Takuya Nagasaka

Vanadium alloys – overview and recent results

Abstract This paper reviews recent progress in research on vanadium alloys with emphasis on V–4Cr–4Ti as a reference composition. New high purity V–4Cr–4Ti ingots and products (NIFS-HEATs) were made. The improved purity of the alloys made a practical demonstration of enhanced feasibility of recycling as a method of handling after use in fusion reactors. Significant progress has been made in the understanding of physical metallurgy of V–4Cr–4Ti and effects of O, N and C on the alloy properties such as low and high temperature mechanical properties, welding properties and low temperature irradiation effects, by means of including the comparison of various large heats and model alloys with different impurity levels. The effects of other trace impurities on some of the properties are also discussed. Other current efforts to characterize V–4Cr–4Ti, to improve its properties and to explore advanced vanadium alloys are reviewed. Issues remaining for the future investigations are discussed.

NIFS program for large ingot production of a V–Cr–Ti alloy

Abstract The National Institute for Fusion Science (NIFS), in collaboration with Japanese industry, has initiated a research program for vanadium alloys, which will be highlighted by the production of a large V–4Cr–4Ti ingot. In this program, the technology and underlying science associated with vanadium alloy fabrication will be enhanced. Effort has been focused on the control of interstitial impurities such as C, O and N. Purification of the present commercial metal vanadium was carried out by improvement of the processes. Medium size (30 kg), high purity V–4Cr–4Ti ingots have been produced, designated as NIFS-HEAT-1, and a large size (200 kg) ingot is planned. In this study, the technology for fabricating large V–Cr–Ti ingots with ∼50 ppm C, ∼180 ppm O and ∼100 ppm N has been demonstrated.

Effect of impurity levels on precipitation behavior in the low-activation V–4Cr–4Ti alloys

Abstract The precipitation behavior of the V–4Cr–4Ti model alloys with different oxygen and nitrogen levels has been investigated. Fine and large precipitates were observed. Large precipitates were formed during the initial fabrication process and were stable to 1373 K. Fine precipitates were formed at 973 K and disappeared at 1373 K. The dependence of the hardness and microstructure on oxygen and nitrogen levels at various annealing temperatures showed that most of nitrogen impurities are included into the large precipitates, and oxygen impurities are partitioned into the large, fine precipitates and the matrix.

Examination of fabrication process parameters for improvement of low-activation vanadium alloys

Abstract Plate products of thickness 0.25–26 mm were evaluated, for the reference V–4Cr–4Ti alloy (NIFS-HEATs), from the viewpoint of recrystallized grain structure, precipitate distribution and fracture behavior at low temperature. 6.6 mm-thick or thinner plates showed relatively homogeneous and fine grain structure, and ductile fracture behavior. For 26 mm-thick plate, however, coarsened grain structure and localized distribution of precipitate clusters suggested that further hot working should be necessary to obtain the similar properties to those of the thinner plates. Grain structure, precipitate distribution, and anisotropy of fracture mode were shown to be influenced deeply by working and heating history at breakdown processes of the ingot. From the results the optimum fabrication parameters for vanadium alloys are discussed.

Recent progress of tungsten R&D for fusion application in Japan

The status of ongoing research projects of tungsten R&D in Japan is summarized in this paper. For tungsten material development, a new improved fabrication technique, the so-called superplasticity-based microstructural modification, is described. This technique successfully improved fracture strength and ductility at room temperature. Recent results on vacuum plasma spray W coating and W brazing on ferritic steels and vanadium alloys are explained. Feasibility of these techniques for the manufacture of the blanket is successfully demonstrated. The latest findings on the effect of neutron damage in tungsten on T retention and on the change in mechanical and electrical properties are described. Retention characteristics for neutron-damaged W were different compared to those for ion-damaged W. Upon neutron irradiation, tungsten alloys containing transmutation elements of W (Re and Os) show changes in properties that are different compared with those shown by pure W. The effects of mixed plasma exposure (D/He/C) are described. Both D/He and D/C mixed ion irradiations significantly affect ion-driven permeation in W. He bubble dynamics play a key role in nano-structure formation on the W surface.

Recovery and recrystallization behavior of vanadium at various controlled nitrogen and oxygen levels

Abstract The effects of nitrogen and oxygen on recovery and recrystallization of vanadium were investigated using high-purity metal vanadium singly doped with nitrogen or oxygen. Nitrogen and oxygen contents ranged from 10 to 500 weight ppm (wppm) and 50 to 1000 wppm, respectively. The hardening coefficient, (Δ H /Δ C ), for nitrogen is estimated to be almost twice that of oxygen. Additional hardening due to annealing at 200–400°C after rolling was observed. The anneal hardening was significantly decreased by restricting nitrogen contents below 100 wppm. On the other hand, oxygen up to 1000 wppm did not strongly affect the anneal hardening.

Effects of post-weld heat treatment conditions on hardness, microstructures and impact properties of vanadium alloys

Abstract Gas-tungsten-arc (GTA) weld joints were made from V–4Cr–4Ti alloys in which oxygen levels at the weld metal ranged from 73 to 355 wppm. Both the weld metal and the base metal of the joints showed hardening after the welding. In this study, the influence of post-weld heat treatment (PWHT) on hardness, microstructure and impact properties were investigated. Change in hardness due to annealing at 673 and 973–1273 K was shown to be caused by release of contaminant hydrogen and precipitate evolution, respectively. Results indicated that the precipitation hardening increased with oxygen level. Charpy impact properties did not change significantly due to annealing at 673 K, while fine precipitation at 1073 K degraded the impact property. No improvement of impact property by PWHT is expected at low oxygen level in the weld metal. Precipitation behavior during aging and irradiation may determine the performance of V–4Cr–4Ti weld joints.

Theromophysical Properties and Microstructure of Plasma-Sprayed Tungsten Coating on Low Activation Materials

Abstract Tungsten (W) coating on various low activation materials, such as ferritic steel (F82H), oxide dispersion strengthened (ODS) steel, and vanadium alloy NIFS-HEAT-2 (NH2) was successfully demonstrated by the vacuum plasma spray (VPS) process. Void and crack-type defects were observed in VPS-W. The mass density of VPS-W at room temperature (RT) was ∼90 % of the bulk W (sintered W). The thermal diffusivity and thermal conductivity of VPS-W from RT to 800 °C were 30∼50 % of the bulk W, while the linear expansion coefficient and specific heat of VPS-W were similar to the bulk W. The thermal conductivity of VPS-W was significantly lower than the bulk W, but was still larger than the NH2 substrate. There was no significant thermal contact resistance at the interface between W coating and NH2 substrate. Thus, the heat transfer properties of NH2 will not be degraded by the W coating with the VPS process.

Sintering Behavior of Metal Powders Involving Microwave-Enhanced Chemical Reaction

Copper powder compacts were sintered by microwave radiation in air. In this procedure, the samples were sintered by microwave in air without using any special atmosphere, only by protecting them in a container filled with ceramic powder. The enhancement of the deoxidation reaction by the microwave was observed. The samples were analyzed by using scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD) analysis. The samples were deoxidized on the surface and were well sintered to the edge from the inside throughout the entire cross section. The tensile strength of the copper samples sintered by the microwave in air was higher than that found in conventional sintering in N2 gas. No other differences were noticed between the samples sintered by the microwave under the air-rich conditions and in the conventional furnace in H2+N2 gas. The microwave-sintered copper samples were of good quality; for example, the tensile strength measured throughout the cross section was the same as that for the samples sintered in H2+N2 gas by the conventional method.

Development of fabrication technology for low activation vanadium alloys as fusion blanket structural materials

Vanadium alloys, candidate fusion reactor blanket materials, consist of intrinsic low activation elements for 14 MeV fusion neutrons (e.g. vanadium, chromium and titanium). From 500 to 1000 K, the tensile strength of vanadium alloys is independent of temperature, enabling the fusion blanket to be operated at least 100 K higher than a blanket made from ferritic steel. However, most of the data have come from laboratory-scale fabricated vanadium alloy, so a feasibility study on large scale production of vanadium alloy products is required. In the present study, high-purity vanadium alloy products (e.g. plates, wires and tubes) were fabricated from reference high-purity V-4Cr-4Ti ingots designated as NIFS–HEAT, with the use of technologies applicable to industrial scale fabrication. A critical issue for NIFS–HEAT large-scale melting was to reduce the levels of interstitial impurities (e.g. C, N and O) that are known to deteriorate mechanical properties before and after neutron irradiation. Impurity behaviour during working and annealing processes and its effect on mechanical properties were investigated. Mechanical properties of the products were significantly improved by the control of Ti-C, N and O precipitation induced during the processes. Good mechanical properties comparable to laboratory-scale alloys were obtained at above 96% in cold working degree.