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Why can grain boundaries weaken a metal at high temperatures?

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Diagram of a zinc-lead blast furnace system showing ore and coke entering through charge hoppers, with labeled furnace, air preheater, blower, taphole for slag and lead bullion, and a connected lead-splash condenser using rotors to separate and cool lead and zinc, with pathways for molten metals and off-gas. — metallurgy A zinc-lead blast furnace and lead-splash condenser. This technology is utilized mainly for producing zinc while managing both lead and zinc from raw materials, a technique that was highly influential in 20th-century smelting.

In metallurgy, grain boundaries are the interfaces formed where small crystals, or grains, with different orientations meet during the solidification or phase transformation of a metal.1

While these boundaries typically strengthen a metal at room temperature, they often become a source of weakness at high temperatures, leading to early structural failure. This occurs because, at elevated temperatures, the cohesion between these crystal grains can diminish, making them susceptible to fracture. Additionally, grain boundaries can serve as sites for localized corrosion, which further compromises the integrity of the material under high-temperature conditions.12

To mitigate these issues, researchers often add small amounts of specific metals to intermetallic compounds to improve the cohesion among crystal grains, thereby enhancing the material's resistance to failure.2