PSI - Issue 13

Naoyuki Tsutsui et al. / Procedia Structural Integrity 13 (2018) 849–854 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

850

2

(Yamamoto et al. 2013). However, transgranular fracturing occurs in some cases, such as Zr alloy solid Hg liquid (Cox and Wong 1997), and steel solid Pb liquid (Legris et al. 2000; Martin et al. 2012). Although polycrystalline Al is embrittled by liquid Ga in intergranular mode, cleavage fracturing occurs on low-index planes in an Al single crystal if its surface is wetted by liquid Ga (Old and Trevena 1979b). It is not yet fully understood whether intergranular or transgranular fracture occurs in a given solid/liquid metal couple. It is important to determine under what conditions intergranular or transgranular fracturing occurs in order to better understand the LME mechanism. The present study investigated the fracture mode of polycrystalline Zn induced by liquid metals. Since it is already known that liquid Ga, In, and Sn cause LME of Zn(Shunk and Warke 1974; Old and Trevena 1979a), we used liquid Ga and In, as well as Bi – In and Bi – Sn eutectic alloys as embrittlers. These eutectic alloys allowed us to obtain liquids at relatively low temperatures (Okamoto 2010). 2. Experimental 2.1. Tensile test The tensile specimens shown in Fig. 1 were wire-cut from a polycrystalline Zn plate (purity 99.5 %; thickness 3 mm). After polishing, the specimens were annealed at 453 K for 3 h. The average grain size was 0.2 mm. A small amount of one of the selected liquid metals (either Ga, In, Bi 21.5 In 78.5 eutectic alloy, or Bi 39 Sn 61 eutectic alloy), was applied to one of the four surfaces of the Zn specimen. In the case of liquid Ga, a droplet of 6 mg was spread across the entire gauge section, while in other cases, 2 mg of liquid was spread within the range of 4 mm on the central part of the gauge section. In some cases, ZnCl 2 solution was used as a soldering flux. Using a needle, the specimen surface was then scratched underneath the liquid metal in order to ensure good contact. Tensile tests were performed in the temperature range between 300 K and 450 K in silicone oil or in air. The strain rate for all tests was 1.2×10 − 2 s − 1 . Then, the fracture surfaces were observed using scanning electron microscopy (SEM). 2.2. Penetration test To examine whether penetration of the liquid metal atoms into the polycrystalline Zn occurred, a droplet of liquid metal was placed on a thin Zn plate; then, the opposite side of the plate was observed using SEM coupled with energy dispersive X-ray spectroscopy (SEM-EDS), as shown in Fig. 2. The Zn plate was 0.8 mm thick and was annealed at 653 K for 2 h to increase the size of the grains. The grain boundaries were revealed beforehand using hydrochloric acid etching.

Fig. 1. Schematic diagram showing the geometry of the tensile specimens.

Fig. 2. Experimental configuration for the penetration tests.