PSI - Issue 13

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

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To examine whether Ga penetration also occurred in polycrystalline Zn, the penetration test was conducted as described in 2.2. After 15 min at room temperature, Ga atoms were observed along the grain boundaries, as shown in Fig. 7. Similar experiments were conducted using Bi – In and Bi – Sn eutectic alloys. Although the Zn plates were maintained above their eutectic temperatures for 1 day, no evidence of In, Sn, or Bi was detected on the opposite surface of the droplet. Therefore, the penetration speed of Ga atoms along the grain boundaries of polycrystalline Zn was of the order of 1  m/s, while that of In and Sn atoms was negligible. 4. Discussion When Sn and In were present on the surface of polycrystalline Zn specimens, the fracture stress was smaller than that of a specimen in an inert environment. The fracture surface showed stepped transgranular cleavage, similar to that of polycrystalline Zn fractured at 77 K. As explained by Hughes et al. (2007), a crack propagates from one grain to the next without grain boundary fracture because of high grain boundary strength. At 77 K, plastic deformation requires a larger stress than that required for a cleavage fracture. In the case of LME, the cleavage strength of Zn is thought to decrease below the plastic deformation stress in the presence of Sn and In, while the grain boundary strength is not affected by the presence of the surface liquid metal. Westwood and Kamdar (1963) found that the cleavage surface energy of the (0001) plane of Zn crystals decreases by 50% in the presence of liquid Ga. Despite the reduction in surface energy, polycrystalline Zn failed in the intergranular mode in contact with liquid Ga. In several LME couples, grain boundary strength is reduced by the presence of liquid metals in the grain boundaries. As shown in Fig. 7, Ga atoms penetrated the grain boundaries of Zn, and the grain boundary strength showed a larger decrease compared to other samples. On the other hand, penetration by In and Sn atoms did not occur, and the grain boundary strength was not affected. In polycrystalline Al, penetration by Ga atoms has been observed using radioactive isotopes (Roques-Carmes et al. 1973), by transmission electron microscopy (TEM) (Hugo and Hoagland 1998, 1999), and by X-ray microtomography (Ludwig and Bellet 2000). In addition, Bi atom penetration into the grain boundaries of polycrystalline Cu was observed by SEM (Joseph et al. 1998). In the case of Ga atoms penetrating the grain boundaries of polycrystalline Al, the penetration speed was estimated as several  m/s (Hugo and Hoagland 1998, 1999; Ina and Koizumi 2004). These solid metals showed significantly reduced UTS in the presence of liquid metals, with intergranular fracture modes. Ina and Koizumi (2004) showed that polycrystalline Al specimens were embrittled even after liquid Ga was removed from their surface. LME is therefore induced by the liquid metal atoms remaining in the grain boundaries.

Fig. 7. Ga observed by SEM-EDS of the Zn plate, with a Ga droplet on the opposite side of the plate.

Observation of Zn fracture surfaces in contact with liquid Ga showed that, with increasing temperature, the ratio of intergranular fracture decreased and cleavage fracture increased (Fig. 6). A concurrent gradual increase in UTS was also observed (Fig. 4). These changes suggested that the strength of the grain boundaries recovered with increasing temperature, probably as the number of Ga atoms in the grain boundaries decreased at high temperatures. The possibility of diffusion of low-melting-point atoms into the grains has been discussed in the case of the Al solid Ga liquid couple (Marya and Wyon 1975), where a similar diffusion mechanism may be active in the present case.