Velocity-dependent Twinning Stress in Magnetic Shape Memory Alloy Ni-Mn-Ga 5M Martensite

The investigation of twin boundary (TB) dynamics on the level of individual TBs has attracted recently more attention because the TB mobility dictates the dynamic properties of magnetic shape memory-based devices. In the present study, we investigated both type 1 and type 2 single TB motion in 5M Ni-Mn-Ga martensite under pulsed magnetic field actuation. A mathematical model was developed to describe the motion and retrieve the twinning stress (TS) values from the experimental data. We found a strong dependence of the dynamic TS on TB velocity and type. For type 1 interface, the TS drastically increases from its quasi-static value of ~0.6 MPa to ~3.2 MPa, thus limiting the TB velocity to 3-4 m/s measured in saturation field. Type 2 TBs show a gradual increase of the TS from ~0.1 MPa to ~2 MPa, reaching saturation velocities of ~33-39 m/s. Our results show that type 2 interfaces are preferable in applications demanding high actuation speeds and frequencies.