Low temperature superplasticity of a fine-grained ZK60 magnesium alloy processed by equal-channel-angular extrusion

Fine-grained ZK60 magnesium alloy with the grain size of 1.4 μm was processed by equal-channel-angular extrusion. The material exhibited low temperature superplasticity. The normalized plot suggested that the present material had equilibrium grain boundaries at the superplastic temperature in contrast to typical materials processed by severe plastic deformation.     

Relation between crystal structure and lattice oxygen content of sintered reaction-bonded silicon nitride

When reaction-bonded silicon nitride containing MgO/Y₂O₃ additives is sintered at three different temperatures to form sintered reaction-bonded silicon nitride (SRBSN), the thermal conductivity increases with sintering temperature. The β-Si₃N₄ (silicon nitride) crystals of SRBSN ceramics were synthesized and characterized to investigate the relation between the crystal structure and the lattice oxygen content. The hot-gas extraction measurement result and the crystal structure obtained using Rietveld analysis suggested that the unit cell size of the β-Si₃N₄ crystal increases with the decrease in the lattice oxygen content. This result is reasonable considering that the lattice oxygen with the smaller covalent radius substitutes nitrogen with the larger one in the β-Si₃N₄ crystals. The lattice oxygen content decreased with increasing sintering temperature which also correlated with increase in thermal conductivity. Moreover, it is noteworthy from the viewpoint that it may be possible to apply the lattice constant analysis for the nondestructive and simple measurement of the lattice oxygen content that deteriorates the thermal conductivity of the β-Si₃N₄ ceramics.     

Stability of protective oxide films in waste incineration environment—solubility measurement of oxides in molten chlorides

Solubility measurements of several oxides in molten NaCl-KCl and NaCl-KCl-Na₂SO₄-K₂SO₄ were conducted in three different levels of basicity. The dissolution behavior of the oxides showed almost the same tendency as that shown by the dissolution behavior of the oxides in molten Na₂SO₄ in literature. In a waste incineration environment, a protective Cr₂O₃ film easily dissolves in molten chlorides as CrO₄²⁻ because pO²⁻ of the molten chlorides tends to have a small value due to the effect of water vapor contained in the combustion gas. From the result of the solubility measurement, the addition of molybdenum and/or silicon was expected to improve the corrosion resistance of alloys. Laboratory corrosion tests confirmed this expectation. However, the scale analysis suggested that the effect of molybdenum could not be explained completely by only the mechanism derived from the result of the solubility measurement.     

Fracture behaviour and numerical study of resistance spot welded joints in high-strength steel sheet

Cross tension tests of resistance spot welded joints with varying nugget diameter were carried out using 980 MPa high strength steel sheet of 1.6 mm thickness. In proportion, as nugget diameter increased from 3√t to 5√t (where t is thickness), cross tension strength (CTS) increased while fracture morphology simultaneously transferred from interface fracture to full plug fracture. In cases of interface fracture, circumferential crack initiation due to separation of the corona bond arose at an early stage of loading. The crack opening process without propagation was recognized until just before fracture and then the crack propagated to the nugget immediately in a brittle manner around CTS. In full plug fracture, main ductile crack initiation from the notch-like part at the end of sheet separation occurred with the sub-crack initiated at an early stage. The ductile crack propagated toward the HAZ and base material to form full plug fracture. The mode I stress intensity factor was considered as a suitable fracture parameter because the circumferential crack behaved pre-crack for brittle fracture in the nugget region at the final stage. Based on the FE analysis, the mode I stress intensity factor was calculated as 116 MPa √m at CTS as fracture toughness for the nugget. With respect to full plug fracture, ductile crack initiation behaviour from the notch-like part was expressed by concentration of equivalent plastic strain. On the assumption that the ductile crack arose in critical value of equivalent plastic strain, the value was calculated as 0.34 by FE analysis. Reasonable interpretation for interface fracture and full plug fracture in the resistance spot welded joint was proposed due to first crack initiation by stress concentration, brittle fracture by using mode I stress intensity factor, and ductile crack initiation by using equivalent plastic strain.     

Crystallographic analysis for acicular ferrite formation in low carbon steel weld metals

Inclusions contributing to acicular ferrite nucleation were investigated from a crystallographic point of view in low carbon low alloy steelweld metals. The samples from electro slag welding (ESW) and submerged arc welding (SAW) deposits with various cooling rates were prepared in this study. In those samples, intragranular acicular ferrite formation was observed from inclusions. The inclusions contributing to acicular ferrite formation were of multi-phase type consisting of amorphous phase, spinel type and MnS. They were surrounded by a Ti-enriched layer. It was confirmed by selected area diffraction patterns and energy-dispersive X-ray spectrometer analyses that the Ti-enriched layer was TiO. The acicular ferrite had a Baker–Nutting orientation relationship with the TiO layer on the inclusion surface. The misfit was 3.0% at the interface between the acicular ferrite and TiO. Therefore, it is considered that TiO on the inclusion surface contributes to the heterogeneous nucleation of acicular ferrite by small lattice misfit. However, themorphologies of ferrite growth which nucleated from inclusions were different in both samples. Whereas the growth of ferrites nucleated from TiO was enough in ESW, the size of nucleated ferrite in SAW was a few hundred millimetres in size. In the early stage of nucleation from TiO, ferrite had small deviation from Kurdjumov–Sachs orientation relationship (K–S relationship) in both ESW and SAW. However, there was a difference in the growth stage of ferrite. The ferrite orientations were gradually changed to fit to the K–S relationship in ESW. On the other hand, the nucleated ferrite in SAW stopped growing and the newly nucleated ferrite which had K–S relationship prior to austenite was formed adjacently because of large super cooling due to small heat input.     

Effect of REM addition of wire on CO2 gas shielded arc phenomenon

Carbon dioxide (CO₂) gas shielded arc welding is the main arc welding method, but it generates a large amount of spatter during welding. The root cause of spatter lies in the fact that the droplet undergoes repeated irregular shaking. To solve this problem, spatter generation modes were clarified and the effects of polarity and rare earth metal (REM) addition of the wire on CO₂ gas shielded arc welding were investigated. As a result, when welding is performed with an electrode negative (DCEN) polarity using REM added wire, it was found that a conical arc plasma is formed, and the droplet which is transferred from the wire tip to the molten pool is fine and continuous, in what is termed ‘spray transfer’. Thus, spatter generation was reduced to 10% of amount of the conventional CO₂ gas shielded arc welding (from 0.058 to 0.005g/s).     

Development of novel temperature-stable Al2O3–TiO2-based dielectric ceramics featuring superior thermal conductivity for LTCC applications

A new composition was developed using sintering to improve the dielectric properties of low-temperature co-fired alumina (LTCA) containing CuO–TiO₂–Nb₂O₅–Ag₂O. By substituting some alumina with rutile TiO₂, the second-phase compound could be changed from AgNbO₃ to the rutile phase. Further, low-temperature sintering at temperatures below 960 °C was possible, suppressing Al₂TiO₅ formation during firing. The dielectric characteristics, particularly the temperature coefficient of the resonant frequency (τ_f) and Q × f values, were improved without significantly reducing the sinterability and thermal conductivity. The dielectric properties of the developed 88Al₂O₃–12TiO₂-based ceramic were ε_r: 14.7, τ_f: +0.8 ppm/K, and Q × f: 13,383 GHz (at ～10 GHz) at a firing temperature of 940 °C. The thermal conductivity was 18.5 W/mK, which is the highest value for reported temperature-stable low-temperature co-fired ceramics (LTCCs). These results provide one of the key technologies for the practical application of LTCCs with superior thermal conductivities.     

An analysis of the length of hospital stay for cataract patients in Japan using the discrete-type proportional hazard model

We analyze the length of hospital stays of patients hospitalized for cataract and related diseases (Diagnosis Related Groups (DRG) 2041) in Japan, utilizing the data pertaining to 3436 patients on whom one-eye lens operations are performed. We use the discrete-type proportional hazard model to analyze variables that may affect the length of stay.     

Analysis of polymer electrolyte fuel cell performance by electrode polarization model

For development of polymer electrolyte fuel cell (PEFC) lifetime estimation method, a high accuracy PEFC electrode polarization model is required. An electrode polarization model which was previously proposed was verified. However, accuracy of the electrode polarization model was not enough to estimate PEFC performance under various conditions. A new high accuracy PEFC electrode polarization model has been developed based on electrochemical consideration and data observed at elevated pressures. In the cathode polarization model, effects of O₂ diffusion and H₂O plugging have to be considered to obtain high accuracy for long-term operation. In addition, PEFC performance degradation was analyzed by the electrode polarization model. Main factors of PEFC performance degradation are OCV drop, the cathodic activation polarization, voltage drops by O₂ diffusion and H₂O plugging.     

Guided-mode resonant grating filter with an antireflection structured surface

We describe a new structure of guided-mode resonant grating (GMRG) filters with low sideband reflectance. This GMRG filter consists of a high-index thin film on an antireflective structured surface called "moth-eye structure." Since the high-index film undulates along the surface structure, the film acts as a modulated optical waveguide. An incident light wave satisfying a resonant condition is reflected by the GMRG filter, and nonresonant light waves pass through the filter. This GMRG filter is valid for reducing reflection of nonresonant light waves in a wide spectral range. The resonant reflection of this new filter was investigated by numerical calculation based on an electromagnetic grating analysis. In the case of a triangular antireflective surface structure whose thickness is 2x greater than its period, the sideband reflectance for nonresonant light waves was lower than 0.5% for TM-polarized light in a wide range of wavelengths.