Twin Boundary Motion in Ni-Mn-Ga

Shilo, Doron

Twin boundary motion is the mechanism responsible for the unique magneto-mechanical response of the ferromagnetic shape memory NiMnGa. Therefore, a quantitative knowledge of the barriers for twin boundary motion, the mechanisms for overcoming them and the kinetic relations between the applied driving force and the resulted velocity of an individual twin boundary are required.

In this presentation, we first identify two different barriers for twin boundary motion:

(1) Long-scale (several tens of micrometers) barriers related to the internal microstructure, which result in a threshold below which there is no twin boundary motion, i.e. the twinning stress.

(2) The Peierls lattice barrier which governs the velocity of the twin boundary at driving force values above the twinning stress.

Our theoretical analysis predicts that twin boundary motion follows thermally activated kinetics at low driving force values and viscous type kinetics at higher driving force values. The transition between the two kinetic behaviors is associated with overcoming the Peierls lattice barrier.

Next, we present two unique methods for studying the twinning dynamics. The long-scale barriers are quantitatively evaluated based on analysis of quasistatic stress-strain curves by means of a new bi-stable chain model. The Peierls lattice barrier and the kinetic relations for twin boundary motion are studied by measuring the distance a boundary pass during the application of a tunable magnetic pulse with almost rectangular shape.

The formulated kinetic relations are validated experimentally, leading to quantitative extraction of all governing material parameters. In particular, nanoscale properties, such as the Peierls lattice barrier and the energy of twinning dislocations are obtained.

Finally, it is shown how the overall obtained information can be used for modeling the frequency response of NiMnGa actuators. In addition, we discuss the importance of the obtained information for the development of improved FSMA crystals for fast magnetically induced actuation applications.