Classical and “Non-Classical” Twins in Ni2MnGa: Phenomenological versus Mechanistic Modelling

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The characteristic crystallography of “classical” twins was set out over 60 years ago by Bilby and Crocker: a twin boundary is an invariant plane, and the adjacent crystals are interrelated by a homogeneous simple shear. In subsequent work, Crocker and Bevis introduced the notion of “non-classical” twinning, where, as before, the adjacent crystals are interrelated phenomenologically by a homogeneous simple shear, but where the misfit-free interface plane is not invariant. Later experimental studies revealed that “classical” twins actually grow by glissile motion of interfacial line-defects with both dislocation and step character, now known as disconnections. This inhomogeneous shear mechanism has been expounded in the topological model of interfaces developed by Pond and Hirth. In the present paper, experimental observations of “non-classical” twins in Ni2MnGa by Seiner et al. are analysed using the phenomenological and topological approaches. It is concluded that the descriptor “non-classical” is not appropriate because physically feasible glissile disconnections cannot arise in these boundaries. Instead, the boundaries are immobile, misfit-free grain boundaries formed at the intersection of “classical” compound twins.