Section 5. Vanadium alloysVanadium alloys – overview and recent results
Introduction
Vanadium alloys are recognized as attractive candidate materials for neutron interactive structural components of fusion energy systems, because of their high temperature strength, high thermal stress factor, low activation property and so on [1], [2], [3], [4], [5], [6]. High compatibility of vanadium alloys with liquid Li makes it possible to design concepts of liquid Li blanket using vanadium alloys, which have the potentiality of high thermodynamic efficiency, high reliability and availability because of high operation temperature and no need for neutron multiplier and ceramic breeder, both of which must be replaced periodically owing to burn-up [7].
Based on past results, a V–4Cr–4Ti ternary alloy is considered to be a leading candidate material. Recent efforts on vanadium alloy development have been focused on characterizing the existing V–4Cr–4Ti heats to establish performance limit and operation window. The efforts include the evaluation of low and high temperature mechanical properties with and without irradiation, improvement of the alloy properties by compositional and microstructural optimization, and exploration of new alloys by changing composition or applying new fabrication processes.
Following the large V–4Cr–4Ti ingot production organized by the US-DOE program (US-HEATs) [8], [9], efforts have been made in Japan (NIFS) [10], [11], [12] and Russia (Bochvar) [13] to produce 30–200 kg V–4Cr–4Ti heats with improved purity. The results from new Japanese heats of V–4Cr–4Ti (NIFS-HEATs) show various benefits by reducing the level of oxygen and other trace impurities. International collaboration is in progress, in which those heats are being characterized by a number of research groups. In relation to the alloy production and characterization activity, understanding of fundamental aspects such as defect formation, evolution and recovery, impurity redistribution and precipitation during heat treatment, plastic deformation and irradiation has been enhanced. Technology for vanadium alloys as materials for components of fusion blanket systems is also in progress, e.g. welding and coating for mitigating magnetohydrodynamic (MHD) pressure drop and for corrosion protection, using the new alloy products.
This paper highlights recent progress in the fundamental and alloy performance knowledge on V–4Cr–4Ti. The MHD coating issues are not covered in this paper because they are overviewed in another paper [14]. The efforts to improve the alloy by modification of the composition or fabrication process and to explore advanced vanadium alloys are also reviewed. Current issues and research requirements are also discussed.
Section snippets
Alloy production, impurity level and low activation properties
In Japan, following the production of high purity 30 kg V–4Cr–4Ti (NIFS-HEAT-1) [10], [11], a pair of V–4Cr–4Ti ingots with a total weight of 166 kg (NIFS-HEAT-2) were produced [12]. The chemical composition of NIFS-HEAT-1, NIFS-HEAT-2 and two large heats produced by the US program (US832665 [15] and US832864 [9]) are compared in Table 1. Plates and sheets with various thicknesses (26, 6.6, 4.0, 1.9, 1.0, 0.5 and 0.25 mm) and wires with diameters of 2 and 8 mm were fabricated from NIFS-HEATs
Behavior of interstitial impurities and influence on properties
Although it was well recognized [22], [23] that interstitial impurities such as O, N and C can have strong impacts on various properties of vanadium alloys, the research for V–4Cr–4Ti was limited partly because of the lack of specimens with systematic variation of the impurity levels. Fig. 2 shows the level of O and N in V–4Cr–4Ti alloys recently available for the property tests. Comparison of US and NIFS-HEATs makes it possible to carry out tests on oxygen effects on various properties
Welding, hydrogen embrittlement and oxidation
Recent efforts devoted to welding technology are focused on gus-tungsten-arc (GTA) welding and laser welding techniques. GTA is a suitable technique for joining large structural components. GTA welding technology for vanadium alloys made a significant progress recently by improvement of the atmospheric control [39]. The oxygen level in the weld metal was controlled by combined use of plates of NIFS-HEAT-1 or US8332665, and filler wire of NIFS-HEAT-1, US8332665 or a high purity model alloy (36
Loss of elongation by irradiation at lower temperatures
A large increase in tensile strength and a drastic decrease in the elongation for V–4Cr–4Ti irradiated below ≃700K was reported [51], [52] and their mechanisms have been investigated. Based on microstructural characterization [53], [54], the high density of defect clusters or radiation-induced precipitates were considered to be responsible for the formation of dislocation channels and the reduction in work-hardening capability.
Limited data suggest that addition of Al, Si and Y, which scavenge
Creep
Creep resistance is an important property for determining the maximum operation temperature of vanadium alloys. Thermal creep property of V–4Cr–4Ti has been investigated by uniaxial tensile [57] and biaxial pressurized tube [58] specimens. The data from the two methods are reasonably consistent with each other showing a 3.7–7 power law for the stress dependence of the creep rate, and suggesting climb-assisted processes. Since these creep experiments were carried out in vacuum, an introduction
Beyond single phase V–4Cr–4Ti
Efforts are being made to improve the V–4Cr–4Ti alloy or to develop alternative alloys by changing minor, major compositions or fabrication processes. The common goal is to enhance the high temperature strength, oxidation resistance and irradiation resistance, which will enable to extend the operation windows of the alloys and to use the alloys in more oxidizing environments such as He cooled systems.
As already mentioned, the addition of Si, Al and Y on V–4Cr–4Ti has been reported to increase
Critical issues remaining for the future research
Although the long term strategy toward fusion materials development is different in different countries, it is commonly accepted that some critical issues should be resolved before proceeding to the advanced step of the development.
As for the remaining critical issues, it should be pointed out that lack of irradiation data is the most serious issue. In the low temperature regime, unified models of deformation and fracture are in progress [67]. Coupling of the models with irradiation experiments
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