Figure 4 summarizes the microstructural evolution during the breakdown process of NIFS-HEAT-2

Hot/cold roll Heat treatment

1373 K/RT 973 K 1273K 1373K 1573K

Ti-rich blocky precipitates (with N, O, C)

Elongation, band structure Dissolution

ingots.4 Bands of small grains aligned along the rolling direction were observed at the annealing temperature below 1223 K. The grains became homogeneous at 1223 K. The examination showed that optimization of size and distribution of Ti-CON precipitates are crucial for good mechanical properties of the V-4Cr — 4Ti products. Two types of precipitates were observed, that is, the blocky and the thin precipitates. The blocky precipitates formed during the initial fabrication pro­cess. The precipitates aligned along the working direc­tion during the forging and the rolling processes forming band structures, and were stable to 1373 K. Since clustered structures of the precipitates result in low impact properties, rolling to high reduction ratio is necessary for making a thin band structure or homo­genized distribution of the precipitates. The thin pre­cipitates were formed at ~-973 K and disappeared at 1273-1373 K. At 1373 K, new precipitates, which were composed of V and C, were observed at grain bound­aries. They seem to be formed as a result of redistri­bution of C induced by the dissolution of the thin precipitates. The impact ofthe inhomogeneous micro­structure can influence the fracture properties.14

Figure 5 shows the hardness as a function of final heat treatment temperature for three V-4Cr-4Ti materials: NIFS-HEAT-1, NIFS-HEAT-2, and US — DOE-832665 (US reference alloy).15 The hardness has a minimum at 1073-1273 K, which corresponds to the temperature range where formation of the thin precipitates is maximized. With the heat treatment higher than this temperature range, the hardness increases and the ductility decreases because the

Annealing temperature (K)

precipitates dissolve enhancing the level of C, N, and O in the matrix. Based on the evaluation of various properties in addition to the hardness as a function of heat treatment conditions, the optimum heat treat­ment temperature of 1173-1273 Kwas suggested.

Plates, sheets, rods, and wires were fabricated mini­mizing the impurity pickup and maintaining grain and precipitate sizes in Japanese, US, and Russian programs. Thin pipes, including those of pressurized creep tube specimens, were also successfully fabricated
in Japan maintaining the impurity level, fine grain size, and straight band precipitate distribution by maintain­ing a constant reduction ratio between the intermedi­ate heat treatments.16 The fine-scale electron beam welding technology was enhanced as a result of the efforts for fabricating the creep tubes, including plug­ging of end caps.17 In the United States, optimum vacuum level was found for eliminating the oxygen pick-up during intermediate annealing to fabricate thin-walled tubing of V-4Cr-4Ti.18 In Russia, fabrica­tion technology is in progress for construction of a Test Blanket Module (TBM) for ITER (International Ther­monuclear Experimental Reactor).19

Joining of V-4Cr-4Ti by gas tungsten arc (GTA) and laser welding methods was demonstrated. GTA

is a suitable technique for joining large structural components. GTA welding technology for vanadium alloys provided a significant progress by improving the atmospheric control. The results are summarized in Figure 6. Oxygen level in the weld metal was controlled by combined use of plates of NIFS — HEAT-1 (181wppm O) or US-8332665 (310wppm O) and filler wire ofNIFS-HEAT-1, US-8332665, ora high-purity model alloy (36 wppm O). As demonstrated in Figure 6, ductile-brittle transition temperature (DBTT) of the joint and the oxygen level in the weld metal had a clear positive relation. This motivated further purification of the alloys for improvement of the weld properties.20 Only limited data on irradia­tion effects on the weld joint are available at present.

The welding results in complete dissolution of Ti — CON precipitates and thus results in significant increase in the level of C, O, and N in the matrix. In such conditions, radiation could cause embrittlement. Some TEM observations showed enhanced defect clus­ter density at the weld metals. However, the overall evaluation of the radiation effects remains to be per­formed. Especially, elimination of radiation-induced degradation by applying appropriate conditions of post­weld heat treatment (PWHT) is the key issue.

For the use of vanadium alloys as the blanket of fusion reactors, the plasma-facing surfaces need to be protected by armor materials such as W layers. Limited efforts are, however, available for developing the coating technology. A low pressure plasma-spraying method was used for coating W on V-4Cr-4Ti for use at the plasma-facing surfaces. The major issue for the fabrication is the degradation of the vanadium alloy substrates by oxidation during the coating processes. Figure 7 shows the result of bending tests of the coated samples. The crack was initiated within the W layer propagating parallel to the interface and followed by cracking across the interface. Thus, in this case, the quality of W coating layer is the issue rather than the property of the V-4Cr-4Ti substrate or the interface. Hardening of substrate V-4Cr-4Ti by the coating occurred but was shown to be in acceptable range.21

Figure 8 is a collection of the products from NIFS-HEAT-2.