13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Recovery of ductility observed in liquid gallium induced embrittlement of polycrystalline silver Kodai Yamamoto, Kohei Arakawa, Hirokazu Koizumi* Department of Physics, School of Science and Technology, Meiji University, Kawasaki 214-8571, Japan * Corresponding author: koizumi@isc.meiji.ac.jp Abstract Polycrystalline silver is embrittled by liquid gallium immediately after intimate contact between the solid and the liquid metals is made. When the tensile test is performed at room temperature, recovery of ductility has been observed if the contact time is long enough. This recovery is induced by formation of an intermetallic compound between silver and gallium. When the specimen is stressed above 420 K, embrittlement is not observed even if the contact time between the solid and liquid metals is short. This is the ductility trough reported in various solid and liquid metal couples that cause the liquid metal embrittlement. Although loss of embrittlement occurs as a function of contact time and as a function of test temperature, it may be difficult to connect the ductility trough with contact time dependent recovery from embrittlement, and there seems two independent recovery processes. Keywords liquid metal embrittlement, ductility trough, sliver, gallium 1. Introduction Liquid metal embrittlement (LME) is a phenomenon that normally ductile metals become brittle when they are stressed in close contact with certain kinds of liquid metals. Embrittlement appears as a decrease in maximum stress and fracture strain in a tensile test. Characteristic features of LME are [1]: (1) LME occurs only in some solid and liquid metal couples. This is called “selectivity” and solid and liquid metal couples that cause LME are listed in ref. [2]. Solid and liquid metal couples such as polycrystalline aluminum and liquid gallium show a significant reduction of the maximum stress and fracture strain, while in case of a steel and liquid lead the maximum stress decreases to only a half of the original maximum stress [3]. The factors that determine the selectivity are not known. (2) The embrittlement takes place immediately after intimate contact between solid and liquid metal is made. For polycrystalline aluminum-liquid gallium and polycrystalline silver-liquid mercury couples, embrittlement occurs even after the surface liquid metal is removed before stressing, if the contact time of the solid and liquid metals is long enough. In these cases, the degree of embrittlement increases with the contact time after a certain incubation time [4]. (3) The degree of embrittlement is severe near the melting temperature of the liquid metal, and the embrittlement disappears at a higher temperature. This temperature range is called “ductility trough”, and is often explained as a result of temperature dependence of the yield stress [3] or stress relaxation at potential crack initiation site [5]. We have reported that liquid gallium induced embrittlement of polycrystalline silver is transitory [6, 7]. The silver tensile specimens with a small amount of gallium on the surface were kept in a furnace at the temperature Th for a period of time th. The tensile test was performed to the specimens at 308 K. When th is small, the specimens undergo severe reduction of maximum stress and fracture strain, while for large th, the specimens show ductile behavior. The duration of brittleness is short when the quantity of gallium is small and the temperature of the furnace is high. For silver-gallium couple ductility trough exists as a function of the contact time. The objective of the present investigation is to show that the ductility trough exits as a function of temperature as many other embrittlement couples, and to discuss differences between the ductility
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