Modeling twin boundary structures in body centered cubic transition metals

The twin boundary structures in the BCC transition metals are determined by analyzing twin boundary generalized stacking fault energy curves and complete structural relaxation of the twinned crystals utilizing both interatomic potentials and density functional theory calculations. In contrast to the crystallographic description of twin elements in BCC metals, the pure reflection twin boundary structure has been found to be unstable in all seven metals considered. Instead, this work identifies three possible metastable twin boundary structures that are distorted forms of the pure reflection twin boundary structure and isosceles twin boundary structure previously proposed: a distorted reflection, distorted isosceles, and scalene structures. The DFT simulations reveal that the stability and energetic preference to form these structures differs between the Group 5 and Group 6 BCC metals despite having the same structure and general type of bonding. This work ates how these equilibrium structures and their relative stability affect the commonly reported twinning generalized stacking fault energy curves and explain why these curves (as previously computed) always have cusps.