Shizuo Nakazawa

NiTi-base intermetallic alloys strengthened by Al substitution

Abstract A series of NiTi-base alloys with Al additions substituting the Ti were designed and evaluated in terms of the microstructure and mechanical properties. It was found that the compression strength is improved drastically by the Al additions, especially when the Al amount is high enough to precipitate Ni 2 TiAl (Heuslar compound) phase which is coherent to the NiTi(B2) phase matrix; an alloy with 8.4 mol.% Al showed compressive yield strengths as high as 2300 and 200 MPa at room and high (1000°C) temperatures, respectively. When the Al content exceeds 11 mol.%, however, Ni 2 TiAl phase started to deposit in a dendritic manner to reduce the strength. Although compressive ductility declined with the increase in Al content, 5.2% deformation was achieved with the 8.4 mol.% Al-containing alloy at room temperature.

Effect of boron microalloying on microstructure, tensile properties and creep behavior of Ti–22Al–20Nb–2W alloy

Abstract The effect of 0.2 at.% boron addition on the microstructure, tensile properties, and creep behavior of Ti–22Al–20Nb–2W were investigated. A TiB phase was observed in the B-doped alloy, which shows that the solubility of boron in this alloy is very low. The addition of 0.2 at.% boron was very effective in reducing the prior β-grain size of the Ti–22Al–20Nb–2W alloy, which is thought to be due to a strong boron segregation at the grain boundaries. The decrease in grain size due to the addition of boron is effective in improving the alloys room temperature ductility. The addition of a small amount of boron was beneficial in lowering the steady-state creep rate at 650°C/310 MPa, but did not improve the creep behavior at 750°C/310 MPa.

The effect of quaternary additions on the microstructures and mechanical properties of orthorhombic Ti2AlNb-based alloys

Abstract In an attempt to enhance the mechanical properties of the orthorhombic Ti 2 AlNb-based (O-based) alloy, six testing alloys were created by adding the quaternary elements to the O-based alloy. The microstructure, creep behavior and tensile properties of these alloys were investigated. The phase constitution was found to be either B2+O, B2+α 2 +O or B2+α 2 , depending on the Nb content and aging temperature. The results of the mechanical test showed that V was effective in improving room temperature ductility, while W was effective in enhancing high temperature tensile strength and creep resistance. Among the six alloys, Ti–22Al–20Nb–2W exhibited the best balance with its high-temperature 0.2% proof stress, room temperature ductility, and 1% creep-strain lifetime.

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.

Mechanical properties of Ir–Nb–Pt–Al quaternary alloys

Abstract Ir–Nb–Pt–Al quaternary alloys had exhibited suitable microstructure for high-temperature usage in the previous study. In this work, investigations were made of the mechanical properties of these quaternary alloys. Compression tests at 1200 °C were carried out for five samples, and compression creep test at 1400 °C under 100 MPa was conducted for one sample. These alloys showed high strength and good creep resistance.

Compressive creep behavior of Ir-base quaternary alloy 76.5Ir–13.5Nb–8.1Ni–1.9Al

Abstract Ir-base quaternary alloy 76.5Ir–13.5Nb–8.1Ni–1.9Al (in atomic percent) with an fcc/L1 2 two-phase structure showed an excellent balance of high-temperature strength and ductility in the previous study. In the present work, the compressive creep behavior of the alloy was characterized within a temperature range of 1500 to 1800 °C and a stress range of 50 to 200 MPa. The microstructure before and after the creep test was investigated. Depending on the steady-state creep rate, the creep stress exponent was calculated.

Microstructures and fracture behaviors of B-free and B-doped Ir3Nb (L12) intermetallic compounds

Abstract The fracture behaviors of the boron-free and boron-doped L1 2 intermetallic compound Ir 3 Nb were investigated by compression tests in air and vacuum (5×10 −4 Pa). High-temperature differential thermal analysis, X-ray diffraction, and scanning electron microscopy were conducted to characterize the microstructure change by the addition of boron in Ir 3 Nb. The results reveal that Ir 3 Nb fractures in intergranular mode in both air and vacuum. Boron doping improves the strength and ductility of doped Ir 3 Nb, and changes the fracture mode to transgranular. But more boron addition, above 500 wppm for Ir 3 Nb, causes a low melting temperature Ir–boride phase to form. On the basis of the results the relationship between boron doping, microstructure development, and the fracture behavior of Ir 3 Nb is discussed.

High Temperature Oxidation of C/C-Composites and Their Components from Viewpoint of Material Design

Aimed at establishing relationship between oxidation kinetics between C/C-composite materials (CCCMs) and the components from view points of material design, rates of oxidation were observed on 3 carbon fibres, 3 carbon-from-resins and 9 CCCMs from each pair of the fibres and the resins. The rates for the CCCMs were almost identical when the CCCMs were composed of an identical origin, although the consumption during the first half period of the oxidation was seen in the matrix carbon. The rate of a CCCMs was not given by a rule of mixture of those of the components, while the activation energy of the rates was given by one. This is explained by assuming that the oxidation mechanism is inherent to each fibre and matrix carbon, that each mechanism operates independently in the CCCMs, and that the number of the active centre in the CCCMs is controlled by a factor of the fibre. The factor to control the number of the active centre remains, however, to be investigated.