Elsevier

Journal of Nuclear Materials

Volumes 386–388, 30 April 2009, Pages 598-601
Journal of Nuclear Materials

The microstructure of laser welded Y doped V–4Cr–4Ti alloys after ion irradiation

https://doi.org/10.1016/j.jnucmat.2008.12.186Get rights and content

Abstract

Laser welded V–4Cr–4Ti–0.15Y alloy, fabricated by National Institute for Fusion Science (NIFS), was used in this study. Copper ion irradiation was carried out with the tandem accelerator at Kyushu University. The TEM samples were sliced from the welded materials and irradiated at 873 K up to the dose of 12 dpa. The microstructure before the irradiation showed that relatively large precipitates, which were commonly observed in the alloy, disappeared in the center of the weld metal. After the ion irradiation, fine titanium oxides with {1 0 0} habit planes were detected. However, in all irradiation doses, the growth of titanium oxides was suppressed by Y addition.

Introduction

The welding procedure is one of the key technologies for use of V–4Cr–4Ti alloys as a large component [1]. But the embrittlement caused by interstitial impurities during welding is highly pronounced. To avoid the pick-up of impurities (e.g. oxygen and nitrogen) from the welding environment, electron beam (EB) and gas tungsten arc (GTA) welding [2], [3] were conducted using vacuum chamber or glove box. Recently, laser welding technology for the alloys was developed by NIFS (National Institute for Fusion Science) by controlling the flow rate of high purity argon gas [4], [5]. Because of flexible, in-field, automated and remote operation, and small weldment and heat affected zone (HAZ), laser welding is an attractive welding technology. However, quite little is knows as to the irradiation effect on the weldment. Our previous studies [6] on neutron irradiated V–4Cr–4Ti alloy (NIFS-HEAT2) revealed that tiny Ti(CON) precipitates were homogenously formed in the weld metal at 673 K and the formation was prominent in comparison with base metal. The effects of post-weld heat treatment (PWHT) on weld metal, effectively improving the CVN impact properties for unirradiated material and for material irradiated at lower temperatures, are not effective or have a very limited effect at higher irradiation temperatures where the growth of Ti(CON) precipitates were prominent. On the other hand, Y addition on V–4Cr–4Ti alloys is expected to reduce the Ti(CON) formation, because oxygen is scavenged by Y. The present paper summarized, therefore, the microstructural evolution of laser welded Y doped V–4Cr–4Ti alloy during ion irradiation.

Section snippets

Experimental procedure

Welded joints used in this study were prepared from V–4Cr–4Ti–0.15Y alloy. Before the YAG laser welding (bead-on-plate welding) in a high purity argon atmosphere, the samples were annealed in a vacuum at 1273 K for 2 h. The detailed welding procedure was described elsewhere [3]. A 2.4 MeV copper ion irradiation was carried out with the tandem accelerator at Kyushu University. The TEM samples were sliced from welded materials and irradiated at 873 K up to the dose of 12 dpa. After the irradiation,

Microstructure of unirradiated samples

Fig. 1 shows microstructural development of the weld metal with a distance from the bead center. In the figure, microstructure of the base metal is also shown. Ti enriched blocky precipitates, which were commonly observed in the base metal and composed of titanium, carbon, nitrogen and oxygen, disappeared in the weld metal (bead center). And relatively higher dislocation density was observed in the weld metal and the heat affected zone (HAZ).

As shown in Fig. 2(a), an increase in hardness

Summary

Copper ion irradiation was carried out on laser welded V–4Cr–4Ti–0.15Y alloy. The main results are summarized as follows.

  • (1)

    The microstructure before irradiation showed that relatively large precipitates disappeared in the center of weld metal.

  • (2)

    After the ion irradiation at 873 K, fine titanium oxides with {1 0 0} habit planes were detected even at the dose of 0.75 dpa. This means that the behaviors of oxygen atoms, which dissolved from the large precipitates during the laser welding, is essential to

Acknowledgement

This work was also supported by NIFS Budget Code NIFS05KFRF021.

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