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 ofthis boundary has aroused great interest since it may provide a definitive answer related to extremely lowtwinning 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 rationalcomponents relaxed by an arrangement of equally spaced disconnections. However, up to now, no one present aclear experimental evidence. This work shows that the macroscopic irrational character of the type II twinboundary is represented by two faceted rational components at the atomistic scale. Complementary, adislocation-free mechanism for easy twin boundary motion, including shuffle of atomic layers due to extremelysoft elastic response along the [110] direction, is proposed. The present model explains very weak or practicallynonexistent temperature sensitivity of twinning stress. The given concept enables breakthrough in technologicalapplications for a broad class of functional materials, including ferroelectrics and multiferroics, exhibiting almostzero lattice friction down to 1.7 K.