In a grain-boundary-engineered microstructure, Σ9 boundaries were the second most abundant boundary type. This article presents data showing that, in grain-boundary-engineered copper, most Σ9s were special. The Σ9 boundaries were also a geometrically necessary component of a grain-boundary-engineered microstructure. It was suggested that there were competing requirements for Σ9s; during iterative grain-boundary-engineered processing, they were required to be mobile, whereas afterward, it was desirable that they were special boundaries.

The Role of Σ9 Boundaries in Grain Boundary Engineering. V.Randle, M.Coleman, M.Waterton: Metallurgical and Materials Transactions A, 2011, 42[3], 582-6