  High mobility of twin boundaries in modulated martensites of Ni-Mn-Ga-based
ferromagnetic shape memory alloys holds a promise for unique magnetomechanical
applications. This feature has not been fully understood so far, and in
particular it has yet not been unveiled what makes the lattice mechanics of
modulated Ni-Mn-Ga specifically different from other martensitic alloys. Here,
results of dedicated laser-ultrasonic measurements on hierarchically twinned
five-layer modulated (10 M) crystals fill this gap. Using a combination of
transient grating spectroscopy and laser-baser resonant ultrasound
spectroscopy, it is confirmed that there is a shear elastic instability in the
lattice, being significantly stronger than in any other martensitic material
and also than what the first-principles calculations for Ni-Mn-Ga predict. The
experimental results reveal that the instability is directly related to the
lattice modulations. A lattice-scale mechanism of dynamic faulting of the
modulation sequence that explains this behavior is proposed; this mechanism can
explain the extraordinary mobility of twin boundaries in 10 M.
