Yutaka Hiraoka

Microstructure and impact properties of ultra-fine grained tungsten alloys dispersed with TiC

Abstract In order to improve both the low temperature toughness and the resistance to embrittlement by recrystallization and irradiation in currently available tungsten and its alloys, ultra-fine grained tungsten alloys with TiC additions of 0.2 and 0.5 wt% were developed by mechanical alloying and hot isostatic pressing. It is shown that the impact toughness of the developed alloys is very sensitive to the magnitude of relative density and is greatly improved by increasing its value. An alloy with 0.2 wt% TiC, which has the highest relative density of 99.5% among the developed alloys, exhibits a much lower ductile-to-brittle transition temperature and higher strength than pure tungsten which has a relative density of 100%. For the alloy, recrystallization and grain growth occur during 1-h heating between 2273 and 2473 K, much higher than the equivalent temperature range for pure tungsten. Increasing the TiC content to 0.5 wt% makes the alloy more resistant to recrystallization and grain growth.

Development of Mo alloys with improved resistance to embrittlement by recrystallization and irradiation

Abstract In order to overcome the recrystallization embrittlement and irradiation embrittlement in Mo and W, which are major problems for their fusion applications, the basic idea of alloy design and microstructure control was presented. By applying the idea to Mo, ultra-fine grained Mo alloys with very fine TiC particles mostly existing at grain boundaries were developed for TiC additions of 0.1 to 1.0 wt%. Impact three-point bending tests showed that before and after recrystallization and fast-neutron irradiations to 0.08 dpa the developed alloys exhibit much lower ductile-brittle transition temperature (DBTT) and higher strength than TZM. The developed alloys also showed much higher resistance to recrystallization and grain growth. Both resistance increased with increasing TiC content; in the irradiated state the 1.0 wt% TiC-added alloy showed a DBTT lower by more than 200 K than TZM. The cause of the observed improvement was discussed.

Preparation of a large-scale molybdenum single-crystal sheet by means of secondary recrystallization

Abstract Secondary recrystallization was used to produce a large-scale molybdenum single-crystal sheet. It was found that the presence of dispersed second phases such as calcium and magnesium oxides plays an important role in the grain growth of the rolled molybdenum, initially restraining normal grain growth and hence substantially promoting abnormal grain growth at higher temperatures. The secondary recrystallization temperature was dependent on the amount of the second phase present. Consequently a large-scale single-crystal sheet (2 mm × 40 mm × 180 mm) can easily be produced from a hot-rolled molybdenum sheet doped with CaO and MgO.

Effect of a small amount of additional carbon on the ductility of recrystallized sintered-molybdenum sheet

Abstract The ductility of sintered-molybdenum sheet, which had relatively fine recrystallized grains, was examined by means of bend tests at low temperature and observations made by optical microscopy, scanning electron microscopy and transmission electron microscopy. In conclusion, it was shown that a small amount of additional carbon remarkably improved the ductility of molybdenum and also changed the brittle fracture mode. Carbon content and strain-rate dependence of ductilebrittle transition behavior were also reasonably interpreted.

Effect of aging after carbon doping on the ductility of molybdenum

Abstract The low temperature tensile properties of undoped specimens and specimens doped with up to 55 wt.ppm C before and after aging at 600 or 1500°C were examined. Carbon doping from 14 to 30 wt.ppm induced a remarkable improvement in ductility, as shown by a sharp increase in σc and a sharp decrease in Tc. Aging at 600°C induced a slight decrease in ductility for all doped specimens, whilst aging at 1500°C induced no monotonie changes in ductility. The latter result was interpreted by considering thermodynamics and decarburizing during aging.

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.

Alloying to improve the properties of welded molybdenum

Abstract Tensile tests at 203 K were performed on electron-beam-welded molybdenum and molybdenum-base alloys. The effects of alloying elements, carbon, zirconium, niobium, vanadium (with or without boron) and rhenium were determined on the low-temperature ductility of weld joints. The results are related to the microstructural effects. The effects of alloying are as follows: (1) Carbon enhanced the fracture strength and improved the ductility due to the grain boundary strengthening. (2) Zr, Nb or V with boron enhanced both the yield and fracture strengths due to the grain refining, but the ductility was only slightly improved. (3) 5% Re induced a high fracture strength due to the refinement of precipitates, an exceptionally low yield strength due to the solution softening, and the ductility was substantially improved.

Effects of rolling procedures on the development of annealing textures in molybdenum sheets

Abstract The effects of rolling procedures on grain growth and on the development of annealing textures at high temperatures in pure and doped molybdenum sheets were investigated. The rolling and primary recrystallization textures in molybdenum were similar to those obtained for other pure b.c.c. metals, i.e. three principal components {001}〈110〉, {111}〈112〉 and {112}〈110〉 for straight rolling and two principal components {001}〈110〉 and {111}〈112〉 for cross rolling. After a high temperature anneal the primary recrystallization texture (normal grain growth) was retained in the pure molybdenum during normal grain growth, regardless of whether straight or cross rolling was employed. In doped molybdenum, however, completely different components were developed as a result of secondary recrystallization (abnormal grain growth), i.e. two strong components {123}〈511〉 and {126}〈811〉 for straight rolling, and two components {123}〈121〉 and {126}〈1081〉 for cross rolling. Most of the components of the secondary recrystallization texture corresponded to very weak components in the primary recrystallization textures and were related by a rotation of approximately 15°–30° about 〈100〉 for straight rolling or about 〈110〉 for cross rolling relative to the strong primary {001}〈110〉 component.

Nitriding of dilute Mo-Ti alloys at a low temperature of 1373 K

Abstract For Mo-0.5 mass% Ti and pure Mo alloy nitrided in a NH 3 gas at a relatively low temperature of 1373 K, microstructural observations through optical and transmission electron microscopes, X-ray diffraction analysis and hardness measurements were carried out. A surface nitriding layer with very high hardness of approximately Hv ~ 1800 consisted of two Mo-nitride regions: an outer one of γ-Mo 2 N and an inner one of β-Mo 2 N. The inward diffusion of nitrogen is a rate-controlling process in the growth of the surface nitriding layer. In Mo-Ti alloy additionally an internal nitriding layer with relatively high hardness of Hv ~ 800 was formed beneath the surface nitriding layer. Such high hardness in the internal nitriding layer was found to result from the uniform dispersion of extremely fine plate-like particles of titanium nitride. The particles are approximately 0.4 nm thick and have coherent strain field in the matrix.

Recrystallization of molybdenum wire doped with lanthanum oxide

Abstract Recrystallization mechanism of La 2 O 3 -doped molybdenum wire was investigated. Samples of 1 mm in diameter were annealed for 1 h at various temperatures. Substructures were observed by transmission electron microscopy. Subgrain structures including dislocations of high density and small particles were stable up to about 1650 °C. Upon annealing at 1800 °C large grains with high aspect ratio along the working direction partly developed, though subgrains composed of dislocations simultaneously remained. Additionally, dislocations in the coarsened grain were not perfectly removed due to the pinning effect of second particles. Upon annealing over 2000 °C, on the other hand, elongated grain structures developed in the whole area. It is concluded that the grain coarsening was attributed to the primary recrystallization (subgrain coalescence) involving a rearrangement of dislocations rather than to the secondary recrystallization involving a nucleation process in primary grains.