The speed limit of martensitic transformations

Applications such as actuation by (magnetic) shape memory effects, magnetocaloric cooling and thermomagnetic energy harvesting benefit from a fast martensitic transformation, because using devices at high frequencies increases their power density. Still, data on the speed limit of theses transformations is scarce, as it is difficult to induce the phase transition in short time spans and measure it non-invasively. Up to now, shape memory wires have been heated with currents by Joule heating, which induces a transformation in as short as 20 µs. With magnetic pulses, magnetic shape memory alloys can be transformed within 13 ms. In both cases, however, transformation dynamics are limited by the duration of the current or magnetic pulse.
To increase knowledge about the speed limit of martensitic transformations, here, we heat a 500 nm thick epitaxial Ni-Mn-Ga film with a 7 ns long laser pulse and probe the martensite to austenite transformation with in situ synchrotron diffraction. With this experimental set-up, we can vary the heating rate by changing the laser fluence and additionally adjust the initial sample temperature with a furnace. We observe a linear dependence of the heating rate on the transformation speed. Thus, the transformation can be completed in as little as 10 ns; however, this requires an overheating of several hundreds of Kelvin. For most applications, only a much smaller overheating is feasible, which gives a switching time in the sub-microsecond range. The austenite to martensite transformation can be completed at least within hundreds of ns. This leaves plenty of room to speed up the performance of all applications based on martensitic transformations.