The discovery of ordered, segregation-induced superstructures at general grain boundaries challenges a traditional view in physical metallurgy

A team of researchers found that randomly selected, high-angle, general grain boundaries in a nickel-bismuth (Ni-Bi) polycrystalline alloy can undergo interfacial reconstruction to form ordered superstructures, a discovery that enriches the theories and fundamental understandings of both grain boundary segregation and liquid metal embrittlement in physical metallurgy.

This discovery shows that segregation-induced ordered superstructures are not limited to special grain boundaries that are inherently periodic, but may exist at a variety of general grain boundaries that were thought to be lacking any long-range order; hence, they can affect the performance of polycrystalline engineering alloys.

The team, including nanoengineering professor Jian Luo here at the University of California San Diego as a co-corresponding author together with Professor Martin Harmer at Lehigh University, lays out their findings in the Oct. 6, 2017 issue of Science.

Prior studies of atomic-level grain boundary and segregation structures have been mostly focused on small-angle or special symmetrical tilt and twist boundaries with high symmetries and well-defined periodicities in artificial bicrystals.

Yet, such general grain boundaries are often significantly weaker mechanically and chemically than the well-studied special grain boundaries, thereby limiting properties and performance of engineered materials.

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