Papers by Author: Kazuya Kurokawa

Abstract: Fe-Si alloys with various concentration of Al (0, 1, 3 and 5 % by mass) were synthesized by a spark plasma sintering technique. The specimens were prepared in an evacuated chamber of less than 4 Pa and under compressive stress of 40 MPa. During spark discharge, the heating rate was fixed at 10°C/min. After the SPS process was completed, the specimen surfaces were ground with silicon carbide papers. The metallographic characterization was performed by mean of X-ray diffraction (XRD) and scanning electron microscope (SEM) equipped with energy dispersive X-ray (EDX) spectroscopy. According to sintering curves, all samples seem to have a similar sintering behavior. The densification of specimens was completed in the temperature range of about 1020-1050°C. Microstructure and phase characterization revealed that the alloys were mainly composed of FeSi₂ and FeSi phases containing oxide inclusions. The SEM images indicated that the fraction of FeSi phase and oxide inclusions appears to decrease with increase in Al concentration in the Fe-Si alloy.

Abstract: High Si steels (base steel and with addition of 0.1, 0.3, 0.5mass% Mo) were oxidized in a N2-3%O2-20%H2O atmosphere at 1150oC for 0.3-10.8ks. The oxidation kinetics showed that base steel has the highest weight gain. In contrast, 0.5Mo has the lowest weight gain. From SEM results, the scale thickness decreased as Mo content increased. Distribution of alloying elements showed that Mo is enriched in internal oxidation zone (IOZ) and played a role in slowing the outward diffusion of Fe, thus, producing a thinner Fe oxide scale and lower weight gain.

Abstract: The high temperature corrosion behavior of Si-containing alloys consisting of Cr-Si-Ni and CoNiCrAlY-Si alloys fabricated by spark plasma sintering technique was investigated in the liquid phase of Na₂SO₄ + 25.7 mass% NaCl at temperatures ranging from 923-1273 K. The purpose of this study is to develop excellent corrosion resistant alloys for coating applications. Our experimental results show the CrSi₂ alloy with 10 mass% Ni content and the CoNiCrAlY alloy with 30 mass% Si content are the most promising materials for applications in this atmosphere. This is due to the formation of a protective SiO₂ and Al₂O₃/SiO₂ scale, respectively. The formation of a dense and continuous oxide layer composed and/or consisted of SiO₂ plays a significant role in hindering the inward diffusion of chlorine and sulfur to the alloys substrate. Particularly, the corrosion mechanism of Cr-Si-Ni alloys and the influence of Ni addition on the corrosion resistance of CrSi₂ alloy are discussed in the present paper.

Abstract: The oxidation behavior of β-SiAlONs (Si_6-zAl_zO_zN_8-z, z = 1, 3, and 4) was investigated at temperature ranging from 1473 to 1673 K in a (N₂-3%O₂)-20%H₂O atmosphere. Oxidation kinetics was followed on the basis of the mass gains, and the oxidized specimens were characterized by FE-SEM, XRD, and EPMA. The mass gain was found to clearly increase with temperature and the z value. For oxidation at 1473 and 1573 K, the gain in mass was observed to be small. During oxidation at 1573 K, mass loss occurred. This loss may be because of the formation of volatile SiO(OH)₂. The changes in mass depended on the z value. On the other hand, the oxidation rates at 1673 K were found to be higher than those at and below 1573 K by more than one order of magnitude. The oxidation kinetics at 1673 K followed an almost linear rate law. The XRD and EPMA results showed that the oxide formed on β-SiAlON (z = 1) was composed of a mixture of amorphous aluminosilicate and mullite, whereas that on β-SiAlONs (z = 3 and 4) was composed only of mullite.

Abstract: The objective of this study is to develop an excellent corrosion resistant alloy for high temperature coating applications. The Si-containing alloys consisting of CoNiCrAlY and CrSi₂ alloys with varying Si and Ni content respectively were prepared by spark plasma sintering (SPS) technique. The corrosion behavior of these alloys was investigated in the gas phase of air-(Na₂SO₄+25.7mass%NaCl) at elevated temperatures of 923, 1073 and 1273K. The results showed that CoNiCrAlY alloy with 30mass% Si content and CrSi₂ alloy with 10mass% Ni content were the most effective materials for application in the gas phase of air-(Na₂SO₄+25.7mass%NaCl) due to the formation of protective Al₂O₃/SiO₂ and SiO₂ scale, respectively. Therefore, it is realized that CoNiCrAlY-30mass% Si and CrSi₂-10mass% Si coating are very effective for improving of high temperature corrosion resistance of STBA21 steel.

Abstract: Low carbon steels containing Si of 0.1 and 1.0 mass%, and 99.5 mass% pure Fe were oxidized in laboratory air and in a H₂O-containing atmosphere at 1173 K. Acoustic emission technique was used to assess the temperature (T_F) at which the first major scale failure takes place during cooling. T_F of 1.0 %Si steel oxidized in the air was found to increase with an increase in the scale thickness and cooling rate, while T_F of 0.1 %Si steel had almost no dependence on these parameters. Moreover, the values of T_F of both the steels oxidized in the H₂O-containing atmosphere are higher than those in the air. These differences are attributable to the cooling rate, scale structure, and eutectoid reaction. In general, higher cooling rate implies a higher strain rate and there may be a larger temperature gradient across the scale thickness, which additionally enhances the scale failure. The metallographic examinations revealed that eutectoid magnetite particles in the scales formed on 0.1 %Si steel coarsen as the cooling rate decreases and magnetite seam was formed at the bottom of the iron oxide layer. It is clear that the influence of magnetite precipitation increases as the cooling rate decreases and thus the stress in the scale increases.

Abstract: For the purpose of the prediction of casting structures, heterogeneous nucleation rate in the undercooled melt of solififying Al-Si alloys were evaluated by comparing experimentally observed macrostructures of solidified ingots with numerically simulated ones. Molten alloys were unidirectionally solidified in an adiabatic mold from a steel chill block located at the bottom of the mold. In the experiment, columnar to equiaxed transition (CET) was observed. A numerical simulation for grain structure formation of the sample ingots was carried out using a cellular automaton (CA) method, and heterogeneous nucleation rate in the solidifying alloys were evaluated by producing the similar structures to experimental ones. An attempt was made to predict the grain structure of conventionally cast ingots using the evaluated heterogeneous nucleation rate. However, the simulation could not predict the structure of ingot with low superheat due to crystal multiplication near the mold wall. The crystal multiplication mechanism, so-called "Big Bang mechanism", was introduced into the simulation and the simulation could predict the grain macrostructure composed of columnar and equiaxed crystals that were similar to experimentally observed one.

Abstract: In this study, the high temperature oxidation behavior of polycrystalline MoSi2 in a low-pressure atmosphere was investigated. Polycrystalline MoSi2 was produced by the spark plasma sintering process. Oxidation tests were carried out at 1500°C at either 10Torr or 760Torr in an Ar-20%O2 atmosphere. For both conditions, the weight change peaked at the initial oxidation stage, and then their weights gradually increased with increasing oxidation time. The sample weight became heavier in the ambient pressure than in the low-pressure, but the evaporation oxidation was not significant in the low-pressure condition. After the low-pressure oxidation tests, the formation of Mo5Si3 in the MoSi2 substrate was identified. The oxidation resistance of MoSi2 at 1500°C is discussed based on the obtained results.

Abstract: Isothermal oxidation behavior of Al added Mo-Si-B in-situ composites was investigated under Ar-20%O2 and air atmosphere over the temperature range of 1073–1673 K. The Al added Mo-Si-B composites ((Mo-8.7mol%Si-17.4mol%B)-1mol%Al) were prepared by arc-melting, and homogenized at 2073 K for 24 h in an Ar-flow atmosphere. The ternary Mo-Si-B in-situ composite exhibited a rapid mass loss at the initial oxidation stage and then the passive oxidation after the substrates were sealed with borosilicate glass in the temperature range of 1173–1473 K, whereas it exhibited a rapid mass gain around 1073 K. On the other hand, the Al addition significantly improved the oxidation resistance of Mo-Si-B in-situ composites at temperatures from 1073–1573 K. These excellent oxidation resistances are considered to be due to the rapid formation of a continuous, dense scale of Al-Si-O complex oxides.