Fragmentation behavior during molten material and coolant interactions

A safe design for a fast breeder reactor (FBR) requires post-accident heat removal (PAHR) for any potential core disruptive accident (CDA). It is important to ensure that the molten core material solidifies in the sodium coolant in the reactor vessel even if all of the core material has melted. In the present experiment, molten material was injected into water to experimentally obtain the information on the molten material jet entering the coolant and its fragmentation. Visual information was obtained with a high-speed video camera, showing that fragmentation behavior on the side of the jet was different from that on the jet front, and that the injection nozzle diameter significantly influenced the jet breakup length, while the molten jet temperature and the coolant temperature did not influence the jet breakup length. Comparison of the diameters of fragments of the solidified molten material thus obtained with fragmentation theory shows that the median fragment diameter is between the critical Weber number theory and the most-unstable wavelength of the instability theory of surface waves at a gas liquid interface.The quench behavior of the molten jet in coolant was calculated for FBR conditions by using the model that reflects actual fragmentation behavior. It was clarified that the mass of molten material in the coolant pool is related to the fragment diameter under FBR conditions.     

Dispersion strengthening in a hypereutectic Al-Si alloy prepared by extrusion of rapidly solidified powder

When a hypereutectic aluminum-silicon alloy containing 16 wt pct silicon was rapidly solidified into powder using the spinning water atomization process, the individual powder grains were predominantly aluminum that was supersaturated with silicon and also contained well-dispersed 0.02-μm silicon particles. Although the silicon particles grew when the powder was extruded into a bar at temperatures from 673 to 803 K at an extrusion ratio of 4.3 and an extrusion speed of 0.9 mm/s, the average diameter was maintained on a submicron level. When the extrusion temperature was decreased from 803 to 673 K, the average diameter of the silicon particles in the extruded bar decreased from 0.8 to 0.5 μm, while the Vickers hardness (HV) and the ultimate tensile strength of the extruded bar increased from 120 to 160 (HV) and from 330 to 500 MPa, respectively. Both the hardness and the tensile strength of the extruded bars were several times higher than those of conventionally cast bars of the same alloy with cooling rates from 10⁻¹ to 10² K/s. On the other hand, the elongation decreased from 5.5 to 3.1 pct when the extrusion temperature was decreased from 803 to 673 K.     

Surface hardening of nickel alloys by means of plasma nitriding

Surface hardening of Ni alloys by plasma nitriding was investigated by using tentative Ni binary alloys contained nitride forming elements such as Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, Mn, Fe, Al, or Si at the nitriding temperature from 673 to 1073 K. Surface hardness was different depending on the types of alloying elements, their contents, and their nitriding temperatures. Higher hardness than HV500 was obtained in Ti, V, Nb, and Cr containing alloys at 823 to 873 K, but other alloys showed lower surface hardness than HV400. The elements Ti, V, Nb, and Cr were the effective alloying elements for the surface hardening of nitrided Ni alloys. From transmission electron microscopy (TEM) and X-ray diffraction (XRD) analysis, the nitrided layer was composed of fine precipitate particles in the matrix of the nitrided layer. At the lower nitriding temperature, these particles were metastable fine particles or Ginier-Preston (GP) zone having coherency with the matrix, and these fine particles induced large microstrain in the matrix. However, at the higher nitriding temperature, equilibrium nitride particles were precipitated and coherency with the matrix was decreased. Therefore, the hardening of Ni alloys by plasma nitriding was due to the microstrain induced in the nitrided layer by the precipitation of metastable particles or GP.     

Plasma-carburization of nickel-based self-fluxing alloy

Surface hardening of Ni-based self-fluxing alloy was carried out using plasma-carburization. The carburization was performed utilizing methane glow discharge plasma on sintered Ni-based self-fluxing alloy (METCO 16C) at 977 °C and 1065 °C for 3.6 ks. The carburized surfaces were characterized by employing optical microscopy, scanning electron microscopy (SEM), electron probe microanalysis (EPMA), transmission electron microscopy and Vickers micro-hardness testing. The SEM and EPMA micrographs show that the thickness of the carburized hard layer increased with increasing temperature, to a depth of approximately 120-200 μm. Hard chromium carbide (Cr₃C₂) was found to precipitate near the surface on carburization at 977 °C more than at 1065 °C. Cr₃C₂ contributes to the surface hardening.     

The influence of investor's behavioral biases on the usefulness of the Dual Moving Average Crossovers

The so called Dual Moving Average Crossovers are said to be useful signals for forecasting trends of stock prices, as one of the technical analysis methods. First, we examined the usefulness of these crossovers by using historical daily closing price data and tick by tick price data of Japanese stocks. The results revealed that these crossovers were useful as confirmatory signals for forecasting market trends. Second, we tried to identify the underlying reasons for the usefulness of the crossovers. A model, which followed the Efficient Market Hypothesis, was found to fail to generate the price fluctuation where the crossovers were useful. We then developed a model that incorporated investor's suspicion about current price validity and two famous behavioral biases: conservativeness and representativeness. We identified the mechanism that those crossovers were closely related to investor's suspicion and the behavioral biases. Kotaro Miwa: He is a Ph.D. candidate at the University of Tokyo. He is also a quantitative financial analyst and fund manager at Tokio Marine Asset Managements. He received his B.A. degree from the Faculty of Engineering at the University of Tokyo in 2001. He also received M.A. degree from the Department of Systems Science at the University of Tokyo in 2003. His current research interests include behavioral finance and financial engineering. Kazuhiro Ueda, Ph.D.: He is an associate professor at the University of Tokyo. He received his B.A. degree from the Faculty of Liberal Arts and Science at the University of Tokyo in 1988. He also received M.A. and Ph.D. degrees in cognitive science from the Department of Systems Science at the University of Tokyo in 1990 and 1993. His current research interests include cognitive analysis on scientific problem solving, adaptive human-machine interface, artificial market and behavioral finance and cognitive robotics.     

Finite element method analysis and control of micro actuator with piezoelectric element for spinstand

A micro actuator was introduced to achieve highly precise track following with an head gimbals assembly (HGA) for a spinstand tester. The actuator constructed with a piezoelectric element and a stroke amplifying mechanism was designed by using finite element method analysis. According to robust controller using H_∞ control theory was designed so as to guarantee the control performance for two kinds of HGAs. As the experimental results of the step response show, it was found that the settling time and the rise time were about 3.0 ms and about 1.0 ms, respectively.     

Linear actuator for precise track following

A new air bearing linear actuator with a Voice Coil Motor (VCM) was investigated for a precise head track following in an Hard Disk Drive (HDD) magnetic recording tester system. The actuator has a servo bandwidth of two times as wide as that of a conventional HDD, due to a high stiffness without any friction. A low-pass filter was introduced to precisely monitor the step response behavior by reducing the relatively large noise of the used optical fiber sensor. The effect of the low-pass filter was investigated comparing with the other method. Track following accuracy was also tested by using a conventional 2.5-inch hard disk drive. The head installed on the actuator could follow on a track by using Position Error Signal (PES) from the servo pattern. When a Double Metal In Gap (D-MIG) head of 4 μm track-width was loaded on a disk rotating at 4200rpm, the tracking error could be compressed down to one-twentieth of the track-width. The tested system did not show any azimuth error of head-tracking due to the linear motion. In conclusion, the air-bearing linear actuator is suitable for a precise track following mechanism of a spin-stand tester for an HDD system.     

Influence of small Cu addition on the crystallization of amorphous alloy Fe−Zr−B

The behavior of Cu during the crystallization processes in Fe?Zr?B amorphous alloy was studied by atom probe field ion microscopy and transmission electron microscopy. It was found that Cu atoms formed clusters prior to the crystallization reaction. During the crystallization, Cu enriched clusters provided nucleation sites for α-Fe phase so that the nucleation density of α-Fe phase is increased.

     

A topology optimization method based on the level set method incorporating a fictitious interface energy

This paper proposes a new topology optimization method, which can adjust the geometrical complexity of optimal configurations, using the level set method and incorporating a fictitious interface energy derived from the phase field method. First, a topology optimization problem is formulated based on the level set method, and the method of regularizing the optimization problem by introducing fictitious interface energy is explained. Next, the reaction–diffusion equation that updates the level set function is derived and an optimization algorithm is then constructed, which uses the finite element method to solve the equilibrium equations and the reaction–diffusion equation when updating the level set function. Finally, several optimum design examples are shown to confirm the validity and utility of the proposed topology optimization method.