Microsecond-scale time-resolved XRD of the martensitic transformation in shape memory alloy wires induced by an abrupt superheating*

The martensitic transformation is a rapid displacive fundamental process in material science, specifically renowned for giving rise to the unique functional properties of shape memory alloys (SMA). Yet, little is known about how fast it can be under high driving forces. We study the transformation kinetics in the large thermodynamic driving force regime, by superheating an SMA wire via an abrupt high-voltage heating-pulse. We track the evolution of the transformation by microsecond-scale multi-frame time-resolved X-ray diffraction at synchrotron radiation with simultaneous high-bandwidth stress measurements. The diffraction data provides local measures for the austenite content with a temporal resolution of one microsecond. We reveal three stages occurring at different times and length-scales. During the dominant stage, a single phase-front propagates from the wire periphery inwards. Fitting a theoretical model to the measured results provides the kinetic relation between the velocity of the phase front and the driving force propagating it. The revealed high-specific-energy front releases energy faster than low-energy fronts forming under low driving forces.