J.M." Chen

M. Kondo

P.F. "Zheng

T. Muroga

T. Nagasaka

Zheng,  P.F.

Nagasaka,  T.

Muroga,  T.

Kondo,  M.

Chen,  J.M."

NIFS-Repository (National Institute for Fusion Science)

   Molten salt such as LiF-BeF2 (Flibe) and LiF-NaF-KF (Flinak) is known as promising tritium (T) breeder and coolant material for LHD type heliotron reactors with self-cooled liquid blanket system with high thermal efficiency in operation above 500˚C.1) It has attractive safety advantages: low tritium solubility, inertness against air, inertness against water, low pressure operation, and low MHD resistance. We previously reported that the chemical stability of AlN, Y2O3, Er2O3 and Al2O3 in Flinak was examined by corrosion test.2) AlN is promising as a tritium barrier material because of better compatibility with Flinak than the other candidates. In the next stage, the formation of nitride layer on the surface of structural materials such as stainless steel and low activation steel is required for the development of the components for the system. In this present study, the nitriding process for the structural material was developed. Considering usage and maintenance of structural components in contact with molten Flibe or Flinak in the blanket system, nitriding treatment in these molten salts might be more advantageous than other vapor phase nitriding treatments such as ion nitriding and radical nitriding. Therefore, in this study, electrochemical nitriding treatment of austenitic steel material was performed in molten fluoride salt with lithium nitride Li3N.  Figure 1 illustrates the experiment setup. A binary eutectic mixture of LiF-KF containing Li3N (mixture ratio of 49mol% LiF, 49mol% KF and 2.0mol% Li3N) was used as a fluoride salt. Rods of 99.99% pure aluminum were employed as counter and reference electrodes. Lithium forms aluminum-lithium alloy. The measured potential was determined by the equilibrium reaction of lithium in an aluminum-lithium alloy. A plate of austenitic stainless steel containing chromium and nickel, stainless steel 316, was employed as a working electrode and specimen for the nitriding treatment. The plate of 5mm x10mm x 1mm was firmly bound to a pure nickel rod with nickel wire. The specimen was characterized before and after the treatment. The surface structure was observed by a scanning electron microscopy (SEM). The cut specimen was embedded into resin after being treated. The cross-section, which was prepared by polishing, was analyzed by an electron probe micro analyzer (EPMA). Depth variation in the chemical composition and valence state was analyzed by an X-ray photoelectron spectroscopy (XPS).  The crystal structure was characterized by an X-ray diffraction (XRD). The electrochemical measurement and treatment were carried out at 873K. The potentiostatic treatment for nitriding was conducted at 1.0V vs. (Li+/Li) at 873K. The cyclic voltammogram suggested that an irreversible nitriding reaction causes between 0.50V vs. (Li+/Li) and 1.06V vs. (Li+/Li) at 873K. It was found from EPMA analysis that nitrogen diffused from the surface to a depth of 35m by the treatment for 100min. The treatment formed multilayered structure near the surface. Chromium nitride CrN was formed from the surface to the depth of 2m and condensed in the layer. Comparing XRD patterns before and after the treatment as seen Fig. 2, it was revealed that the surface structure changed drastically in composition and crystal structure with nitrogen diffusion. Iron nitride -Fex(x>8)N was also formed as a solid solution that nitrogen atoms are inserted between iron atoms. It was formed from the surface to a depth of 35m as an inside diffusion layer.  Although several nitride phases of chromium and iron were considered, formation of CrN and -Fex(x>8)N was confirmed by XRD measurement and XPS analysis.  Therefore, it can be concluded that this electrochemical nitriding treatment made selective nitriding of chromium at the surface layer and deep diffusion layer of nitrogen to the depth over a few tens of mirometers.3)  Fig.  1. Experiment setup for electrochemical measurement and treatment.  Fig. 2. XRD patterns. (a) initial specimen. (b)treated specimen. 1) Sagara, A. et al.: Nuclear Fusion 45 (2005) 258. 2) Watanabe, T. et al.: J. Plasma Fusion Res. Ser.  9(2010)342.  3) Watanabe, T. et al.: submitted to Electrochimica Acta.   V-4Cr-4Ti alloy is a leading candidate structural material for self-cooled liquid Li blanket of a fusion reactor.1) The interactions of liquid Li and the V-4Cr-4Ti alloy are of concern, because the mass transfer in Li/V system can influence the mechanical properties of V-4Cr-4Ti, thus change its service performance. As Li exposures for V-4Cr-4Ti alloy at 700oC which is the designed operation temperature, and at 800oC have been researched in some previous studies,2) this study deals with the mechanical properties of V-4Cr-4Ti after Li exposure at 650oC.      The V-4Cr-4Ti alloy is thermomechanically treated using standard heat treatment (STD: 1000oC for 2hrs), precipitation hardening (SAA: solid solution annealed at 1100oC for 1hr +aging at 600oC for 20hrs) and a work hardening plus precipitation hardening (SACWA: solid solution annealed at 1100oC for 1hr + 20% cold work + aging at 600oC for 20hrs). Pure vanadium is used for investigation on the effects of the alloying elements, e.g. Ti, on the mass transfer. Then the specimens are exposed to liquid Li and vacuum at 650oC for 248hrs. Mechanical properties are measured at RT and high temperatures. Microstructures are observed to study the mechanism of the mechanical property change.       The present Li exposure has softened pure vanadium because Li absorbs O from vanadium matrix.3) However, the Li exposure strengthens the V-4Cr-4Ti alloy. A significant strengthening appears near the surface areas, which agrees with the diffusion range of N in vanadium matrix (shown as Fig. 1) based on the equation:   � � √� � �         (1)  where d is the diffusion range, D is diffusion coefficient and t is diffusion time. The mechanism of the strengthening is considered to be precipitation hardening induced by Ti precipitates, which contain N and C transferred from the liquid Li. And this strengthening can be more effective than the softening due to loss of O and decrease in dislocation and precipitate number density during the aging.     1) Muroga, T. et al.: J. Nucl. Mater. 307-311 (2002) 457. 2) Li, M. et al.: J. Nucl. Mater. 367-370 (2007) 788. 3) Nagasaka, T. et al.: Fus. Eng. Des. 81 (2006) 312.                           Fig. 1.  Distribution of cross section hardness for pure V and V-4Cr-4Ti in different states。 0 50 100 150 200 2506080100120140160180200220240260Diffusion range of Nsample surface Hardness/HvPosition x/µm  Initial V Initial STD Initial SAA Initial SACWA Li-V Li-STD Li-SAA Li-SACWA Vacuum-V Vacuum-STD Vacuum-SAA Vacuum-SACWAsample surfaceDiffusion range of N297§19. Effect of Li Exposure on the Mechanical Properties of Pure V and V-4Cr-4Ti AlloyZheng, P.F. (Grad. Univ. Advanced Studies),  Nagasaka, T., Muroga, T., Kondo, M.,  Chen, J.M. (Southwestern Institute of Physics)

Effect of Li Exposure on the Mechanical Properties of Pure V and V-4Cr-4Ti Alloy

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Effect of Li Exposure on the Mechanical Properties of Pure V and V-4Cr-4Ti Alloy

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