Papers by Author: Yuji Kawakami

Abstract: This paper describes the fatigue crack propagation behavior of extruded AZ31B magnesium alloys (average grain size: approximately 15 and 119 μm, respectively). Fatigue crack propagation tests were performed on center cracked tension (CCT) specimens at a stress ratio of R=0.1 and a frequency of 10 Hz at room temperature. Loading axis was parallel to the extrusion direction; crack face was perpendicular to basal plane of each grain. The crack growth rate (da/dN) of the coarse-grained specimen was approximately 5 times higher than that of the fine-grained specimen. Fracture surfaces of the fine-grained and coarse-grained specimens showed various directional steps independent of macroscopic crack growth direction.

Abstract: This paper deals with fatigue crack propagation behavior of rolled AZ31B magnesium alloy. Two types of specimens with the loading axis parallel to rolling direction were machined; fatigue crack propagation direction was parallel to transverse direction (L-T specimen), and short transverse direction (L-S specimen). Fatigue crack propagation tests were performed with center cracked plate tension specimen with stress ratio R=0.1 and frequency of 10Hz at room temperature. Crack propagation rate of L-T specimen was approximately 10 times higher than that of L-S specimen. SEM-EBSD observations revealed that the c-axis direction is unfavorable for the fatigue crack propagation in textured polycrystalline magnesium alloy.

Abstract: Pseudoelastic behaviors were observed in compressive and tensile loading-unloading tests at room temperature. The large anelastic strains were observed in compressive stress-strain hysteresis loops. The fatigue limit of axial load-controlled fatigue test at 107 cycles was 90MPa at room temperature. The deformation twins were observed in the specimen subjected to the higher stress amplitude of fatigue limit and free deformation twins were observed in the specimen subjected to the lower stress amplitude of fatigue limit. Stress-strain hysteresis loops were linear in tensile and compressive phases at the lower stress amplitude of fatigue limit and the complicated pseudoelastic deformations were observed in tensile and compressive phases at the higher stress amplitude of fatigue limit of axial load-controlled fatigue tests. Compressive mean strain generated by cyclic pseudoelastic deformations at the higher stress amplitude of fatigue limit. Fatigue cracks initiated at the secondary particle/matrix interface or broken secondary particle near the surface. Subsequently, small cracks tended to grow through transgranular.

Abstract: ZnO is heat-resistant and inexpensive, and the raw material of which is abundant, it is considered to be a good candidate thermoelectric material. Usually, a low-resistance n-type ZnO sintered body is obtained by doping 0.5-5 mol% Al2O3 followed by burning at a high temperature of approximately 1673K. However, this high-temperature burning has drawbacks, such as high power consumption and an increase in thermal conductivity with grain growth. Under these circumstances, we attempted to address these disadvantages. When ZnO was burned with Al as a dopant in an electric furnace at a temperature as low as approximately 1473K, ZnO with relatively good thermoelectric properties was obtained. In addition, the Al-doped sample showed lower electric resistance (332 K: 6.85×10-4Ωcm) than the Al2O3-doped sample, as determined on the basis of the resistance temperature characteristics of these samples. The causes of this low resistance may be as follows: 1) the metal-Al-mediated sample was densified by burning at a low temperature of approximately 1473K and 2) the Al distribution to the entire region of the ZnO bulk, resulting in the spread of Al solid-solution regions in the ZnO. We also found that the thermal conductivity decreased (973 K: 3.66 Wm-1K-1) in the Al-doped sample.

Abstract: To clarify the influence of internal pulsed current upon the sintering behavior of powder materials during spark plasma sintering processing, simultaneous measurement of internal current using magnetic probe was carried out. Magnetic probe is installed to the side of the sintering ZnO powder material through the carbon graphite sintering die, and detects the magnetic field generated by internal current which flow through the specimen. By magnetic probe measurement, the internal current that flows through the specimen during SPS process was several hundred ampere, and the ratio of the internal current to the total current was found to be dependent on the electrical conductivity, diameter of powder material and the progress of SPS process. The measurement and estimation of an internal pulsed current using a magnetic probe in the specimen is very useful for in situ observation of the sintering behavior during the SPS process.

Abstract: Titanium has widely been used as a biomaterial because of its excellent corrosion resistance and biocompatibility. However, problems with respect to biological reaction and fitness of elastic modulus for human bone or tooth have yet to be solved. Porous titanium is expected to be a promising material to solve these problems. The aim of this study is to clarify the effect of the porous structure of this material on the biomechanical compatibility. The spherical pure titanium powder, with an average particle size of 100 µm, was sintered by spark plasma sintering. The sintered porous titanium compacts had a porosity of 33 %. The specimens were machined from the sintered compacts for the evaluation of the mechanical properties. The elastic modulus indicated a value close to human bone, while the tensile and compressive strengths showed lower values than those of human bone.

Abstract: In this investigation, functionally graded technique has been applied to prepare tungsten Carbide alloys (WC-Co alloys)-austenitic stainless steel to improve corrosion and wear resistance and reduce their fabrication costs. Two types of specimens were prepared by Pulsed Current Sintering (PCS) method. One is fabricated from WC alloy (WC-20 wt% Co, WC-20 wt% Fe-46Ni) powder and SUS316L stainless steel powder, and is designated “powder / powder process“. The other is fabricated from pre-sintered WC alloy and SUS316L plate, and is designated “bulk / bulk process”. Both types of specimens have four functionally graded layers. Mechanical properties of these specimens were evaluated by three points bending test, and microscopy observations were made to their microstructures. Although the proper bending strength of bulk / bulk process specimen was obtained, the optimum sintering condition is sensitive to the sintering temperature and function graded layers.

Abstract: Austenitic stainless steel has been used as a corrosion resistance material in tough corrosive circumstance. However, austenitic stainless steel has poor wear resistance property due to its low hardness. Tungsten Carbide alloys (WC) are widely used as tooling materials, because of their high hardness and excellent wear resistance property. In this investigation, we apply powder composite process to obtain hard layer of Stainless steel. The composite material was fabricated from planetary ball milled WC powder and SUS316L stainless steel powder and sintered by Pulsed Current Sintering (PCS) method. We also added TiC powder as a hard particle in WC layer. Evaluations of wear properties were performed by pin-on-disk wear testing machine, and a remarkable improvement in wear resistance property was obtained. The weight loss rate of the composite was 1/10 of SUS316L. In addition, it was found that TiC hard particle addition has a positive effect on the wear resistance property. EPMA investigation showed good dispersion of WC hard phase and TiC hard particle with SUS316L matrix.

Abstract: Thermoelectric elements using environment-friendly materials with high thermoelectric conversion efficiency and of these thermoelectric elements can be increased by using a structure combining n-type and p-type semiconductors. From the above point of view, attention was directed at ZnO as a candidate n-type semiconductor material and investigations were made. As the result, a dimensionless figure of merit ZT close to 0.28 (1073K) was obtained for specimens produced by the PCS (Pulse Current Sintering) method with addition of specified quantities of TiO2, CoO, and Al2O3 to ZnO. It was found that the interstitial TiO2 in the ZnO restrains the grain growth and CoO acts onto the bond between grains.

Abstract: Pulsed Current Sintering (PCS) process possesses some problems that need to be resolved. We, therefore aims at understanding phenomena of PCS process by presenting some basic data on in situ sintering behavior of PCS. In order to observe in situ sintering behavior of PCS, a special graphite mold equipped with thermo couple and electrodes were designed to measure the temperature, electric current and voltage inside the powder during PCS process. We apply three types of raw materials, especially for ZnO (thermoelectric material) as semiconductor, Al2O3 as non-conductor and WC (Tungsten Carbide) as good conductor. The observation succeeded and some valuable data were obtained. The results showed that the temperature in the Al2O3 powder is 100 K higher than the graphite mold at the temperature of 1473K and ZnO powder is 150 K higher than the graphite mold at the temperature of 1373K. The electric current and voltage were measured for each powder during PCS process. In addition, their electric resistance properties were calculated. The electric resistance showed different behavior.