Papers by Author: Tetsuo Shoji

Abstract: Effects of Zr addition and annealing on the magnetostriction of Tb-Dy-Fe alloy crystals were investigated. The (Fe_1.9Tb_0.27Dy_0.73)_1-xZr_x (x: 0, 0.0125, 0.025, 0.05, 0.075) crystal fibers about 2 mm in diameter were grown by the micro-pulling-down (μ-PD) method. The grown crystals showed the low chemical segregation along longitudinal direction and the growth direction was oriented in the <311> direction. The saturation magnetostriction of the fiber increases with increasing the Zr content, annealing temperature and annealing time. The fiber-shape Tb-Dy-Fe crystals prepared by the μ-PD method are promising for the applications as sensors and actuators.

Abstract: The mechanism of the degradation process of nickel based superalloy CM247LC under creep loading was analyzed by considering the microscopic chemical composition of the material such as phase boundary of γ and γ’ phases. In this study, a Directionally Solidified (DS) nickel-based superalloys CM247LC was used as test specimens. The creep test was performed at 900oC under an uni-axial stress of 216 MPa. Each specimen was creep ruptured and interrupted at different hours and then analyzed. The change of chemical composition around the interface analyzed precisely by used Auger Electron Spectroscopy (AES). It was found that the local enrichment or lack of cobalt and chromium was found at the interface. This tendency was not found in the stress free area. Since chromium is the well-known element that dominates the degradation of this material, such an enrichment or lack of cobalt and chromium may play an important role for forming a crack propagation path near the interface. This local segregation should be analyzed further to make clear degradation mechanism of this material.

Abstract: Three types of austenitic stainless steels JK2, JJ1 and JN1 were isothermally aged at temperatures from 600 to 900°C for 10 to 1000 minutes in order to study the microstructural evolution and its effect on the fracture toughness at cryogenic temperatures. The Charpy V-Notch fracture energy at 77 K showed a significant decrease with aging time in JJ1 and JN1 steels because of their higher contents of C and N. In contrast, the fracture energy corresponding to the aged JK2 steel decreased gradually with aging time. The abundant intergranular precipitation of carbides and nitrides seems to be the responsible for the fracture toughness deterioration in the aged JJ1 and JN1 steels. On the other hand, the intergranular precipitation of carbides was less abundant in the aged JK2 steel. The scanning electron microscope fractographs of the CVN test specimens corresponding to the aged JJ1 and JN1 steels showed mainly an intergranular brittle fracture and its fraction increased with aging time and temperature. In general, the presence of a more abundant intergranular precipitation resulted in a more rapid decrease in toughness with aging time.

Abstract: Magnetic-optical film (hereafter, MO film) is a bismuth-doped iron garnet film grown on a gadolinium gallium garnet substrate by liquid phase epitaxial method. Because an MO film has physical characteristics such as Faraday rotation and magnetic anisotropy, the distribution of its external magnetic field can be visualized. The MOI (Magneto-Optic/Eddy Current Imager) was developed to detect far-side cracks and corrosions in aluminum alloy materials on an aircraft. The detection probability of 2.5mm-length cracks is 90/95% when the MO film and the induced area current are used. However, the quantitative evaluation algorithm for quantifying crack shapes and sizes has yet to be proposed. This paper investigates a quantitative evaluation algorithm for magnetic optical eddy current imaging by using the finite element analysis methods (FEM) and experimental results. The MOI including a primary induction coil was simulated by FEM software. The position, depth, shape, and the induction current frequency, lift-off were simulated and examined. A simplified quantitative evaluation algorithm for MOI is proposed by using the analysis results.

Abstract: The magneto-optical nondestructive inspection system (hereafter refer to as RMO system) using magneto-optical sensor (hereafter refer to as MO sensor) offers the benefits of providing image data and LMF information at the same time. Therefore this system makes it possible to carry out remote and high speed inspection of cracks from the intensity of the reflected light and to estimate the shape of a crack more effectively than by already existing methods. In other words, the shape of crack could be evaluated using image data, and crack depth can be determined by calculating the intensity of reflected light. The purposes of this study were to confirm the vertical components of leakage magnetic flux from a crack using RMO system and to verify the effects of MO sensor using the finite element method and dipole model calculation. The effectiveness of these analysis methods was compared with experiments using a RMO system and several types and sizes of the crack on plate specimens. The volume of a crack could be estimated using the optical intensity regardless of the shape of cracks.

Abstract: The effects of cold working and heat treatment on caustic stress corrosion cracking (SCC) of mill annealed (MA) alloy 800M in boiling solution of 50%NaOH+0.3%SiO2+0.3%Na2S2O3 were investigated by means of microstructure examination, tensile test, X-ray stress analysis, SCC testing of C-rings, Auger electron spectroscopy (AES), scanning electron microscopy (SEM) and metallography. The microstructure of alloy 800M under tested conditions was austenite. With a strain of 25% by cold working, the grains of alloy 800M became longer, yield strength (YS) and ultimate tensile strength (UTS) increased, elongation (δ ) decreased, residual stress and the susceptibility to SCC increased. With increasing temperature of heat treatment of alloy 800M with cold working, the grains became bigger , residual stress, YS and UTS decreased and δ increased, the susceptibility to SCC of alloy 800M decreased. In boiling caustic solution, SCC cracks on the surfaces of C-ring specimens polarized potentiostatically at –20mV/SCE initiated from pitting and propagated along grain boundaries. AES analysis indicated that the surface films on MA alloy 800M were enriched in nickel and depleted in iron and chromium.

Abstract: We are developing a Small-Punch SCC testing apparatus with an in-situ observation system applied to high temperature water and high pressure. This new testing apparatus is used to evaluate the Environmentally Assisted Cracking effect. A constant loading rate was carried out with different values of speed, of water flow rate and different polishing of the sample. In BWR (Boiling Water Reactor) conditions (9MPa and 288°C) it was found that the loading rate, water flow rate and surface preparation of the sample have some influence if the test is performed during a short time (maximum 4 days). After the test, by SEM observation, studying the secondary microcracks, it is possible to find all the sequences of EAC process. The in-situ observation system permits to study the strain of the sample during the test.

Abstract: In general, the S-N curve in railway axles was mainly carried out under 107-108 cycles, while the service area of a railway axle is 108-109 cycles. The strain gages using electrical resistance have been used to measure stresses in railway vehicle wheelsets. However, there are some problems with strain gages using electrical resistance for railway axles. For example, the measured data is for special or limited intervals only. Strain gage installation is complicated, that is, it requires lead wires for measurement. The design of railway axles makes use of data that was obtained many years ago. The applied stresses in wheelsets running for a long time and in new railway vehicle wheelsets have not been studied clearly yet. It is necessary to carry out stress monitoring for more than 108 cycles to evaluate the safety of railway wheelset. Therefore, it is necessary to develop new stress monitoring techniques that can easily measure the working stress of the wheelset. In the present paper, the stress measurement technique of copper electroplating is considered because of its high potential for this purpose.

Abstract: The effect of grain boundary character distribution (GBCD) on intergranular stress corrosion cracking (IGSCC) in austenitic stainless steels in high temperature water was verified experimentally. GBCD control using the strain annealing method increased the fraction of low- S coincidence site lattice (CSL) boundaries and the segmentalized network of random grain boundaries in austenitic stainless steels. The fractions of low- S CSL boundaries of GBCD controlled steels were 75–85%, while those of uncontrolled steels were 60–70%. Creviced bent beam tests were conducted at 561 K in pure water containing 8 ppm dissolved oxygen for stress corrosion cracking (SCC) evaluation. The tests revealed that GBCD control suppressed IGSCC initiation or propagation and that cracks were predominantly propagated along random grain boundaries. It is considered that induced lower- S CSL boundaries result in high resistance to IGSCC.