Low-temperature densification of titanium powder was attempted via pulse-current sintering under cyclic uniaxial pressure. The sintering was performed under cyclic or constant uniaxial pressure of 100 MPa and 200 MPa at temperatures of 400°C and 500°C. The effect of the cyclic pressure on the densification of the titanium powder was confirmed under all sintering conditions. When the sintering was performed at 400°C under a uniaxial pressure of 100 MPa, the relative density was 92.2% for sintering under cyclic pressure and as low as 77.3% for sintering under constant pressure. The relative density reached 97.7% through sintering at 500°C under a cyclic uniaxial pressure of 200 MPa. Thus, applying the cyclic uniaxial pressure during the pulse-current sintering is effective for preparing dense sintered titanium at lower temperatures.
Solubility of Yb to the Si clathrates Ba8Ni3.8Si42.2 and Ba8Cu4.9Si41.1 with the type-I clathrate structure has been studied with polycrystalline samples by the electron micro probe analysis. It is revealed that most of the doped Yb is precipitated as secondary phases and the Yb solubility limit is about 1% of the Ba site or less. Thermoelectric properties of those composite samples are also investigated. Yb-doped samples show lower electrical resistivity and Seebeck coefficient, compared to the pristine compounds. Thermal conductivity increases with the addition of Yb, which is attributed to the increase of the electronic thermal conductivity. Overall thermoelectric figure of merit is not improved for the Ni-system, while a slight enhancement is observed for the Cu-system above 800 K. The best performance is zT = 0.21 for the nominal composition Ba7.8Yb0.2Cu4.9Si41.1 at T = 1000 K.
Powder metallurgy (PM) Al3Zr reinforced aluminum (Al) composites were fabricated by in-situ reaction during spark plasma sintering (SPS) using the pre-mixed Al and zirconium hydride (ZrH2) powders. The diffusion behavior of Al and zirconium (Zr) elements in Al matrix were studied. It was clarified that ZrH2 decomposed at around 773 K and dissolved Zr elements reacted with Al matrix to form Al3Zr layers around ZrH2 particle. Al3Zr compounds grew toward the center of ZrH2 particle due to a faster diffusion rate of Al compared to Zr element. The tribological behavior of the composite material was investigated by using pin-on-disk wear test under lubricated condition. Friction coefficient and wear volume of Al-Al3Zr composite were much lower than those of pure Al. SEM observation and SEM-EDS analysis on wear track of pin and disk specimens were investigated. The sliding surface of SUS304 stainless steel counterpart after wear test showed very slight damages and no seizure phenomenon with the pin specimen. It is concluded that PM Al-Al3Zr composite had a good tribological property and wear resistance due to hard Al3Zr particles dispersed in Al matrix preferentially contacting the counterpart material to prevent the soft Al matrix in direct contact with SUS steel surface.
A functionally graded material (FGM) is a compound of two different materials with gradient composition. In addition to combining the characteristic of two materials, FGM show new characteristic. The concept of FGM was applied to hard material. The material which consists of ceramic surface layer and cemented carbide core and an intermediate layer with graded two materials showed excellent performance for cutting tool. And we studied on FGM which is composed of cobalt composition graded cemented carbide layers and steel substrate for oil drilling stabilizer. FGM stabilizer showed excellent wear resistance compared with conventional type. The idea of FGM was applied for ceramic coating technology, too. We succeeded balancing breakage resistance and adhesion by FGM coating technology. This paper introduces these materials feature and performance.
Cemented carbides have been expanded for cutting tools and wear-resistant parts. Recently, the application has been expanded to pump and chemical plants that require corrosion resistance, and WC-Ni-Cr cemented carbides have been developed. In this study, microstructures, some mechanical properties and electrochemical corrosive behavior of WC-(0-16)mass%Cr3C2-15 mass%Ni alloys were studied as a function of carbon and Cr3C2 contents of these alloys. At the stage of practical application, the effects of carbon and Cr3C2 contents on their corrosion resistance were investigated by both immersion test and electrochemical polarization test in 3%NaCl neutral and 3%NaCl + 0.1NH2SO4 acid solutions as simulated sea water to clarify the corrosion reaction. Further, Galvanic corrosion between cemented carbides and steel, and the NaCl concentration in brackish-water region were investigated for standardization of WC-Ni-Cr corrosion resistant carbides for using seawater pump industries.
TiC-Ti-W-Mo alloy and TiC-Ti-(Ti,W)C-Mo2C alloy were prepared through powder metallurgy. The microstructure of the TiC-Ti-(Ti,W)C-Mo2C alloy was significantly smaller than that of the TiC-Ti-W-Mo alloy. It was considered as follows. Firstly, carbon in W and/or Mo containing carbides is diffused into α-Ti, and then W and Mo phases with vacancies precipitate from carbides. Secondly, they are easily diffused into α-Ti, and then α-Ti is transformed to β-Ti at lower temperature than 1155 K, at which pure Ti transforms. Thirdly, when TiCx particles precipitate in β-Ti, their growths are suppressed by dissolved W and Mo in β-Ti. Furthermore, in the first step above, (Ti,W)C particles are decomposed into super fine (Ti,W)Cx and W phases. From these, it was considered that the carbides grain size of the TiC-Ti-(Ti,W)C-Mo2C alloy is significantly smaller than that of the TiC-Ti-W-Mo alloy. Sintered and HIP treated TiC-Ti-(Ti,W)C-Mo2C alloy has better mechanical properties than those of TiC-Ti-W-Mo alloy.
TiC-47 mass% Ti-20 mass% Mo (Mo added alloy) and TiC-52 mass% Ti-21 mass% Mo2C (Mo2C added alloy) were prepared through powder metallurgy. The TiC grain size of Mo2C added alloy sintered at 1673 K was about 3 μm, which was smaller than that of the Mo added alloy. The mechanism was considered as follows. First, when C diffuses from Mo2C to α-Ti, the Mo phase containing defects such as vacancies precipitates. Second, the solid solution of Mo and Ti is promoted by the mutual diffusion between the precipitated Mo phase and α-Ti, and the transformation from α-Ti to β-Ti occurs. Third, the solute diffusion coefficient of β-Ti is much larger than that of α-Ti, so C diffuses from TiC to β-Ti, producing TiC0.6. Meanwhile, Mo has a smaller free energy for carbide formation than Ti has and then TiC0.6 cannot contain Mo, so Mo is concentrated in β-Ti adjacent to precipitated TiC0.6. This suppresses the growth of TiC0.6, and then the grain size of TiC0.6 becomes smaller. The transverse-rupture strength of the Mo2C added alloy was 20% higher than that of the Mo added alloy, reaching 0.95 GPa. At that time, both alloys have almost the same hardness.
AlN–WC and AlN–Y2O3 composite ceramics were prepared by the resistance-heated hot pressing method at the sintering temperatures of 1873 K and 1973 K. The AlN grains were well-bonded each other by the addition of WC into AlN. W2C-type phase was produced by the addition of WC into AlN ceramics. The bulk density of AlN–WC ceramics was increased with increasing the WC amount, and was in good agreement with the calculated density in ceramics containing 10–50 mol% WC at 1873 K and 1–50 mol% WC at 1973 K. The Young’s modulus for AlN–WC ceramics was increased with increasing the WC amount, reaching for 470 GPa at 50 mol% WC. The addition of WC into AlN ceramics improved mechanical properties, both Vickers hardness and fracture toughness. The AlN–50 mol% WC ceramic had the highest Vickers hardness and fracture toughness value of 17 GPa and 5.9 MPa m0.5, respectively. The thermal conductivity of AlN–WC ceramics was virtually constant at 1–50 mol% WC, ranging between 40 and 60 W m-1 K-1.