Transforming martensite in NiTi within nanoseconds

Abstract

Martensitic transformations enable various emerging applications like the shape memory effect and elastocaloric applications in NiTi. Increasing the speed of this transformation can shorten the response time for actuation and increase the power density of caloric cooling systems. Up to now, research on the speed and possible time limits of the martensitic transformation in NiTi has been limited to milli- and microsecond experiments. The dynamics of the transformation for shorter time scales are therefore unknown. Here, we report the fastest transformations in NiTi so far by heating an epitaxial NiTi film with a ns laser pulse and tracking the martensitic transition with in-situ synchrotron X-ray diffraction. We find that the martensite to austenite transition upon heating can proceed within the 7 ns pulse duration of the laser, but it requires substantial overheating as the rate of the transformation increases with the driving energy. The austenite to martensite transition is slower because cooling proceeds by conductive heat transfer, but with appropriate undercooling, the complete transformation from martensite to austenite and back only takes 200 ns. We compare our results to previous experiments on the Heusler alloy Ni-Mn-Ga and (K,Na)NbO3 and find very similar trends, which reveal that fast martensitic transformations in general follow a universal scaling law.