Atomic structures of symmetric tilt grain boundaries in hexagonal close packed (hcp) crystals

Using molecular dynamics (MD) simulations, the dislocation structures of [1 (2) over bar 1 0] symmetric tilt grain boundaries (STGBs) in hexagonal close packed (hcp) crystal structures are studied. STGBs over the entire range of possible rotation angles theta from 0 degrees to 90 degrees are found to have an ordered atomic structure. Formation energy calculations reveal four local minimum-energy boundaries that correspond to coherent grain boundaries (GBs). Deviations in tilt from the basal plane (theta = 0 degrees, P-B((1))), prismatic plane (theta = 90 degrees, P-B((6))), or one of these four minimum-energy boundaries, P-B((2)), P-B((3)), P-B((4)), P-B((5)), result in the formation of a tilt wall (edge-type grain boundary dislocations, GBDs) superimposed on the nearest GB structure P-B((i)) in theta-space. As theta deviates far from the rotation angle of one P-B((i)) and draws closer to that of an adjacent P-B((j)), an abrupt transition in STGB base boundary structure and GBD Burgers vector occurs. For all theta, the sign and spacing of GBDs depend on theta, and their Burgers vector is either one or two times the interplanar spacing of P-B. We present a simple model that generalizes the results to other c/a ratios. Subsequent MD simulations show that (1) the model forecasts the STGB structure to first-order and (2) STGBs with two distinct atomic structures can have remarkably different responses when interacting with basal lattice dislocations originating from the adjoining crystals.