The martensitic transformation appears in many natural and engineering materials, and proceeds through several sub-processes at various length and time scales. We study the transformation kinetics in the large thermodynamic driving force regime, by inducing a rapid heating-pulse in a shape memory alloy wire. We track the evolution of the transformation by 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 third and 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.

Unraveling the rate of the martensitic transformation: A microsecond-scale time-resolved X-ray diffraction study

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