Extremely low stress and faceted nature of type II twinning in Ni-Mn-Ga shape memory alloys

The origin of extraordinarily low twinning stresses, at the level of 0.02 MPa, for type II twin boundaries in Ni-Mn-Ga single crystals has been a matter of discussion for at least three decades. In this sense, the atomic structure of this boundary has aroused great interest since it may provide a definitive answer related to extremely low twinning stress, practically no temperature dependence, irrational twinning plane, and a high migration rate. This points to fundamentally different dynamics of type II boundaries compared to other twin interconnections. For this reason, its irrational twin-plane character was supposed to decompose into arrays of facets with rational components relaxed by an arrangement of equally spaced disconnections. However, up to now, no one present a clear experimental evidence. This work shows that the macroscopic irrational character of the type II twin boundary is represented by two faceted rational components at the atomistic scale. Complementary, a dislocation-free mechanism for easy twin boundary motion, including shuffle of atomic layers due to extremely soft elastic response along the [ 1 ¯ 10] direction, is proposed. The present model explains very weak or practically nonexistent temperature sensitivity of twinning stress. The given concept enables breakthrough in technological applications for a broad class of functional materials, including ferroelectrics and multiferroics, exhibiting almost zero lattice friction down to 1.7 K.