Twin boundaries and incoherent steps on twin boundaries in body-centered-cubic metals

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Abstract
The atomistic structure of coherent {112} twin boundaries and of incoherent steps on these boundaries is studied by computer simulation. The interatomic interaction is described by four different central force laws. It is found that for strong binding the boundary for which the first layer of the twin is sheared by 1/12⟨111⟩ is preferred to the boundary where all the layers of the twin are sheared by ⅙⟨111⟩. For weaker binding no pronounced preference for either of these boundaries exists. The incoherent steps appear to be best described by two 1/12⟨111⟩ twinning dislocations situated in two adjacent {112} layers. Their cores are confined to these layers and are generally wider than the cores of lattice dislocations. The mechanism of twin growth by spontaneous nucleation of these twinning dislocations is discussed. It is shown that rather high stress is needed for the operation of the mechanism but this can still be substantially lower than the stress needed for twin nucleation.