Papers by Author: Keizo Hashimoto

Abstract: In this study, WS₂-dispersed Al composite material was fabricated by Compression Shearing Method at Room Temperature, using various WS₂ content ratios. The mechanical and friction properties of the WS₂-dispersed Al composites were measured. As a result, the density measurements showed that the compacted WS₂-dispersed aluminum composite had a relative density of 95 to 99%. Tensile strength of WS₂-dispersed Al has 200 MPa. The friction coefficient of Al/0.5vol.%WS₂ was 0.14, a reduction of 83%, in comparison with the 1.0 friction coefficient of the pure Al matrix material. The addition of WS₂ to the matrix systems used reduced the friction coefficient. Therefore, WS₂-dispersed Al composite material is useful for maintenance-free material of slide member.

Abstract: The mechanical properties of g-TiAl at elevated temperatures have been investigated extensively over the last 30 years. Designed alloys have been proposed from the first generation alloy (Ti-48Al-2Cr-2Nb) to the second, the third and the fourth generations. However, a decisive chemical composition of g-TiAl has not been agreed among researchers yet. The main reasons for this situation are difficulties in compositional control of Ti-Al-X-Y. In this paper, the high temperature tensile properties of g-TiAl alloy with lots of different composition have been examined from the room temperature to 1200C and the tensile strength data of those specimens have been summarized. It is clear that Ti/Al atomic ratio plays an important role on the behaviors of the high temperature strength since the Ti/Al atomic ratio is strongly related to the phase stabilities between g and a₂ phases in the binary Ti-Al phase diagram. A very narrow confine of a/a₂ atomic ratio exists in the specimens having the comparatively high tensile strength at the elevated temperatures. Moreover, additions of the third elements such as Cr, Nb, Ta and W to g-TiAl contribute on the increase of the tensile strength and the shift of the phase stability among a₂, b and g phases. In order to utilize g-TiAl alloys in the various machine components at high temperatures, the severe process controls of melting, casting, thermo-mechanical treatments and heat treatments are indispensable.

Abstract: Gamma titanium aluminides (γ-TiAl) have been investigated extensively for more than 25 years, since they are considered to be candidate materials for advanced jet engine components, automobile exhaust valves, turbo-chargers, and so on. Many researchers have reported that the mechanical properties of γ-TiAl have been improved by micro-alloying and thermo-mechanical microstructure control. Recently, γ-TiAl entered a new era by being applied to low-pressure turbine blades in newly developed commercial jet engines. In order to spread their applications further, material durability and affordability have become key issues. The tensile properties of the Ti-Al-X (X=Cr or W) have been studied intensively at various strain rates and test temperatures in a vacuum atmosphere. It has been demonstrated that the additions of a few atomic percent of Cr or W to γ-TiAl shifts the phase stability drastically and creates relatively fine-grain microstructures consisting of α₂+β+γ in three phases. Although the microstructures of Ti-46at%Al-2.7at%Cr and Ti-45at%Al-1.9at%W show similar morphology, the high-temperature mechanical properties of each indicate distinguishable properties. The former specimens have demonstrated the capability of super-plastic deformation at temperatures above 1323K; the latter specimens, however, have showed relatively higher tensile strength than those of the other specimens having ternary compositions (Ti-Al-X). The differences in the tensile properties of Ti-Al-X (X=Cr or W) have been discussed in conjunction with microstructures and the effects of solid-solution hardening due to W atoms.

Abstract: The mechanical properties and the oxidation resistance of -TiAl at elevated temperatures have to be improved to be used in the severe environmental conditions. It has become clear that the addition of more than 4at.%Ta in TiAl demonstrates a superior oxidation to the other TiAl-X compounds, according to the weight gain results of cyclic oxidation experiments at 1173 and 1273K. Oxidation behaviors are strongly influenced by the Ta concentration in TiAl. XRD, SEM-EDS, and TEM-EDS observations have been carried out to determine the microstructures and the surface compositions of multi-layered oxide scales. It was revealed that a protective intermediate phase simultaneously formed between the substrate γ-TiAl and the oxide scale layer. The Ti53Al32Ta15 ternary compound exists as an equilibrium phase at 1373K, according to the published Ti-Al-Ta ternary phase diagram. This ternary compound can work as a barrier to some extent. It contributes to decelerating the diffusion of Ti and Al atoms and to decreasing the oxidation rate. The formation mechanism of the intermediate phase has been discussed in conjunction with diffusion in TiAl.

Abstract: SiC fiber reinforced intermetallic is one of the promising candidate material for the next generation space plane, because of its excellent high temperature specific strength and elastic modulus. Oxidation behavior of the fiber-reinforced intermetallic (FRIM) is one of the most important properties for the practical use in the severe environment. Recently fabrication process of CVD-SiC fiber reinforced γ-TiAl matrix composite has been developed. Oxidation behavior of SiC/γ-TiAl and γ-TiAl was studied. Cyclic oxidation experiments were executed at 900°C under the dry airflow for 200 hours. Mass gains of the specimens were measured. The cross sections of specimen were observed by optical microscope. Mass gain of the SiC/γ-TiAl composite material was two times larger than that of γ-TiAl. Surface of the SiC/γ-TiAl composite was covered with a comparatively thick oxide scale. Furthermore, formation of the oxide at the vicinity of interface between SiC fiber and γ-TiAl matrix was observed. Oxidation mechanism of SiC/γ-TiAl composite was discussed.

Abstract: In Pt-10mass%Rh alloys which were recrystallized using full annealing at or above 1273K after the cold rolling with the reduction ratio of both 90% and 98%, the formation of cube texture has already been found. When these primary recrystallized alloys are subjected to the further annealing at higher temperatures, the occurrence of the secondary recrystallization can be expected. In this study, the development of the cube texture and the process in the coarsening of crystal grains during the secondary recrystallization were investigated. In addition, the creep tests were carried out for the secondary recrystallized alloys and resultant creep properties were compared with those for the primary recrystallized alloys. It became clear that the cube texture which further developed during the secondary recrystallization in 98% rolled specimens exhibits the greater thermal stability than that in secondary recrystallized 90% rolled ones. During the secondary recrystallization, it was confirmed that the grains coarsened according to the law of tm, where t and m show time and a constant, respectively. The higher development of the cube texture and the coarsening of grains during the secondary recrystallization were found to have remarkable effects of improving the creep resistance.

Abstract: In this study, Nb doped γ-TiAl is designed and examined their tensile properties at elevated temperatures. Small compositional changes cause drastic changes of the mechanical properties at 1273K. In order to clarify the deformation mechanism at elevated temperatures, dislocation structures have been observed using transmission electron microscope (TEM). All of the specimens observed by TEM show that at least two slip systems are operating in each grain. The specimen having relatively lower strength and higher tensile elongations shows more than four operating slip systems in the grains. The specimen having a medium strength shows many super-dislocations and their dissociations that block the other dislocation motions. The specimen having the highest strength shows many deformation twins. These observations suggest that deformation twins become the strong obstacles against moving dislocations at elevated temperatures. The relationships between observed dislocation structures and mechanical properties of γ-TiAl are discussed.