Yoko Yamabe-Mitarai

Rh-base refractory superalloys for ultra-high temperature use

In the previous paper, the authors proposed a new class of superalloys, namely, refractory superalloys. This new concept is defined as alloys with fcc and L1{sub 2} coherent two phase structures similar to Ni-base superalloys, and yet with considerably higher melting temperatures. In this paper, Rh-Nb and Rh-Ti systems were selected to compare with Ir-Nb and Ir-Ti systems which were shown in the previous paper. Rh-Ta system was also selected because of its highest melting temperatures among above binary systems. The microstructure evolution and high temperature strengths of these Rh-base alloys were investigated.

Pt3Ti Nanoparticles: Fine Dispersion on SiO2 Supports, Enhanced Catalytic CO Oxidation, and Chemical Stability at Elevated Temperatures

A platinum-based intermetallic phase with an early d-metal, Pt(3)Ti, has been synthesized in the form of nanoparticles (NPs) dispersed on silica (SiO(2)) supports. The organometallic Pt and Ti precursors, Pt(1,5-cyclooctadiene)Cl(2) and TiCl(4)(tetrahydrofuran)(2), were mixed with SiO(2) and reduced by sodium naphthalide in tetrahydrofuran. Stoichiometric Pt(3)Ti NPs with an average particle size of 2.5 nm were formed on SiO(2) (particle size: 20-200 nm) with an atomically disordered FCC-type structure (Fm3m; a = 0.39 nm). A high dispersivity of Pt(3)Ti NPs was achieved by adding excessive amounts of SiO(2) relative to the Pt precursor. A 50-fold excess of SiO(2) resulted in finely dispersed, SiO(2)-supported Pt(3)Ti NPs that contained 0.5 wt % Pt. The SiO(2)-supported Pt(3)Ti NPs showed a lower onset temperature of catalysis by 75 degrees C toward the oxidation reaction of CO than did SiO(2)-supported pure Pt NPs with the same particle size and Pt fraction, 0.5 wt %. The SiO(2)-supported Pt(3)Ti NPs also showed higher CO conversion than SiO(2)-supported pure Pt NPs even containing a 2-fold higher weight fraction of Pt. The SiO(2)-supported Pt(3)Ti NPs retained their stoichiometric composition after catalytic oxidation of CO at elevated temperatures, 325 degrees C. Pt(3)Ti NPs show promise as a catalytic center of purification catalysts for automobile exhaust due to their high catalytic activity toward CO oxidation with a low content of precious metals.

Stress-induced α″ martensitic (110) twinning in β-Ti alloys

A fully transformed α″ martensite with stress-induced nanoscale {110}, {021}-type compound twin or a 90° rotation twin has been experimentally explored and unambiguously characterized in traditional β-type Ti alloys, which usually undergo a partial martensitic transformation [a β grain partially transformed into α″ with internal (111) twin] by quenching. This newly observed twinning, which matches with the predication based on the deformation twinning theory of Bilby and Crocker [Proc. R. Soc. 288, 241 (1965)], can help to explain the deformation mechanism and aid future development of advanced materials.

High-temperature mechanical properties of Ir-Al alloys

Abstract To design an alloy with high strength around 1773 K and good ductility at room temperature, the microstructure, the compression strength and the creep properties at 1773 K of the Ir–Al alloys with an fcc and B2 two-phase structure were investigated. High-temperature mechanical properties are discussed in terms of microstructure.

Investigations into Ti-(Nb,Ta)-Fe alloys for biomedical applications

UNLABELLED In this study, a Ti-(Ta,Nb)-Fe system was investigated with aims toward the development of high strength, biocompatible titanium alloy suitable for the development of porous orthopedic biomaterials with minimal processing. Notable findings include yield strengths of 740, 1250 and 1360 MPa for the Ti-12Nb-5Fe, Ti-7Ta-5Fe and Ti-10Ta-4Fe alloys, respectively, with elastic moduli comparable to existing Ti-alloys, yielding admissible strains of 0.9 ± 0.3, 1.2 ± 0.2 and 1.13 ± 0.02% for the Ti-12Nb-5Fe, Ti-7Ta-5Fe and Ti-10Ta-4Fe alloys, respectively; more than twice that of human bone. Observed microstructure varied significantly depending on alloy; near pure β-phase was seen in Ti-12Nb-5Fe, β with some ω precipitation in Ti-10Ta-4Fe, and a duplex α+β structure was observed throughout the Ti-7Ta-5Fe. In addition to suitable mechanical parameters, all investigated alloys exhibited promising corrosion potentials on the order of -0.24 V SCE, equalling that seen for a C.P.-Ti control at -0.25V SCE, and substantially more noble than that seen for Ti-6Al-4V. Electrochemical corrosion rates of 0.5-3 μm/year were likewise seen to agree well with that measured for C.P.-Ti. Further, no statistically significant difference could be seen between any of the alloys relative to a C.P.-Ti control regards to cell proliferation, as investigated via MTS assay and confocal microscopy. As such, the combination of high admissible strain and low corrosion indicate all investigated alloys show significant promise as potential porous biomaterials while in the as-cast state, with the Ti-10Ta-4Fe alloy identified as the most promising composition investigated. STATEMENT OF SIGNIFICANCE The findings of this paper are of significance to the field of metallic biomaterials as they detail the development of alloys of satisfactory biocompatibility and electrochemical behaviour, that furthermore display exceptional mechanical properties. Notably, both extremely high compressive yield strengths and admissible strains, up to 1.36 GPa and 1.2% respectively, are reported, exceeding or rivalling that seen in traditional alloys such as Ti-6Al-4V, which typically displays compressive yield strengths and admissible strains on the order of 895 MPa and 0.81% respectively, as well as modern alloys such as Gum Metal or TNZT. That this is achieved in the absence of thermomechanical processing represents a significant and novel outcome of substantial benefit for application as a porous biomaterial.

Precipitation hardening of Ir-Nb and Ir-Zr alloys

The authors previously developed refractory superalloys based on platinum-group metals with fcc and L1{sub 2} two-phase coherent structures. The refractory superalloys are potentially useful at ultra-high temperature, at which Ni-based superalloys can not be used. From among the platinum-group metals, they selected Ir as the base material because its melting temperature (2447 C) is higher than that of Ni (1455 C) and because Ir, with an fcc structure, can be equilibrated with the L1{sub 2} structure phase. Preliminary results demonstrated the superior strength of Ir-based refractory superalloys above 1200 C. Those results also revealed that precipitation hardening occurs and is affected by the shape of the precipitate. The precipitate shape of Ir-based alloys heat-treated at 1200 C is affected by the lattice misfits between the matrix and precipitates. In this study, the authors investigated the precipitation hardening mechanism by observing dislocation structures in deformed samples of Ir-Nb and Ir-Zr alloys using bright-field imaging and dark-field weak-beam imaging techniques with a transmission electron microscope (TEM).

An assessment of Ir–Pt–Al alloys for high-temperature materials

Abstract To design high-temperature materials with suitable ductility and high strength at high temperatures, Ir–Pt–Al alloys were investigated. It is expected that precipitation hardening by the B2 phase and solid-solution hardening of Ir by Pt and Al occur in the Ir–Pt–Al alloys. The phase constitution and microstructure were investigated by X-ray diffraction analysis (XRD), scanning electron microscopy (SEM) and electron probe microscopy analysis (EPMA). Compression creep tests were then carried out with some of these alloys at 1773 K under 30 MPa. The creep mechanism is discussed in terms of the microstructure and phase constituents. Finally, the potential of Ir–Pt–Al alloys as high-temperature materials is discussed.

Creep behavior of Ni-added Ir85Nb15 two-phase refractory superalloys at 1800 °C

Abstract To provide information relevant to Ir-based two-phase alloys for future ultra-high-temperature applications, the compression creep properties for the Ni-added Ir 85 Nb 15 alloys were investigated at 1800 °C under 137 MPa. The results show that Ni addition has a significant effect on the creep resistance of the Ir 85 Nb 15 two-phase refractory superalloy.

Solid Solution Hardening Effect of Ir

Solid solution hardening effects of Ir was investigated to develop high temperature materials at 2223 K. Pt, Rh, Hf, and Zr were chosen as second elements because their solubility into Ir at 2223 K is over 2at% and the melting temperatures of Ir solid solution are above 2273 K. Compressive strength of Ir solid solution at 2223K were investigated. Solid solution hardening effect of Ir is discussed in terms of lattice parameter change and solubility,

The stabilization of Pt3Al phase with L12 structure in Pt–Al–Ir–Nb and Pt–Al–Nb alloys

Abstract It has been reported that the Pt3Al phase has two crystal structures: the high-temperature cubic form of L12 and the low-temperature tetragonal form of the distorted L12 structure D0’c. In this study, the structure of the Pt3Al phase in quaternary Pt–Al–Ir–Nb and ternary Pt–Al–Nb alloys was investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD), differential thermal analysis (DTA), and transmission electron microscopy (TEM) technologies. The structure form was determined to be cubic L12. The stabilization of the L12-Pt3Al structure in room temperature was attributed to the effect of Nb.