In manufacturing process of semiconductor, blue light emitting diode (LED), and lithium ion battery, detection and monitoring of trace water vapor (<1 µmol/mol) are significantly important techniques. We have been researching about detection of trace water vapor with high speed and sensitivity by using ball surface acoustic wave (SAW) sensor.
In this paper, we tried to detect trace water vapor by coating silicon oxide (SiO_x) on ball SAW sensor using sol–gel method. It was confirmed that SiO_x-coated ball SAW sensor could detect water vapor in a wide range from −95 to 0°C in frost point (0.035–6.0 × 10³ µmol/mol in concentration) and its response time to frost point change from −70 to −60°C was one half of that of a chilled-mirror hygrometer.

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The laser ultrasonic technique has been used to determine grain sizes in Interstitial Free (IF) steels. With an analysis method that employs scattering theory, absolute values of the average grain size have been calculated directly from the ultrasonic traces. These results indicate that the laser ultrasonic technique may be incorporated on-line for direct measurements of grain size during steel production.

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A new approach of non-destructive testing for thick welded structural materials based on laser-ultrasonic technique is investigated. In this study, weld part of structural materials, which should be conventionally inspected after welding, is inspected during welding process in order to save time and cost of manufacturing. The laser-ultrasonic is a method to generate and detect ultrasonic signals by laser beams and has potential to be applied to remote inspection/monitoring of materials under welding at elevated temperature. Bulk longitudinal acoustic wave generated by a Q-switched Nd:YAG laser irradiation and detected as surface vibration by laser interferometer coupled with a long pulse detection laser is used to detect defects around the weld. To overcome the lack of sensitivity of laser-ultrasonic testing on thick welded part having a thickness of more than 100 mm at higher temperature, we have originally developed a modified synthesis aperture focus signal processing technique (m-SAFT). The in-process testing with actual piping weld having a thickness of 150 mm with high temperature more than 200 degrees C. was demonstrated. By using m-SAFT, an actual weld defect of 1.5 mm in diameter at 106 mm depth in the specimen was clearly observed. The measurement result well agreed with the result of conventional ultrasonic testing conducted after weld process and also the cross-sectional observation of the specimen.

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High-selectivity ultrasonic imaging of closed crack is an important subject for avoiding not only underestimation but also misidentification of closed cracks. Thus far, the combination of load difference phased array (LDPA) and the crack opening method (GPLC) that combines global preheating (GP) and local cooling (LC) has been verified. Here the LDPA utilized the subtraction of the phased array (PA) images before and after the application of thermal stress induced by GPLC. However, as the cooling time increased, the change in wave velocity depending on the temperature within specimen is not negligible. This results in ghosts that degrade the selectivity of closed crack in subtracted images. In this study, on the basis of the finding that the change in crack response is faster than that in the wave velocity during GPLC, we propose a short time interval subtraction (STIS) method as an option in LDPA. This is based on the subtraction of a PA image from that obtained before a short time interval. It was experimentally verified in a closed fatigue crack specimen made of aluminum alloy.

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Laser-ultrasonic is one of the non-contact and non-destructive inspection methods for hot materials, complicated shapes and minute materials. It has possibilities to measure dimensions and inspect metallographic structures and defects. It is difficult to measure or inspect them by traditional methods such as an ultrasonic diagnostics. Though laser-ultrasonic can be applicable to many industrial fields, we have studied the practical measures to apply the technology to a thickness measurement system for hot materials, such as robustness of optical systems and safety measures for high-power laser. The thickness measuring techniques for cold and hot bulk steels using laser-ultrasonic were tested in our laboratory. The measuring accuracy was within 2% for cold bulk steels and 4% for hot bulk steels (compared with measured value by gauges or X-ray system). Then an experimental system has been installed in a hot rolling process line and verified in terms of the measuring accuracy, safety counter-measures and durability for high temperature or dust. As the results of this experiment, the measuring accuracy is within 4% compared with X-ray system, dust on optical devices was free due to air curtains and an air jet nozzle for more than 5 months and laser reflection are shielded completely with fireproof metallic fiber curtains.

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For ultrasonic imaging closed cracks, unable by traditional ultrasonic techniques, the subharmonic phased array for crack evaluation (SPACE) has been developed. However, a simulation model for optimizing inspection conditions and for scientific understanding of subharmonic waves is required. In this paper, by extending the finite-difference time-domain (FDTD) model with damped double nodes (DDNs) to realistic testing conditions and comparing the simulation with the experiment, the availability of this model is demonstrated.
To model a compact tension specimen with a fatigue crack extended from a notch, the DDN model was extended to the vertical closed crack mode. When the tensile stress of incident ultrasonic wave at the closed crack is larger than the compression residual stress T_th at the crack, the crack changes to the open state with particle velocity nodes separated to dual nodes. Moreover, to suppress the numerical noise due to the contact vibration, the damping term is introduced.
In the experiment with small amplitude incident waves, only the notch response was observed, whereas in the experiment with large amplitude incident waves, the crack response was observed. The latter behavior was reproduced by the simulation when the tensile stress of the incident wave was larger than T_th. The experimental image of the crack tip obtained by the SPACE was also reproduced by the simulation, indicating the usefulness of the DDN model.
Moreover, in both experiment and simulation, a scattered wave was observed at the crack face between the crack tip and the notch in the subharmonic images. Since only “reflection” takes place on a crack face with no singularity, whereas “scattering” takes place at a crack tip with singularity, the scattering at a crack face with no singularity is surprising. Therefore, we examined the cause of this phenomenon and found that the crack was continuously opened until the crack closure point (CCP) where the tensile stress of incident wave became less than T_th. The scattered wave was generated at the CCP as a new singularity formed at the crack face. This is a novel finding and will be useful in evaluation of local compression residual stress at cracks, which plays important role in prediction of lifetime of structures.

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Ultrasonic waves generated by a pulsed Nd-YAG laser are being adopted to evaluate the cavitation damage, so-called pitting, caused by proton beam injection in pulsed neutron sources. The wave’s propagation behavior depends on the density and depth of the pits. To quantitatively understand the relationship between the pits and the wave propagation behavior, the artificial pits were imposed on the evaluated specimen while controlling the density and depth. A laser Doppler vibrometer was used to remotely detect the ultrasonic waves generated by the Nd-YAG laser. It was found that the two parameters, namely, the maximum negative peak value and the attenuation of received waveforms were useful for quantitatively evaluating the damage. As a result, cavitation damage with a peak-to-peak roughness of more than 15 µm was successfully evaluated.

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An absolute hydrophone calibration technique using ultrasonic far-field with optical interferometry was extended to 40 MHz to evaluate the sound pressure amplitude generated by medical ultrasound equipment. A 1-mm nominal radius plane transducer was the calibration sound source for an ultrasonic far-field at 50 mm propagation distance at 40 MHz. The transducer’s effective radius was 0.88 mm at 40 MHz, with a range of 10–40 MHz. A coplanar membrane hydrophone with a 0.2 mm radius active element was calibrated using the technique at frequencies of 10–40 MHz with a 1 MHz frequency interval. Calibration results obtained using the developed system were compared with those obtained using our current calibration system. The discrepancy between those results was within the uncertainty of our current calibration system for frequencies of 10–20 MHz.

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Since the dental structure and the mechanical parameters of dental hard tissues would affect the propagation features of ultrasonic waves, laser ultrasonic nondestructive evaluation (NDE) technique can be used to assess human teeth. With the application of an expansion Laguerre polynomial technique, surface acoustic waves (SAW) in human incisors are calculated, and the effects of inhomogeneous elastic properties of enamel and initial dental caries on SAW are also discussed. An experimental setup to generate and detect SAW in incisor by noncontact and nondestructive manner is established. A focused laser line source is used to generate broadband SAW, which is detected by laser Dopple vibrometer on healthy incisor and initial dental carious, respectively. The results demonstrate that some dental parameters such as dental structures and initial dental carious can affect the phase velocities of laser induced SAW. Laser ultrasonic NDE methods have the potential for evaluation of human teeth in vivo.

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In order to commonly use explosive H₂ gas as an energy source, fast, sensitive, and low-power consumption sensor is required. We developed a ball surface acoustic wave (SAW) sensor with porous Pd–Pt alloy film to realize such a sensor. The sensor with 20% Pt alloying film was useful for the suppression of a phase transition of Pd and the detection of ppm order concentration at 35°C. The amplitude response was proportional to the square root of the concentration, which was demonstrated for the first time in the field of SAW sensor, resulting in the detection limit of 3.7 ppm at signal to noise ratio of 3. The response time decreased to 1/5, compared with those of pioneering H₂ sensors working at room temperature. From these results, it was shown that the fastest and most sensitive hydrogen sensor working at room temperature could be realized using the ball SAW sensor with porous Pd alloy film.

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Previously, we used contact receiving transducers in the scanning laser source (SLS) technique for imaging defects on a plate in order to achieve a high signal-to-noise (SN) ratio. Herein, we developed a fast non-contact defect imaging technique that employs the SLS technique for in-line product inspection. Leaky Lamb waves from a plate were detected with a sufficiently large SN ratio by using low-frequency air-coupled transducers. Spurious images caused by reflected waves in the plate were removed by synthesizing images from multiple receivers. Defect images were compared for different repetition frequencies of laser emission. Images were found to be distorted at high repetition frequencies (2/3 kHz) owing to reverberations in the plate.

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In this work, the early stage of precipitation have been investigated in a Mg–2.9 at% Gd–0.8 at% Y by high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) and high resolution transmission electron microscopy (HRTEM).
At the underaged condition, precipitates observed by HRTEM were classified as follows; mono-layer, a part of β″, β′. By HAADF-STEM, zig-zag structure, small hexagonal network, and β′ could be recognized. The small “super hexagons” are the first precipitates in this alloy. This structure, referred to as the pre β″-phase, displays a short range similar to the one present in the D0₁₉ structure. This phase is formed during quenching and prevails over the zig-zag structure at the beginning of aging. The zig-zag contrast is one of the variations of this small hexagonal network or heterogeneous nucleation during aging. Finally, we concluded that the proposed precipitation sequence is as follows: SSSS → pre β″-phase having D0₁₉-SRO → β″-phase → β′-phase. The comparison of HRTEM, HAADF-STEM and simulated images have helped to clarify the meaning of HRTEM features.

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To examine growth behavior of α-CoSn₃ at solid-state temperatures, kinetics of reactive diffusion between Co and Sn was experimentally observed using sandwich Sn/Co/Sn diffusion couples prepared by a diffusion bonding technique. The diffusion couples were isothermally annealed in the temperature range of 433–473 K for various times up to 744 h. Owing to annealing, an intermetallic layer consisting of CoSn₃ was formed at the original interface in the diffusion couple. The mean thickness of the intermetallic layer increases in proportion to a power function of the annealing time. The exponent of the power function takes values of 0.67 and 0.62 at 433–453 and 473 K, respectively. These values of the exponent indicate that volume diffusion predominantly controls the layer growth and interface reaction partially contributes to the rate-controlling process.

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Brass was treated after 4 GPa high pressure heat treatment at 700°C insulated for 15 min, and the effects of 4 GPa high pressure heat treatment on micro-mechanical properties of the brass were discussed by optical microscope, transmission electron microscopy, scanning electron microscope and nanoindenter. The results showed that 4 GPa high pressure heat treatment could increase the hardness, elastic modulus and elastic recovery coefficient, and decrease its friction coefficient. They were attributed to the microstructure of brass that became finer and more uniform compact after 4 GPa high pressure heat treatment.

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We investigated a composition-dependent tempering parameter for more accurate prediction of tempered martensite hardness in low alloy steels. The proposed composition-dependent tempering parameter decreased with the addition of alloying elements related to the activation energy for tempering. The tempered martensite hardness calculated using the composition-dependent tempering parameter showed a reliable accuracy compared with experimental data.

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Tribological behavior of Ti-5Al-2.5Sn, Ti-10V-2Fe-3Al and Ti-38Al carburized via current heating technique with graphite powders was studied. In carburizing, the direct current was applied across the graphite powders and alloys with a fixed electric power from 120 to 180 W for 20 min in argon atmosphere. The alloys were characterized using XRD, SEM, and hardness tester. Tribological behavior of the alloys was examined by pin-on-disk tribological test. Formation of TiC, TiC with VC and Ti₂AlC with TiC on Ti-5Al-2.5Sn, Ti-10V-2Fe-3Al and Ti-38Al, respectively, could improve surface hardness and reduce friction coefficient leading to improve wear resistance of the alloys. Coatings of TiC totally exhibited higher wear resistance than that of Ti₂AlC. Improvement of wear resistance was influenced by quality of coatings which could be sorted in descending as TiC, TiC with VC and Ti₂AlC with TiC in the carburized Ti-5Al-2.5Sn, Ti-10V-2Fe-3Al and Ti-38Al, respectively.

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With the aim of minimizing the loss of platinum into slags by controlling the slag composition during the high-temperature recycling process, the effects of representative slag components, namely, Al₂O₃, MgO, Fe₂O₃, and CuO_x, on the dissolution behavior of platinum into Na₂O–SiO₂-based slags were investigated. The solubility of platinum in the slags was measured by equilibrating the Na₂O–SiO₂-based slags with pure solid platinum at 1473 K in air. The dissolution of platinum in the slags was found to be suppressed by the addition of Al₂O₃, MgO, and Fe₂O₃. Al₂O₃ and Fe₂O₃ behaved as acidic oxides, whereas MgO behaved as a diluent and decreased the solubility of platinum in the slags. CuO_x behaved as a weakly basic oxide and slightly enhanced the dissolution of platinum into the slags. The correlation between the platinate capacity of the slag, which is a parameter proposed in this paper, and the optical basicity enabled the content of platinum in slags at 1473 K to be estimated from the slag composition.

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The microstructure and mechanical properties of TiAl₃/Al₂O₃ in situ composite by combustion process are investigated in details. Pure Al powder and TiO₂ powder were mechanically milled with a molar ratio of 13 : 3 at 300 rpm for 18 ks. This milling powder’s green compact cold-pressed at 200 MPa for 0.3 ks sintered at 923 to 1173 K for 1.8 ks under Ar atmosphere. The microstructure of the sintered compact was identified by XRD, SEM and TEM/EDS. Mechanical properties were evaluated by Vickers hardness and indentation fracture toughness tests. The compacts at less than 973 K consist of a TiAl₃ phase and remained phases of Al and TiO₂, while the compact at more than 1023 K consist of TiAl₃ matrix, dispersed Al₂O₃ particles and some plate like Ti₃Al phase. The compacts at more than 1023 K have the higher hardness around 800 Hv compared to TiAl₃ single phase compact with 450 Hv. The fracture toughness of TiAl₃/Al₂O₃ composite is about two times TiAl₃ single phase compact. This improvement of hardness and toughness is attributed to the effect of the microstructure formation of dispersed Al₂O₃ particles and Ti₃Al plate like phase in the TiAl₃ matrix.

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A mixed potential sensor based on yttria-stabilized zirconia (YSZ) and a spinel-type oxide sensing electrode (SE) ZnFe₂O₄ is fabricated and examined for NO₂ detection at high temperatures. The devices give a linear correlation between EMF and the logarithm of NO₂ concentration from 100 to 1000 ppm in the temperature 450 and 500°C. The mechanism of the sensor has been discussed on the basis of the particle size of sensing electrode-ZnFe₂O₄.

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The work hardening behavior and the developments of microstructure and crystallographic texture during high-pressure torsion (HPT) in pure iron were investigated. A set of the 3D electron backscatter diffraction (EBSD) measurements were also performed to characterize the microstructure in 3D using an orthogonally arranged focused ion beam-scanning electron microscope (FIB-SEM) instrument. It was found that the image quality (IQ) values of the EBSD data obtained by the FIB-SEM are better compared to the conventional mechanical polishing procedure. A detailed analysis on the distribution of misorientation angle was performed, being fitted with the Mackenzie plot. The use of the Hencky equivalent strain made it possible to compare the work hardening behaviors and the microstructural evolutions observed in the current investigation with those reported for accumulative roll bonding (ARB). This comparison has revealed that the Hall–Petch coefficient k obtained by HPT deformation drastically increases with the grain refinement by HPT deformation.

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Effects of magnesium on wear resistance of H13 steel have been investigated. A disc-on-disc configuration with load of 200 N was employed to study the wear behavior. In order to understand wear mechanisms, wear tracks were studied by scanning electron microscopy. The experimental results show that after magnesium treatment finer carbides were precipitated and the hardness increased with the increasing of magnesium content, which can improve the wear resistance. The specimen without magnesium treatment has the lowest wear resistance and the mode of wear is adhesive. With increase of magnesium content, the degree of oxide patches coverage increased result in the wear rate decreased gradually. Wear mechanism has been changed from adhesive to oxidative after the magnesium content increased to 10 ppm. In this case, the friction coefficient exhibits a higher fluctuation resulted from the alternative formation and delamination of oxide layers.

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Nanopowders of MgO, Al₂O₃ and SiO₂ were made by high energy ball milling. The simultaneous synthesis and consolidation of nanostuctured MgSiO₃–Mg₃Al₂Si₃O₁₂ composites from milled 4MgO, Al₂O₃ and 4SiO₂ powders was investigated by the high-frequency induction heated sintering process. The advantage of this process is that it allows very quick densification to near theoretical density and inhibition of grain growth. Highly dense nanostructured MgSiO₃–Mg₃Al₂Si₃O₁₂ composites were produced with a simultaneous application of 80 MPa pressure and an induced current within 1 min. The microstructure, sintering behavior and mechanical properties of MgSiO₃–Mg₃Al₂Si₃O₁₂ composite were evaluated.

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