*Invited* The Mechanical Response and Phase Transformation Kinetics of NiTi Under a Rapid Heating Pulse

Shape memory alloy actuators are very promising due to their large strain and work-output, but are considered to be very slow. In this study we demonstrate an ultra-fast one-directional actuation mode based on one-occasional heating of detwinned NiTi wires by an electric pulse of few microseconds. Recently, we significantly improved the experimental system to allow measuring both the force and displacement in the micro-second scale. In addition, high speed photography with up to 200,000 frames per second has been applied to track local shape changes of the wire.

The improved experimental system allows exploring a unique dynamic behavior, which has not been observed previously in studies of the shape memory effect. In particular, stress levels around 2 GPa and elastic strain rates of about 1000 1/s are obtained. A comparison of actuation performances demonstrates that our actuation experiments are significantly advantageous over other fast actuation methods in almost every actuation aspect reviewed.

The unique experimental conditions allow studying the kinetics of the reverse martensitic phase transformation under conditions at which it is not dictated neither by the kinematics of the experimental setup nor by the rate of heat transfer. Several different characteristic times are identified and their origin as either a material property or a structural (wire related) property is distinguished. In particular, a nucleation time of about 20 microseconds and a phase transformation characteristic time of about 30 microseconds are evaluated. In addition, the relation between the equilibrium stress and the temperature is studied. The measured equilibrium stress values are much higher than obtained in quasi-static tests and the source for this behavior is explained. Finally, we show how the phase transformation kinetic laws can be used for obtaining approximated design rules for the prediction of actuation performances under different conditions.