base alloy in air and in a diesel engine exhaust gas atmosphere up to 1223 K. This effect is attributed to the formation of an Al2O3 layer in an oxidizing atmosphere. The above layer is supposed to be formed as follows: Since the WO3 powder is shot repeatedly during the surface treatment, a WO3 layer adhering to the substrate is formed on the surface. This layer causes the preferential oxidation of aluminum at the early stage of oxidation, resulting in the formation of a sound and continuous Al2O3 layer. The developed surface treatment gives the same oxidation resistance as that of nickel-base superalloy Inconel 713C up to 1223 K, and is applicable to complex-shape parts with as-cast skin surfaces, such as turbocharger wheels.View full abstractDownload PDF (1389K) -
Masuo Hagiwara, Satoshi Emura, Feng Tang2000 Volume 64 Issue 11 Pages 1033-1040
Published: 2000
Released on J-STAGE: April 24, 2008
JOURNAL FREE ACCESSThe ordered orthorhombic Ti2AlNb(O phase)-based alloys are considered as candidate materials for high temperature application. A preliminary study was conducted to improve room and high temperature mechanical properties of an O phase-based Ti-22Al-27Nb alloy by microstructural and compositional modifications.
The spherical α2 particles which were formed during the hot rolling in the (α2+B2) two phase region were found to be very stable when the hot-rolled material was reheated in the B2 single phase region, and therefore relatively small prior B2 grains ranging from 50 μm to 80 μm in diameter were obtained by the pinning effect of these undissolved α2 particles. This fine-grained microstructure showed an excellent combination of room temperature tensile strength and ductility.
The transition metal elements such as Mo, W and V were substituted for a portion of Nb in a Ti-22Al-27Nb alloy. The substitution was made so that the β phase stability in the modified alloy is equal to that in Ti-22Al-27Nb. It was found that the substitution of 2%W for 7%Nb (Namely Ti-22Al-20Nb-2W) was quite effective in increasing the tensile strength at temperatures above 923 K and reducing the steady state creep rate and primary creep strain.View full abstractDownload PDF (1059K) -
Hiroshi Mabuchi, Hiroshi Tsuda, Toshiyuki Matsui, Kenji Morii2000 Volume 64 Issue 11 Pages 1041-1047
Published: 2000
Released on J-STAGE: April 24, 2008
JOURNAL FREE ACCESSTo investigate the effects of vanadium addition on the formation and material properties of the L12-type intermetallic compounds in the Al-Ti-Cr system, button ingots of Al-25Ti-5Cr-4V and Al-xTi-yCr-3V(x=25, 27, 29, y=6, 8, 10, 12, 14) alloys were prepared by arc-melting. The L12-type phase is formed in the (Al, Cr)3Ti-base alloys containing vanadium content up to 4 mol%. The vanadium atom, as well as chromium atom, substantially occupies the Al-site of the L12 structure. The Vickers hardness increases slightly by the addition of vanadium, and is nearly independent on the Cr+V contents, although the higher titanium alloys have larger hardeness. The bend ductility at ambient temperature is not improved by the addition of vanadium. The best ductility is about 0.5% (plastic bend strain) for the Al-25Ti-12Cr-3V alloy. Serrations in the compressive stress-strain curve are observed at 673 and 873 K. The high temperature yielding phenomenon is also observed at 1473 K. This softening behavior is thought to be caused by dynamic recrystallization. The compressive yield strength (0.2% offset) gradually decreases from around 300 MPa at ambient temperature to about 190 MPa at 1300 K, and then decreases rapidly. From tests performed at strain rate varying from 1×10−4 to 2×10−2 s−1, the data reveal that the compressive strength is independent of strain rate at 1173 and 1273 K; although at 1373 and 1473 K strain rate dependency is observed. Thus the L12-type Al-25Ti-5Cr-4V alloy retains a high temperature strength level of about 200 MPa at 1173 K. The L12-type alloys in the Al-Ti-(Cr+V) system have a smaller linear expansion coefficient and an excellent oxidation resistance as well as the L12 Al-Ti-Cr alloys without vanadium.View full abstractDownload PDF (852K) -
Effect of Ir Addition on the High-Temperature Creep Strength of B2-Type NiAl Intermetallic CompoundsTsukasa Ono, Akihiko Chiba, Shuji Hanada2000 Volume 64 Issue 11 Pages 1048-1055
Published: 2000
Released on J-STAGE: April 24, 2008
JOURNAL FREE ACCESSIn order to study the mechanisms of the improvement in high temperature strength of B2-type NiAl intermetallic compound by Ir addition, compressive creep tests and stress-relaxation tests of (Ni, Ir)Al alloys have been conducted in the temperature range from 1273 to 1473 K. The 5 mol%Ir addition increases the creep resistance of NiAl by a factor of approximately 3 at 1273 K. Ir addition significantly enhances the internal stress of the flow stress, and led to a significant improvement in the high-temperature creep strength of NiAl. An effective stress component comprising a flow stress of the (Ni, Ir)Al alloy is caused by the dragging resistance to the jogged screw dislocations. To understand the strengthening mechanisms of the high-temperature strength of NiAl by Ir addition, it is necessary to consider a decrease in self-diffusion coefficient and a significant increase in shear modulus in NiAl by Ir addition. Such variations in the self-diffusion coefficient and the shear modulus, caused by a decrease in vacancy concentration of NiAl, suppress the recovery process and promote the work-hardening, resulting in an enhancement in the high-temperature creep strength of NiAl.View full abstractDownload PDF (1783K) -
Nobuaki Sekido, Yoshisato Kimura, Fu-Gao Wei, Seiji Miura, Yoshinao Mi ...2000 Volume 64 Issue 11 Pages 1056-1061
Published: 2000
Released on J-STAGE: April 24, 2008
JOURNAL FREE ACCESSEffects of lamellar spacing on the mechanical properties of (Nb)/(Nb, Ti)5Si3 two-phase alloys with a fine lamellar structure have been investigated for an improvement in room-temperature toughness and high-temperature strength. Lamellar spacing of the alloy are controlled, without changing the volume fraction and composition of each phase, by varying annealing temperature for the decomposition of high-temperature phase (Nb, Ti)3Si into (Nb) and (Nb, Ti)5Si3. A Time-Temperature-Transformation diagram (TTT diagram) was determined for a Nb-25 mol%Si-10 mol%Ti alloy. It was found that the decomposition of (Nb, Ti)3Si in the Nb-Si-Ti system is kinetically faster than that of Nb3Si in the binary Nb-Si system. Average lamellar spacing, λ , of the alloy is characterized by the degree of super-cooling, ΔT, following a relationship as “λ∝1⁄ΔT”. Coarse lamellar structure shows better room temperature compressive plastic strain before fracture and higher elevated temperature strength. The alloys investigated in this study show superior compressive strength at over 1000°C as compared with some commercial Ni-based superalloys.View full abstractDownload PDF (920K) -
Tetsuro Hayashi, Toshio Maruyama2000 Volume 64 Issue 11 Pages 1062-1067
Published: 2000
Released on J-STAGE: April 24, 2008
JOURNAL FREE ACCESSIsothermal oxidation of NbAl3 dispersed with MgO or Mg2Si was carried out at 1473 K in a stream of Ar+O2(20%) in order to examine the effect of Mg on the oxidation resisitance of NbAl3. In the case of NbAl3 with MgO, a continuous Al2O3 scale with a thin outermost scale containing Mg is formed. When breakaway occurs after longer oxidation, the alloy is oxidized severely. In the case of NbAl3 with Mg2Si, a thick protective scale is formed, which is composed of an outer MgO layer and an inner MgAl2O4 layer. This sample maintains a protective scale even after oxidation for 1036.8 ks. At the very initial stage of oxidation, an MgO layer is formed on NbAl3 with MgO or Mg2Si. The oxygen partial pressure beneath this layer is low and helps the selective formation of the Al-rich protective scale.View full abstractDownload PDF (1524K) -
Yoko Yamabe-Mitarai, Yoshikazu Ro, Shizuo Nakazawa, Hiroshi Harada2000 Volume 64 Issue 11 Pages 1068-1075
Published: 2000
Released on J-STAGE: April 24, 2008
JOURNAL FREE ACCESSThe purpose of this paper is to understand the high-strength and deformation mechanism of new-generation “refractory superalloys”. Refractory superalloys are defined as alloys that have an fcc and L12 two-phase coherent structure and yet considerably high melting points. This paper examines compressive strength up to 1800°C and creep behavior at 1500°C of Ir-V, -Ti, -Nb, -Ta, -Hf, and -Zr binary refractory superalloys. It is indicated that the precipitation-hardening effect depends on L12 precipitate morphology. Plate-like precipitates are more effective for precipitation hardening than cuboidal precipitates. Shearing of precipitates was observed in the alloy with plate-like precipitates using TEM. When plate-like precipitates form, a dislocation cannot bypass around a plate, thus being forced to shear a precipitate. However, high-coherency strain energy at the interface prevents the dislocation motion. Therefore, large precipitation hardening appeared in the alloy with plate-like precipitates. On the other hand, the creep resistance of the alloy with plate-like precipitates was smaller than that of the alloy with cuboidal precipitates because discontinuous coarsening occurs and the microstructure becomes coarse in the alloy with plate-like precipitates.View full abstractDownload PDF (1337K) -
Manabu Enoki, Atsuko Ohta, Dong Seok Chung, Makoto Watanabe, Teruo Kis ...2000 Volume 64 Issue 11 Pages 1076-1081
Published: 2000
Released on J-STAGE: April 24, 2008
JOURNAL FREE ACCESSTi-Al intermetallic compounds are promising materials for structural application. Processing of Ti/Ti-Al layered materials by self-propagating high temperature synthesis was tried in order to enhance the crack propagation resistance of intermetallics. Tensile test, three-point fracture toughness test and in-situ observation of crack propagation were employed in these materials, and the effect of microstructure and volume fraction of intermetallic layer on the crack propagation behavior was investigated and this paper was aimed at understanding the fracture mechanism of these materials. Tensile strength was increased with the decrease of volume fraction of intermetallics layer, and the materials with many microcracks during tensile test demonstrated higher elongation regardless of the volume fraction of intermetallic layer. The crack propagation resistance curve was quantitatively predicted from the contribution of bridging mechanism in the materials where crack propagates forward during fracture toughness test. The proposed criterion for crack propagation orientation could also predict whether crack propagates forward or deflects in the intermetallic layer.View full abstractDownload PDF (1341K) -
Teppei Suzuki, Naoyuki Nomura, Kyosuke Yoshimi, Shuji Hanada2000 Volume 64 Issue 11 Pages 1082-1088
Published: 2000
Released on J-STAGE: April 24, 2008
JOURNAL FREE ACCESSFor high temperature structural applications such as turbine blades, high temperature strength and fracture toughness at room temperature of Mo-ZrC in-situ composites with hyper-eutectic structure were investigated. Samples of Mo-30, 40, 50 mol%ZrC were prepared by arc-melting in an argon gas atmosphere, followed by annealing at 1673 K for 252 ks. Microstructures of all the annealed samples are found to consist of primary ZrC and fine Mo/ZrC eutectic region. In Mo/ZrC eutectic region, very fine ZrC precipitates are in Moss matrix. The results of compression tests indicate relatively weak temperature dependence of yield stress. Particularly, the yield stresses of Mo-40 mol%ZrC and Mo-50 mol%ZrC at 1773 K are higher than that of monolithic ZrC reported previously. It is considered that ZrC phase in Mo-ZrC in-situ composite is remarkably solid-solution-strengthened by Mo.
From 3-point-bending tests examined at room temperature, fracture toughness of Mo-30 mol%ZrC and Mo-40 mol%ZrC was evaluated to be 14.7 MPa·m1⁄2 and 13.7 MPa·m1⁄2, respectively. In Mo-30 mol%ZrC, it is revealed that cleavage fracture occurs in primary ZrC particles while two types of intergranular fracture at Moss grain boundaries and Mo/ZrC phase boundaries are observed in Mo/ZrC eutectic region. These intergranular fractures in eutectic region are associated with improvement in the fracture toughness of this composite by crack deflection and ligament formation.View full abstractDownload PDF (1156K) -
Wan Jiang2000 Volume 64 Issue 11 Pages 1089-1093
Published: 2000
Released on J-STAGE: April 24, 2008
JOURNAL FREE ACCESSMoSi2-oxide composites fabricated by using fine aluminosilicate powder (<0.2 μm) have demonstrated excellent low temperature oxidation and thermal shock resistance. Considering the heat-resistance characteristic of the MoSi2-oxide composites, the composition of 20 and 25 vol% oxide was selected and evaluated as a heating element being used for thermal treatment equipment of fast thermal processing(FTP). It is found that the working life(with 10% change of resistance) of low-temperature oxidation resistance of the composite is over 126 times of the MoSi2 heating element currently available on the market, and the life of the accelerated heating evaluation carried out repeatedly between 573-1673 K is above 110000 cycles. The working life of MoSi2-oxide composites can be expected sufficient as a heating element for thermal treatment equipment of FTP.View full abstractDownload PDF (768K) -
Toshio Shimoo, Wataru Muto, Kiyohito Okamura2000 Volume 64 Issue 11 Pages 1094-1099
Published: 2000
Released on J-STAGE: April 24, 2008
JOURNAL FREE ACCESSElectron-beam-irradiation-cured polycarbosilane fiber was fired for 0-36 ks at 1273-1673 K in a tube evacuated to 1.33 Pa, and then exposed at 1873 K in argon. The optimum firing condition for the thermal stability of the fiber was investigated by examining gas evolution, grain growth, resistivity, morphology and strength. The strength was low for the fibers fired below 1373 K, because of imperfect ceramization. The firing of 3.6 ks at 1473 K yielded the highest strength (3.5 GPa). The high strength of 1.9 GPa was retained when exposed at 1873 K in argon. Above 1573 K, the fiber strength was degraded by the thermal decomposition of amorphous SiCXOY phase. The residual oxygen in the tube caused the active-oxidation of β-SiC crystals during firing at 1673 K, resulting in the loss of fiber strength.View full abstractDownload PDF (811K) -
Kiyoshi Kumagawa, Hiroyuki Yamaoka, Masaki Shibuya, Kiyohito Okamura2000 Volume 64 Issue 11 Pages 1100-1105
Published: 2000
Released on J-STAGE: April 24, 2008
JOURNAL FREE ACCESSA new SiC-based continuous fiber containing a small amount of sol-gel-derived oxide material(zircon) has been developed. The new inorganic continuous fiber has a better oxidation resistance in dry and wet air at high temperatures than those of the current SiC-based continuous fibers (Tyranno ZMI). The precursor polymer was prepared by polymerizing the current precursor polymer with a sol-gel-derived alcoholic solution. The production process of the newly developed Si-M-C-(O) fiber (Tyranno ZX) from its precursor polymer was almost the same as that of the currently available Tyranno Fiber (ZMI). Although the productivity is open to improvement, fiber diameter, tensile strength and tensile modulus of Tyranno ZX fiber were 11 μm, 3.0 GPa and 190 GPa, respectively. The oxidation resistance of Tyranno ZX at 1273-1773 K in wet air was excellent, compared to the current polymer-derived SiC-based fiber (Tyranno ZMI). Since the addition of an oxide material to the SiC-based fibers was shown to improve the fiber properties, we are currently investigating the addition of other oxide material, Al2O3, in order to achieve fibers with much higher thermal stability by using the sol-gel method.View full abstractDownload PDF (977K) -
Katsuyoshi Kondoh, Atsushi Kimura, Yoshinobu Takeda, Ryuzo Watanabe2000 Volume 64 Issue 11 Pages 1106-1112
Published: 2000
Released on J-STAGE: April 24, 2008
JOURNAL FREE ACCESSBehavior of magnesium and tin on the particle surface has been examined by in-situ XPS, when Al-Si-Mg-Sn alloy powder is heated in a high vacuum chamber. Tin resolved in the powder particle was found to concentrate as a liquid below the Al2O3 surface film of the particle at the temperature above 505 K. This phenomenon has no relation with the magnesium content of the powder. Magnesium helps tin located below the oxide film in appearing at the particle surface, because magnesium is used for reducing the Al2O3 film in the temperature range from 670 K to 700 K. When Al-Si-Mg-Sn alloy powder was sintered in nitrogen gas atmosphere, the nitrogen content of the sintered material was less than 1/10 of that of Al-Si-Mg sintered material without tin. Thus, it can be concluded that liquid tin covered the surface of aluminum particles and prevented the formation of AlN during sintering.View full abstractDownload PDF (642K) -
Atsushi Honda, Katsuyuki Matsunaga, Hideaki Matsubara2000 Volume 64 Issue 11 Pages 1113-1119
Published: 2000
Released on J-STAGE: April 24, 2008
JOURNAL FREE ACCESSMolecular dynamics simulations were performed to study grain boundaries of α-alumina (Al2O3) with a glassy phase of anorthite (CaAl2Si2O8). We calculated atomic structures and excess energies of the grain boundaries with different thicknesses of the glassy film. It was found that the grain boundary energies readily decreased with increasing film thickness, while increased for thicknesses of more than 2 nm. In other words, excess energies exhibit a minimum at a thickness around 1 nm. In this range of film thicknesses, the atoms in the glassy film show a short-range ordered structure and slow diffusion rather than the random structure and rapid diffusion expected for a liquid phase. These results are thought to correspond to an observation of an equilibrium thickness for intergranular glassy films in ceramics.View full abstractDownload PDF (812K) -
Masashi Takahashi, Equo Kobayashi, Hisashi Doi, Takayuki Yoneyama, Hit ...2000 Volume 64 Issue 11 Pages 1120-1126
Published: 2000
Released on J-STAGE: April 24, 2008
JOURNAL FREE ACCESSPhase stability and mechanical properties of Ti-Zr based alloys containing Nb as β stabilizer were investigated. Phase stability of annealed and quenched alloys with various Nb concentration were examined by optical microscopy and X-ray diffractometry while mechanical properties of the alloys were evaluated by hardness and tensile tests. Young’s moduli were also measured by free-free beam resonance method.
Because Widmanstatten structure is observed in annealed 0-4 mol%Nb alloys while a two-phase structure is seen in annealed alloys containing more than 8 mol%Nb, it is considered that of α transus lays around 4-8 mol%Nb at room temperature. In the case of quenched alloys, alloys containing 0-4 mol%Nb exhibit martensitic transformed structure while alloys with more than 8 mol%Nb remain in a β single phase structure. Through microscopy and X-ray diffractometry, β phase stability in Ti-Zr-Nb system was considered and a Ti-Zr equiatomic isoplethal toward Nb corner in the Ti-Zr-Nb ternary phase diagram showing MS temperature was established.
Hardness and strength of β phase alloys are lower than those of martensitic alloys. Ductility of the former is larger than the latter. This might be caused by the deformability of bcc structure. Young’s moduli of β alloys are 74-90 GPa which are smaller than those of CP titanium (105 GPa). Those moduli correspond well with data in references.
From the results above, it is concluded that development of biomedical alloys with low elastic modulus is promising using titanium and zirconium as base materials and niobium as β stabilizer.View full abstractDownload PDF (1189K) -
Takayuki Yoshioka, Osamu Ohashi2000 Volume 64 Issue 11 Pages 1127-1132
Published: 2000
Released on J-STAGE: April 24, 2008
JOURNAL FREE ACCESSTiAl intermetallic compound has attracted considerable interest for their potential as new heat-resisting material, because of its lightness and high strength at elevated temperature. Using TiAl as automobile engine parts and so on, it is necessary to modify the oxidation resistance. On the other hand, TiAl3 has more excellent oxidation resistance than TiAl, but it is too brittle to be a structural material. It has been reported that the crystal structure of TiAl3 changes from D022 type into L12 type due to the addition of the third element X.
This study was carried out considering that L12 type Ti(Al, X)3 is used as a coating material on the TiAl. The purpose of this study is to investigate the effects of the additional third element X on the properties of coating layer that provides the TiAl with high temperature oxidation resistance.
Manganese, chromium, silver and copper were chosen as the additional element in this study.
Vickers hardness of the Ti(Al, X)3, in which X is Mn, Cr or Ag, becomes similar value to that of TiAl. From this result, these materials may be good for the coating layer.
From the oxidation test, it was found that all the four materials have more excellent oxidation resistance than TiAl and they can play a role of coating layer that provides the TiAl with high temperature oxidation resistance. Especially, Mn or Ag added material has good oxidation resistance.
From the observation of diffusion behavior of TiAl/Ti(Al, X)3 diffusion couple, it was found that phase separation occurs in the Ti(Al, X)3 material added with Ag or Cu. In the case of Mn or Cr added Ti(Al, X)3, the concentration of each element changes gradually and no intermediate phase forms at the interface. The growth rate of the diffusion layer of Mn added Ti(Al, X)3 material is lower than that of Cr added material. The lower the growth rate of the diffusion layer is, the more effective it is for the coating layer.
From these results, it is considered that Mn added Ti(Al, X)3 material is most suitable for the coating layer that provides the TiAl with high temperature oxidation resistance among the four materials.View full abstractDownload PDF (918K) -
Feng Wang, Kiyoshi Itoh, Tohru Watanabe2000 Volume 64 Issue 11 Pages 1133-1139
Published: 2000
Released on J-STAGE: April 24, 2008
JOURNAL FREE ACCESSThe crystallographic structure and morphology of electrodeposited Fe-W films under different bath conditions were studied in detail by using SEM, XRD and Heat-treatment method. The crystallographic structures of the Fe-W electrodeposits were also compared with the Fe-W thermal equilibrium diagram. The results indicated that the crystallographic structure of electrodeposited Fe-W alloy film gradually changed from microcrystalline to amorphous with increasing W content. The crystallographic structure of the electrodeposited Fe-W alloy film is closely related to its thermal equilibrium diagram, but does not always agree with it.View full abstractDownload PDF (998K)
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