Which substance is noted for rapidly penetrating the grain boundaries of aluminum alloys and causing embrittlement?

• A. Acids and Acid Solutions
• B. Iodine
• C. Gallium
• D. Mercury

Answer: (C)

Liquid metal embrittlement occurs when a liquid metal can enter the grain boundaries of a solid metal under stress, weakening cohesion and enabling intergranular cracking. Gallium is particularly problematic for aluminum alloys because it readily wets aluminum and diffuses quickly along grain boundaries. This penetration disrupts boundary integrity and reduces the energy required to propagate cracks under load, leading to brittle failure even at relatively low stresses or temperatures. The other substances listed don’t produce this rapid grain-boundary penetration in aluminum to the same extent, so gallium is the best explanation for this embrittlement behavior.