FINE-TUNING AT SIGMA-3(N) BOUNDARIES IN NICKEL

The mechanisms for evolution or a grain boundary population are invest igated in sequentially strain annealed superpure nickel. Whereas the o verall population of coincidence site lattice (CSL) boundaries does no t increase with extended annealing times, the proportions of Sigma 3(n )s which approach the exact Sigma 3 configuration increases considerab ly which is shown to be due mainly to local lattice rotations. The ter m fine tuning is coined to describe this type of grain boundary engine ering. In general, fine tuning is not observed at other CSLs, and this is considered to be because the potential for energy reduction is far greater at Sigma 3s than at other CSLs. The fine tuning that occurs a t Sigma 9s and Sigma 27s is con sidered to be ''geometrically necessar y'' because of the relationship between Sigma 3, Sigma 9 and Sigma 27 at grain junctions. (C) 1997 Acta Metallurgica Inc.