Gaseous hydrogen embrittlement of a hydrided zirconium alloy
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TRODUCTION
ZIRCONIUM alloys are generally employed as fuel element cladding. The environment in the reactor causes hydriding and consequently degrading of the mechanical properties of the cladding materials. Meanwhile, the cladding material may be exposed to hydrogen gas produced from the reaction of water with the alloy. This may result in further hydriding of the material; moreover, the hydrogen gas may facilitate the growth of existing microcracks. Since the safety of the core of a reactor is strongly dependent on the integrity of the cladding material, it is necessary to investigate the change of fracture behavior on the hydrided zirconium alloy in the gaseous hydrogen environment. Hydrogen embrittlement of zirconium alloys has been extensively studied in recent decades.[1–6] A few studies[4,5,6] were conducted to investigate the effect of hydrogen content on the mechanical properties of hydrided ZIRCALOY4.* Among these investigations, an interesting ‘‘ductile*ZIRCALOY-4 is a trademark of Westinghouse Electric Company, Pittsburgh, PA.
brittle transition’’ phenomenon was found by a few authors. Lin et al.,[4] by tensile testing ZIRCALOY-4 at room temperature, found that there is a ductile-brittle transition when the hydrogen content is between 400 and 800 ppm. Bai et al.[5] observed that the reduction of area (RA) for the recrystallized ZIRCALOY-4 at room temperature is drastically reduced when the hydrogen concentration reaches 760 ppm. In both these works, the definition of ductile-brittle transition was taken to be the abrupt change of the value
J.-H. HUANG, Professor, and M.-S. YEH, Graduate Assistant, are with the Department of Engineering and Systems Science, National Tsing Hua University, Hsinchu, Taiwan 300, Republic of China. Manuscript submitted February 20, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
of RA. Since the effect of hydrogen on RA is more distinct than on elongation, the RA has been used as a parameter to exhibit hydrogen effects. In the present work, we have adopted the definition used by the previous workers.[4,5] Since notched specimens provide a triaxial state of stress, which will enhance brittle fracture, they are more suitable to be used to study the ductile-brittle transition of metals than smooth tensile specimens. Huang and Huang[6] performed tensile tests on notched specimens of hydrided ZIRCALOY-4. They found that a room-temperature ductilebrittle transition in the RA occurs with hydrogen contents between 30 and 140 ppm for the hydrided specimens tested in air. The ductile-brittle transition disappears as the temperature increases to 100 7C and above. This is attributed to the improved ductility of the zirconium matrix with increasing temperature. On the other hand, the effect of hydrogen gas on the mechanical properties of zirconium alloys is less well known. Dissolved hydrogen has been found to increase plasticity and result in fracture of many metals, such as Ni and iron.[7,8] Even the hydride formation metal, a-Ti, was reported to be able to fracture by the mechanism
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