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High temperature strength and room temperature fracture toughness of Nb-based alloy system were investigated to explore a candidate material for gas turbine use at 1773 K as a function of chemical compositions. 0.2% proof stress at 1773 K and the fracture toughness were studied for the solid-solution hardening Nb-Mo-W alloys and the Nb5Si3-reinforcing Nb-Si-Mo-W-Hf alloys, and were found that the 0.2% proof stress linearly increases with increasing (Mo+1.5W) content and also increases with volume fraction of Nb5Si3, while the fracture toughness decreases with (Mo+1.5W) content in solid-solution. The critical content for a ductile-to-brittle transition to occur is at (Mo+1.5W) content of 33.5 mol% together with molybdenum content of 15 mol%. Nb-16Si-5Mo-15W-5Hf alloy with a microstructure consisting of 50 vol% solid-solution and 50 vol% Nb5Si3, obtained by optimizing the alloy chemistry without a significant loss of high temperature capability and ductility, and by modifying with 5Hf-5C addition, was found to demonstrate an unprecedentedly excellent creep resistance and also a creep rupture strength exceeding the target strength of 150 MPa and above at 1773 K for 100 h.

Fatigue tests of plane-woven SiC/SiC composites, which had carbon layer as the interphase on the surface of the fiber, were carried out at room temperature in air and at 1473K in vacuum and also creep tests at 1473K in vacuum. At room temperature, the S-N relation was almost flat and the level was slightly lower than the static strength, showing the fracture mechanism was similar to that of static fracture. On the other hand, at 1473K, fracture behavior was mainly caused by time-dependent creep mechanism. The fracture surface showed that debonding occurred between matrix and carbon interphase in the case of static tests at room temperature. On the other hand, in the cases of static, creep and creep fatigue tests at 1473K, debonding occurred between fiber and carbon interphase because of creep deformation of fiber. Delamination was also dominant for all cases. The length of delamination and debonding was longer in creep fatigue test at 1473K than in fatigue test at room temperature and creep test at 1473K because of reduction in sliding resistance of interface caused by cyclic loading. Thus, stress concentration caused by breakage of fiber and matrix was weaker in creep fatigue test than in creep test. The relation between the test time and the creep strain in creep tests could be evaluated quantitatively using the stress power law.

Flexural and interlaminar shear properties of carbon fiber reinforced carbon (C/C) composite, fabricated by “the preformed yarn method”, were evaluated at elevated temperatures up to 2300°C in vacuum. Four point flexure test and short beam test were carried out on the C/C composites of unidirectional, cross-ply and quasi-isotropic laminates. Effect of absorbed gas on the composite strength was also studied. The results revealed that flexural strength increase with increasing temperature, and was not profoundly affected by absorbed gas. On the other hand, interlaminar shear strength (ILSS) was remarkably lowered by absorbed gas at room temperature, and the test on the degased specimen showed that ILSS was kept constant up to 2300°C. Hence the absorbed gas is shown to lower matrix strength but not fiber strength. Young's modulus of C/C composites was kept constant up to 1500°C, then decreased at 2000 and 2300°C, that might be originated form temperature dependence of the Young's modulus of the carbon fiber.

Mechanical properties of carbon fiber reinforced carbon (C/C) composites, fabricated by the preformed yarn method, were studied at elevated temperature up to 2300℃ in vacuum. Various mechanical tests including tensile, flexural, interlamina shear and fracture toughness tests were carried out to evaluate temperature dependence of strength, Young's modulus and fracture toughness for three kinds of laminate C/C composites with uni-directional, 0°/90° and O°/90°/±45° fiber orientations. The failure processes of C/C composites were examined mainly focused on the influence of "transverse-cracks", which bad introduced during processing.

Structural investigation was made to explain corrosion resistance of two phase ferrite-austenite stainless steels SUS329J1 and DSD1H containing Nb+B which were reheated over the temperature range of 400∼750°C for 2hr after solution treatment.The results are as follows:(1) The corrosion rate of SUS329J1 and DSD1H after solution treatment was 1.24g/m2·hr and 2.36g/m2·hr respectively.(2) The corrosion rate of SUS329J1 increased extremely at around reheating temperature of 600°C and 800°C. In the case of DSD1H the peak of corrosion rate was at around only 800°C.(3) In the reheated specimen at 600°C of SUS329J1, film-like carbide Cr23C6 was observed at ferrite-austenite interface, but at the ferrite-ferrite and austenite-austenite interface, no chromium carbide was observed. Preferential corrosion was observed near the ferrite-austenite interface of the reheated specimen at 600°C of SUS329J1, and then corrosion proceeded to the austenite phase.(4) Corrosion resistance of the reheated specimen at 600°C of SUS329J1 was recovered by heating at 700°C.(5) In the reheated specimen at 600°C of DSD1H containing Nb+B, chromium carbide was not observed at the ferrite-austenite interface and also decrease in corrosion resistance was not observed.

In order to develop a series of austenitic manganese nonmagnetic steels, a systematic investigation has been made on the effects of chemical compositions including impurities and also heat treatments on the magnetic permeabilities, mechanical properties, machinabilities, and weldabilities.The magnetic permeability depends on the austenite stability, that of strained specimen was expressed as linear function of Md temperature and the increase due to isothermal austenite decomposition was found to be suppressed by vanadium addition. A linear regression equation relating 0.2% proof stress to chemical composition indicated a great strengthening effect of vanadium comparable to that of carbon. Isothermal decomposition of austenite also lowers the Charpy impact toughness. It was found that the 0.7C-15Mn-1Ni steel was the optimum base composition. A modified grade of the base steel to possess both improved machinabilities and weldabilities was developed by lowering the phosphorus level and also adding free-machining elements as calcium and sulfur.

A study was made about some influences of alloying elements and impurities upon tensile ductility and also critical strain in upset test.It was confirmed that RK001, RK002 and RK007 whose P, S and Si amount of S45C, SCM3 and SCR4 were reduced down to 0.010, 0.010 and 0.10 respectively had, comparing them with the base steels, good forgeability, duplicate punch life, 20% less extrusion energy and also good flowability into die.

ローピング発生の度合いはホットコイルの時点でほぼ決まってしまい,その後の冷延焼鈍の過程で耐ローピング性を大巾に改良することは事実上不可能である。したがって,ホットコイル集合組織の検討および圧延焼鈍によるその変化を調査し次の結果を得た。イ) ホットコイルの表層部は中心部と集合組織が著しく異なり,中心部では(100)～(211) 〔011〕方位であるが,表層部はランダムに近く弱い(100) 〔011〕, (110) 〔001〕方位もみとめられる。ロ) ホットコイルはδフェライトとマルテンサイトからなる著しいbanded structureを示し,主方位である(100) 〔011〕方位はマルテンサイト,δフェライトの共通方位である。ハ) 圧延焼鈍によってホットコイルの組織はランクォフード値が著しく異なるマルテンサイトの分解,再結晶領域と粗大なδフェライト領域とからなるbanded structureに変化する。

Adding copper up to 2.5% to low nickel stainless steel whose base composition is 17Cr-5.5Ni-5Mn, there was made a tensile test at temperature of -100°C to +80°C.Also, an investigation was made about the influence of copper on the stability of austenite. The tensile strength becomes smaller as copper content and testing temperature increase. However, the elongation reaches a peak at a certain temperature and this peak temperature becomes low as copper content increase. It was +50°C when no copper was added, but it was -30°C in the 2.5% copper bearing steel. The lowering effect of copper in this peak temperature is bigger than that of nickel.Relation between martensite content fα and true strain e, there is recognized such a relation as fα=m.ek, m, k being constants. The constant m becomes bigger as temperature decreases, but k is maximum at the peak temperature. Comparing the decreasing effect in stacking fault probability of copper with that of nickel, we obtained modified formula of Angel's Md30, M*d30=Md30 -28.7 (% Cu), considering the term of copper.

After investigating the generating tendency of δ ferrite making Ni of SUS 27 replaced with Mn and N up to 4% there made a tensile test about the steels whose ferrite amount is similar to that of SUS 27 within the range of between -50°C and 100°C. The generating tendency of martensite had been checked simultaneously.Within the composition range of the said test Mn acted as the ferrite former and δ ferrite amount was found much in those steels having much Mn.On the other hand, there recognized that N was effective to make the stacking fault energy raised and the work hardening reduced.The peak elongation in the sub-zero temperature was found in the case when the martensite amount (mostly α' martensite) obtained after deformation was between 15% and 30%.N, in opposition to Ni, makes the tensile strength at room temperature bigger, but it makes both the raising degree of the tensile strength obtained in the case when the temperature is reduced to the sub-zero range and the generating tendency of martensite smaller.

The exhaust valves of 21-4N or 21-2N generally have hardfaced heads for heavy-duty engines. Hardfacing alloys contain a large amount of cobalt (Co), which sometimes becomes very scarce and outrageously expensive. Therefore, some automakers have adopted nickel (Ni)-base superalloys as unfaced exhaust valves, but continued to search for less expensive heat resisting alloy. The authors had succeeded in developing an iron (Fe)-base superalloy, RS 914, containing 40% Ni, 19% Cr, 2.8% Ti and 0.7% Al. The mechanical and chemical properties of this alloy had proved to be comparable with those of Ni-base Inconel 751. And unfaced exhaust valves of RS 914 passed endurance tests in turbo-charged diesel engines with minimal damage. This alloy is relatively low cost and easy to fabricate, so that it can be applied for unfaced exhaust valves substituting for Ni-base superalloys.