Zr55Al10Ni5Cu30 bulk metallic glass composite containing 10 vol% crystalline ZrC particles was prepared by an in-situ reaction between Zr-based melt and graphite powder being followed by an injection casting into a copper mould. The crystallization process of the composites during annealing at 723 K has been investigated by using DSC, TEM and microhardness test. It is found that an unknown crystalline phase appears first along the interface between ZrC particle and the matrix of the composite, and then a homogeneous nanocrystallization occurs in the matrix. The nanocrystalline phase precipitated in the matrix is determined to have orthorhombic structure by using TEM diffraction. It has a higher hardness than the hardness of the amorphous phase which remained in the matrix. Hardness of the matrix increases with increasing the volume fraction of the nanocrystalline phases. After annealed at 723 K for 40 min, most of the matrix has been transformed into crystalline phases with a final average grain size of about 120 nm.
In order to improve the mechanical properties of MoSi2 alloys, alloys with Al, B or Nb addition were prepared by an advanced consolidation process that combined mechanical alloying with pulse discharge sintering (MA-PDS). Their microstructure and mechanical properties were investigated. The microstructure of MoSi2 alloys fabricated by the MA-PDS process was much finer than that of the sample sintered from the commercial MoSi2 powders. Alloys made from powders milled in Ar gas had fewer silica or alumina phases as compared to their counterparts sintered from powders milled in air, presumably because of the suppression of the oxidation process during milling in Ar gas. Both Vickers hardness and tensile tests indicated that the alloys fabricated by the MA-PDS process showed better performance than the sample sintered from commercial MoSi2 powders because of the finer grain sizes. The Al-added alloy sintered from the powders milled in air gave the best mechanical properties due to the suppression of SiO2 and formation of fine Al2O3 particles.
In-service degradation of metallurgical and mechanical properties of aluminized CoCrAlY coatings and Ni-base superalloy substrates in advanced gas turbine blades has been studied. The aluminized coatings of the unexposed and in-service exposed blades consisted of four layers with different microstructure and chemical composition. In-service environmental attack led to the deposition of Fe oxides on the top aluminized coating and formation of a thin-layered Al2O3. While in-service, Ni diffused extensively from the substrate into the near-surface coating region. The interdiffusion of Co/Ni resulted in the formation of Al/Ni rich precipitates in all the coating regions, except a near-surface coating region indicating Cr rich precipitates. A number of Cr rich precipitates were found in the substrate near the interdiffusion zone. The near-interface coating region and substrate softened at room and elevated temperatures. The ductility and low cycle fatigue life of the internal coating region at room temperature was not degraded. However, the ductility of the internal and near-interface coating regions and substrate at elevated temperatures was substantially degraded. In-service mechanical degradation of the aluminized CoCrAlY coatings is discussed in light of the metallurgical evolution.
Si3N4–O’SiAlON composites using waste-Si-sludge was fabricated by gas-pressure-sintering (GPS) process at 1950°C for 3.5 h. The percent nitridation of waste Si compacts showed a lower value than that of using commercial Si powders caused by the high contents of oxygen in the waste Si particles. Some amounts of Si2N2O and Y2Si2O7 phases were also detected in the reaction-bonded Si3N4 (RBSN) although main components were α-Si3N4, β-Si3N4 and residual Si phases. In the post-sintered body by GPS, no residual Si, α-Si3N4, Si2N2O and Y2Si2O7 phases were detected except the β-Si3N4 and O’SiAlON. The grain growth of rod-like Si3N4 grains was inhibited by the dispersion of fine O’-SiAlON particles. The fracture toughness and strength of Si3N4–O’SiAlON composites were 5.6 MPa·m1⁄2 and 456 MPa, respectively.
A new process for continuous semi-solid casting of billets of aluminum alloys was developed. Round aluminum alloy billets 75 mm and 150 mm in diameter are continuously cast in a semi-solid state by agitating the alloy in the agitating vessel with a mechanical screw and/or an electromagnetic stirrer. The solidification structure of the billets obtained by this process is a mixed structure of granular particles and a fine eutectic structure, except in a thin chill layer about 2 mm in thickness, which shows a dendrite structure. It was possible to use billets of AC4C alloy obtained by this process in the thixoforming process without surface conditioning.
Mechanical alloying followed by pulse-discharge sintering (MA-PDS) is used to fabricate bulk (Bi0.25Sb0.75)2Te3 polycrystals dispersed with metal Ag, ceramic BN or both. During sintering, the dispersed second phase particles, especially BN, effectively hinder the growth of the (Bi, Sb)2Te3 matrix phase, giving rise to a refined microstructure in compacts. Property measurements demonstrate that the metal Ag is effective in raising electrical conductivity, while the ceramic BN is effective in decreasing thermal conductivity through reinforced phonon scattering. By adding Ag alone, we have identified a large potential for further improving the room-temperature figure of merit of the (Bi0.25Sb0.75)2Te3 mother alloy. The maximum figure of merit appears at 0.02 mass% Ag with value in the order of 3.4×10−3 K−1, which is 18% higher than that of the mother alloy. This result is of practical significance. Because of the deterioration of Seebeck coefficient, addition of BN either alone or in combination with Ag does not produce a favorable figure of merit.
Fe–15 mass%Al (Fe3Al-based) and Fe–25 mass%Al (FeAl-based) alloys with various carbon contents (0, 0.5, 1 and 2 mass%) were produced to study the effect of carbon addition on microstructure, mechanical properties and tribological properties of Fe–Al intermetallics. Carbon addition to the Fe–15Al alloy led to the formation of perovskite-type Fe3AlC0.5 carbides in the matrix, while carbon was present in the form of graphite in the Fe–25Al alloy. The formation of Fe3AlC0.5 carbides resulted in a significant strengthening effect on the Fe–15Al alloy, but the strengthening was accompanied by a great loss in ductility. In the Fe–25Al alloy, the strength and ductility were slightly reduced by the formation of graphite. The tribological properties were determined using ball-on-disk sliding wear test in the range of room temperature to 773 K. It was demonstrated that the carbon addition significantly improved the room temperature friction coefficient and wear rate of both Fe–15Al and Fe–25Al alloys, but it tended to decrease the wear rate of these alloys at elevated temperatures. The tribological properties were discussed in terms of microstrutural features and mechanical properties.
The effect of morphology and Si content on SiO2 particle erosion wear of full pearlitic spheroidal graphite cast iron was studied. Experimental data shows the specimens posses more area fraction of cementite for an identical silicon content, that contribute to improve erosion resistance. However, the specimens posses larger interlamellar spacing (λ) at increased Si content, even if the matrix hardness is higher that still debase the erosion resistance. The 2.1Si-air(930°C) specimens not only possess finer λ value and larger cementite fraction but also with better ductility, which could improve erosion resistance.
Molybdate conversion coatings were deposited on zinc substrate in the presence of Al to improve the corrosion resistance and their structural and electrochemical properties were characterized with various analytical techniques. Infrared reflectance spectroscopy analysis indicated that Al existed as Al(OH)3 in the molybdate conversion coatings. It was found that the addition of Al to molybdate conversion coatings hindered the oxygen reduction and hydrogen evolution reactions effectively compared with pure molybdate conversion coatings, resulting in the enhanced anticorrosive property.
A large amount of neodymium magnet sludge is generated during the manufacture process. Because the sludge is considerably contaminated by oxygen, it is difficult to reuse it as it is. The present basic study has been carried out to establish efficient recycling process of the sludge. The rare earths in the neodymium magnet sludge were extracted by chlorination with FeCl2. An activated carbon was used as a de-oxidation reagent. Metallic iron in the sludge was not chlorinated because the iron monochloride is not stable. The extracted rare earth chlorides were easily separated from Fe-alloy and the excess of FeCl2 by vacuum distillation. In this study, 96% of neodymium and 94% of dysprosium in the sludge were extracted into chloride phase. By the vacuum distillation, a mixture of neodymium and dysprosium trichlorides of 99.2% purity was recovered with the rare earth element’s yield of 76% for the charged sludge. In addition, it was confirmed that the rare earth chlorides can be converted to the corresponding oxides by a pyrohydrolysis reaction accompanied by a formation of HCl gas. The HCl gas can chlorinate metallic iron to FeCl2. Therefore, a new recycling process for rare earth magnet waste can be realized as a chlorine circulation type process. During the process, only carbon and water are consumed, and there are no toxic pollutants. Moreover, the obtained rare earth oxide can be directly used as raw material in the oxide electrolysis, which is the conventional industrial reduction process for rare earth metal production.
Ni–57 mass%Sn alloy has been examined as a new anode material for Li-ion batteries. Li ions can reversibly intercalate and de-intercalate in this alloy. Ball milled nanocrystalline and subsequently annealed microcrystalline Ni–57 mass%Sn alloy showed very high initial discharge capacity. The cell capacity decayed rapidly after the first discharge. The capacity of the ball milled Ni–57 mass%Sn alloy faded continuously on cycling, while the annealed alloy exhibited good cyclic properties. Therefore, Ni–57 mass%Sn alloy is an attractive intercalation host for Li.
In this paper we will provide a prescription for evaluating properties of liquid metals from measured diffraction data. When a liquid metal structure factor is of hard-sphere form, there is a correlation between structure factor and entropy. This is investigated for liquid alkali, noble and typical polyvalent metals. First of all, we calculate the pair and triplet correlation entropies (S(2) and S(3)) from the measured diffraction data. To evaluate the triplet correlation entropy S(3), we do not use the superposition approximation of Kirkwood. Next, assuming that the excess entropy (i.e., ion configurational entropy) SE is given as S(2)+S(3)+S(x), where S(x) denotes an entropy contribution arising from the four-body and higher-order terms of correlations, we calculate the self-diffusion coefficient D based on Dzugutov’s scaling law. By means of D, we estimate the viscosity coefficient η and the surface tension γ for these liquid metals using the Stokes-Einstein relation, the Born-Green equation and Fowler’s formula for the surface tension. We can find there exists a clear relationship between SE, D, η and γ. Predicted values of D, η and γ are reasonable in size in comparison with the available experimental data.
Magnetization of MnAs1−xSbx was measured as functions of temperature and magnetic field for 0≤x≤0.4. The entropy change caused by a magnetic field, ΔSmag, was estimated on the basis of the Maxwell relation. The ΔSmag for 0≤x≤0.3 in a field change of 5 T reaches 25–30 J/K kg, which exceeds that of other materials by a factor of 2–4. The substitution of Sb for As can tune the Curie temperature between 230 K and 315 K without any significant reduction of ΔSmag. The large ΔSmag originates in a paramagnetic to ferromagnetic transition induced by a magnetic field. These results indicate that MnAs1−xSbx is a promising material for a working substance in magnetic refrigeration near room temperature.
High-strain-rate superplastic magnesium alloy, AZ91, was processed through the ingot metallurgy route. The relationship between working temperature and resulting grain size indicated that the grain size tends to decrease with decreasing working temperature in AZ91. Based on this preliminary result, the ingot was hot extruded at a relatively low temperature of 523 K with a reduction ratio of 44. A very fine grain size of 1.7 \\micron was attained only by hot extrusion. The fine-grained structure was stable blow 573 K. Owing to the fine grain size, high-strain-rate superplasticity was observed at temperatures of ∼548 K.
New bulk glassy alloys were formed in the Ca–Mg–Cu system by the copper mold casting method. The maximum rod diameter (dmax) for the formation of a glassy phase was 2 mm for Ca67Mg19Cu14 and above 4 mm for Ca57Mg19Cu24. The glass transition temperature (Tg), crystallization temperature (Tx), supercooled liquid region (ΔTx=Tx−Tg) and reduced glass transition temperature (Tg⁄Tm) are 387 K, 407 K, 20 K and 0.60, respectively, for the former alloy and 404 K, 440 K, 36 K and 0.64, respectively, for the latter alloy. There is a tendency for dmax to increase with increasing ΔTx and Tg⁄Tm. Young’s modulus (E) and compressive fracture strength are 38 GPa and 545 MPa, respectively, for the Ca57Mg19Cu24 alloy rod with a diameter of 2 mm. The success of synthesizing bulk glassy alloys in the simple metal (Ca) base system makes it important as a basic alloy system for examining fundamental chemical and physical properties of bulk glassy alloys.
The aim of the present paper is to demonstrate the feasibility of investment process in producing single-cell materials (hollow spheres) from copper alloys. Spheres were formed from Cu2O powders by the conventional investment shell mold manufacturing process of investment casting and then reduced to metallic copper hollow spheres in a hydrogen atmosphere. Optimum processing variables such as slurry viscosity were investigated and the mass ratio, hardness, and electrical conductivity of the hollow spheres were evaluated with respect to oxide reduction temperature and ZnO content.