Phase Evolution During the Liquid-Phase Bonding of Zirconium and Austenitic Stainless Steel with Zinc Insertion
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I.
INTRODUCTION
HETEROGENEOUS bonding has been of constant interest in recent years, because combining the properties of two base materials by forming a strong and ductile bond has a wide range of applications in industry. In particular, zirconium exhibits significantly better corrosion resistance than stainless steels, whereas steels have superior mechanical properties. The joining of these two materials is thus of particular interest to a number of industries. Direct bonding, mainly by mechanical techniques such as explosion bonding and roll bonding, is the technique currently favored by industry. Various techniques, nevertheless, have been used already in an attempt to join zirconium alloys to stainless steel. Welding leads to the formation of brittle intermetallics such as Zr(Cr,Fe)2 compounds as well as Fe-Ni-Zr eutectic phases in the molten zone.[1–3] The difference in thermal expansion coefficients also leads to the formation of large residual stresses at the interface during the cooling of the weld, thus reducing the strength of the bonding. Diffusion bonding was also attempted, using either a pure-metal interlayer (iron,[4] titanium,[5] and platinum[6]) or an alloy interlayer (Fe-Ti,[2] Cr-V-Ti, and Cu-V-Ti,[7] Nb-Ni-Cu[8]). Furthermore, only one study on the direct diffusion bonding of
zirconium alloy to stainless steel at a temperature below the eutectic point could be found in the literature,[9] but no mechanical strength testing was conducted in that study. In all the preceding studies, the bonding temperature was higher than 1173 K (900 C) and the bonding time was longer than 1 hour. The goal of this research is to achieve the bonding of zirconium alloy and stainless steel at relatively low temperatures by a liquid-phase bonding technique using zinc as an interlayer. Liquid-phase bonding with a lowmelting-point metallic interlayer enables bonding at low temperatures and also allows a complete reorganization of the interface to form more stable compounds and a more uniform bonding zone, given that the interfaces are first melted and then solidify into a new shape. Zinc was selected as a suitable metal for liquid-phase bonding with steel since it does not form any compounds with iron above 1055 K (782 C),[10] and all experiments were conducted at 1073 K (800 C). The evolution of the microstructure near the interface was thoroughly studied to fully understand the bonding process and the formation and disappearance of phases at the interface.
II.
EXPERIMENTAL PROCEDURE
ZIRCALOY-2* and type 316 austenitic stainless steel GUILLAUME REBOUL, Graduate Student, SHOICHI NAMBU, Assistant Professor, JUNYA INOUE, Associate Professor, and TOSHIHIKO KOSEKI, Professor, are with the Department of Materials Engineering, Graduate School of Engineering, The University of Tokyo, Tokyo 113-8656, Japan. Contact e-mail: inoue@ material.t. u-tokyo.ac.jp Manuscript submitted December 6, 2010. Article published online February 14, 2012 2366—VOLUME 43A, JULY 2012
*ZIRCALOY-2 is a trademark of Westinghouse Electric Company, Pitts
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