Energy pathways and directionality in deformation twinning

We present ab initio density functional theory calculations of twinning energy pathways for two opposite twinning modes, (111)[11 (2) over bar] and (111)[(1) over bar(1) over bar2], in fcc materials to examine the directional nature of twinning which cannot be explained by classical twin nucleation models or the twinnability criterion. By accounting for these energy pathways in a multiscale model, we quantitatively predict the critical twinning stress for the (111)[(1) over bar(1) over bar2] mode to be substantially higher compared to the favorable (111)[11 (2) over bar] mode (whose predicted stresses are in agreement with experiment), thus, ruling out twinning in the (111)[(1) over bar(1) over bar2] mode. (C) 2007 American Institute of Physics.