Gallium penetration into aluminum alloys detected using laser-generated Rayleigh waves

We have studied penetration processes of liquid gallium into 2017 aluminum alloys by measuring Rayleigh-wave velocity and attenuation using laser-generated ultrasound. The gallium penetration is the initial stage of liquid-metal embrittlement, which is reduction in the elongation to failure that can be produced when normally ductile solid metals are stressed while in contact with a liquid metal such as gallium. The results for samples without annealing showed that the Rayleigh-wave velocity decreased a few percent in the time scale of 103 s, suggesting an extraordinarily rapid penetration of gallium along subsurface. The results for annealed samples showed decrease in the time scale of 104 s. The difference in the penetration rates suggests that residual stress may facilitate the penetration of gallium.