Twin boundary mobility and twin microstructures: The MIR and related magnetomechanical effects are mediated by the motion of martensite twin boundaries [Ulllakko_1996, Heczko_MIR]. Consequently, the high mobility of twin boundaries or low twinning stress is the crucial property of MSMAs. The overall mobility and related material performance is controlled by two major factors i) by the intrinsic mobility of twin boundaries, ii) by twin-twin and twin-defect interaction and the arrangement of twin boundaries within the twin microstructure. The influence of the second point is often ignored, but it is very strong [Straka_APL_2010-1, Straka_tailoring]. It can change the apparent mobility more than tenfold, can alter the character of the response e.g. from magnetoplastic to magnetoelastic, or can prevent the MIR completely [Straka_JAP]. An important practical viewpoint is that the twin microstructure also heavily influence the fatigue life of the material. With particular microstructure, the fatigue life can be more than hundred times improved [Straka_actuator_2012, 2018_Zhang_Surface].
Illustration how twin microstructure control could drastically improve the actuating response in Ni-Mn-Ga MSMAs. Response of the material with type 2 twins is superior to that obtained in no-twin-control material or with type 1 twins.