Keizo Hashimoto

Alloy design of gamma titanium aluminides based on phase diagrams

Abstract Phase stability in the TiAlX (X = Cr, Mo and W) system has been investigated at 1473 and 1573 K by the following methods: microstructure observation of quenched specimens, diffusion coupling experiments, thermal analysis of DTA and in situ observation of X-ray diffraction. The ternary phase diagrams of TiAlX system are proposed; additions of Cr, Mo and W stabilize the β phase in the ternary phase diagrams. The tendency of ‘β stabilizer’ is in the following order: 6 at% Cr = 3 at% Mo = 1 at% W ≒ 10 vol% β phase. In these Ti-Al-X ternary systems, the α + β + γ three-phase coexisting region is close to the TiAl binary line and sifts slightly toward a higher concentration of Al as temperature increases. Based on the ternary phase diagrams, (γ + β) TiAl having a super-plastic capability and partial lamellar (P-L) microstructure which shows relatively well balanced mechanical properties from room temperature to elevated temperatures have been rationalized.

Study on phase stability in TiAlX systems at high temperatures

Abstract The effect of additional elements X(XV, Cr, Mn, Nb, Mo and Ta) on γ(TiAl)-based titanium aluminides has been investigated. The addition of elements X caused a change in phase equilibria among the γ, α 2 (Ti 3 Al) and β (b.c.c. Ti) phases; the β phase is stabilized by the addition (except XTa), resulting in a drastic change in microstructures. The TiAlMo system has shown unique behavior; the β phase region protrudes toward the α phase region around 1200°C in the ternary phase diagram. The β phase contained in specimens quenched from high temperature has a long-range ordering structure that is different from the B2 structure. The chemical potentials of the β, α and γ phases may have close values around 1200°C and their phase stabilities are strongly influenced by processing or heat treatment.

Study on phase stability in Ti–Al–X systems at high temperatures

Abstract The effect of additional elements X(X = V, Cr, Mn, Nb, Mo and Ta) on γ(TiAl)-based titanium aluminides has been investigated. The addition of elements X caused a change in phase equilibria among the γ, α2(Ti3Al) and β (b.c.c. Ti) phases; the β phase is stabilized by the addition (except X = Ta), resulting in a drastic change in microstructures. The Ti–Al–Mo system has shown unique behavior; the β phase region protrudes toward the α phase region around 1200 °C in the ternary phase diagram. The β phase contained in specimens quenched from high temperature has a long-range ordering structure that is different from the B2 structure. The chemical potentials of the β, α and γ phases may have close values around 1200 °C and their phase stabilities are strongly influenced by processing or heat treatment.

Microstructure and phase stability of TiAl–W ternary alloy

Abstract This paper discusses microstructure control of a TiAl–W ternary alloy with emphasis on the phase stability. As a necessary prerequisite for study of the phase stability in the present study, uniformity of the samples is assured by using a well homogenized Ti–W master alloy and breaking down cast structure through thermomechanical processing. The sample, Ti–47Al–3W (at.%) alloy, is prepared by isothermal forging with a reduction of 70% in a temperature range between 1373 and 1573 K, in order to form a fine γ+β micro duplex structure similar to the TiAl–Cr system having good superplastic forming and diffusion bonding ability. Water quenching from 1473 K produces a γ+β+α 2 phase in the TiAl–W system and a γ+α 2 binary phase in the TiAl–Cr system. Tungsten is enriched in the β phase and the volume fraction of β phase is significantly higher in the TiAl–W than the TiAl–Cr ternary system, suggesting that stronger β stabilization is achieved by W alloying than Cr alloying.

Fabrication of WS2-Dispersed Al Composite Material by Compression Shearing Method at Room Temperature

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.

High-Temperature Tensile Properties of Ti-Al-X (X=Cr,W) Consisting of α2, β and γ in Three Phases

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.

Formation of Protective Intermediate Phase in Ta Addition TiAl during High Temperature Oxidation

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.

Role of Ti/Al Ratio of Ti-Al-X (X=Cr, Nb, Ta and W) Intermetallics on High Temperature Tensile Properties

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.

Oxidation Behavior of SiC/TiAl Composite Material

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.

Influence of the Cube-Texture and the Grain Growth on the Creep Behavior in Platinum-10%Rhodium Alloy

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.