The evolution of the martensitic transformation at the high-driving-force regime: A microsecond-scale time-resolved X-ray diffraction study

The reverse martensitic transformation proceeds through several sub-processes at various length and time scales. We study the transformation kinetics at the high thermodynamic driving force regime, by inducing a rapid heating-pulse in a shape memory alloy wire. We track the evolution of the transformation at the microsecond scale by multi-frame time-resolved X-ray diffraction at synchrotron radiation with simultaneous high-bandwidth force measurements. The x-ray probed a narrow region at the periphery of the wire, while the force was a measure for the transformation-induced stress in the bulk of the wire. We reveal three stages occurring at different times and length-scales in the material. During the first 5 μs the transformation is undetectable. Then, a rapid transformation occurs near the surface and saturates after 9 μs. The evolution of the transformation in the bulk becomes discernible only after the transformation near the surface completes and lasts for approximately 40 μs.