Tadashi Igarashi

Deformation behavior and strengthening mechanisms at intermediate temperatures in W-La2O3

Abstract Tensile properties have been investigated at 1273–1973 K in rolled and annealed pure W and W-La 2 O 3 sheets. The as-rolled W-La 2 O 3 showed almost the same strength as the as-rolled pure W. On the other hand, the annealed W-La 2 O 3 exhibited higher strength than the annealed pure W. The higher strength of the annealed W-La 2 O 3 is attributed not only to the dislocation-particle interaction but also to the substructure which is stabilized by the La 2 O 3 particles.

Tensile properties at elevated temperature of W-1%La2O3

The tensile properties were investigated from 1273 to 1973 K for recrystallized pure W and W-1%La2O3. The WLa2O3 showed much higher strength than the pure W, but slightly lower elongation, except at 1273 K where the pure W suffered from intergranular fracture. The excellent mechanical properties of the WLa2O3 are probably the result of the structure, consisting of elongated grains, which resists grain boundary sliding.

R&D of a MW-class solid-target for a spallation neutron source

Abstract R&D for a MW-class solid target composed of tungsten was undertaken to produce a pulsed intense neutron source for a future neutron scattering-facility. In order to solve the corrosion of tungsten, tungsten target blocks were clad with tantalum by means of HIP’ing, brazing and electrolytic coating in a molten salt bath. The applicability of the HIP’ing method was tested through fabricating target blocks for KENS (spallation neutron source at KEK). A further investigation to certify the optimum HIP conditions was made with the small punch test method. The results showed that the optimum temperature was 1500 °C at which the W/Ta interface gave the strongest fracture strength. In the case of the block with a hole for thermocouple, it was found that the fabrication preciseness of a straight hole and a tantalum sheath influenced the results. The development of a tungsten stainless-steel alloy was tried to produce a bare tungsten target, using techniques in powder metallurgy. Corrosion tests for various tungsten alloys were made while varying the water temperature and velocity. The mass loss of tungsten in very slow water at 180 °C was as low as 0.022 mg/y, but increased remarkably with water velocity. Simulation experiments for radiation damage to supplement the STIP-III experiments were made to investigate material hardening by hydrogen and helium, and microstructures irradiated by electrons. Both experiments showed consistent results on the order of the dislocation numbers and irradiation hardness among the different tungsten materials. Thermal-hydraulic designs were made for two types of solid target system of tungsten: slab and rod geometry as a function of the proton beam power. The neutronic performance of a solid target system was compared with that of mercury target based on Monte Carlo calculations by using the MCNP code.

Strength proof evaluation of diffusion-jointed W/Ta interfaces by small punch test

Abstract For the development of tantalum-clad tungsten targets for spallation neutron sources, the bonding strength of tantalum–tungsten interface was investigated by means of an easy-to-use and miniaturized small punch (SP) test, in which a punching load is vertically applied to the center of a jointed disk. Cracks initiated and propagated in the tungsten side for all the samples hot-isostatically pressed (HIPed) at temperatures from 1673 to 2073 K, whereas no crack and debonding were observed in the interface, indicating that the jointed interface is strongly bonded. The recrystallization of tungsten occurs and results in its strength reduction, consequently the crack-initiating load decreases with HIPing temperature. The finite element analysis of the measured SP testing results shows that the maximum bonding strength can exceed 1000 MPa. The present study shows that SP test is suitable for the strength evaluation of jointed tantalum–tungsten interfaces.

Mechanical properties of a ZrC-dispersed Mo alloy processed by mechanical alloying and spark plasma sintering

A ZrC particle-dispersed Mo processed by mechanical alloying showed much higher strength than a recrystallized pure Mo at room temperature and showed a large elongation of 180% at 1970 K. These excellent properties of the particle-dispersed Mo are attributed to a very small grain size of 3 μm.

Corrosion resistance of refractory metals in high-temperature water

Abstract The high-temperature water corrosion resistance of refractory transition metals of group IVa, Va and VIa was studied. Corrosion tests were conducted for 17 kinds of tungsten, 12 kinds of molybdenum and other 9 kinds of metals in flowing water with a dissolved oxygen content of 400 ppb and pH 6–7 at 180, 260 and 320 °C for 200 h. It is shown that the corrosion rate of W and Mo depends strongly on temperature and material. W and Mo are not corroded severely at 180 °C, the corrosion rates are 0.001–0.047 mm/y for W and 0.022–0.029 mm/y for Mo. The heavy alloys are corroded severely at 260 °C, the corrosion rates are 0.141–0.214 mm/y. At 320 °C, significant corrosion attack occurs depending on the materials. CrN coated W shows excellent corrosion resistance, the rate is as low as ≃0.004 mm/y. W–Re and Mo–Re alloys are corroded severely, with a maximum rate of 1.656 mm/y for W–2%Re and 0.405 mm/y for Mo–17.7%Re. Mo shows a considerably higher corrosion resistance than W. All group IVa and Va metals except Nb, as well as Cr, Re and SUS316 at 320 °C show better resistance than Mo.

Optimization method of FGM compositional distribution profile design by genetic algorithm

The progress of technologies requires high performance materials. However it is sometimes difficult to achieve the required high performance by homogeneous material. Therefore composite materials have been developed so far. Functionally graded materials (FGM), which is a kind of composite materials, can change the material property in each area for its objective such as heat transfer property, stiffness, and so on, by controlling a compositional distribution ratio of materials. Therefore we can make a product which has two or more different desirable properties in one body. However it is difficult to design a compositional distribution ratio of an FGM as a desired one, because material properties of one part relates to the other part and its shapes and surroundings also affect the properties. In this paper, we propose the automatic compositional distribution profile design system for FGM. The proposed system optimizes the compositional distribution profile to satisfy requirements. The design system consists of two parts: Analyzer and Optimizer. We employ the Finite Element Method as Analyzer for analysis of the state of the product and Genetic Algorithms as Optimizer for optimization of the compositional distribution profile. To show the effectiveness of the proposed design system, we apply the proposed system to thermal stress relaxation problem.